Super-Strong and Super-Stiff Chitosan Filaments with Highly Ordered Hierarchical Structure

Currently, the world is facing the problems of the gradual depletion of non-renewable fossil resources and the severe harm of non-degradable plastic waste to the land and marine ecological environment. Because of the rapid increase in the demand for fiber materials, the development of high-performance biomass-based fibers has emerged as an important research topic to reduce the reliance on petroleum-based synthetic fibers. In this study, a novel green wet-spinning strategy is used for the fabrication of super-strong and super-stiff chitosan filaments from an aqueous KOH/urea solution using a two-step drawing process. The highly ordered hierarchical structure of the resulting filaments contributes to their excellent mechanical properties. The tensile strength and Young's modulus of the chitosan filaments are 878 ± 123 MPa and 44.7 ± 12.3 GPa, respectively, and these values are comparable to those of spider silk and bacterial cellulose. The chitosan filaments prepared in this study are superior to low-density steel in terms of the specific strength and modulus. The green and scalable strategy proposed in this study will broaden the application range of chitosan filaments in flexible bioelectronics, biomaterials, and textiles.     

Compliant and Robust Tissue-Like Hydrogels via Ferric Ion-Induced of Hierarchical Structure

It is a challenge to synthesize materials that possess biological tissue-like properties: strain-stiffening, robust yet compliant, sensitive, and water-rich. Herein, a ferric ion-induced salting out and coordination cross-linking strategy is presented to create a hierarchical hydrogel network, including dipole–dipole interactions connected curved chains, acrylonitrile (AN)-rich clusters, and homogeneous iron-ligand interactions. The design allows the network to deform stress-free under small strain by unfolding the curved segments with the elastic deformation of the AN-rich clusters, and sequentially breaking the dipole–dipole interactions and iron-ligand interactions from weak to strong ones under large strain. As a result, the hydrogel exhibits tissue-like mechanical properties: low elastic modulus (0.06 MPa), high strength (1.4 MPa), high toughness (5.1 MJ m⁻³), intense strain-stiffening capability (27.5 folds of stiffness enhancement), excellent self-recovery ability and fatigue resistance. Moreover, the hydrogel exhibits high water content (≈84%), good biocompatibility and multi-sensory capabilities to strain, pressure and hazardous chemicals stimuli. Therefore, this work offers a novel strategy to prepare hydrogel that can mimic the diverse functions of tissues, thereby expanding advanced applications of hydrogel in soft robotics, wearable devices, and biomedical engineering.     

Nitridation and Layered Assembly of Hollow TiO2 Shells for Electrochemical Energy Storage

The nitridation of hollow TiO₂ nanoshells and their layered assembly into electrodes for electrochemical energy storage are reported. The nitridated hollow shells are prepared by annealing TiO₂ shells, produced initially using a sol–gel process, under an NH₃ environment at different temperatures ranging from 700 to 900 °C, then assembled to form a robust monolayer film on a water surface through a quick and simple assembly process without any surface modification to the samples. This approach facilitates supercapacitor cell design by simplifying the electrochemical electrode structure by removing the need to use any organic binder or carbon‐based conducting materials. The areal capacitance of the as‐prepared electrode is observed to be ≈180 times greater than that of a bare TiO₂ electrode, mainly due to the enhanced electrical conductivity of the TiN phase produced through the nitridation process. Furthermore, the electrochemical capacitance can be enhanced linearly by constructing an electrode with multilayered shell films through a repeated transfer process (0.8 to 7.1 mF cm^–2, from one monolayer to 9 layers). Additionally, the high electrical conductivity of the shell film makes it an excellent scaffold for supporting other psuedocapacitive materials (e.g., MnO₂), producing composite electrodes with a specific capacitance of 743.9 F g^–1 at a scan rate of 10 mV s^–1 (based on the mass of MnO₂) and a good cyclic stability up to 1000 cycles.     

