CARBON AEROGELS GO TO THE (VERY) DARK SIDE Scientists have long sought a “perfect black” material that can absorb all light that hits its surface, a quality that could be a boon to applications including solar energy conversion, optical instruments, thermal detectors, and pyroelectric sensors. Recent studies have reported significant headway in this area using subwavelength two-dimensional (2D) patterns, which have been shown to be highly effective at reducing light reflectance. However, this concept has yet to be translated into three-dimensional materials. To make this leap, Sun et al. (DOI: 10.1021/ acsnano.6b02039) investigated the use of low-density carbon aerogels (CAs), whose pores and skeletons are much smaller than the wavelength of visible light. The researchers synthesized several different CAs with a range of densities and skeleton sizes by adjusting the concentrations of precursors and catalyst used to make these materials. Initial tests showed that as density increased and skeleton size decreased, reflectance values also decreased to as low as 0.19%. However, the relationship with skeleton size was not strictly linear; further investigation showed that the more micropores in a sample, the lower its reflectance. To explain this phenomenon, the researchers constructed a model that suggests that this microstructure is key to lowering reflectance by causing a decrease of the amplitude of electron oscillation caused by the subwavelength physical structures. The authors suggest that these findings could aid in designing future similar or even darker materials.
material is a TiN nanowire array. These nanowires were coated with an ultrathin amorphous carbon shell to prevent oxidation. Alongside this supercapacitor is a solar-energy-harvesting module based on ZnO nanowires sensitized with N719 dye. These groups of nanowire “threads” are integrated into cloth by weaving them together with cotton yarns using flying shuttle industrial looms. Tests showed that neither the supercapacitor nor the solar cell threads lost performance with bending or cutting. The entire device could be fully charged to 1.2 V in 17 s with solar energy and then fully discharged in 78 s at a discharge current density of 0.1 mA. The authors suggest that this device could eventually make it possible to produce smart energy garments that can gather and store energy while also leaving room for fashion design.
CARBON NANOTUBE-COATED SPANDEX: NOT YOUR GRANDMOTHER’S YARN Smart textiles with a variety of functionalities are currently under development, including those that might harvest or store energy; generate pressure or force; change in porosity or color; or sense movement, temperature, or chemicals. To make production of these materials friendly for the consumer market, their manufacturing methods must easily integrate into existing methods without major modification to production protocols and rates. In a recent study, Foroughi et al. (DOI: 10.1021/ acsnano.6b04125) detail a novel textile they created by coating Spandex threads with carbon nanotube (CNT) sheaths, imbuing the popular elastomeric copolymer with electrical conductivity. The fabrication process is facile and couples easily with existing textile manufacturing techniques. Essentially, a CNT aerogel sheet is drawn from a CNT forest and wrapped around a continuous supply of Spandex filaments fed to an interlocking circular knitting machine. Fabrics could then be composed either of single Spandex fibers or yarns with 4, 8, or 12 filaments. The resulting textiles were highly stretchable and electrically conductive. Tests showed that they could serve as actuators or artificial muscles, exhibiting contraction with electrothermal heating that was completely reversible with passive cooling. The gravimetric work and power outputs of this material are 16 and 4 times higher, respectively, than mammalian skeletal muscle. Additionally, because stretching increased electrical resistance, these textiles could be used as strain sensors. The authors suggest that the unique
SOLAR ENERGY HARVESTING AND STORAGE, CUT TO FIT Wearable “smart” electronics have attracted growing attention for use in a variety of consumer applications. To avoid the need for frequent recharging, recent research has focused on integrating the need for simultaneous energy harvesting and storage. These devices have a long list of requirements to fulfill, including being lightweight, flexible, easily integrated into clothing using existing manufacturing techniques, and tailorable, maintaining their performance after being cut into arbitrary lengths and shapes. Thus far, no devices have been developed that meet all of these criteria. In a recent study, Chai et al. (DOI: 10.1021/ acsnano.6b05293) report an all-solid energy textile that can harvest and store solar energy while also being tailorable. The active material in the fiber supercapacitor portion of this © 2016 American Chemical Society
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CRYSTALLIZING KNOWLEDGE ON DNA TRIANGLE CRYSTAL GROWTH Researchers have made tremendous progress in using nucleic acids for programmable self-assembly, creating increasingly large and more complex structures over the past several years. Being able to use DNA origami or tile-brick objects for the bottom-up assembly of three-dimensional (3D) crystalline materials could be a boon for fields such as photonics, plasmonics, and catalysis, with the tailorable nature of these building blocks offering the potential for designer macroscopic functionalities. However, crystallizing most DNA origami structures into 3D lattices remains a challenge. Hindrances to successful crystallization could be multifold, including heterogeneity, both from partially folded or geometrically different building blocks and chemical imperfections in the DNA oligonucleotides, or interactions that are too weak or too strong between particles. In a recent study, Stahl et al. (DOI: 10.1021/ acsnano.6b04787) investigated both of these factors in the crystallization of 38 kDa DNA triangles, a system already shown in previous studies to crystallize into 3D lattices readily. By varying sample chemistry, geometry, and thermodynamics, the researchers show that crystallization in this system is extremely robust despite sometimes massive heterogeneity, with triangles forming crystals over time if concentrations of the original building block were high enough. However, by varying the sequences of base pairs on the sticky ends of the triangle, the researchers show that crystal growth was extremely sensitive to the strength of Watson−Crick interactions. The authors suggest that these findings could help guide efforts to crystallize larger multichain DNA objects.
