Minoru Miyahara

Kyoto University, Kioto, Kyōto, Japan

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Publications (70)142.82 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Gold nanoshells with tunable surface plasmon resonances are a promising material for optical and biomedical applications. They are produced through seed-mediated growth, in which gold nanoparticles (AuNPs) are seeded on the core particle surface followed by growth of the gold seeds into a shell. However, synthetic gold nanoshell production is typically a multistep, time-consuming batch-type process, and a simple and scalable process remains a challenge. In the present study, a continuous flow process for the seed-mediated growth of silica–gold nanoshells is established by exploiting the excellent mixing performance of a microreactor. In the AuNP-seeding step, the reduction of gold ions in the presence of core particles in the microreactor enables the one-step flow synthesis of gold-decorated silica particles through heterogeneous nucleation. Flow shell growth is also realized using the microreactor by selecting an appropriate reducing agent. Because self-nucleation in the bulk solution phase is suppressed in the microreactor system, no washing is needed after each step, thus enabling the connection of the microreactors for the seeding and shell growth steps into a sequential flow process to synthesize gold nanoshells. The established system is simple and robust, thus making it a promising technology for producing gold nanoshells in an industrial setting.
    Particle and Particle Systems Characterization 08/2014; · 0.86 Impact Factor
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    ABSTRACT: Zeolitic imidazolate framework-8 (ZIF-8) has a “gate-opening” framework with narrow pore apertures that swing open by reorientation of 2-methylimidazolate (MeIM) linkers enforced by guest adsorption. The present study aimed to employ free energy analysis to provide insight into the mechanism of the adsorption-induced structural transition that results from the reorientation of the MeIM linkers. We combined experimental Ar adsorption at cryogenic temperatures with grand canonical Monte Carlo simulations to determine the free energy profiles as functions of the rotational angle of the MeIM linker (θIM) and bulk gas pressure. We also estimated the energy fluctuation of the system, which is crucial to discussing the structural transition from a metastable state. The results from the free energy analysis, for example, at 91 K, suggest the following conclusions: A gradual reorientation of the MeIM linkers up to θIM = 10.5° occurs with increasing gas pressure that is followed by a spontaneous structural transition to θIM = 25.5° during the adsorption process (gate opening), and then, during the desorption process, an equilibrium structural transition occurs with the opposite reorientation of the MeIM linkers from θIM = 25.5° to θIM = 10.5° (gate closing).
    The Journal of Physical Chemistry C. 04/2014; 118(16):8445–8454.
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    ABSTRACT: We have analyzed various phenomena that occur in nanopores, focusing on elucidating their key mechanisms, to advance the effective engineering use of nanoporous materials. As ideal experimental systems, molecular simulations can effectively provide information at the molecular level that leads to mechanistic insight. In this short review, several of our recent results are presented. The first topic is the critical point depression of Lennard-Jones fluid in silica slit pores due to finite size effects, studied by our original Monte Carlo (MC) technique. We demonstrate that the first layers of adsorbed molecules in contact with the pore walls act as a “fluid wall” and impose extra finite size effects on the fluid confined in the central portion of the pore. We next present a new kernel for pore size distribution (PSD) analysis, based entirely on molecular simulation, which consists of local isotherms for nitrogen adsorption in carbon slit pores at 77 K. The kernel is obtained by combining grand canonical Monte Carlo (GCMC) method and open pore cell MC method that was developed in the previous study. We show that overall trends of the PSDs of activated carbons calculated with our new kernel and with conventional kernel from non-local density functional theory are nearly the same; however, apparent difference can be seen between them. As the third topic, we apply a free energy analysis method with the aid of GCMC simulations to investigate the gating behavior observed in a porous coordination polymer, and propose a mechanism for the adsorption-induced structural transition based on both the theory of equilibrium and kinetics. Finally, we construct an atomistic silica pore model that mimics MCM-41, which has atomic-level surface roughness, and perform molecular simulations to understand the mechanism of capillary condensation with hysteresis. We calculate the work required for the gas–liquid transition from the simulation data, and show that the adsorption branch with hysteresis for MCM-41 arise from spontaneous capillary condensation from a metastable state.
