Bench‐Top Fabrication of Hierarchically Structured High‐Surface‐Area Electrodes

Fabrication of hierarchical materials, with highly optimized features from the millimeter to the nanometer scale, is crucial for applications in diverse areas including biosensing, energy storage, photovoltaics, and tissue engineering. In the past, complex material architectures have been achieved using a combination of top‐down and bottom‐up fabrication approaches. A remaining challenge, however, is the rapid, inexpensive, and simple fabrication of such materials systems using bench‐top prototyping methods. To address this challenge, the properties of hierarchically structured electrodes are developed and investigated by combining three bench‐top techniques: top‐down electrode patterning using vinyl masks created by a computer‐aided design (CAD)‐driven cutter, thin film micro/nanostructuring using a shrinkable polymer substrate, and tunable electrodeposition of conductive materials. By combining these methods, controllable electrode arrays are created with features in three distinct length scales: 40 μm to 1 mm, 50 nm to 10 μm, and 20 nm to 2 μm. The electrical and electrochemical properties of these electrodes are analyzed and it is demonstrated that they are excellent candidates for next generation low‐cost electrochemical and electronic devices.     

Multidimension‐Controllable Synthesis of Ant Nest‐Structural Electrode Materials with Unique 3D Hierarchical Porous Features toward Electrochemical Applications

Hierarchical porous materials (HPM) have been widely used to enhance electrochemical performance in different fields of application, since their porous structures benefit electrolyte infiltration and ion diffusion. However, the realization of multidimension‐controllable synthesis of HPM, including material category, material components, supporting substrates, as well as pore sizes/distributions, is still a huge challenge. Herein, a novel concept is proposed, for the first time, on the geometry structure of HPM bioinspired by natural ant nests, which features 3D interlaced and well‐interconnected porous structures. Moreover, a facile and universal approach is developed to the multidimension‐controllable synthesis of ant nest‐structural HPM. Further investigation shows that the in situ construction of carbon‐based ant nests onto porous current collectors to fabricate the integrated electrode for supercapacitors could induce nearly 70% and 45% enhancement on the specific capacitance compared to the common powder and freestanding materials, respectively. Moreover, this synthesis route can be facilely extended to obtain the ant nest‐structural CuO_x, which exhibits fivefold enhancement in sensitivity for glucose detection. Such biomimetic hierarchical porous architectures are of great significance in the field of electrochemical applications.     

Cover Picture: Sequential Nucleation and Growth of Complex Nanostructured Films (Adv. Funct. Mater. 3/2006)

A sequential nucleation and growth process has been developed to construct complex nanostructured films step‐by‐step from aqueous solutions, as reported by Liu, Voigt, and co‐workers on p. 335. This method can be applied to a wide range of materials, and can be combined with top–down techniques to create spatially resolved micropatterns. The cover figure shows images of oriented nanowires, nanoneedles, nanotubes, nanoplates and stacked columns, wagon‐wheels, hierarchical films based on wagon‐wheels, hierarchically ordered mesophase silicate, and micropatterned flower‐like structures. Nanostructured films with controlled architectures are desirable for many applications in optics, electronics, biology, medicine, and energy/chemical conversions. Low‐temperature, aqueous chemical routes have been widely investigated for the synthesis of continuous films, and arrays of oriented nanorods and nanotubes. More recently, aqueous‐phase routes have been used to produce films composed of more complex crystal structures. In this paper, we discuss recent progress in the synthesis of complex nanostructures through sequential nucleation and growth processes. We first review the use of multistage, seeded‐growth methods to synthesize a wide range of nanostructures, including oriented nanowires, nanotubes, and nanoneedles, as well as laminated films, columns, and multilayer heterostructures. We then describe more recent work on the application of sequential nucleation and growth to the systematic assembly of large arrays of hierarchical, complex, oriented, and ordered crystal architectures. The multistage aqueous chemical route is shown to be applicable to several technologically important materials, and therefore may play a key role in advancing complex nanomaterials into applications.     

Towards the Next Level of Bioinspired Dry Adhesives: New Designs and Applications

This Feature Article aims to highlight our recent efforts to develop more robust gecko‐inspired dry adhesives and their applications. Due to recent progress in micro‐ and nanofabrication techniques, it is possible to fabricate highly sophisticated multiscale, hierarchical structures using various polymer materials. In addition, the adhesion strength of synthetic dry adhesives has been shown to be similar to or exceed that of real gecko foot‐hair by several times. Therefore, it is timely and appropriate to drive the research of gecko‐inspired dry adhesives into a new epoch by investigating more robust dry adhesive structures, efficient detachment mechanisms, and new applications. In this Feature Article, we present a series of our recent achievements to overcome some of the limitations of gecko‐like hair structures such as rough surface adaptation, durability, and controlled geometry, with particular emphasis on materials issues and detachment mechanism. For potential applications, a clean transportation device and a biomedical skin patch are briefly described to expand the application realm from the well‐known wall climbing robot.     

