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

A brief review of preparation and applications of monolithic aerogels in atmospheric environmental purification

Monolithic aerogels are promising candidates for use in atmospheric environmental purification due to their structural advantages, such as fine building block size together with high specific surface area, abundant pore structure, etc. Additionally, monolithic aerogels possess a unique monolithic macrostructure that sets them apart from aerogel powders and nanoparticles in practical environmental clean-up applications. This review delves into the available synthesis strategies and atmospheric environmental applications of monolithic aerogels, covering types of monolithic aerogels including SiO2, graphene, metal oxides and their combinations, along with their preparation methods. In particular, recent developments for VOC adsorption, CO2 capture, catalytic oxidation of VOCs and catalytic reduction of CO2 are highlighted. Finally, challenges and future opportunities for monolithic aerogels in the atmospheric environmental purification field are proposed. This review provides valuable insights for designing and utilizing monolithic aerogel-based functional materials.

Alumina Incorporation in Self-Supported Poly(ethylenimine) Sorbents for Direct Air Capture

Self-supported branched poly(ethylenimine) scaffolds with ordered macropores are synthesized with and without Al2O3 powder additive by cross-linking poly(ethylenimine) (PEI) with poly(ethylene glycol) diglycidyl ether (PEGDGE) at -196 °C. The scaffolds' CO2 uptake performance is compared with a conventional sorbent, i.e., PEI impregnated on an Al2O3 support. PEI scaffolds with Al2O3 additive show narrow pore size distribution and thinner pore walls than alumina-free materials, facilitating higher CO2 uptake at conditions relevant to direct air capture. The PEI scaffold containing 6.5 wt % Al2O3 had the highest CO2 uptake of 1.23 mmol/g of sorbent under 50% RH 400 ppm of CO2 conditions. In situ DRIFT spectroscopy and temperature-programmed desorption experiments show a significant CO2 uptake contribution via physisorption as well as carbamic acid formation, with lower CO2 binding energies in PEI scaffolds relative to conventional PEI sorbents, likely a result of a lower population of primary amines due to the amine cross-linking reactions during scaffold synthesis. The PEI scaffold containing 6.5 wt % Al2O3 is estimated to have the lowest desorption energy penalty under humid conditions, 4.6 GJ/tCO2, among the sorbents studied.

Biomimetic Conductive Hydrogel Scaffolds with Anisotropy and Electrical Stimulation for In Vivo Skeletal Muscle Reconstruction

The nature of the hydrogel scaffold mimicking extracellular matrix plays a crucial role in tissue engineering like skeletal muscle repair. Herein, an anisotropic and conductive hydrogel scaffold is fabricated using gelatin methacryloyl (GelMA) as the matrix hydrogel and silver nanowire (AgNW) as the conductive dopant, through a directional freezing technique for muscle defect repair. The scaffold has an anisotropic structure composed of a directional longitudinal section and a honeycomb cross-section, with high mechanical strength of 10.5 kPa and excellent conductivity of 0.26 S m-1 . These properties are similar to native muscle extracellular matrix (ECM) and allow for cell orientation under the guidance of contact cues and electrical stimulation synergistically. In vitro experiments show that the scaffold's oriented structure combined with electrical stimulation results in enhanced myotube formation, with a length of up to 863 µm and an orientation rate of 81%. Furthermore, the electrically stimulated scaffold displays a promoted muscle reconstruction ability when transplanted into rats with muscle defects, achieving a muscle mass and strength restoration ratio of 95% and 99%, respectively, compared to normal levels. These findings suggest that the scaffold has great potential in muscle repair applications.

Structural investigations of Au-Ni aerogels: morphology and element distribution

The physical properties of nanomaterials are determined by their structural features, making accurate structural control indispensable. This carries over to future applications. In the case of metal aerogels, highly porous networks of aggregated metal nanoparticles, such precise tuning is still largely pending. Although recent improvements in controlling synthesis parameters like electrolytes, reductants, or mechanical stirring, the focus has always been on one particular morphology at a time. Meanwhile, complex factors, such as morphology and element distributions, are studied rather sparsely. We demonstrate the capabilities of precise morphology design by deploying Au-Ni, a novel element combination for metal aerogels in itself, as a model system to combine common aerogel morphologies under one system for the first time. Au-Ni aerogels were synthesized via modified one- and two-step gelation, partially combined with galvanic replacement, to obtain aerogels with alloyed, heterostructural (novel metal aerogel structure of interconnected nanoparticles and nanochains), and hollow spherical building blocks. These differences in morphology are directly reflected in the physisorption behavior, linking the isotherm shape and pore size distribution to the structural features of the aerogels, including a broad-ranging specific surface area (35-65 m2 g-1). The aerogels were optimized regarding metal concentration, destabilization, and composition, revealing some delicate structural trends regarding the ligament size and hollow sphere character. Hence, this work significantly improves the structural tailoring of metal aerogels and possible up-scaling. Lastly, preliminary ethanol oxidation tests demonstrated that morphology design extends to the catalytic performance. All in all, this work emphasizes the strengths of morphology design to obtain optimal structures, properties, and (performances) for any material application.

Chitosan-Based Hydrogels for Bioelectronic Sensing: Recent Advances and Applications in Biomedicine and Food Safety

Due to the lack of efficient bioelectronic interfaces, the communication between biology and electronics has become a great challenge, especially in constructing bioelectronic sensing. As natural polysaccharide biomaterials, chitosan-based hydrogels exhibit the advantages of flexibility, biocompatibility, mechanical tunability, and stimuli sensitivity, and could serve as an excellent interface for bioelectronic sensors. Based on the fabrication approaches, interaction mechanisms, and bioelectronic communication modalities, this review divided chitosan-based hydrogels into four types, including electrode-based hydrogels, conductive materials conjugated hydrogels, ionically conductive hydrogels, and redox-based hydrogels. To introduce the enhanced performance of bioelectronic sensors, as a complementary alternative, the incorporation of nanoparticles and redox species in chitosan-based hydrogels was discussed. In addition, the multifunctional properties of chitosan-based composite hydrogels enable their applications in biomedicine (e.g., smart skin patches, wood healing, disease diagnosis) and food safety (e.g., electrochemical sensing, smart sensing, artificial bioelectronic tongue, fluorescence sensors, surface-enhanced Raman scattering). We believe that this review will shed light on the future development of chitosan-based biosensing hydrogels for micro-implantable devices and human-machine interactions, as well as potential applications in medicine, food, agriculture, and other fields.