    Adsorption 01/2014; · 1.55 Impact Factor
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    ABSTRACT: Microwire networks composed of noble metal particles are promising for the use of transparent conductive films. Bottom-up approaches can offer a route to establishing a fabrication technique that is robust and cost-effective, and template-assisted self-assembly techniques are widely used. However, they require additional processes to prepare templates and generally suffer from the difficulty in a large-scale fabrication. A template-free technique thus waits to be developed.In the present study, we explore a template free technique to fabricate colloidal networks of Au nanoparticles. We combine the convective self-assembly method with a liquid-level manipulation scheme in which the suspension is periodically pumped out. By using the technique, we successfully fabricate stripe, grid, and triangle patterns with controlled periodicity and examine the relationship between operation parameters and the resultant structures. We then measure the transparency and conductivity of a grid pattern to demonstrate the property as the transparent conductive film.
    Advanced Powder Technology 01/2014; · 1.65 Impact Factor
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    ABSTRACT: Porous coordination polymers (PCPs) with soft frameworks show a gate phenomenon consisting of an abrupt structural transition induced by adsorption of guest molecules. To understand the dependence of the gating behavior on the host framework structure, we conduct grand canonical Monte Carlo simulations and a free-energy analysis of a simplified model of a stacked-layer PCP. The interlayer width of the rigid layers composing the simplified model can be changed by guest adsorption and by varying the initial interlayer width h0, which is controlled by the length of pillars between the layers. We introduce three types of gating behavior, one-step gating, filling and gating, and double gating, which depend on three parameters: the initial interlayer width h0; the interaction parameter ∊ss, which determines the host-guest framework interaction as well as the inter-framework interaction; and the elastic modulus of the framework, which depends on the stiffness of the pillars. We show that the one-step gating and the filling and gating behaviors depend strongly on h0 rather than on ∊ss, and thus a transformation from filling and gating to double gating can be achieved by reducing the stiffness of the host framework. This study should be a guideline for controlling the gating pressure of PCPs by modifying their chemical components.
    12/2013; 140(4).
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    ABSTRACT: Nanoporous materials, such as zeolites, activated carbons, and metal-organic frameworks (MOFs), are peculiar platforms in which a variety of guest molecules are stored, reacted, and/or separated. The size of the nanopores is essential to realize advanced functions. In this work, we demonstrate a very simple but innovative method for the control of nanopore size, that is, reversible and continuous control by mechanical force loaded to soft nanoporous materials. The elastic properties of several microporous materials, including zeolites, zeolite-templated carbon (ZTC), activated carbon, and MOFs (e.g., ZIF-8), are examined and it is found that ZTC is a material that is suitable for the aforementioned idea thanks to its extraordinary soft properties compared to the others. The original pore size of ZTC (1.2 nm) can be contracted to 0.85 nm by using a relatively weak loading force of 135 MPa, whereas the other microporous materials barely contracted. To demonstrate the change in the physical properties induced by such artificial deformation, in situ gas adsorption measurements were performed on ZTC with and without loading mechanical force, by using CO2 , CH4 , and H2 , as adsorbates. Upon the contraction by loading 69 or 135 MPa, CO2 adsorption amount is increased, due to the deepening of the physisorption potential well inside the micropores, as proved by the increase of the heat of adsorption. Moreover, the adsorption amount is completely restored to the original one after releasing the mechanical force, indicating the fully reversible contraction/recovery of the ZTC framework against mechanical force. The experimental results are theoretically supported by a simulation using Grand Canonical Monte Carlo method. The similar adsorption enhancement is observed also on CH4 , whereas H2 is found as an exception due to the weak interaction potential.