Full 3D Printing of Stretchable Piezoresistive Sensor with Hierarchical Porosity and Multimodulus Architecture

A soft piezoresistive sensor with its unique characteristics, such as human skin, light weight, and multiple functions, yields a variety of possible practical applications to skin‐attachable electronics, human–machine interfaces, and electronic skins. However, conventional filler‐matrix piezoresistive sensors often suffer from unsatisfactory sensitivity or insufficient measurement range, as well as significant cross‐correlation between out‐of‐plane pressure and in‐plane extension. Here, a stretchable piezoresistive sensor (SPS) is realized by combining a hierarchically porous sensing element with a multimodulus device architecture via a full 3D printing process. As a result, the sensor exhibits high sensitivity (5.54 kPa^?1), large measurement range (from 10 Pa to 800 kPa), limited cross‐correlation, and excellent durability. Meanwhile, benefiting from the porous structure and mechanical mismatch design, which efficiently distributes the stress away from the sensing element, the device experiences only 7% resistance change at 50% stretching. This approach is employed to rapidly program and readily manufacture stylish, all‐in‐one, functional devices for various applications, demonstrating that the technique is promising for customized stretchable electronics.     

Emergence of New Mechanical Functionality in Materials via Size Reduction

Julia R. Greer received her S.B. in Chemical Engineering from the Massachusetts Institute of Technology (1997) and a Ph.D. in Materials Science from Stanford University, where she worked on the nanoscale plasticity of gold with W. D. Nix (2005). She also worked at Intel Corporation in Mask Operations (2000–03) and was a post‐doctoral fellow at the Palo Alto Research Center (2005–07), where she worked on organic flexible electronics with R. A. Street. Greer is a recipient of TR‐35, Technology Review's Top Young Innovator award (2008), a NSF CAREER Award (2007), a Gold Materials Research Society Graduate Student Award (2004), and an American Association of University Women Fellowship (2003). Julia joined Caltech's Materials Science department in 2007 where she is developing innovative experimental techniques to assess mechanical properties of nanometer‐sized materials. One such approach involves the fabrication of nanopillars with different initial microstructures and diameters between 25 nm and 1 µm by using focused ion beam and electron‐beam lithography microfabrication. The mechanical response of these pillars is subsequently measured in a custom‐built in situ mechanical deformation instrument, SEMentor, comprising a scanning electron microscope and a nanoindenter. Read our interview with Prof. Greer on MaterialsViews.com

     

Hierarchical Assemblies of Polymer Particles through Tailored Interfaces and Controllable Interfacial Interactions

Hierarchical assembly architectures of functional polymer particles are promising because of their physicochemical and surface properties for multi-labeling and sensing to catalysis and biomedical applications. While polymer nanoparticles’ interior is mainly made up of the cross-linked network, their surface can be tailored with soft, flexible, and responsive molecules and macromolecules as potential support for the controlled particulate assemblies. Molecular surfactants and polyelectrolytes as interfacial agents improve the stability of the nanoparticles whereas swellable and soft shell-like cross-linked polymeric layer at the interface can significantly enhance the uptake of guest nano-constituents during assemblies. Besides, layer-by-layer surface-functionalization holds the ability to provide a high variability in assembly architectures of different interfacial properties. Considering these aspects, various assembly architectures of polymer nanoparticles of tunable size, shapes, morphology, and tailored interfaces together with controllable interfacial interactions are constructed here. The microfluidic-mediated platform has been used for the synthesis of constituents polymer nanoparticles of various structural and interfacial properties, and their assemblies are conducted in batch or flow conditions. The assemblies presented in this progress report is divided into three main categories: cross-linked polymeric network's fusion-based self-assembly, electrostatic-driven assemblies, and assembly formed by encapsulating smaller nanoparticles into larger microparticles.     

Creating Directionality in Nanoporous Carbon Materials: Adjustable Combinations of Structural and Chemical Gradients

The properties of porous materials benefit from hierarchical porosity. A less noted element of hierarchy is the occurrence of directionality in functional gradient materials. A sharp boundary is replaced by a transition from one feature to the next. The number of cases known for porous materials with either structural or chemical gradients is small. A method capable of generating combinations of structural and chemical gradients in one material does not exist. Such a method is presented with a focus on silver and nitrogen containing carbon materials because of the potential of this system for electrocatalytic CO₂ reduction. A structural gradient results from controlled separation using ultracentrifugation of a binary mixture of template particles in a resorcinol–formaldehyde (RF) sol as carbon precursor. A new level of complexity can be reached, if the surfaces of the template particles are chemically modified. Although the template is removed during carbonization, the modification (Ag, N) becomes integrated into the material. Understanding how modified and unmodified large and small particles sediment in the RF sol enables almost infinite variability of combinations: chemically graded but structurally homogeneous materials and vice versa. Ultimately, a material containing one structural gradient and two chemical gradients with opposing directions is introduced.     

