Chain-like One-Dimensional Assembly of Mesoporous Silica Nanoparticles: An Approach To Improve Hydrogel Adhesion.

Mesoporous silica nanoparticles (MSNPs) are well known for their adhesive properties with hydrogels and living tissues. However, achieving direct contact between the silica nanoparticle surface and the adherend necessitates the removal of capping agents, which can lead to severe aggregation when exposed to wet surfaces. This aggregation is ineffective for simultaneously bridging the two adherends, resulting in a reduced adhesive strength. In this study, we designed and synthesized mesoporous silica nanochains (MSNCs) to enhance the interactions with hydrogels by promoting the formation of coarser structures with increased nanopore exposure. Chain-like one-dimensional assemblies in the MSNCs were generated by depleting the capping ligand, cetyltrimethylammonium bromide, from the surface of the MSNPs. To quantify the porous areas of the MSNCs, we analyzed scanning electron microscopy (SEM) images using an in-house SEM image analysis algorithm. Additionally, we conducted a comparative assessment of the adhesion energies of MSNCs and MSNPs on a poly(dimethylacrylamide) hydrogel using a universal testing machine. The MSNCs exhibited a maximum adhesion energy of 13.7 ± 0.7 J/m2 at 3 wt %, surpassing that of MSNPs (10.9 ± 0.3 J/m2) at 2 wt %. Moreover, the unique stacking structure of the MSNCs enabled them to maintain an adhesion energy of 13.4 ± 1.0 J/m2 at a high concentration of 9 wt %, whereas the adhesion energy of MSNPs decreased to 8.2 ± 0.4 J/m2. This underscores their potential as superior hydrogel adhesives in challenging wet tissue-like environments.

High-Efficiency Anion-Exchange Membrane Water Electrolyzer Enabled by Ternary Layered Double Hydroxide Anode.

Developing high-efficiency and low-cost oxygen-evolving electrodes in anion exchange membrane (AEM) water electrolysis technology is one of the major challenges. Herein, it is demonstrated that the surface corrosion of a conventional Ni foam electrode in the presence of Fe3+ and V3+ cations can transform it into an electrode with a high catalytic performance for oxygen evolution reaction (OER). The corroded electrode consists of a ternary NiFeV layered double hydroxide (LDH) nanosheet array supported on the Ni foam surface. This NiFeV LDH electrode achieves an OER current density of 100 mA cm-2 at an overpotential of 272 mV in 1 m KOH, outperforming the IrO2 catalyst by 180 mV. Density functional theory calculations reveal that the unique structure and the presence of vanadium in NiFeV LDH play a key role in achieving improved OER activity. When coupled with a commercial Pt/C cathode catalyst, the resulting AEM water electrolyzer achieves a cell current density as high as 2.1 A cm-2 at a voltage of only 1.8 Vcell in 1 m KOH, which is similar to the performance of the proton exchange membrane water electrolyzer obtained from the IrO2 and Pt/C catalysts pair.

Direct Chemical Synthesis of Plasmonic Black Colloidal Gold Superparticles with Broadband Absorption Properties.

Self-assembly of plasmonic metal nanoparticles can provide an opportunity of creating colloidal superparticles with fascinating optical properties arising from interparticle plasmonic coupling, but typically requires multiple steps involving solvent and/or ligand exchange. We developed a direct, one-step chemical synthesis of plasmonic black colloidal Au superparticles with broadband absorption in visible and near-infrared regions. During the synthesis, the Au superparticles were formed through self-assembly of in-situ-formed Au nanoparticles driven by solvophobic interactions between nanoparticles and solvent. These superparticles could be solution-processed to fabricate a thin film, which exhibited near-perfect absorption over a broad range from 400 nm to 2.5 µm as well as the excellent antireflective property. Thanks to their broadband absorption property, the Au superparticles showed good performances for near-infrared surface-enhanced Raman spectroscopy and light-to-heat conversion.