Recent progress in fiber-based soft electronics enabled by liquid metal

Soft electronics can seamlessly integrate with the human skin which will greatly improve the quality of life in the fields of healthcare monitoring, disease treatment, virtual reality, and human-machine interfaces. Currently, the stretchability of most soft electronics is achieved by incorporating stretchable conductors with elastic substrates. Among stretchable conductors, liquid metals stand out for their metal-grade conductivity, liquid-grade deformability, and relatively low cost. However, the elastic substrates usually composed of silicone rubber, polyurethane, and hydrogels have poor air permeability, and long-term exposure can cause skin redness and irritation. The substrates composed of fibers usually have excellent air permeability due to their high porosity, making them ideal substrates for soft electronics in long-term applications. Fibers can be woven directly into various shapes, or formed into various shapes on the mold by spinning techniques such as electrospinning. Here, we provide an overview of fiber-based soft electronics enabled by liquid metals. An introduction to the spinning technology is provided. Typical applications and patterning strategies of liquid metal are presented. We review the latest progress in the design and fabrication of representative liquid metal fibers and their application in soft electronics such as conductors, sensors, and energy harvesting. Finally, we discuss the challenges of fiber-based soft electronics and provide an outlook on future prospects.

Self-assembly, alignment, and patterning of metal nanowires

Armed with the merits of one-dimensional nanostructures (flexibility, high aspect ratio, and anisotropy) and metals (high conductivity, plasmonic properties, and catalytic activity), metal nanowires (MNWs) have stood out as a new class of nanomaterials in the last two decades. They are envisaged to expedite significantly and even revolutionize a broad spectrum of applications related to display, sensing, energy, plasmonics, photonics, and catalysis. Compared with disordered MNWs, well-organized MNWs would not only enhance the intrinsic physical and chemical properties, but also create new functions and sophisticated architectures of optoelectronic devices. This paper presents a comprehensive review of assembly strategies of MNWs, including self-assembly for specific structures, alignment for anisotropic constructions, and patterning for precise configurations. The technical processes, underlying mechanisms, performance indicators, and representative applications of these strategies are described and discussed to inspire further innovation in assembly techniques and guide the fabrication of optoelectrical devices. Finally, a perspective on the critical challenges and future opportunities of MNW assembly is provided.

Sources, Chemical Functionalization, and Commercial Applications of Nanocellulose and Nanocellulose-Based Composites: A Review

Nanocellulose is the most abundant material extracted from plants, animals, and bacteria. Nanocellulose is a cellulosic material with nano-scale dimensions and exists in the form of cellulose nanocrystals (CNC), bacterial nanocellulose (BNC), and nano-fibrillated cellulose (NFC). Owing to its high surface area, non-toxic nature, good mechanical properties, low thermal expansion, and high biodegradability, it is obtaining high attraction in the fields of electronics, paper making, packaging, and filtration, as well as the biomedical industry. To obtain the full potential of nanocellulose, it is chemically modified to alter the surface, resulting in improved properties. This review covers the nanocellulose background, their extraction methods, and possible chemical treatments that can enhance the properties of nanocellulose and its composites, as well as their applications in various fields.

Hybrid Silver Nanowire-CMC Aerogels: From 1D Nanomaterials to 3D Electrically Conductive and Mechanically Resistant Lightweight Architectures

The directed assembly of nanomaterials into 3D architectures is a powerful tool to produce macroscopic materials with tailored physical properties. We show in this article that such a process can be advantageously performed for the fabrication of lightweight electrically conductive materials. Silver nanowire aerogels (AgNWAs) with very low densities (down to ∼6 mg cm^-3) were ice-templated and freeze-dried, leading to 3D shaped cellular materials based on one-dimensional nanoscopic building blocks. Due to their intrinsic moderate mechanical resistance, the potential use of pure AgNWAs in real life applications appears rather limited. We demonstrate that the addition of carboxymethylcellulose (CMC) in a 1:1 weight ratio leads to the fabrication of hybrid aerogels with highly improved mechanical properties. The molecular weight of the CMC is shown to be a critical parameter to ensure a good dispersion of the AgNWs, and thus to reach excellent performances such as a very low resistivity (0.9 ± 0.2 Ω·cm at 99.2 vol % porosity). The combination of silver nanowires with CMC-700k results in a gain higher than 7100% of the Young's modulus, from 10.4 ± 0.9 kPa (at very low density, i.e., 12 mg cm^-3) for the AgNWAs to 740 ± 40 kPa for the AgNW:CMC aerogel. Electromechanical characterizations allowed us to quantify the piezoelectric properties of these hybrid aerogels. The very good elasticity and the piezoelectric behavior stability up to 100 cycles of compression under high (50%) deformation were revealed, which may be of interest for various applications such as pressure sensors.

Ultra-light 3D MnO2-agar network with high and longevous performance for catalytic formaldehyde oxidation

Under the background of indoor air formaldehyde decontamination, a freestanding ultra-light assembly was fabricated via ice-templating approach starting from MnO₂ nanoparticles and environmentally benign agar powder. The 3D composite of agar and MnO₂ (AM-3D) was comparatively studied with powdered counterparts (including pure MnO₂ and mixture of agar and MnO₂) and the 3D-structured agar for formaldehyde oxidation, and their physicochemical properties were examined with XRD, ATR, SEM, XPS, isothermal N₂ adsorption, ESR, Raman, CO-TPR and O₂-TPD. For the single test of formaldehyde oxidation, the AM-3D catalyst exhibited 62.0%-67.0% removal percentage for ~400 mg/m³ formaldehyde, which did not demonstrate significant advantage over the control samples. However, thanks to the porous 3D agar scaffold with large spatial volume that could promote a rapid gas-phase formaldehyde concentration reduction, and the strong interaction between the dispersed MnO₂ particles and agar substrate that could afford a large amount of reactive oxygen species to further oxidize the adsorbed formaldehyde, the AM-3D composite was a much better HCHO-to-CO₂ converter and possessed much more advantageous stability for repeated cycles of formaldehyde oxidation: even after ten cycles, there was still 41.7% of formaldehyde removed. Furthermore, the viable sunlight irradiation could easily restore the activity of the used AM-3D catalyst back to the level approaching that of the fresh one. Finally, reaction pathways were put forward via the infrared spectroscopic and ion chromatographic investigations on the surface intermediates of the spent materials.