General Formation of MS (M = Ni,Cu, Mn) Box‐in‐Box Hollow Structures with Enhanced Pseudocapacitive Properties

Complex hollow structures of metal sulfides could be promising materials for energy storage devices such as supercapacitors and lithium‐ion batteries. However, it is still a great challenge to fabricate well‐defined metal sulfides hollow structures with multi‐shells, hierarchical architectures, and non‐spherical shape. In this work, a template‐engaged strategy is developed to synthesize hierarchical NiS box‐in‐box hollow structures with double‐shells. The NiS box‐in‐box hollow structures constructed by ultrathin nanosheets are evaluated as electrode materials for supercapacitors. As expected, the NiS box‐in‐box hollow structures exhibit excellent rate performance and impressive cycling stability due to their unique nano‐architecture. More importantly, the synthetic method can be easily extended to synthesize other transition metal sulfides box‐in‐box hollow structures. For example, we have also successfully synthesized similar CuS and MnS box‐in‐box hollow structures. The present work makes a significant contribution to the design and synthesis of transition metal sulfides hollow structures with non‐spherical shape and complex architecture, as well as their potential applications in electrochemical energy storage.     

Hierarchical Carbon–Nitrogen Architectures with Both Mesopores and Macrochannels as Excellent Cathodes for Rechargeable Li–O2 Batteries

Lithium–oxygen batteries are attracting more and more interest; however, their poor rechargeability and low efficiency remain critical barriers to practical applications. Herein, hierarchical carbon–nitrogen architectures with both macrochannels and mesopores are prepared through an economical and environmentally benign sol–gel route, which show high electrocatalytic activity and stable cyclability over 160 cycles as cathodes for Li–O₂ batteries. Such good performance owes to the coexistence of macrochannels and mesopores in C–N hierarchical architectures, which greatly facilitate the Li⁺ diffusion and electrolyte immersion, as well as provide an effective space for O₂ diffusion and O₂/Li₂O₂ conversion. Additionally, the mechanism of oxygen reduction reactions is discussed with the N‐rich carbon materials through first‐principles computations. The lithiated pyridinic N provides excellent O₂ adsorption and activation sites, and thus catalyzes the electrode processes. Therefore, hierarchical carbon–nitrogen architectures with both macrochannels and mesopores are promising cathodes for Li–O₂ batteries.     

Hierarchically Structured Porous Materials for Energy Conversion and Storage

Materials with hierarchical porosity and structures have been heavily involved in newly developed energy storage and conversion systems. Because of meticulous design and ingenious hierarchical structuration of porosities through the mimicking of natural systems, hierarchically structured porous materials can provide large surface areas for reaction, interfacial transport, or dispersion of active sites at different length scales of pores and shorten diffusion paths or reduce diffusion effect. By the incorporation of macroporosity in materials, light harvesting can be enhanced, showing the importance of macrochannels in light related systems such as photocatalysis and photovoltaics. A state‐of‐the‐art review of the applications of hierarchically structured porous materials in energy conversion and storage is presented. Their involvement in energy conversion such as in photosynthesis, photocatalytic H₂ production, photocatalysis, or in dye sensitized solar cells (DSSCs) and fuel cells (FCs) is discussed. Energy storage technologies such as Li‐ions batteries, supercapacitors, hydrogen storage, and solar thermal storage developed based on hierarchically porous materials are then discussed. The links between the hierarchically porous structures and their performances in energy conversion and storage presented can promote the design of the novel structures with advanced properties.     