Binary FeCo Oxyhydroxide Nanosheets as Highly Efficient Bifunctional Electrocatalysts for Overall Water Splitting.

Bifunctional catalysts that are highly active toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are attractive for efficient electrochemical water splitting. Herein, we report a bifunctional FeCoOOH nanosheet catalyst for highly efficient electrochemical water splitting in an alkaline electrolyte. The FeCoOOH nanosheet arrays were grown directly on the surface of a porous Ni foam by using a simple hydrothermal method. Because of their binary oxyhydroxide structure and high electrical conductivity intrinsic to direct growth, these FeCoOOH nanosheets exhibited excellent activities toward both the HER and OER. With the use of this bifunctional FeCoOOH catalyst, an alkaline water electrolyzer in a two-electrode configuration achieved 10 mA cm-2 only at a cell voltage of 1.62 V without iR compensation in 1 m KOH, which outperformed that based on the combination of commercial IrO2 and Pt/C catalysts.

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

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

Shape-controlled synthesis of Pd nanocrystals and their catalytic applications.

Palladium is a marvelous catalyst for a rich variety of reactions in industrial processes and commercial devices. Most Pd-catalyzed reactions exhibit structure sensitivity, meaning that the activity or selectivity depends on the arrangement of atoms on the surface. Previously, such reactions could only be studied in ultrahigh vacuum using Pd single crystals cut with a specific crystallographic plane. However, these model catalysts are far different from real catalytic systems owing to the absence of atoms at corners and edges and the extremely small specific surface areas for the model systems. Indeed, enhancing the performance of a Pd-based catalyst, in part to reduce the amount needed of this precious and rare metal for a given reaction, requires the use of Pd with the highest possible specific surface area. Recent advances in nanocrystal synthesis are offering a great opportunity to investigate and quantify the structural sensitivity of catalysts based on Pd and other metals. For a structure-sensitive reaction, the catalytic properties of Pd nanocrystals are strongly dependent on both the size and shape. The shape plays a more significant role in controlling activity and selectivity, because the shape controls not only the facets but also the proportions of surface atoms at corners, edges, and planes, which affect the outcomes of possible reactions. We expect catalysts based on Pd nanocrystals with optimized shapes to meet the increasing demands of industrial applications at reduced loadings and costs. In this Account, we discuss recent advances in the synthesis of Pd nanocrystals with controlled shapes and their resulting performance as catalysts for a large number of reactions. First, we review various synthetic strategies based on oxidative etching, surface capping, and kinetic control that have been used to direct the shapes of nanocrystals. When crystal growth is under thermodynamic control, the capping agent plays a pivotal role in determining the shape of a product by altering the order of surface energies for different facets through selective adsorption; the resulting product has the lowest possible total surface energy. In contrast, the product of a kinetically controlled synthesis often deviates from the thermodynamically favored structure, with notable examples including nanocrystals enclosed by high-index facets or concave surfaces. We then discuss the key parameters that control the nucleation and growth of Pd nanocrystals to decipher potential growth mechanisms and build a connection between the experimental conditions and the pathways to different shapes. Finally, we present a number of examples to highlight the use of these Pd nanocrystals as catalysts or electrocatalysts for various applications with structure-sensitive properties. We believe that a deep understanding of the shape-dependent catalytic properties, together with an ability to experimentally maneuver the shape of metal nanocrystals, will eventually lead to rational design of advanced catalysts with substantially enhanced performance.

Reverse micelle synthesis of colloidal nickel-manganese layered double hydroxide nanosheets and their pseudocapacitive properties.

Colloidal nanosheets of nickel-manganese layered double hydroxides (LDHs) have been synthesized in high yields through a facile reverse micelle method with xylene as an oil phase and oleylamine as a surfactant. Electron microscopy studies of the product revealed the formation of colloidal nanoplatelets with sizes of 50-150 nm, and X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy studies showed that the Ni-Mn LDH nanosheets had a hydrotalcite-like structure with a formula of [Ni3 Mn(OH)8 ](Cl(-) )⋅n H2 O. We found that the presence of both Ni and Mn precursors was required for the growth of Ni-Mn LDH nanosheets. As pseudocapacitors, the Ni-Mn LDH nanosheets exhibited much higher specific capacitance than unitary nickel hydroxides and manganese oxides.