Recent Development of Morphology-Controlled Hybrid Nanomaterials for Triboelectric Nanogenerator: A Review

Being cognizant of modern electronic devices, the scientists are continuing to investigate renewable green-energy resources for a decade. Amid different energy harvesting systems, the triboelectric nanogenerators (TENGs) have been found to be the most promising mechanical harvesting technology and have drawn attention to generate electrical energy. Thanks to its instant output power, choice to opt for wide-ranging materials, low maintenance cost, easy fabrication process and environmentally friendly nature. Due to numerous working modes of TENGs, it is dedicated to desired application at ambient conditions. In this review, an advance correlation of TENGs have been explained based on the variety of nanostructures, including 0D, 1D, 2D, 3D, metal organic frameworks (MOFs), coordination polymers (CPs), covalent organic frameworks (COFs), and perovskite materials. Moreover, an overview of previous and current perspectives of various nanomaterials, synthesis, fabrication and their applications in potential fields have been discussed in detail.

Self-Assembly of Nanowires: From Dynamic Monitoring to Precision Control

ConspectusNatural biomaterials often show ordered nanowire structures (ONWS) which display unique structural color or superior mechanical performance. Meanwhile, plenty of modern nanodevices with ONWS have flourished with activities focused on both basic and applied research. Manipulating synthetic nanowire (NW) from a disordered state to a hierarchically ordered structure via various assembly strategies brings about intriguing and exotic chemical/physical properties. In the past decades, many methods have been developed to assemble NWs and fabricate organized architectures, such as Langmuir-Blodgett interfacial assembly, spin-coating assembly, fluid-flow-induced assembly, and ice-template assembly. Nevertheless, for practical applications, large-scale and high-efficiency assembly strategies toward precise controlled architectures are largely limited by the lack understanding of assembly mechanisms. Especially, the manipulation principles and driving forces behind the state-of-art assembly strategies are still unclear. Besides, the lesser research attention on dynamic kinetics also impedes the revelation of the NW self-assembly mechanism. With the emergence of advanced in situ techniques, such as synchrotron-based X-ray techniques and in situ transmission electron microscopy (TEM), the dynamic monitoring of NW behavior in many practical environments becomes possible. In addition, the alignment direction and the stacking manner of NW film are of significance to the final performance. There is a lack of connection between the properties of one-dimensional nanoscale building blocks and the functionalities of the macro-assembly structures. To this end, dynamic monitoring is highly desired, which enables the precision modulation of NW assembly structure, leading to the discovery or prediction of new structures, novel properties, and performance optimization.In this Account, we aim to uncover the underlying kinetics of NW assembly or local reaction and mass transportation processes, as well as to build a solid connection from individual NWs to NW assembly structures with enhanced properties and eventually to macroscopic materials application. We first review the recent progress in state-of-art NW assembly strategies for diverse aligned structures according to the manipulation principle and the driving forces. To systematically review the NW self-assembly strategies, we categorize these strategies into three states: NWs on the liquid interface via surface tension, NW assembly in liquid via solution-shearing flow field, and NW assembly at the solid interval via physical repulsive force. Then, we introduce the existing advanced characterization techniques, including synchrotron-based X-ray scattering and in situ TEM, to dynamically monitor the intermediate states of the NW assembly and transport processes. The comprehensive understanding of this thermodynamic and kinetic mechanism facilitates the rational design, large scale, and high-efficiency fabrication of NW assemblies, thus promoting their applications in tailored optical-electrical electronics, smart electrochromic devices, electrocatalysis, structural materials, and chiral photonic crystals.

Bioinspired Techniques in Freeze Casting: A Survey of Processes, Current Advances, and Future Directions

Freeze casting, popularly known as ice templating or freeze gelation, is a mechanical method to fabricate scaffolds of desirable properties and materials. Aerospace engineering, the healthcare sector, manufacturing department, and automotive industries are the different fields where freeze casting has been used. Bioinspiration refers to the translation of biological systems into new and innovative creations. Bioinspired materials are extensively used in freeze casting methods such as ceramide, spines of porcupine fish, and collagen. Due to the tunable properties and production of complex structures with ease, biomaterials have found numerous applications in the ice templating method. This review rigorously explains the freeze casting process and the effect of thermal conductivity, stress, and electrostatic repulsion on the porous materials. Also, we have discussed the different biomaterial polymers used in freeze casting along with different methods involved.

Ultrastretchable and Self-Healing Conductors with Double Dynamic Network for Omni-Healable Capacitive Strain Sensors

Skin-mountable capacitive-type strain sensors with great linearity and low hysteresis provide inspiration for the interactions between human and machine. For practicality, high sensing performance, large stretchability, and self-healing are demanded but limited by stretchable electrode and dielectric and interfacial compatibility. Here, we demonstrate an extremely stretchable and self-healing conductor via both hard and soft tactics that combine conductive nanowire assemblies with double dynamic network based on π-π attractions and Ag-S coordination bonds. The obtained conductor outperforms the reported stretchable conductors by delivering an elongation of 3250%, resistance change of 223% at 2000% strain, high durability, and multiresponsive self-healability. Especially, this conductor accommodates large strain of 1500% at extremely knotted and twisted deformations. By sandwiching hydrogel conductors with a newly developed dielectric, ultrahigh stretchability and omni-healability are simultaneously achieved for the first time for a capacitive strain sensor inspired by metal-thiolate coordination chemistry, showing great potentials in wearable electronics and soft robotics.