    Chemistry 08/2013; · 5.93 Impact Factor
  • Minoru T Miyahara, Hideki Tanaka
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    ABSTRACT: We present a modification of the molecular dynamics simulation method with a unit pore cell with imaginary gas phase [M. Miyahara, T. Yoshioka, and M. Okazaki, J. Chem. Phys. 106, 8124 (1997)] designed for determination of phase equilibria in nanopores. This new method is based on a Monte Carlo technique and it combines the pore cell, opened to the imaginary gas phase (open pore cell), with a gas cell to measure the equilibrium chemical potential of the confined system. The most striking feature of our new method is that the confined system is steadily led to a thermodynamically stable state by forming concave menisci in the open pore cell. This feature of the open pore cell makes it possible to obtain the equilibrium chemical potential with only a single simulation run, unlike existing simulation methods, which need a number of additional runs. We apply the method to evaluate the equilibrium chemical potentials of confined nitrogen in carbon slit pores and silica cylindrical pores at 77 K, and show that the results are in good agreement with those obtained by two conventional thermodynamic integration methods. Moreover, we also show that the proposed method can be particularly useful for determining vapor-liquid and vapor-solid coexistence curves and the triple point of the confined system.
    The Journal of Chemical Physics 02/2013; 138(8):084709. · 3.12 Impact Factor
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    ABSTRACT: We conduct grand canonical Monte Carlo simulations and a free-energy analysis for a simplified model of a stacked-layer porous coordination polymer to understand the gate phenomenon, which is a structural transition of a host framework induced by the adsorption of guest particles. Our calculations demonstrate that stabilization of the system due to the guest adsorption causes host deformation under thermodynamic equilibrium. We also investigate spontaneous transition behaviors (gate opening and closing under metastable conditions). The structural transition should occur when the required activation energy, which is determined using the free-energy analysis, becomes equal to the system energy fluctuation. To estimate the system energy fluctuation, we construct a kinetic transition model based on the transition state theory. In this model, the system energy fluctuation can be calculated by setting the adsorption time and transition domain size of the host framework. The model demonstrates that a smaller domain size results in a gate-opening transition at lower pressure. Furthermore, we reveal that the slope of the logarithm of the equilibrium structural transition pressure versus reciprocal temperature shows transition enthalpy, and that slopes of the gate-opening and -closing transition pressures versus reciprocal temperature show activation enthalpies.
    The Journal of Chemical Physics 02/2013; 138(5):054708. · 3.12 Impact Factor
  • Satoshi Watanabe, Minoru T. Miyahara
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    ABSTRACT: Bottom-up self-organization approaches are promising for fabricating higher-order patterned surfaces composed of colloidal particles. The first example among the patterns that have been extensively studied would be stripes; however, the formation of stripe patterns has so far been confined to partially or fully hydrophobic surfaces. By contrast, we have succeeded in preparing well-defined stripe patterns even on strongly hydrophilic substrates via a convective self-assembly technique. By using this technique, a stripe pattern was produced simply by suspending a substrate in a dilute suspension, without any complicated procedure; the stripes spontaneously aligned parallel to the contact line. Driven by this finding, we further investigate this self-assembly process, and find out that the convective self-assembly is quite promising as a template-free pattern formation technique. In the present paper, we first overview the convective self-assembly technique which is originally developed for uniform film formation, and then present our recent results on the pattern formation of colloidal particles through the convective self-assembly. This technique can produce various patterns including stripes, cluster arrays, and grids in response to macroscopic experimental parameters such as particle concentration and temperature.
    Advanced Powder Technology 01/2013; 24(6):897–907. · 1.65 Impact Factor
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    ABSTRACT: Zeolite‐templated carbon is extraordinarily flexible and behaves as a kind of elastic microporous material, unlike other microporous materials, such as activated carbons, zeolites, and metal–organic frameworks. Its uniform micropores (1.2 nm) can be contracted/recovered reversibly just by loading mechanical force, and such pore‐size change induces noticeable increase in gas‐physisorption amount. For more details see the Full Paper by H. Nishihara et al. on page 13009 ff.