Synthesis of copper nanoparticles with controlled sizes by reverse micelle method.

Copper (Cu) nanoparticles with controllable sizes were successfully synthesized by using a modified reverse micelle method, in which copper(II) acetate was reacted with L-ascorbic acid in a solution containing water and xylene in the presence of oleic acid and oleylamine as surfactants. The as-synthesized Cu nanoparticles had a nearly spherical profile and multiple-twinned structures, whose surface was enclosed by {111} facets. Our synthetic procedure provides a simple and readily scalable route to the preparation of Cu nanoparticles with controllable sizes.

Quick, controlled synthesis of ultrathin Bi2Se3 nanodiscs and nanosheets.

Ultrathin (4-6 nm) single-crystal Bi(2)Se(3) nanodiscs and nanosheets were synthesized through a simple and quick solution process. The growth mechanism was investigated in detail. Crystal seeds grew via 2D self-attachment of small nanoparticles followed by epitaxial recrystallization into single crystals. The lateral dimension of the nanodiscs increased and their shape changed from circles to hexagons as the reaction temperature increased. Positively charged polymer surfactants greatly enlarged the lateral dimension to produce nanosheets with uniform thickness.

Self-Assembled 4-Aminopyridine Monolayer as a Nucleation-Inducing Layer for Transparent Silver Electrodes.

The role of a self-assembled monolayer obtained by vacuum deposition of 4-aminopyridine (4-AP), a small organic molecule having amine and pyridine groups, as a metal nucleation inducer and adhesion promoter was verified, and the applicability was evaluated. 4-AP deposited to an extremely thin thickness effectively changed the substrate surface properties, increasing the nucleation density of silver (Ag) more than 3 times and eventually forming a more transparent, low-resistance Ag thin film. The optical transmittance of the Ag thin film, which was less than 60% when 4-AP was not applied, could be increased to about 77% by simply applying 4-AP, and the electrical resistance could be lowered from 37 to 14 Ω/square at the same time. Transmittance could be further improved to higher than 90% by depositing an antireflection layer for use as a transparent Ag electrode. It was also verified that 4-AP not only serves as a nucleation inducer but also contributes to improving interfacial adhesion. The Ag transparent electrode using 4-AP provided the improved performance of the organic light-emitting device due to higher transmittance, lower resistance, and surface roughness. Small organic molecules including functional groups that can be vacuum deposited, such as 4-AP, are expected to be used as surface pretreatment materials for various depositions because they can be easily patterned and can efficiently modify the surface even with extremely thin thickness.

Tannic acid modified antifreezing gelatin organohydrogel for low modulus, high toughness, and sensitive flexible strain sensor.

Current hydrogel strain sensors have met assorted essential requirements of wearing comfort, mechanical toughness, and strain sensitivity. However, an increment in the toughness of a hydrogel usually leads to an increase in elastic moduli that could be unfavorable for wearing comfort. In addition, traits of biofriendly and sustainability require synthesis of the hydrogels from natural polymer-based networks. We propose a novel strategy to fabricate an ionic conductive organohydrogel from natural biological macromolecule "gelatin" and polyacid "tannic acid" to resolve these challenges. Tannic acid modified the structure of the gelatin network in the ionic conductive organohydrogels, that not only led to an increase in toughness accompanying a decrease in elastic moduli but also headed to higher strain sensitivity and tunability. The proposed methodology exhibited tunable tensile modulus from 27 to 13 kPa, tensile strength from 287 to 325 kPa, elongation at fracture from 510 to 620%, toughness from 500 to 550 kJ/m3, conductivity from 0.29 to 0.8 S/m, and strain sensitivity (GF = 1.4-6.5). Moreover, the proposed organohydrogel exhibited excellent freezing tolerance. This study provides a facile yet powerful strategy to tune the mechanical and electrical properties of organohydrogels which can be adapted to various wearable sensors.