Synthesis of Free-Standing Silver Foam via Oriented and Additive Nanojoining

Silver foams with high porosity and electrical conductivity have many potential applications in energy storage, catalysis, and fuel cells. However, its application is largely hindered by the low efficiency of complicated synthesis processes. In this work, a facile and rapid bottom-up fabrication of silver foams in an aqueous solution allowing large-scale production through oriented and additive nanojoining of silver nanoplate building blocks is reported. Self-assembling of as-grown silver nanoplates facilitates the oriented nanoscale joining to align the atomic lattice, and the local additive of silver promotes diffusion and interconnection at room temperature to realize a rapid synthesis process. The freeze-dried silver foam exhibits a porosity of 95.45%, an ultralow density of 61 mg·cm^-3, low thermal conductivity of 0.29 W·m^-1·K^-1, and high electrical conductivity of 8086 S·m^-1. This oriented and locally additive nanojoining process presents a new strategy to fabricate silver foams that may also inspire the fabrications of other metal foams.

Kirigami-Inspired Highly Stretchable, Conductive, and Hierarchical Ti3C2Tx MXene Films for Efficient Electromagnetic Interference Shielding and Pressure Sensing

Although Ti₃C₂T_x MXene sheets are highly conductive, it is still a challenge to design highly stretchable MXene electrodes for flexible electronic devices. Inspired by the high stretchability of kirigami patterns, we demonstrate a bottom-up methodology to design highly stretchable and conductive polydimethylsiloxane (PDMS)/Ti₃C₂T_x MXene films for electromagnetic interference (EMI) shielding and pressure sensing applications by constructing wrinkled MXene patterns on a flexible PDMS substrate to create a hierarchical surface with primary and secondary surface wrinkles. The self-controlled microcracks created in the valley domains of the hierarchical film via a nonuniform deformation during prestretching/releasing cycles endow the hierarchical PDMS/MXene film with a high stretchability (100%), strain-invariant conductivity in a strain range of 0%-100%, and stable conductivities over an 1000-cycle fatigue measurement. The stretchable film exhibits a highly stable EMI shielding performance of ≈30 dB at a tensile strain of 50%, and its EMI shielding efficiency increases further to 103 dB by constructing a two-film structure. Furthermore, a highly stretchable and sensitive iontronic sensor array with integrated MXene-based electrodes and circuits is fabricated by a stencil printing process, exhibiting high sensitivity (66.3 nF kPa^-1), excellent dynamic cycle stability over 1000 cycles under different frequencies, and sensitive pressure monitoring capability under a tensile strain of 50%.

Recent Advances in Natural Functional Biopolymers and Their Applications of Electronic Skins and Flexible Strain Sensors

In order to replace nonrenewable resources and decrease electronic waste disposal, there is a rapidly rising demand for the utilization of reproducible and degradable biopolymers in flexible electronics. Natural biopolymers have many remarkable characteristics, including light weight, excellent mechanical properties, biocompatibility, non-toxicity, low cost, etc. Thanks to these superior merits, natural functional biopolymers can be designed and optimized for the development of high-performance flexible electronic devices. Herein, we provide an insightful overview of the unique structures, properties and applications of biopolymers for electronic skins (e-skins) and flexible strain sensors. The relationships between properties and sensing performances of biopolymers-based sensors are also investigated. The functional design strategies and fabrication technologies for biopolymers-based flexible sensors are proposed. Furthermore, the research progresses of biopolymers-based sensors with various functions are described in detail. Finally, we provide some useful viewpoints and future prospects of developing biopolymers-based flexible sensors.

Polymer-Assisted Fabrication of Silver Nanowire Cellular Monoliths: Toward Hydrophobic and Ultraflexible High-Performance Electromagnetic Interference Shielding Materials

Metal nanofibers with excellent electrical conductivity and superior mechanical flexibility have great potentials for fabrication of lightweight, flexible, and high-performance electromagnetic interference (EMI) shielding architectures. The weak interactions and large contact resistance among the wires, however, hinder their assembly into robust and high-performance EMI shielding monoliths. In this work, we used low fractions of polymers to assist the construction of lightweight, flexible, and highly conductive silver nanowire (AgNW) cellular monoliths with significantly enhanced mechanical strength and EMI shielding effectiveness (SE). The normalized surface specific SE of our AgNW-based cellular monoliths can reach up to 20522 dB·cm²/g, outracing that of most shielding materials ever reported. Moreover, this robust conductive framework enabled the successful fabrication of hydrophobic, ultraflexible, and highly stretchable aerogel/polymer composites with outstanding EMI SE even at an extremely low AgNW content. Thus, this work demonstrated a facile and efficient strategy for assembling metal nanofiber-based functional high-performance EMI shielding architectures.

Highly conductive, stretchable, and breathable epidermal electrode based on hierarchically interactive nano-network

Stretchable electrodes have a crucial impact on the development of flexible electronic systems. Most conventionally blended nanocomposite electrodes are incapable of achieving high stretchability, breathability, or durability. In this work, a highly conductive, breathable, and stretchable epidermal electrode (SEE) is demonstrated by designing a hierarchically interactive nano-network that is composed of elastic polymer nano-fibers and multi-level silver nano-wires (AgNWs). The elastic polymer nano-fibers act as a continuous scaffold, and multi-level AgNWs embedded in the nano-fibers form branched conductive pathways. This structure enables high conductivity of the SEE at 4800 S cm-1 (at a significantly low AgNW content of 1.59 vt%), with high stretchability and excellent durability. For example, the SEE remained conductive even at a high strain of 500%, and it also maintained its initial resistance even after 30 000 cycles of strain at 50% or being washed with water for 100 000 cycles. The SEE was prepared by a facile in situ nonequilibrium fabrication process, and can easily be produced into an elastic circuit on a large scale, which provides a foundation for integrated and multifunctional electronic skins. The SEE possesses superior mechanical conformability and permeability of gas and liquid, and therefore, it was successfully applied in measuring electrocardiogram signals and thermal therapy, and exhibited highly robust and comfortable performances even while being washed with water or undergoing complex deformations.