    Chemistry 01/2013; 19(39). · 5.83 Impact Factor
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    ABSTRACT: Cluster arrays composed of metal nanoparticles are promising for application in sensing devices because of their interesting surface plasmon characteristics. Herein, we report the spontaneous formation of cluster arrays of gold colloids on flat substrates by vertical-deposition convective self-assembly. In this technique, under controlled temperature, a hydrophilic substrate is vertically immersed in a colloid suspension. Cluster arrays form when the particle concentration is extremely low (in the order of 10(-6)-10(-8) v/v). These arrays are arranged in a hierarchically ordered structure, where the particles form clusters that are deposited at a certain separation distance from each other, to form "dotted" lines that are in turn aligned with a constant spacing. The size of the cluster can be controlled by varying the particle concentration and temperature while an equal separation distance is maintained between the lines formed by the clusters. Our technique thus demonstrates a one-step, template-free fabrication method for cluster arrays. In addition, through the direct observation of the assembly process, the spacing between the dotted lines is found to result from the "stick-and-slip" behavior of the meniscus tip, which is entirely different from the formation processes observed for the striped patterns, which we reported previously at higher particle concentrations. The difference in the meniscus behavior possibly comes from the difference in colloidal morphology at the meniscus tip. These results demonstrate the self-regulating characteristics of the convective self-assembly process to produce colloidal patterns, whose structure depends on particle concentration and temperature.
    Langmuir 08/2012; 28(36):12982-8. · 4.38 Impact Factor
  • Yasushi Mino, Satoshi Watanabe, Minoru T Miyahara
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    ABSTRACT: We describe a template-free technique for arranging colloidal particles into a stripe pattern on a large scale. A simple liquid-level manipulation system was incorporated into the vertical-deposition convective self-assembly (CSA) technique. By periodically pumping a colloidal dispersion out of or into a reservoir to manipulate the liquid level, we successfully fabricated stripe patterns with various periodicities (i.e., line widths and spacings) that are unachievable with the normal CSA technique. We developed a simple model to predict the periodicity of the resultant colloidal stripes that enables the tailored fabrication of colloidal stripes with the desirable periodicity for a practical application. This technique has the advantages of versatility and scalability. By combining this technique with the two-step CSA technique (Mino et al., Langmuir2011, 27(9), 5290-5295), we fabricated a large-sized colloidal grid network pattern of silver nanoparticles.
    ACS Applied Materials & Interfaces 05/2012; · 5.01 Impact Factor
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    ABSTRACT: Specific types of coordination polymers show an adsorption-induced structural transition, or so-called "gate adsorption", in which a host framework is said to change its structure from a "closed" nonporous phase to an "open" porous one for guest molecules. To identify the pathway for such a structural transition, we perform grand canonical Monte Carlo simulations for the adsorption of guest molecules in a host interpenetrated framework and calculate the free energy profiles of the structural changes in a complete three-dimensional space. In addition to the open phase found in our previous analyses along a fixed one-dimensional path, we reveal the existence of another open configuration. Each of the two open phases yields the status of global minimum to the other depending on the external pressure, resulting in a two-step isotherm. Moreover, the shape of adsorption hysteresis associated with the structural transition can change depending on the energy barrier between a metastable and a stable state that the system can overcome. Our simulations not only give a comprehensive understanding of stepped isotherms observed empirically but also suggest that isotherms with hysteretic gate adsorption is closely related to the thermal fluctuation of the system.
    Langmuir 03/2012; 28(11):5093-100. · 4.38 Impact Factor
  • Daigo Yamamoto, Satoshi Watanabe, Minoru T. Miyahara
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    ABSTRACT: Dendrimer-encapsulated Pt nanoparticles can be synthesized by Pt2+ coordination with dendrimers followed by reduction. In the present study, we have systematically investigated the coordination kinetics of PtCl42– with fourth-generation hydroxyl-terminated poly(amidoamine) (PAMAM G4-OH) dendrimers using UV–vis spectroscopy measurements. Our experimental investigation clarifies that Pt2+ coordination with dendrimers occurs after the H2O ligand exchange (aquation) of PtCl42– and that a resultant species PtCl2(H2O)2 predominantly coordinates with the dendrimers. From these results, we have proposed a simple dynamic model that describes the Pt2+ coordination as a consecutive reaction composed of the aquation reaction of PtCl42– and a subsequent coordination reaction of the resultant PtCl2(H2O)2 with the dendrimers. Our proposed model is in good agreement with the experimental results at a low concentration condition of PtCl42–, validating its performance. Furthermore, we suggest that the proposed scheme can be generalized and applied to metal species other than Pt2+.
