Nanocrystals Assembled by the Chemical Reaction of the Dispersion Solvent.

The development of methods to pattern nanocrystals with different sizes and shapes remains a challenge. In this study, we demonstrate a unique class of bottom-up approaches to assemble nanocrystals into patterns. Our approach for patterning nanocrystals focuses on the utilization and control of the chemical reaction of solvents surrounding nanocrystals. The photopolymerization of solvent molecules through a photomask creates time-dependent concentration gradients of the solvents. Dispersed nanocrystals such as silver nanowires (AgNWs) migrate and are gradually organized and integrated into the polymerizing films based on the concentration gradients. The AgNW-embedded film properties are determined by the organized AgNW structures and include light transmission and electrical conductivity. Overall, the demonstrated method is very simple, widely applicable to various nanocrystals and solvents, and can thus contribute to the development of a new class of nanocrystal patterning methods.

Effects of Nanowire Length on Charge Transport in Vertically Aligned Gold Nanowire Array Electrodes.

In this study, we demonstrate that vertically aligned gold nanowire array electrodes provide rapid ion and electron transport to the electrode-electrolyte interface. The charge-transport properties of the nanowire electrodes were investigated through cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy under a constant-volume device configuration. The total charge stored in the corresponding devices increases monotonically with the length of the nanowires owing to the concomitant increase in the electroactive real surface area of the electrode. A remarkable feature of the electrodes is that the internal resistance associated with charge transport decreases with increasing nanowire length. The electric double-layer capacitance per unit electroactive surface area remains constant up to high charge/discharge rates. Our results demonstrate that charge migration occurs rapidly on the surfaces of the nanowires regardless of their length and the charge/discharge rate used. Thus, vertically aligned nanowire array electrodes show promise as current collectors for next-generation electrochemical energy-storage devices.

Polymer networks with bicontinuous gradient morphologies resulting from the competition between phase separation and photopolymerization.

Poly(ethyl acrylate)/poly(methyl methacrylate) (PEA/PMMA) polymer networks (IPNs) with spatially graded bicontinuous morphology were designed and controlled by taking advantage of the spinodal decomposition process induced by photopolymerization of the MMA monomer. Spatial gradients of the quench depth, induced by the gradients of light intensity, were generated along the path of the excitation light travelling through the mixture. Bicontinuous structures with uniaxial gradient of characteristic length scales were obtained by two different methods: simply irradiating the mixture with strong light intensity along the Z-direction and using the so-called computer-assisted irradiation (CAI) method with moderate intensity to generate the light intensity gradient exclusively in the XY plane. These experimental results suggest that the combination of these two irradiation methods could provide polymer materials with biaxially co-continuous gradient morphology. An analysis method using the concept of spatial correlation function was developed to analyze the time-evolution of these graded structures. The experimental results obtained in this study suggest a promising method to design gradient polymers in the bulk state (3D) as well as on the surface (2D) by taking advantage of photopolymerization.

Interfacial structures of particle-stabilized emulsions examined by ultrasonic scattering analysis with a core-shell model.

Pickering emulsions comprising liquid droplets stabilized by solid microparticles have gained much attention in the field of cosmetics, inks, and drug delivery systems. To ensure that microparticles in Pickering emulsions are localized at the surface of liquid droplets, ultrasonic spectroscopy analysis combined with scattering function theory was conducted in this study. Two specific cases were investigated: (1) silica particles and liquid droplets independently dispersed in liquid and (2) silica particles effectively localized at the surface of the droplets. It was found that the core-shell model was effective for analyzing nanoparticles anchored at the surface of oil droplets. Conversely, it was found that an effective shell comprised of solid particles was no longer observed as the particle size or the distance between solid particles increased. When a large solid particle was applied, the ultrasonic spectra resembled those of conventional surfactant-stabilized emulsions without solid stabilizers.

Patterning Silver Nanowires by Inducing Transient Concentration Gradients in Reaction Mixtures.