Segregated and Non-Settling Liquid Metal Elastomer via Jamming of Elastomeric Particles

Liquid metal elastomer (LME)—that is, liquid metal particles dispersed in elastomer—is a soft material that has useful electric, dielectric, and thermal properties. Two issues with LME are sought to be addressed: 1) the dense liquid metal (LM) particles can settle before curing of the elastomer, and 2) the LM particles are separated by a thin layer of insulating elastomer and therefore require some “mechanical sintering” to break this layer to create conductive paths. These issues are addressed using an LME containing elastic particles (LMEP). Elastic polydimethylsiloxane particles (PPs) and LM particles jam to prevent particle settling. Meanwhile, the PPs reduce the loading necessary to create conductive paths, thus decreasing the density and cost relative to LME. Surprisingly, the particles percolate into conductive paths prior to curing the LMEP but not in LME. The dielectric constant, electrical conductivity, and thermal conductivity of LMEPs are investigated by changing the volume fraction of LM particles, polydimethylsiloxane pre-polymer and PPs, and propose an LMEP with the optimal ratio. In addition, LMEP-based sensors and circuits are demonstrated for wearable electronics.     

Electric Field‐Directed Convective Assembly of Ellipsoidal Colloidal Particles to Create Optically and Mechanically Anisotropic Thin Films

A method of simultaneous field‐ and flow‐directed assembly of anisotropic titania (TiO₂) nanoparticle films from a colloidal suspension is presented. Titania particles are oriented by an alternating (ac) electric field as they simultaneously advect towards a drying front due to evaporation of the solvent. At high field frequencies (ν > ～25 kHz) and field strengths (E > 300 V cm^?1), the particles orient with their major axis along the field direction. As the front recedes, a uniform film with thicknesses of 1–10 µm is deposited on the substrate. The films exhibit a large birefringence (Δn ≈ 0.15) and high packing fraction (? = 0.75 ± 0.08), due to the orientation of the particles. When the frequency is lowered, the particle orientation undergoes a parallel–random–perpendicular transition with respect to the field direction. The orientation dependence on field frequency and strength is explained by the polarizability of ellipsoidal particles using an interfacial polarization model. Particle orientation in the films also leads to anisotropic mechanical properties, which are manifested in their cracking patterns. In all, it is demonstrated that the field‐directed assembly of anisotropic particles provides a powerful means for tailoring nanoparticle film properties in situ during the deposition process.     

Synthesis of Hierarchically Porous Hydrogen Silsesquioxane Monoliths and Embedding of Metal Nanoparticles by On‐Site Reduction

A facile synthesis of a new class of reactive porous materials is reported: hierarchically porous hydrogen silsesquioxane (HSiO_1.5, HSQ) monoliths with well‐defined macropores and mesopores. The HSQ monoliths are prepared via sol‐gel accompanied by phase separation in a mild condition, and contain micrometer‐sized co‐continuous macropores and high specific surface area reaching up to 800 m² g^?1 because of the small mesopores. A total preservation of Si–H, which is always an issue of HSQ materials, is confirmed by ²⁹Si solid‐state NMR. The HSQ monolith has then been subjected to reduction of noble metal ions to their corresponding metal nanoparticles in simple aqueous solutions under an ambient condition. The nanoparticles produced in this manner are immobilized on the HSQ monolith and are characterized by X‐ray diffraction (XRD) and high angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM). Both the bare HSQ and nanoparticles‐embedded HSQ are promising as heterogeneous catalysts, exhibiting reusability and recyclability.     

Synthesis and Assembly of Gold Nanoparticles in Quasi‐Linear Lysine–Keggin‐Ion Colloidal Particles

The formation of fiber‐like colloidal particles of the amino acid lysine complexed with Keggin ions is demonstrated. The lysine–phosphotungstic acid (PTA) colloidal particles act as excellent templates for the synthesis and assembly of gold nanoparticles wherein the lysine‐PTA complex acts as a UV‐switchable reducing agent for gold ions. This novel bio‐organic–inorganic template shows excellent potential as a regulated nanoreactor for application in programmed nanoparticle synthesis and assembly in a single step.     