Improved Interfacial Floatability of Superhydrophobic/Superhydrophilic Janus Sheet Inspired by Lotus Leaf

Interfacial materials exhibiting superwettability have emerged as important tools for solving the real‐world issues, such as oil‐spill cleanup, fog harvesting, etc. The Janus superwettability of lotus leaf inspires the design of asymmetric interface materials using the superhydrophobic/superhydrophilic binary cooperative strategy. Here, the presented Janus copper sheet, composed of a superhydrophobic upper surface and a superhydrophilic lower surface, is able to be steadily fixed at the air/water interfaces, showing improved interfacial floatability. Compared with the floatable superhydrophobic substrate, the Janus sheet not only floats on but also attaches to the air–water interface. Similar results on Janus sheet are discovered at other multiphase interfaces such as hexane/water and water/CCl₄ interfaces. In accordance with the improved stability and antirotation property, the microboat constructed by a Janus sheet shows the reliable navigating ability even under turbulent water flow. This contribution should unlock more functions of Janus interface materials, and extend the application scope of the binary cooperative materials system with superwettability.     

Three‐Dimensional Printing of Elastomeric,Cellular Architectures with Negative Stiffness

Three‐dimensional printing of viscoelastic inks to create porous, elastomeric architectures with mechanical properties governed by the ordered arrangement of their sub‐millimeter struts is reported. Two layouts are patterned, one resembling a “simple cubic” (SC)‐like structure and another akin to a “face‐centered tetragonal” (FCT) configuration. These structures exhibit markedly distinct load response with directionally dependent behavior, including negative stiffness. More broadly, these findings suggest the ability to independently tailor mechanical response in cellular solids via micro‐architected design. Such ordered materials may one day replace random foams in mechanical energy absorption applications.     

Wood‐Derived Materials for Advanced Electrochemical Energy Storage Devices

Over the past decade, wood‐derived materials have attracted enormous interest for both fundamental research and practical applications in various functional devices. In addition to being renewable, environmentally benign, naturally abundant, and biodegradable, wood‐derived materials have several unique advantages, including hierarchically porous structures, excellent mechanical flexibility and integrity, and tunable multifunctionality, making them ideally suited for efficient energy storage and conversion. In this article, the latest advances in the development of wood‐derived materials are discussed for electrochemical energy storage systems and devices (e.g., supercapacitors and rechargeable batteries), highlighting their micro/nanostructures, strategies for tailoring the structures and morphologies, as well as their impact on electrochemical performance (energy and power density and long‐term durability). Furthermore, the scientific and technical challenges, together with new directions of future research in this exciting field, are also outlined for electrochemical energy storage applications.     

Bioinspired Assembly of Hierarchical Light‐Harvesting Architectures for Improved Photophosphorylation

Molecular assembly offers a bottom‐up way to construct biomimetic architectures with unique structures and properties. Although artificial photophosphorylation systems have long been developed, their microstructures have yet to achieve the sophisticated order and hierarchy of natural organisms. Herein, by utilizing principles in the natural plant leaves, it is shown that a biomimetic system with hierarchically ordered and compartmentalized structures, combining photosystem II (PSII) and adenosine triphosphate (ATP) synthase, can be obtained through template‐directed layer‐by‐layer assembly. Under light illumination, PSII in such a highly ordered light‐harvesting array, splits water to produce protons and electrons. Furthermore, a remarkable proton gradient is created across the covering ATP synthase‐reconstituted lipid membrane. As a consequence, highly efficient photophosphorylation is achieved. Outstandingly, the rate of ATP production in this hierarchical light‐harvesting architecture is enhanced 14 times, compared to that in the nature. This study paves a new way to assemble bioinspired systems with enhanced solar‐to‐chemical energy conversion efficiency.     

Gecko‐Inspired Dry Adhesive for Robotic Applications

Most geckos can rapidly attach and detach from almost any kind of surface. This ability is attributed to the hierarchical structure of their feet (involving toe pads, setal arrays, and spatulae), and how they are moved (articulated) to generate strong adhesion and friction forces on gripping that rapidly relax on releasing. Inspired by the gecko's bioadhesive system, various structured surfaces have been fabricated suitable for robotic applications. In this study, x–y–z asymmetric, micrometer‐sized rectangular flaps composed of polydimethylsiloxane (PDMS) were fabricated using massively parallel micro‐electromechanical systems (MEMS) techniques with the intention of creating directionally responsive, high‐to‐low frictional‐adhesion toe pads exhibiting properties similar to those found in geckos. Using a surface forces apparatus (SFA), the friction and adhesion forces of both vertical (symmetric) and angled/tilted (x–y–z asymmetric) microflaps under various loading, unloading and shearing conditIons were investigated. It was found that the anisotropic structure of tilted microflaps gives very different adhesion and tribological forces when articulated along different x–y–z directions: high friction and adhesion forces when articulated in the y–z plane along the tilt (+y) direction, which is also the direction of motion, and weak friction and adhesion forces when articulated against the tilt (–y) direction. These results demonstrate that asymmetric angled structures, as occur in geckos, are required to enable the gecko to optimize the requirements of high friction and adhesion on gripping, and low frictional‐adhesion on releasing. These properties are intimately coupled to a (also optimum) articulation mechanism. We discuss how both of these features can be simultaneously optimized in the design of robotic systems that can mimic the gecko adhesive system.     