Patterning nanocrystals in polymer films is essential for the widespread use of nanocrystals in various fields from optics to electronics; therefore, the development of patterning methods for nanocrystals is an important task. Here, we report a unique approach for patterning silver nanowires (AgNWs) using a thermodynamic driving force induced by transient concentration gradients in reaction mixtures. The procedure starts with the preparation of a photocurable monomer solution containing homogeneously dispersed AgNWs. Ultraviolet illumination through a straight-line mask reduces the polymerization rate of monomers in the masked area, decreasing the polymer concentration in comparison with that in the unmasked area. Such transient polymer concentration gradients yield imbalances in the chemical potentials of AgNWs, inducing the migration of AgNWs to form a straight-line pattern of AgNWs. The pattern of AgNWs was visualized via photoluminescence imaging under a laser scanning confocal microscope and compared with the light patterns applied to the mixture. These observations revealed that the magnitude of the AgNW migration is enhanced as the transient concentration gradient increases by thickening the mask to decrease the intensity of light passing through the mask. The structural features of the AgNW pattern were reproduced using numerical simulations based on a set of reaction-diffusion equations, which suggested the key role of the polymerization kinetics characterized by the Trommsdorff-Norrish effect. Moreover, as the AgNW pattern becomes clearer, the electrical resistance along the patterns decreases and more complex patterns can be produced, indicating the potential of the method. Overall, the present patterning method constitutes a simple approach that only requires illumination through a mask to generate the AgNW pattern, which renders it a promising alternative for patterning nanocrystals in polymer films.

Particle size distribution analysis of oil-in-water emulsions using static and dynamic ultrasound scattering techniques.

Dynamic ultrasound scattering allows us to investigate the particle motion and its average size via the time-evolution analysis of the scattering amplitude in optically turbid media. Recently, we proposed a novel particle sizing method that simultaneously analyzes the depth dependences on the sedimentation velocity and the scattered intensity without prior knowledge about the shape of the size distribution (Ultrasonics, 65 (2016) 59-68). In this study, the applicability of the technique to Fluorinert/water dilute and concentrated emulsions (up to 40 vol%) was examined. For the bimodal distribution of the emulsion, the size distribution of the particles was successfully obtained by directly probing the particle motion with different sizes as a function of the sample depth. The validity of the analysis was also investigated by comparing with conventional ultrasonic spectroscopy.

Ultrasound attenuation and phase velocity of moderately concentrated silica suspensions.

Ultrasound attenuation coefficient and phase velocity of moderately concentrated suspensions of charged silica particles were measured as a function of frequency. The attenuation coefficients were found to be significantly smaller than the theoretical prediction, and such a difference did not appear in the neutral particle suspensions under the corresponding concentrations. In this study, we have investigated the acoustic spectra of silica particles with different particle sizes, concentrations as well as salt concentrations. It was revealed that the noticeable deviation from the theoretical estimation only appeared in the case of large particle sizes close to the wavelength of ultrasound, and could be circumvented by addition of small amount of salt to suppress the electrostatic interactions of the charged silica particles.

Structures and dynamics of carbon-black in suspension probed by static and dynamic ultrasound scattering techniques.

Carbon black (CB) suspension exhibits various structures depending on the properties of solvent and dispersant as well as the preparation process of suspension. In most cases, CB particles do not exist as independent nanoparticles but as aggregates or agglomerates. In order to evaluate the size distribution at different level of hierarchal structure, we carried out static/dynamic ultrasound scattering analysis for the CB suspensions in alcohol and/or water with or without Nafion, a perfluorinated polymer. The potential of the dynamic ultrasound scattering technique was demonstrated by discriminating diffusing nanoparticles and micron-sized aggregates/agglomerates without dilution of the sample. Particularly, suppression of large agglomerates by addition of Nafion was clearly observed. Phosphotungstic acid (PWA), a family of polyoxometalate, was also employed to obtain smaller unit structures of the CB particles without formation of aggregation after decomposition of CB. The possible structures of the CB/PWA suspensions with and without Nafion were also discussed.