Metal nanocrystals with highly branched morphologies.

Metal nanocrystals with highly branched morphologies are an exciting new class of nanomaterials owing to their unique structures, physicochemical properties, and great potential as catalysts, sensing materials, and building blocks for nanoscale devices. Various strategies have recently been developed for the solution-phase synthesis of metal nanocrystals with branched morphologies, such as multipods and nanodendrites. In this Minireview, the procedures and mechanisms underlying the formation of branched metal nanocrystals are presented in parallel with recent advances in synthetic approaches based on kinetically controlled overgrowth, aggregation-based growth, heterogeneous seeded growth, selective etching, and template-directed methods, as well as their properties for catalytic or electrocatalytic applications.

High-Performance Non-Volatile InGaZnO Based Flash Memory Device Embedded with a Monolayer Au Nanoparticles.

Non-volatile memory (NVM) devices based on three-terminal thin-film transistors (TFTs) have gained extensive interest in memory applications due to their high retained characteristics, good scalability, and high charge storage capacity. Herein, we report a low-temperature (<100 °C) processed top-gate TFT-type NVM device using indium gallium zinc oxide (IGZO) semiconductor with monolayer gold nanoparticles (AuNPs) as a floating gate layer to obtain reliable memory operations. The proposed NVM device exhibits a high memory window (ΔVth) of 13.7 V when it sweeps from -20 V to +20 V back and forth. Additionally, the material characteristics of the monolayer AuNPs (floating gate layer) and IGZO film (semiconductor layer) are confirmed using transmission electronic microscopy (TEM), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS) techniques. The memory operations in terms of endurance and retention are obtained, revealing highly stable endurance properties of the device up to 100 P/E cycles by applying pulses (±20 V, duration of 100 ms) and reliable retention time up to 104 s. The proposed NVM device, owing to the properties of large memory window, stable endurance, and high retention time, enables an excellent approach in futuristic non-volatile memory technology.

Transparent Molecular Adhesive Enabling Mechanically Stable ITO Thin Films.

With rapid advances in flexible electronics, transparent conductive electrodes (TCEs) have also been significantly developed as alternatives to the conventional indium tin oxide (ITO)-based material systems that exhibit low mechanical flexibility. Nanomaterial-based alternating materials, such as graphene, nanowire, and nanomesh, exhibit remarkable properties for TCE-based applications, such as high electrical conductivity, high optical transparency, and high mechanical stability. However, these nanomaterial-based systems lack scalability, which is a key requirement for practical applications, and exhibit a size-dependent property variation and inhomogeneous surface uniformity that limit reliable properties over a large area. Here, we exploited a conventional ITO-based material platform; however, we incorporated a transparent molecular adhesive, 4-aminopyridine (4-AP), to improve mechanical flexibility. While the presence of 4-AP barely affected optical transmittance and sheet resistance, it improved interfacial adhesion between the substrate and ITO as well as formed a wavy surface, which could improve the mechanical flexibility. Under various mechanical tests, ITO/4-AP/poly(ethylene terephthalate) (PET) exhibited remarkably improved mechanical flexibility as compared with that of ITO/PET. Furthermore, ITO/4-AP/PET was utilized for a flexible Joule heater application having spatial uniformity of heat generation, voltage-dependent temperature control, and mechanical flexibility under repeated bending tests. This molecular adhesive could overcome the current limitations of material systems for flexible electronics.

Synthesis of Pd-Au bimetallic nanocrystals via controlled overgrowth.