Wood-Inspired Anisotropic Cellulose Nanofibril Composite Sponges for Multifunctional Applications

Nanocellulose-based porous materials have been recently considered as ideal candidates in various applications. However, challenges on performances remain owing to the disorderly structure and the limited transport specificity. Herein, wood-inspired composite sponges consisting of cellulose nanofibrils (CNFs) and high-aspect-ratio silver nanowires (AgNWs) were generated with anisotropic properties by the directional freeze-drying. The obtained composite sponges exhibited attractive features, such as an excellent compressive stress of 24.5 kPa, low percolation threshold of 0.1 vol % AgNWs, and high electrical conductivity of 1.52 S/cm. Furthermore, the self-assembled ordered structure in the longitudinal direction and synergistic effect between CNFs and AgNWs benefited the sponge interesting anisotropic electrical conductivity, thermal diffusivity, ultrafast electrically induced heating (<5 s), sensitive pressure sensing (errors <0.26%), and electromagnetic interference (EMI) shielding for special practical demands. This multifunctional material inspired by natural woods is expected to broaden new applications as electronic devices for an intelligent switch or EMI shielding.

Ice-assisted synthesis of functional nanomaterials: the use of quasi-liquid layers as nanoreactors and reaction accelerators

The discovery of peculiar quasi-liquid layers on ice surfaces marks a major breakthrough in ice-related sciences, as the facile tuning of the reactions and morphologies of substances in contact with these layers make ice-assisted chemistry a low-cost, environmentally benign, and ubiquitous methodology for the synthesis of nanomaterials with improved functionality. Ice-templated synthesis of porous materials offers the appealing features of rapid self-organization and remarkable property changes arising from confinement effects and affords materials that have found a diverse range of applications such as batteries, supercapacitors, and gas separation. Moreover, much attention has been drawn to the acceleration of chemical reactions and transformations on the ice surface due to the freeze concentration effect, fast self-diffusion of surface water, and modulated surface potential energy. Some of these results are related to the accumulation of inorganic contaminants in glaciers and the blockage of natural gas pipelines. As an emerging theme in nanomaterial design, the dimension-controlled synthesis of hybrid materials with unprecedentedly enhanced properties on ice surfaces has attracted much interest. However, a deep understanding of quasi-liquid layer characteristics (and hence, the development of cutting-edge analytical technologies with high surface sensitivity) is required to achieve the current goal of ice-assisted chemistry, namely the preparation of tailor-made materials with the desired properties.

Anisotropic and hierarchical SiC@SiO2 nanowire aerogel with exceptional stiffness and stability for thermal superinsulation

Ceramic aerogels are promising lightweight and high-efficient thermal insulators for applications in buildings, industry, and aerospace vehicles but are usually limited by their brittleness and structural collapse at high temperatures. In recent years, fabricating nanostructure-based ultralight materials has been proved to be an effective way to realize the resilience of ceramic aerogels. However, the randomly distributed macroscale pores in these architectures usually lead to low stiffness and reduced thermal insulation performance. Here, to overcome these obstacles, a SiC@SiO₂ nanowire aerogel with a nanowire-assembled anisotropic and hierarchical microstructure was prepared by using directional freeze casting and subsequent heat treatment. The aerogel exhibits an ultralow thermal conductivity of ~14 mW/m·K, an exceptional high stiffness (a specific modulus of ~24.7 kN·m/kg), and excellent thermal and chemical stabilities even under heating at 1200°C by a butane blow torch, which makes it an ideal thermally superinsulating material for applications under extreme conditions.

Flexible and Ultrathin Waterproof Cellular Membranes Based on High-Conjunction Metal-Wrapped Polymer Nanofibers for Electromagnetic Interference Shielding

Ultrathin, lightweight, and flexible electromagnetic interference (EMI) shielding materials are urgently demanded to address EM radiation pollution. Efficient design to utilize the shields' microstructures is crucial yet remains highly challenging for maximum EMI shielding effectiveness (SE) while minimizing material consumption. Herein, novel cellular membranes are designed based on a facile polydopamine-assisted metal (copper or silver) deposition on electrospun polymer nanofibers. The membranes can efficiently exploit the high-conjunction cellular structures of metal and polymer nanofibers, and their interactions for excellent electrical conductivity, mechanical flexibility, and ultrahigh EMI shielding performance. EMI SE reaches more than 53 dB in an ultra-broadband frequency range at a membrane thickness of merely 2.5 µm and a density of 1.6 g cm^-3 , and an SE of 44.7 dB is accomplished at the lowest thickness of 1.2 µm. The normalized specific SE is up to 232 860 dB cm² g^-1 , significantly surpassing that of other shielding materials ever reported. More, integrated functionalities are discovered in the membrane, such as antibacterial, waterproof properties, excellent air permeability, high resistance to mechanical deformations and low-voltage uniform heating performance, offering strong potential for applications in aerospace and portable and wearable smart electronics.

A review of multifunctional nacre-mimetic materials based on bidirectional freeze casting

Nacre has achieved an excellent combination of strength and toughness through its unique brick-and-mortar structure of layered aragonite platelets bonded with biopolymers. Mimicking nacre has been considered as a practical way for the development of high-performance structural composites. Over the past years, many techniques have been developed to fabricate multifunctional nacre-mimetic materials, including freeze casting, layer-by-layer assembly, vacuum filtration, 3D printing and so on. Among them, freeze casting, especially bidirectional freeze casting, as an environmentally friendly and scalable method, has attracted extensive attention recently. In this review, we begin with the introduction and discussion of various fabrication techniques comparing their advantages and disadvantages, focusing on the most recent advances of the bidirectional freeze casting technique. Then, we summarize representative examples of applying the bidirectional freeze casting technique to assemble various building blocks into multifunctional nacre-mimetic materials and their wide applications. At the end, we discuss the future direction of using bidirectional freeze casting to make nacre-mimetic materials.