Simultaneous measurements of ultrasound attenuation, phase velocity, thickness, and density spectra of polymeric sheets.

The size distribution and mechanical properties of microparticle dispersed in liquid can be characterized by ultrasonic spectroscopy with the aid of acoustic scattering theories. In order to carry out the accurate analysis of the particles, the basic properties, such as the density, viscosity, longitudinal and shear velocities and intrinsic attenuation coefficient of the particle must be known prior to the analysis. Particularly, for soft elastomers or rubbers which exhibit complex mechanical properties with comparable real and imaginary parts, such fundamental information should be provided prior to the particle analysis to minimize the uncertainty of estimation associated with the number of adjustable parameters. In this study, we examined the acoustical properties of poly(methyl methacrylate)(PMMA) and cross-linked poly(dimethyl siloxane) sheets having different cross-linker concentrations by Multiple-Echo Reflection Ultrasonic Spectroscopy which simultaneously enabled us to acquire 4 fundamental properties, the ultrasound attenuation coefficient, phase velocity, density, and thickness (MERUS4 for solid plate). In addition, it was confirmed that the acoustic spectra of PMMA particles dispersed in water were reproduced well with the physical properties determined by MERUS4 using the PMMA plates.

Hexagonal phase induced by a reversible photo-cross-link reaction in a polymer mixture.

A hexagonal phase was found during the synthesis of interpenetrating polymer networks composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA). By using confocal microscopy, it was found that the regularity of this hexagonal phase further increases upon de-cross-linking of the PS networks in the matrix phase by irradiation with shorter uv wavelengths. We conclude that the cooperation between the cross-link-induced suppression of phase separation and the elastic repulsion between the dispersed PMMA-rich domains is responsible for the emergence of this hexagonal phase.

Ultrasound attenuation and phase velocity of micrometer-sized particle suspensions with viscous and thermal losses.

The attenuation coefficient and the phase velocity of micrometer-sized polydivinylbenzene particles in water were investigated by ultrasound spectroscopy equipped with 10-30MHz longitudinal wave transducers. While the surrounding liquid could be assumed to be inviscid for large particles with the size comparable to the wavelength of longitudinal ultrasound, the viscous and thermal waves were considered to have important roles with decreasing the particle size because the particle size becomes comparable with those wavelengths. In this study, these contributions were systematically investigated by changing the particle-size.

Size distribution and elastic properties of thermo-responsive polymer gel microparticles in suspension probed by ultrasonic spectroscopy.

The size distribution and the elasticity of thermo-responsive gel particles dispersed in water were examined in-situ by ultrasonic spectroscopy (US). Poly(N-isopropylacrylamide) (PNIPAM) gel particles, undergoing volume phase transition at 34°C were synthesized by redox polymerization in emulsion, and the temperature dependence of the acoustic properties was examined. The longitudinal velocity and the attenuation coefficient of particle exhibited a drastic change at the transition temperature, indicating the increase in the mechanical properties, such the longitudinal, bulk and shear moduli, due to the shrinkage of particles upon temperature stimulation. The shear modulus of monodisperse particles, and also the stiffness of the gel particles were evaluated.

Metal Nanowire-Based Hybrid Electrodes Exhibiting High Charge/Discharge Rates and Long-Lived Electrocatalysis.