Flexible and Highly Sensitive Pressure Sensors Based on Bionic Hierarchical Structures

The rational design of high‐performance flexible pressure sensors attracts attention because of the potential applications in wearable electronics and human–machine interfacing. For practical applications, pressure sensors with high sensitivity and low detection limit are desired. Here, ta simple process to fabricate high‐performance pressure sensors based on biomimetic hierarchical structures and highly conductive active membranes is presented. Aligned carbon nanotubes/graphene (ACNT/G) is used as the active material and microstructured polydimethylsiloxane (m‐PDMS) molded from natural leaves is used as the flexible matrix. The highly conductive ACNT/G films with unique coalescent structures, which are directly grown using chemical vapor deposition, can be conformably coated on the m‐PDMS films with hierarchical protuberances. Flexible ACNT/G pressure sensors are then constructed by putting two ACNT/G/PDMS films face to face with the orientation of the ACNTs in the two films perpendicular to each other. Due to the unique hierarchical structures of both the ACNT/G and m‐PDMS films, the obtained pressure sensors demonstrate high sensitivity (19.8 kPa^?1, <0.3 kPa), low detection limit (0.6 Pa), fast response time (<16.7 ms), low operating voltage (0.03 V), and excellent stability for more than 35 000 loading–unloading cycles, thus promising potential applications in wearable electronics.     

Sequential Nucleation and Growth of Complex Nanostructured Films

Nanostructured films with controlled architectures are desirable for many applications in optics, electronics, biology, medicine, and energy/chemical conversions. Low‐temperature, aqueous chemical routes have been widely investigated for the synthesis of continuous films, and arrays of oriented nanorods and nanotubes. More recently, aqueous‐phase routes have been used to produce films composed of more complex crystal structures. In this paper, we discuss recent progress in the synthesis of complex nanostructures through sequential nucleation and growth processes. We first review the use of multistage, seeded‐growth methods to synthesize a wide range of nanostructures, including oriented nanowires, nanotubes, and nanoneedles, as well as laminated films, columns, and multilayer heterostructures. We then describe more recent work on the application of sequential nucleation and growth to the systematic assembly of large arrays of hierarchical, complex, oriented, and ordered crystal architectures. The multistage aqueous chemical route is shown to be applicable to several technologically important materials, and therefore may play a key role in advancing complex nanomaterials into applications.     

From Passive Inorganic Oxides to Active Matters of Micro/Nanomotors

Controllable actuation and coordinating motion of artificial self‐propelled micro/nanomotors to mimic the motile natural microorganism systems are of great significance for constructing intelligent nanoscale machines. In particular, inorganic oxide particles have shown considerable promise in implementation of synthetic micro/nanomotors, due to their unique features and active response to environmental stimuli. This work critically reviews the recent progress in inorganic oxide‐based micro/nanomotors and focuses on their propulsion response to chemical and physical stimuli, especially emphasizing and discussing operating principles in the single engine, adaptive navigation under composite‐driven powers, and intriguing collective behaviors. The impact of oxide structure, multiple fields in motion controllability, and interaction between grouped micro/nanomotors are explored. Practical applications of individual and assembled micro/nanomotors in environmental and biomedical fields are demonstrated, including the removal of pollutants, drug delivery, cancer therapy, and in vivo imaging. Finally, current challenges for the development of novel micro/nanomotors and possible constraints toward the defined structure and accumulated toxicity are discussed along with future opportunities and directions. Owing to their facile synthesis, impressive physicochemical performances, high biocompatibility, and versatile actuations, it is expected that the association of inorganic oxides with micro/nanomotors will bring new and unique capabilities to the field of active matter.     