Freeze-Thaw-Promoted Fabrication of Clean and Hierarchically Structured Noble-Metal Aerogels for Electrocatalysis and Photoelectrocatalysis

Noble‐metal aerogels (NMAs) have drawn increasing attention because of their self‐supported conductive networks, high surface areas, and numerous optically/catalytically active sites, enabling their impressive performance in diverse fields. However, the fabrication methods suffer from tedious procedures, long preparation times, unavoidable impurities, and uncontrolled multiscale structures, discouraging their developments. By utilizing the self‐healing properties of noble‐metal aggregates, the freezing‐promoted salting‐out behavior, and the ice‐templating effect, a freeze–thaw method is crafted that is capable of preparing various hierarchically structured noble‐metal gels within one day without extra additives. In light of their cleanliness, the multi‐scale structures, and combined catalytic/optical properties, the electrocatalytic and photoelectrocatalytic performance of NMAs are demonstrated, which surpasses that of commercial noble‐metal catalysts.

Freeze Casting: From Low-Dimensional Building Blocks to Aligned Porous Structures-A Review of Novel Materials, Methods, and Applications

Freeze casting, also known as ice templating, is a particularly versatile technique that has been applied extensively for the fabrication of well-controlled biomimetic porous materials based on ceramics, metals, polymers, biomacromolecules, and carbon nanomaterials, endowing them with novel properties and broadening their applicability. The principles of different directional freeze-casting processes are described and the relationships between processing and structure are examined. Recent progress in freeze-casting assisted assembly of low dimensional building blocks, including graphene and carbon nanotubes, into tailored micro- and macrostructures is then summarized. Emerging trends relating to novel materials as building blocks and novel freeze-cast geometries-beads, fibers, films, complex macrostructures, and nacre-mimetic composites-are presented. Thereafter, the means by which aligned porous structures and nacre mimetic materials obtainable through recently developed freeze-casting techniques and low-dimensional building blocks can facilitate material functionality across multiple fields of application, including energy storage and conversion, environmental remediation, thermal management, and smart materials, are discussed.

Multiscale Soft-Hard Interface Design for Flexible Hybrid Electronics

Emerging next-generation soft electronics will require versatile properties functioning under mechanical compliance, which will involve the use of different types of materials. As a result, control over material interfaces (particularly soft/hard interfaces) has become crucial and is now attracting intensive worldwide research efforts. A series of material and structural interface designs has been devised to improve interfacial adhesion, preventing failure of electromechanical properties under mechanical deformation. Herein, different soft/hard interface design strategies at multiple length scales in the context of flexible hybrid electronics are reviewed. The crucial role of soft ligands and/or polymers in controlling the morphologies of active nanomaterials and stabilizing them is discussed, with a focus on understanding the soft/hard interface at the atomic/molecular scale. Larger nanoscopic and microscopic levels are also discussed, to scrutinize viable intrinsic and extrinsic interfacial designs with the purpose of promoting adhesion, stretchability, and durability. Furthermore, the macroscopic device/human interface as it relates to real-world applications is analyzed. Finally, a perspective on the current challenges and future opportunities in the development of truly seamlessly integrated soft wearable electronic systems is presented.

Ultralight, Flexible, and Biomimetic Nanocellulose/Silver Nanowire Aerogels for Electromagnetic Interference Shielding

Ultralight and highly flexible biopolymer aerogels, composed of biomimetic cellular microstructures formed from cellulose nanofibers and silver nanowires, are assembled via a convenient and facile freeze-casting method. The lamellar, honeycomb-like, and random porous scaffolds are successfully achieved by adjusting freezing approaches to modulate the relationships between microstructures and macroscopic mechanical and electromagnetic interference (EMI) shielding performances. Combining the shielding transformation arising from in situ compression and the controlled content of building units, the optimized lamellar porous biopolymer aerogels can show a very high EMI shielding effectiveness (SE), which exceeds 70 or 40 dB in the X-band while the density is merely 6.2 or 1.7 mg/cm³, respectively. The corresponding normalized surface specific SE (defined as the SE divided by the material density and thickness) is up to 178235 dB·cm²/g, far surpassing that of the so-far reported shielding materials. Antibacterial properties and hydrophobicity are also demonstrated extending the versatility and application potential of the biopolymer hybrid aerogels.

Superelastic EGaIn Composite Fibers Sustaining 500% Tensile Strain with Superior Electrical Conductivity for Wearable Electronics

Stretchable conductive fibers have gained significant attention in the field of wearable and flexible electronics because of their inherited unique properties. Up to now, there are few reports regarding the highly stretchable fibers with excellent electronic properties. In this work, a highly stretchable fiber with superior electrical conductivity is fabricated, which contains a core fiber, an intermediate modified layer, and an outer eutectic-gallium-indium liquid metal layer. The fiber demonstrates an excellent electrical conductivity of over 10³ S cm^-1 when stretched up to 500% strain, which is far superior to the existing stretchable conductive fiber. The stretchable conductive fiber shows excellent thermostability with a maximum operating temperature of nearly 250 °C. Such unique fibers can be applied as highly stretchable, deformable conductor to charge a mobile phone, and sensor to monitor human activities. This work offers promising application in the areas of flexible and wearable electronics.

Carbon Microtube Aerogel Derived from Kapok Fiber: An Efficient and Recyclable Sorbent for Oils and Organic Solvents

A carbon microtube aerogel (CMA) with hydrophobicity, strong adsorption capacity, and superb recyclability was obtained by a feasible approach with economical raw material, such as kapok fiber. The CMA possesses a great adsorption capacity of 78-348 times its weight. Attributed to its outstanding thermal stability and excellent mechanical properties, the CMA can be used for many cycles of distillation, squeezing, and combustion without degradation, which suggests a potential practical application in oil-water separation. In addition, the adsorption capacity still retained 98% by distillation, 97% by squeezing, and 90% by combustion after 10 cycles. Therefore, the obtained CMA has a broad prospect as an economical, efficient, and environmentally friendly adsorbent.