Nanostructured electrodes are at the forefront of advanced materials research, and have been studied extensively in the context of their potential applications in energy storage and conversion. Here, we report on the properties of core-shell (gold-polypyrrole) hybrid nanowires and their suitability as electrodes in electrochemical capacitors and as electrocatalysts. In general, the specific capacitance of electrochemical capacitors can be increased by faradaic reactions, but their charge transfer resistance impedes charge transport, decreasing the capacitance with increasing charge/discharge rate. The specific capacitance of the hybrid electrodes is enhanced due to the pseudocapacitance of the polypyrrole shells; moreover, the electrodes operate as an ideal capacitive element and maintain their specific capacitance even at fast charge/discharge rates of 4690 mA/cm3 and 10 V/s. These rates far exceed those of other types of pseudocapacitors, and are even superior to electric double layer-based supercapacitors. The mechanisms behind these fast charge/discharge rates are elucidated by electrochemical impedance spectroscopy, and are ascribed to the reduced internal resistance associated with the fast charge transport ability of the gold nanowire cores, low ionic resistance of the polypyrrole shells, and enhanced electron transport across the nanowire's junctions. Furthermore, the hybrid electrodes show great catalytic activity for ethanol electro-oxidation, comparable to bare gold nanowires, and the surface activity of gold cores is not affected by the polypyrrole coating. The electrodes exhibit improved stability for electrocatalysis during potential cycling. This study demonstrates that the gold-polypyrrole hybrid electrodes can store and deliver charge at fast rates, and that the polypyrrole shells of the nanowires extend the catalytic lifetime of the gold cores.

Metal-Organic Coaxial Nanowire Array Electrodes Combining Large Energy Capacity and High Rate Capability.

Pseudocapacitors have been widely studied in the context of their potential applications in portable electronics and energy regeneration. However, the internal resistance within these devices hampers charge transport and limits their performance. As a result, maximum charge/discharge rates are typically limited to a few hundred mV s-1 for pseudocapacitors. Beyond this limit, capacitance rapidly decreases and devices become incapable of storing energy. Here, we design electrodes in which coaxial nanowires made of highly conductive metal cores and pseudocapacitive organic shells are fabricated into a seamless, monolithic, and vertically aligned structure. The design of this structure reduces its internal resistance, and devices fabricated using these electrodes exhibit excellent energy capacity even when charged/discharged at high rates of more than a few hundred mV s-1 . The energy density obtained in these devices corresponds to the maximum energy density predicted by the Trasatti method, and the coaxial-nanowire structure of the electrodes enhances the charge storage capacity and rate capability simultaneously.

Autocatalytic phase separation and graded co-continuous morphology generated by photocuring.

Interpenetrating polymer networks (IPNs) with spatially graded co-continuous structures were constructed by photo-cross-linking a homogeneous mixture of photo-reactive polystyrene and methyl methacrylate monomer. For a given thickness, irradiation with weak ultraviolet (UV) light results in a co-continuous morphology uniform throughout the sample. However, as the irradiation intensity increases to some certain extent, spatially graded co-continuous morphology in the micrometre scales emerges due to the significant effect of the gradient of the light intensity along the irradiation direction. These 3-dimensional graded structures were observed by using laser scanning confocal microscopy (LSCM) and were analyzed by digital image analysis. The depth dependence of these graded structures can be well expressed by a power law with an exponent depending strongly on the irradiation intensity. The time-evolution of the graded morphology was monitored at different depths of the same irradiated sample. It was found that the phase separation does not follow conventional laws of kinetics, but instead exhibits the autocatalytic behavior, reflecting the effects of the heat produced by the photopolymerization of MMA monomer on the phase separation process. The experimental data obtained in this study suggest a method of producing polymeric materials with spatially graded structures in the micrometer scales by solely changing the irradiation intensity.

Dynamics of micron-sized particles in dilute and concentrated suspensions probed by dynamic ultrasound scattering techniques.

A novel ultrasound technique called Frequency-Domain Dynamic ultraSound Scattering (FD-DSS) was employed to determine sedimentation velocities and the diameters of microparticles in a highly turbid suspension. The paper describes the importance of the scattering vector q for dynamic scattering experiments using broadband ultrasound pulses because q (or frequency) corresponds to the spatial length scale whereas the pulses involve inevitable uncertainty in the time domain due to the frequency distribution of broadband pulse. The results obtained from Stokes velocity of monodispersed silica and polydivinylbenzene (PDVB) particles were compared to those obtained by a Field Emission Scanning Electron Microscope (FE-SEM). A novel method to extract the particle size distribution is also demonstrated based on an ultrasound scattering theory.