characteristics of these fabrics could lead to use in smart clothing, robotics, and medical devices.
SMALL METAL−ORGANIC FRAMEWORK ACCELERATES NERVE AGENT HYDROLYSIS Because nerve agents can kill after a short exposure, there is significant interest in developing prophylactic agents that can circulate in the blood to provide pre-exposure protection. Catalytic enzymes such as organophosphorous acid anhydrolase (OPAA) can hydrolyze nerve agents such as Soman. However, when distributed as free enzymes in the bloodstream, they are rapidly cleared by the immune system. Biodegradable liposomes have shown some utility as carriers for highly purified enzymes. However, these vehicles require handling and storage procedures that can be difficult in the field where these enzymes are likely to be administered. Seeking another vehicle for stable circulation of nerve agent hydrolyzing enzymes, Li et al. (DOI: 10.1021/ acsnano.6b04996) looked to metal−organic frameworks (MOFs), materials composed of metal ions or cluster nodes connected by a network of organic linkers. The researchers investigated the feasibility of immobilizing OPAA onto the MOF NU-1003, composed of zirconium-based nodes and tetracarboxylate linkers. With both micro- and mesopores, this MOF offers the largest pore aperture among all zirconiumbased MOFs reported to date. Synthesis produced hexagonal cylinder-shaped crystals with lengths ranging from 300 to 10000 nm. After soaking these crystals in OPAA, the researchers found that the smallest crystals had hydrolysis rates faster than the free enzyme in a buffer with a pH close to human blood for both Soman and a nerve agent simulant, diisopropyl fluorophosphates. The authors suggest that this nanosized OPAA carrier could have potential for use as an injectable antidote for nerve agent hydrolysis.
FOCUS ON THE DEFECTS FOR MXENES An increasing amount of research has focused on twodimensional (2D) materials over the past decade. These materials include MXene phases, materials that have shown significant promise for energy storage applications. MXene phases are formed by etching bulk layered MAX phases in which the “M” is a transition metal, “A” is a group A element, and “X” is either C or N, using hydrofluoric acid (HF) or a HFcontaining etchant. Although defects have been studied extensively in other 2D materials such as graphene, BN, and MoS2, showing their effects on electronic, electrical, and optoelectronic properties, little is known about defects in MXene phases. To investigate, Sang et al. (DOI: 10.1021/acsnano.6b05240) used atomic-resolution scanning transmission electron microscopy imaging on MXene phase Ti3C2Tx (where T stands for a surface termination group, such as −OH). After preparing monolayer flakes using a minimally intensive layer delamination method, the researchers characterized various types of Ti vacancies. They show that the higher the etchant concentration during preparation, the higher the Ti vacancy concentration. 9065
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These vacancies formed as either single point defects or clusters. Density functional theory-based calculations suggested that these defects significantly influenced the surface morphology of the flakes as well as their termination groups but did not affect the material’s metallic conductivity. Experiments in which the researchers used flakes to construct field-effect transistors showed that the flake thickness was more important for conductivity, with resistivity increasing as thickness increased. The authors suggest that this characterization could aid in positioning Ti3C2Tx as a promising 2D material for future nanoelectronic devices and catalytic applications.