Promoting the Electrocatalytic Performance of Noble Metal Aerogels by Ligand-Directed Modulation

Noble metal aerogels (NMAs) are an emerging class of porous materials. Embracing nano-sized highly-active noble metals and porous structures, they display unprecedented performance in diverse electrocatalytic processes. However, various impurities, particularly organic ligands, are often involved in the synthesis and remain in the corresponding products, hindering the investigation of the intrinsic electrocatalytic properties of NMAs. Here, starting from laser-generated inorganic-salt-stabilized metal nanoparticles, various impurity-free NMAs (Au, Pd, and Au-Pd aerogels) were fabricated. In this light, we demonstrate not only the intrinsic electrocatalytic properties of NMAs, but also the prominent roles played by ligands in tuning electrocatalysis through modulating the electron density of catalysts. These findings may offer a new dimension to engineer and optimize the electrocatalytic performance for various NMAs and beyond.

Nanosheet-assembled microflower-like coordination polymers by surfactant-assisted assembly with enhanced catalytic activity

Nanosheet-assembled microflower-like coordination polymers were successfully prepared by a surfactant-assisted approach and used as an efficient heterogeneous catalyst to execute the conversion reactions of nitromethylbenzenes into benzoic acids.

Enhanced Photothermal Conversion by Hot-Electron Effect in Ultrablack Carbon Aerogel for Solar Steam Generation

Ecofriendly, highly effective, and low-cost solar steam generation has great potential in the applications of power generation, seawater desalination, and industry wastewater treatment. Solar steam generation requires an evaporator that has strong light absorption over a wide-frequency band (200-3000 nm), high photothermal conversion efficiency, and good thermal insulation to avoid excessive heat loss. Herein, foam-strengthened ultrablack carbon aerogels (CAs) with micropores, mesopores, and macropores were prepared using freeze drying. The small-size conductor effect in ultrablack CAs could increase the surface electron concentration, leading to the increase of light absorption and ultimately enhancing photothermal conversion efficiency. Consequently, under 1 sun illumination, dried CA-5 exhibited a fast temperature rise rate and the highest thermal equilibrium temperature of 87.6 °C among CAs, the equilibrium temperature is 20.8 °C higher than that of the foam. Besides, CA-5 exhibited a high water evaporation rate of 1.29 kg m^-2 h^-1 under only 1 sun illumination. To further increase the hot-electron effect, CO₂ activation was implemented. The highest water evaporation rate among activated CAs (ACAs) was 1.37 kg m^-2 h^-1, which is about 2.85 times higher than that of pure water. The highest water evaporation efficiency was 87.51%, which is better than most of the previously reported evaporation efficiencies. Besides the hot-electron enhancement effect, ACAs with thermal insulation, mechanical strength, and thermal stability show great potential for producing fresh water from seawater, industrial wastewater, and even concentrated acidic or alkaline solutions.

Plasmon-coupled 3D porous hotspot architecture for super-sensitive quantitative SERS sensing of toxic substances on real sample surfaces

This paper reports a facile, fast, and cost-effective method for the synthesis of three-dimensional (3D) porous AgNPs/Cu composites as SERS substrates for the super-sensitive and quantitative detection of food organic contaminations. Due to the 3D porous hotspot architecture and the strong plasmonic coupling between Ag and Cu, the porous AgNPs/Cu substrate achieves ultrasensitive detection of multiple analytes as low as 10^-11 M (crystal violet, CV), 10^-9 M (malachite green, MG), 10^-11 M (acephate), and 10^-9 M (thiram) even with a portable Raman device. Moreover, this 3D solid substrate has good signal uniformity (RSD < 11%) and superior stability (<14% signal loss), allowing for practical SERS detections. Importantly, by simply wiping the real sample surface using the substrate, it successfully detects CV and MG residues on crayfish, and the limit of detection (LOD) of CV and MG is determined to be 1.14 × 10^-9 M and 0.94 × 10^-7 M, respectively. Further, the substrate can also be applied to detect acephate on eggplant with a LOD of 1.41 × 10^-9 M and thiram on an apple surface with a LOD of 1.04 × 10^-7 M. Note that all these SERS detections on real samples have a broad dynamic concentration range and a good linear dependence. As a "proof of concept", multi-component detection on a real sample has also been demonstrated. This 3D solid substrate possesses excellent detection sensitivity, diversity, and accuracy, which allows rapid and reliable determination of toxic substances in foods.

Highly Compressive Boron Nitride Nanotube Aerogels Reinforced with Reduced Graphene Oxide

Boron nitride nanotubes (BNNTs), structural analogues of carbon nanotubes, have attracted significant attention due to their superb thermal conductivity, wide bandgap, excellent hydrogen storage capacity, and thermal and chemical stability. Despite considerable progress in the preparation and surface functionalization of BNNTs, it remains a challenge to assemble one-dimensional BNNTs into three-dimensional (3D) architectures (such as aerogels) for practical applications. Here, we report a highly compressive BNNT aerogel reinforced with reduced graphene oxide (rGO) fabricated using a freeze-drying method. The reinforcement effect of rGO and 3D honeycomb-like framework offer the BNNTs/rGO aerogel with a high compression resilience. The BNNTs/rGO aerogels were then infiltrated with polyethylene glycol to prepare a kind of phase change materials. The prepared phase change material composites show zero leakage even at 100 °C and enhanced thermal conductivity, due to the 3D porous structure of the BNNTs/rGO aerogel. This work provides a simple method for the preparation of 3D BNNTs/rGO aerogels for many potential applications, such as high-performance polymer composites.