Noble Metal Ion-Directed Assembly of 2D Materials for Heterostructured Catalysts and Metallic Micro-Texturing

Assembling 2D-material (2DM) nanosheets into micro- and macro-architectures with augmented functionalities requires effective strategies to overcome nanosheet restacking. Conventional assembly approaches involve external binders and/or functionalization, which inevitably sacrifice 2DM's nanoscale properties. Noble metal ions (NMI) are promising ionic crosslinkers, which can simultaneously assemble 2DM nanosheets and induce synergistic properties. Herein, a collection of NMI–2DM complexes are screened and categorized into two sub-groups. Based on the zeta potentials, two assembly approaches are developed to obtain 1) NMI-crosslinked 2DM hydrogels/aerogels for heterostructured catalysts and 2) NMI–2DM inks for templated synthesis. First, tetraammineplatinum(II) nitrate (TPtN) serves as an efficient ionic crosslinker to agglomerate various 2DM dispersions. By utilizing micro-textured assembly platforms, various TPtN–2DM hydrogels are fabricated in a scalable fashion. Afterward, these hydrogels are lyophilized and thermally reduced to synthesize Pt-decorated 2DM aerogels (Pt@2DM). The Pt@2DM heterostructures demonstrate high, substrate-dependent catalytic activities and promote different reaction pathways in the hydrogenation of 3-nitrostyrene. Second, PtCl₄ can be incorporated into 2DM dispersions at high NMI molarities to prepare a series of PtCl₄–2DM inks with high colloidal stability. By adopting the PtCl₄–graphene oxide ink, various Pt micro-structures with replicated topographies are synthesized with accurate control of grain sizes and porosities.     

Rapid Fabrication of Two‐ and Three‐Dimensional Colloidal Crystal Films via Confined Convective Assembly

We have developed a self‐assembly method for fabricating well‐ordered two‐dimensional (2D) and three‐dimensional (3D) colloidal crystal films. With a minute amount of a polystyrene colloidal suspension and without any special equipment, the proposed method can be used to rapidly deposit high‐quality colloidal crystal films over a large surface area. By controlling the lift‐up rate of the substrate, we modulate the meniscus thinning rate, which determines whether the colloidal particles are assembled into two or three dimensions. The proposed method can be used to fabricate not only monolayered colloidal crystals with colloidal particles of various sizes, but also multilayered colloidal crystals. In addition, the method enables us to fabricate binary colloidal crystals by consecutively depositing large and small particles.     

多孔硅与有机发光材料复合的光电特性

用旋涂法实现了多孔硅(PS)与有机发光材料聚乙烯咔唑(PVK)和八羟基喹啉(Alq3)的复合，研究了PS／(PVK，Alq3)复合体系的光学性能和电学性能。PL谱的测试发现，PS／PVK复合体系的PL同时具有PS和PVK的峰；在485nm的位置出现了1个新峰，讨论了这个峰的来源。研究了n型单晶Si制备的PS与PVK复合(n-PS／p-PVK)和P型单晶Si制备的Alq3复合(p-PS／n-Alq3)后的}y特性。测试表明，这两个复合体系的I-V曲线都显示了良好的整流特性。结果表明，PS／(PVK，Alq3)复合体系适合用来制备PS基的发光二极管。最后借助无机半导体的能带理论对此进行了分析。     

纳米金属及无机材料的绿色合成

高聚物模板是用来合成纳米材料的重要方法之一,但在制备纳米材料的过程中,这些高聚物往往难以被大自然所降解。而淀粉类生物多糖物质来源广泛,并具有很好的生物降解性和相容性,以淀粉类生物多糖为模板合成纳米材料已经引起了广泛的重视。现报道以淀粉类生物多糖作为模板,来引导纳米金属及无机材料合成的最新研究进展。     

Rapid Photochemical Synthesis of Sea‐Urchin‐Shaped Hierarchical Porous COF‐5 and Its Lithography‐Free Patterned Growth