ROPING IN A BETTER UNDERSTANDING OF CLOSED GRAIN BOUNDARY LOOPS Defects in graphene can detrimentally affect the exceptional electronic, mechanical, and chemical properties of this material. Of all possible defects, grain boundaries (GBs) have attracted increasing attention due to their inevitability with chemical vapor deposition synthesis, one of the most common methods to produce large-area graphene. Transmission electron microscopy and scanning tunneling microscopy studies have shown that this type of defect consists of a chain of adjacent pentagon−heptagon dislocation cores. Grain boundaries in graphene are most often classified as symmetric or asymmetric, which have been characterized both theoretically and experimentally in multiple studies. However, few studies have focused on a third type of GB known as the closed GB loop, in which the pentagon−heptagon pairs join end-to-end to generate a loop with the inner honeycomb lattice rotated. To characterize closed GB loops, Gong et al. (DOI: 10.1021/ acsnano.6b04959) used an in situ heating holder within an aberration-corrected transmission electron microscope to study these defects in graphene at the atomic level. After inducing closed GB loops either through high heat or focused electron beam radiation, the researchers investigated their structure and dynamics over time. Their findings show that large, closed GB loops induced by the electron beam relax into several isolated and distant dislocations. Line defects composed of multiple adjacent excess-atom clusters often appear during this reconfiguration process. Reduction in loop size was aided by dislocation ejection, which the researchers saw in real time. The authors suggest that this work could offer insights into defects that commonly arise in graphene at elevated temperatures.
WATER UNDER THE BRIDGE: ELECTROSTATICS IN TWO-DIMENSIONAL MOS2 WETTABILITY Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides (TMDs) including MoS2, have attracted increasing attention in recent years due to their unique and potential electrical and mechanical qualities. Because these materials come into contact with liquid media in several proposed applications, such as desalination membranes or surface coatings, developing a better understanding of how they interact with liquids will be key for engineering their synthesis and designing applications. Pristene graphene’s homopolar, covalent bonds preclude electrostatic effects that could affect wetting and adsorption phenomena. However, liquid interaction with TMDs could be significantly more complicated due to the presence of polar, partially ionic metal−chalcogen bonds in these materials. In a recent study, Rajan et al. (DOI: 10.1021/ acsnano.6b04276) use molecular dynamics simulations to investigate wettability and friction in MoS2. Their findings show that electrostatic interactions play only a minor role in determining the equilibrium contact angle on the MoS2 basal plane as well as on the friction coefficient and the slip length. Rather, these values appear instead to be determined by the exponential decay of the electrical potential above the MoS2 surface and dispersion interactions in the lateral direction caused by the trilayer structure of the MoS2 monolayer. In addition, these simulations show that MoS2 is only nonpolar in the low-energy, 2D basal plane and that other planes, such as the zigzag plane, are polar during interactions with water. Their findings further indicate that entropy plays an important role in the wettability of this material. The authors suggest that these findings offer important insights to understand TMD−liquid interactions better under diverse conditions.
NOT JUST A PHASE: CHEMICAL LITHIATION FOR BETTER ENERGY STORAGE Transition-metal dichalcogenides (TMDs) can exist in different polymorphs that can be influenced by charge transfer from intercalants, such as lithium and sodium ions. For example, Li intercalation can switch MoS2 from a 2H to an 1T′ phase, converting this material from semiconducting to metallic 9066
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character. Although this phase change has been studied for its potential use in catalyzing H2 evolution and electrical conductivity, less is known about the crystal domain size and defect generation that accompanies this change. In a recent study, Leng et al. (DOI: 10.1021/ acsnano.6b05746) examined the effects of Li intercalation on MoS2 using multiple analytical techniques. The researchers show that by controlling the relative concentrations of Li to bulk MoS2 during chemical intercalation, it is possible to produce a nearly 100% 1T′ phase. Microscopy after conversion showed that the original crystalline domains became interleaved nanosized domains distorted in three primary directions, creating a mosaic-like structure with numerous domain boundaries and extended defects. Additionally, the interlayers expanded considerably compared to bulk MoS2, giving a quasitwo-dimensional structure. When used as an electrode in a battery coin cell, the researchers found that the interconnected nanocrystals were able to reform after thousands of discharge/ charge cycles, showing high stability and high capacity. In contrast, when bulk MoS2 was electrochemically intercalated, it cracked and was pulverized after just a few cycles. Importantly, chemical intercalation provided the same benefits for WS2, suggesting general applicability to other TMDs. The authors suggest that this strategy could lead to novel electrode materials with high capacity and stability.
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