Enhanced arsenite removal from water by radially porous Fe-chitosan beads: Adsorption and H2O2 catalytic oxidation

Although Fe-chitosan adsorbents are attractive for removing arsenite from water, the practical applications of these granular adsorbents are mainly limited by slow adsorption kinetics. In this study, radially porous Fe-chitosan beads (P/Fe-CB) were prepared using freeze-casting technique. The P/Fe-CB particles possess radially aligned micron-sized tunnels from the surface to the inside as well as excellent acid resistance. Kinetic studies show that the adsorption equilibrium time of P/Fe-CB to 0.975 mg/L As(III) (within 240 min) is considerably shorter than that of compact Fe-chitosan beads (over 600 min). The maximal adsorption capacity of P/Fe-CB for As(III) is 52.7 mg/g. It can work effectively in a wide pH range from 3 to 9, and the coexisting sulfate, carbonate, silicate and humic acid have no significant effect on As(III) removal. The addition of H₂O₂ can further accelerate and promote the As(III) removal except at high pH (11) and phosphate concentration (50 mg/L). The fixed-bed experiments demonstrate that the P/Fe-CB column can effectively treat about 3000 bed volume (BV) of simulated As(III)-containing groundwater to meet the drinking water standard (<10 μg As/L). This study would extend the potential applicability of porous Fe based chitosan adsorbent and millimeter-sized adsorbent combined with H₂O₂ to a great extent.

Specific ion effects directed noble metal aerogels: Versatile manipulation for electrocatalysis and beyond

Noble metal foams (NMFs) are a new class of functional materials featuring properties of both noble metals and monolithic porous materials, providing impressive prospects in diverse fields. Among reported synthetic methods, the sol-gel approach manifests overwhelming advantages for versatile synthesis of nanostructured NMFs (i.e., noble metal aerogels) under mild conditions. However, limited gelation methods and elusive formation mechanisms retard structure/composition manipulation, hampering on-demand design for practical applications. Here, highly tunable NMFs are fabricated by activating specific ion effects, enabling various single/alloy aerogels with adjustable composition (Au, Ag, Pd, and Pt), ligament sizes (3.1 to 142.0 nm), and special morphologies. Their superior performance in programmable self-propulsion devices and electrocatalytic alcohol oxidation is also demonstrated. This study provides a conceptually new approach to fabricate and manipulate NMFs and an overall framework for understanding the gelation mechanism, paving the way for on-target design of NMFs and investigating structure-performance relationships for versatile applications.

Pressure-Responsive Hierarchical Chiral Photonic Aerogels

Pressure-responsive chiral photonic aerogels are fabricated by combining liquid crystal self-assembly and ice-templating processes. The aerogels have a hierarchical structure in which the primary 2D chiral nematic structured walls of cellulose nanocrystals form ribbons that support a secondary 3D cellular network. Owing to the flexibility of the aerogels in solvent, the 3D structure of the aerogel can easily be transformed to a 2D structure by pressure-induced rearrangement. The aerogels vary from white in color, which arises from light scattering, to a reflective photonic crystal displaying bright iridescent colors that depend on the immersed solvent. A solvent-sensitive ink that shows quick color response to different solvents is designed using the pressure-responsive photonic aerogel. This material demonstrates a new response mechanism for the design of smart and mechanoresponsive photonic materials.

A Directional Strain Sensor Based on Anisotropic Microhoneycomb Cellulose Nanofiber-Carbon Nanotube Hybrid Aerogels Prepared by Unidirectional Freeze Drying

Aerogels are one of the most popular composite reinforcement materials because of their high porosity and their continuous and homogeneous network. Most aerogels are isotropic, thus leading to isotropic composites when they are used as fillers. This fundamentally limits their applications in areas where anisotropy is needed. Here, an anisotropic microhoneycomb cellulose nanofiber- (CellF)-carbon nanotube (CNT) aerogel (denoted MCCA) is reported that contains unidirectionally aligned penetrating microchannels, which is prepared by a unidirectional freeze-drying method, using the structure-directing function of the CellFs. Due to its anisotropic nature, MCCA-reinforced polydimethylsilexane (denoted MCCA/PDMS) shows distinct anisotropic behavior, with the electrical conductivity and Young's modulus along the direction of penetrating microchannels being approximately twice those in the orthogonal direction. MCCA/PDMS is used to make "directional" strain sensors with electrical resistance as the output signal. They demonstrate a 92% sensitivity difference between the microchannel direction and its orthogonal direction. This approach can be used to prepare anisotropic MCCA-based composites with other polymers for different applications.

Commercial application of cellulose nano-composites - A review

Cellulose is the biosynthetic product from plants, animals and bacteria. Cellulose is the most abundant polymer having long linear chain like structure composed of (1,4) linked β-D glucopyranosyl units assembled into hierarchical structures of microfibrils with excellent strength and stiffness. And 'nanocellulose' refers to the cellulosic materials with defined nano-scale structural dimensions. They may be cellulose nanocrystal (CNC or NCC), cellulose nanofibers (CNF) or bacterial nanocellulose. Nanocellulose is non-toxic, biodegradable and biocompatible with no adverse effects on health and environment. Due to its low thermal expansion coefficient, high aspect ratio, better tensile strength, good mechanical and optical properties, they find many applications in thermo-reversible and tenable hydrogels, paper making, coating additives, food packaging, flexible screens, optically transparent films and light weight materials for ballistic protection, automobile windows. It also find potential in biopharmaceutical applications such as in drug delivery and for fabricating temporary implants with PHB like sutures, stents etc.

Elastomer-Free, Stretchable, and Conformable Silver Nanowire Conductors Enabled by Three-Dimensional Buckled Microstructures

Many three-dimensional (3D) nanomaterial-based assemblies need incorporation with elastomers to attain stretchability-that also compromises their pristine advantages for functional applications. Here, we show the design of elastomer-free, highly deformable silver nanowire (AgNW) conductors through dip-coating AgNWs on a 3D polymeric scaffold and following a simple triaxial compression approach. The resulting 3D AgNW conductors exhibit good stability of resistance under multimodal deformation, such as stretching, compressing, and bending as well as comparable conductivity with those elastomer-based ones. Moreover, the buckled structures endow our 3D conductors with novel negative Poisson's ratio behavior, which can offer good comfortability to curvilinear surfaces. The combination of mechanical properties, conductive performance, and unique deformation characteristics can satisfy multiscale conformal mechanics with a soft, curvilinear human body.