Sound velocity and attenuation coefficient of hard and hollow microparticle suspensions observed by ultrasound spectroscopy.

Size and elastic properties of micro-particles suspended in liquid can be acoustically determined by ultrasound attenuation and velocity measurements with the aid of elastic scattering theories and a dispersion relation. While quantitative evaluation for hard micron-sized spheres using the theories is available in literature, that for hollow particles is not yet achieved. In this study, we show that the shell thickness and the elastic modulus of hollow particles can be quantitatively evaluated by ultrasound spectroscopy. Several kinds of microparticles including polystyrene rigid particles, polydivinylbenzene rigid particles, borosilicate hollow particles, and phenolic-resin hollow particles were examined as a function of the particle concentration.

Origin of the anomalous decrease in the apparent density of polymer gels observed by multi-echo reflection ultrasound spectroscopy.

Multi-echo reflection ultrasound spectroscopy (MERUS), which enables one to simultaneously evaluate the attenuation coefficient α, the sound velocity v and the density ρ, has been developed for measurements of elastic moduli. In the present study, the technique was further developed to analyze systems undergoing gelation where an unphysical decrease in the apparent density was previously observed after polymerization. The main reason for this problem was that the shrinkage accompanying the gelation led to a small gap between the cell wall and the sample, resulting in the superposition of the reflected signals which were not separable into individual components. By taking into account the multiply reflecting echoes at the interface of the gap, the corrected densities were systematically obtained and compared with the results for the floating test. The present technique opens a new route to simultaneously evaluate the three parameters α, v and ρ and also the sample thickness for solid thin films.

Collective motion of microspheres in suspensions observed by phase-mode dynamic ultrasound scattering technique.

Compared with a nano-sized particle, dynamics of a micron-sized particle in a liquid is often associated with sedimentation (or floating) due to its relatively large mass. The motion of more than two particles is dominated by the hydrodynamic interactions, which are known to persist over a fairly long range, e.g., several millimeters, in suspensions. The particle size may be obtained from the dynamic ultrasound scattering (DSS) technique by the analysis of velocity fluctuations, whose origin is believed to take root in the particle-number fluctuations among temporally formed domains involving collective motion of particles with a certain cut-off length. In this study, such collective particle motion in highly turbid solutions was visualized by means of the phase-mode DSS technique with a single element transducer. Quantitative agreement between the velocity fluctuations obtained by the phase- and conventional amplitude-mode analyses was confirmed, followed by examination of the concentration and the particle size dependences on the dynamic structures induced by the long-ranged interactions. It was found that the phase mode-DSS was a promising method to evaluate the time-dependent structures of the micro-particles in highly turbid suspensions.

Simultaneous evaluation of ultrasound velocity, attenuation and density of polymer solutions observed by multi-echo ultrasound spectroscopy.

Ultrasound spectroscopy is a powerful tool to investigate the viscoelastic properties of materials. The longitudinal elastic moduli M' and M(″), or the adiabatic compressibility κ(S) can be evaluated from ultrasound velocity v and attenuation coefficient α via the relation M'=ρv(2) and M(″)=2ραv(3)/ω, where ρ is the density and ω is the angular frequency. So far, the density was independently measured by other equipments or its variation during the chemical reaction has been ignored in the previous literatures. Here we propose a multiple echo method to simultaneously evaluate α, v, ρ, from a single acquisition, enabling us to monitor the polymerization process of acrylamide, where the three parameters vary independently during the reaction. This allows us to analyze the time evolution of the acoustic parameters for polymeric or gelling systems with the better understanding.