This paper describes the synthesis of Pd-Au bimetallic nanocrystals with controlled morphologies via a one-step seeded-growth method. Two different reducing agents, namely, L-ascorbic acid and citric acid, were utilized for the reduction of HAuCl(4) in an aqueous solution to control the overgrowth of Au on cubic Pd seeds. When L-ascorbic acid was used as the reducing agent, conformal overgrowth of Au on the Pd nanocubes led to the formation of Pd-Au nanocrystals with a core-shell structure. On the contrary, localized overgrowth of Au was observed when citric acid was used as the reducing agent, producing Pd-Au bimetallic dimers. Through this morphological control, we were able to tune the localized surface plasmon resonance peaks of Pd-Au bimetallic nanostructures in the visible region.

Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics?

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Unveiling the composite structures of emissive consolidated p-i-n junction nanocells for white light emission.

Hybrid organic-Red-Green-Blue (RGB) color quantum dots were incorporated into consolidated p(polymer)-i(RGB quantum dots)-n(small molecules) junction structures to fabricate a single active layer for a light emitting diode device for white electroluminescence. The semiconductor RGB quantum dots, as an intrinsic material, were electrostatically bonded between functional groups of the p-type polymer organic material core surface and the n-type small molecular organic material shell surface. The ZnCdSe/ZnS and CdSe/ZnS quantum dots distributed uniformly and isotropically surrounding the polymer core which in turn was surrounded by small molecular organic materials. In the present study, we have identified the mechanisms of chemical synthesis and interactions of the p-i-n junction nanocell structure through modeling studies by DFT calculations. We have also investigated optical, structural and electrical properties along with the carrier transport mechanism of the light emitting diodes which have a single active layer of consolidated p-i-n junction nanocells for white electroluminescence.

Five-minute synthesis of silver nanowires and their roll-to-roll processing for large-area organic light emitting diodes.

Silver (Ag) nanowires (NWs) are promising building blocks for flexible transparent electrodes, which are key components in fabricating soft electronic devices such as flexible organic light emitting diodes (OLEDs). Typically, Ag NWs have been synthesized using a polyol method, but it still remains a challenge to produce high-aspect-ratio Ag NWs via a simple and rapid process. In this work, we developed a modified polyol method and newly found that the addition of propylene glycol to ethylene glycol-based polyol synthesis facilitated the growth of Ag NWs, allowing the rapid production of long Ag NWs with high aspect ratios of about 2000 in a high yield (∼90%) within 5 min. Transparent electrodes fabricated with our Ag NWs exhibited performance comparable to that of an indium tin oxide-based electrode. With these Ag NWs, we successfully demonstrated the fabrication of a large-area flexible OLED with dimensions of 30 cm × 15 cm using a roll-to-roll process.

Electrostatically regulated ternary-doped carbon foams with exposed active sites as metal-free oxygen reduction electrocatalysts.

Pt, a representative electrocatalyst for the oxygen reduction reaction (ORR), has suffered from high cost and poor stability, and thus it is essential to develop alternative electrocatalyst with a high catalytic activity comparable to Pt. Herein, we propose a rationally designed metal-free electrocatalyst with exposed active sites using an N, P, and S ternary-doped and graphene-incorporated porous carbon foam. We developed a novel template-free synthetic approach wherein the electrostatically-mediated complexation of graphene oxide (GO) with 2-aminothiazole (2AT) and branched polyethylenimine (PEI) in the presence of phytic acid (PA) was first induced, followed by a carbonization process to drive the formation of a three-dimensionally interconnected porous carbon foam. The resulting electrocatalyst exhibited a high pore volume and greatly extended specific surface area along with exposed active sites. Benefiting from these properties, the synthesized ternary-doped carbon foam displayed an outstanding electrocatalytic activity for the oxygen reduction ORR through four-electron transfer pathways. We observed that the remarkably improved ORR performance of the synthesized materials manifested an onset and a half-wave potential, mostly close to those of the commercially available ORR electrocatalyst of 20 wt% Pt/C while securing a greater stability in alkaline media.