Bioinspired Hierarchical Liquid‐Metacrystal Fibers for Chiral Optics and Advanced Textiles

The organization of nanoparticles in constrained geometries has attracted increasing attention due to their promising structures and topologies. However, the control of hierarchical structures with tailored periodicity at different length scales and topology stabilization in a dynamic environment are very limited and challenging. Herein, through self‐assembly of cellulose nanocrystals (CNCs) within an in situ formed hydrogel sheath using a simple microfluidic strategy, a new breed of liquid crystal (LC) fibers with hierarchical core–sheath architectures, metaperiodic cholesteric alignments, and 3D topological defects, termed as liquid metacrystal (LMC) fibers, is created. The resulting LMC fibers not only exhibit vivid, tunable interference colors, and even inverse optical activity but also have a unique ability to precisely regulate linearly and circularly polarized light in a half‐sync/half‐async form. Furthermore, robust hydrogel sheath enables the LMCs with alignment stability and configuration programmability during drying, which endows the unprecedented freedom to tailor different optical appearances for polarization‐based encryption and recognition. This work opens an avenue toward the fabrication of length‐scale colloidal LCs with continuous and stable topologies and expands the application regimes of LC materials in chiral optics and smart textiles.     

Hierarchical Self‐Assembly of Peptide‐Coated Carbon Nanotubes

Numerous applications, from molecular electronics to super‐strong composites, have been suggested for carbon nanotubes. Despite this promise, difficulty in assembling raw carbon nanotubes into functional structures is a deterrent for applications. In contrast, biological materials have evolved to self‐assemble, and the lessons of their self‐assembly can be applied to synthetic materials such as carbon nanotubes. Here we show that single‐walled carbon nanotubes, coated with a designed amphiphilic peptide, can be assembled into ordered hierarchical structures. This novel methodology offers a new route for controlling the physical properties of nanotube systems at all length scales from the nano‐ to the macroscale. Moreover, this technique is not limited to assembling carbon nanotubes, and could be modified to serve as a general procedure for controllably assembling other nanostructures into functional materials.     

Media access control for high-speed local area and metropolitanarea communications networks

The features, architectures, and principles of key media access control (MAC) schemes for high-speed LAN and MAN systems are categorized and reviewed. These architectures are related to the hierarchical structure of a telecommunications network. An overview is given of the MAC protocol operation of key local- and metropolitan-area network systems, as defined by standards committees, covering current methods as well as approaches for future broadband integrated services digital networks. Modeling and analysis techniques are then reviewed for key classes of relevant MAC schemes, including fixed-assignment time-division-multiaccess (TDMA) schemes; demand-assignment reservation schemes, involving, in particular, pure packet-switched, pure circuit-switched and hybrid-switched integrated-services demand-assignment TDMA and time-division-multiplexing structures; demand-assigned polling procedures; and random-access policies     

Dual‐Activity Controlled Asymmetric Synthesis of Superconducting Lead Hemispheres**

A binary surfactant mixture of cetyltrimethylammonium bromide and polyvinyl pyrrolidone is used as the tailoring agent in the fabrication of lead micro/nanostructures. Electron microscopy studies indicate that the morphologies of the products can be efficiently controlled in this simple one‐step synthetic procedure. Intriguingly, well‐defined asymmetric functional colloids, Pb hemispheres, are obtained for the first time, and a dual‐activity‐controlled growth process is proposed to explain their formation. The magnetization measurements show that the as‐prepared samples are superconducting with the same transition temperature as bulk Pb. These findings prove the unique morphology tailoring efficacy of mixed surfactants, which could be used to obtain more variform structures or architectures in the fabrication of advanced materials.     

Assembly of Molecular Building Blocks into Integrated Complex Functional Molecular Systems: Structuring Matter Made to Order

Function‐inspired design of molecular building blocks for their assembly into complex systems has been an objective in engineering nanostructures and materials modulation at nanoscale. This article summarizes recent research and inspiring progress in the design/synthesis of various custom‐made chiral, switchable, and highly responsive molecular building blocks for the construction of diverse covalent/noncovalent assemblies with tailored topologies, properties, and functions. Illustrating the judicious selection of building blocks, orthogonal functionalities, and innate physical/chemical properties that bring diversity and complex functions once reticulated into materials, special focus is given to their assembly into porous crystalline networks such as metal/covalent–organic frameworks (MOFs/COFs), surface‐mounted frameworks (SURMOFs), metal–organic cages/rings (MOCs), cross‐linked polymer gels, porous organic polymers (POPs), and related architectures that find diverse applications in life science and various other functional materials. Smart and stimuli‐responsive or dynamic building blocks, once embedded into materials, can be remotely modulated by external stimuli (light, electrons, chemicals, or mechanical forces) for controlling the structure and properties, thus being applicable for dynamic photochemical and mechanochemical control in constructing new forms of matter made to order. Then, an overview of current challenges, limitations, as well as future research directions and opportunities in this field, are discussed.