Despite potential advantages of covalent organic frameworks (COFs) in wide area applications, several limitations in conventional solvothermal synthesis, such as long reaction time and high reaction temperature, reduce reaction efficiency and prohibit technical processes for practical applications. Therefore, the development of a novel synthesis method that provides better reaction efficiency and spatial controllability has become a critical challenge. Herein, a photochemical synthesis of C₉H₄BO₂ (COF‐5) is demonstrated for the first time, by which “sea urchin‐shaped” COF‐5 (UV‐COF‐5) with uniform size is synthesized with a highly enhanced growth rate, ≈48 times faster than that of the solvothermal method for 75% yield. In addition, an enlarged surface area is measured from UV‐COF‐5, which originates from its hierarchical morphology. The selectively increased growth rate of UV‐COF‐5 in the [001] direction observed by microscopic analysis results in the local 1D morphology of the hierarchical structure. Density functional theory calculations determine that the enhanced growth rate along the [001] direction can be understood by the characteristic of the interlayer orbital coupling at the frontier energy region. In addition, this study successfully demonstrates the preparation of COF‐5 patterns without any complicated postsynthesis lithography process, but simply by utilizing optical masks during the photochemical method.     

One‐Pot Synthesis and Hierarchical Assembly of Hollow Cu2O Microspheres with Nanocrystals‐Composed Porous Multishell and Their Gas‐Sensing Properties

Hierarchical assembly of hollow microstructures is of great scientific and practical value and remains a great challenge. This paper presents a facile and one‐pot synthesis of Cu₂O microspheres with multilayered and porous shells, which were organized by nanocrystals. The time‐dependent experiments revealed a two‐step organization process, in which hollow microspheres of Cu₂(OH)₃NO₃ were formed first due to the Ostwald ripening and then reduced by glutamic acid, the resultant Cu₂O nanocrystals were deposited on the hollow intermediate microspheres and organized into finally multishell structures. The special microstructures actually recorded the evolution process of materials morphologies and microstructures in space and time scales, implying an intermediate‐templating route, which is important for understanding and fabricating complex architectures. The Cu₂O microspheres obtained were used to fabricate a gas sensor, which showed much higher sensitivity than solid Cu₂O microspheres.     

Highly Emissive,Water‐Repellent,Soft Materials: Hydrophobic Wrapping and Fluorescent Plasticizing of Conjugated Polyelectrolyte via Electrostatic Self‐Assembly

Sulfonated poly(diphenylacetylene) (SPDPA) is used as an anionic conjugated polyelectrolyte to examine stoichiometric electrostatic self‐assembly with homologous cationic surfactants (octadecyl)_X(methyl)_Y ammonium bromides (O_XM_YABs) having different numbers of long hydrophobic tails. The SPDPA–O_XM_YAB complexes formed show significantly increased water contact angle and enhanced fluorescence (FL) emissions compared with the pristine SPDPA. The complexes exist in a gum state at room temperature owing to the plasticizer effect of the hydrophobic tails, hence they are very soft and highly stretchable. The hydrophobicity, softness, and FL quantum efficiency of the SPDPA–O_XM_YAB complexes increase as the number of hydrophobic tails increases. SPDPA adsorbs uniformly onto filter papers to produce fluorescent papers. The SPDPA‐adsorbed papers have many unique applications, including FL image writing, fingerprinting, stamping, and inkjet printing using the surfactant solutions as an ink to reveal high‐resolution FL images. In particular, multideposit inkjet‐printing using SPDPA and O_XM_YAB solutions as inks produces water‐resistant, embedded figures in paper currency.     

Improved Thermoelectric Performance of Eco‐Friendly β‐FeSi2–SiGe Nanocomposite via Synergistic Hierarchical Structuring,Phase Percolation,and Selective Doping

A β‐FeSi₂–SiGe nanocomposite is synthesized via a react/transform spark plasma sintering technique, in which eutectoid phase transformation, Ge alloying, selective doping, and sintering are completed in a single process, resulting in a greatly reduced process time and thermal budget. Hierarchical structuring of the SiGe secondary phase to achieve coexistence of a percolated network with isolated nanoscale inclusions effectively decouples the thermal and electrical transport. Combined with selective doping that reduces conduction band offsets, the percolation strategy produces overall electron mobilities 30 times higher than those of similar materials produced using typical powder‐processing routes. As a result, a maximum thermoelectric figure of merit ZT of ≈0.7 at 700 °C is achieved in the β‐FeSi₂–SiGe nanocomposite.