    Industrial & Engineering Chemistry Research. 05/2011; 50(12).
  • Yasushi Mino, Satoshi Watanabe, Minoru T Miyahara
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    ABSTRACT: We explored a "template-free" approach to arranging colloidal particles into a network pattern by a convective self-assembly technique. In this approach, which we call "two-step convective self-assembly," a stripe pattern of colloidal particles is first prepared on a substrate by immersing it in a suspension. The substrate with the stripes is then rotated by 90° and again immersed in the suspension to produce stripes perpendicular to the first ones, resulting in a grid-pattern network of colloidal arrays. The width of the colloidal grid lines can be controlled by changing the particle concentration while maintaining an almost constant spacing between the lines. On the basis of these results, we propose a mechanism for grid pattern formation. Our method is applicable to various types of particles. In addition, the wide applicability of this method was employed to create a hybrid grid pattern.
    Langmuir 04/2011; 27(9):5290-5. · 4.38 Impact Factor
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    ABSTRACT: The thermal transitions of confined polymers are important for the application of polymers in molecular scale devices and advanced nanotechnology. However, thermal transitions of ultrathin polymer assemblies confined in subnanometre spaces are poorly understood. In this study, we show that incorporation of polyethylene glycol (PEG) into nanochannels of porous coordination polymers (PCPs) enabled observation of thermal transitions of the chain assemblies by differential scanning calorimetry. The pore size and surface functionality of PCPs can be tailored to study the transition behaviour of confined polymers. The transition temperature of PEG in PCPs was determined by manipulating the pore size and the pore-polymer interactions. It is also striking that the transition temperature of the confined PEG decreased as the molecular weight of PEG increased.
    Nature Communications 10/2010; 1:83. · 10.02 Impact Factor
  • Daigo Yamamoto, Satoshi Watanabe, Minoru T Miyahara
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    ABSTRACT: We synthesized Pt nanoparticles encapsulated in poly(amidoamine) (PAMAM) dendrimers by Pt(2+) coordination and subsequent reduction by NaBH(4). To optimize the experimental conditions for the Pt nanoparticle synthesis, we systematically examined the effects of pH, temperature, coordination time, and surface functional groups of the dendrimers on coordination and NaBH(4) reduction by UV-vis spectroscopy and transmission electron microscopy (TEM) measurements. We used generation-4 dendrimers (hydroxyl-terminated PAMAM dendrimers; G4-OH) and generation-4.5 dendrimers (carboxyl-terminated PAMAM dendrimers; G4.5-COO(-)). According to our results, dendrimer-encapsulated Pt nanoparticles with a narrow size distribution were obtained at high Pt(2+) coordination ratios (alpha), while nonencapsulated Pt nanoparticles were formed at low alpha values. To enhance alpha, it was necessary to use a neutral G4-OH solution or an acidic G4.5-COO(-) solution. Temperature had a marked effect on the coordination rate, with an increase in the temperature from room temperature to 50 degrees C, and the coordination time decreased from 10 days to 1-2 days.
    Langmuir 02/2010; 26(4):2339-45. · 4.38 Impact Factor
  • Satoshi Watanabe, Minoru T. Miyahara
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    ABSTRACT: We examine the adsorption process and order formation of colloidal nanoparticles on a planar surface with friction. We perform Brownian dynamics simulations with a three-dimensional cell model in which the particle–particle and particle–substrate interactions are modeled on the DLVO theory, and examine the effects of the friction acting between the adsorbed particles and the substrate on the adsorbed structure formed on the substrate. The results obtained are as follows: when the friction is so strong that the adsorbed particles are stuck to the substrate, ordered structures never form, which seems to be quite natural. However, when the magnitude of the frictional force is moderate, an ordered structure can form even with low coverage because the frictional force aids order formation. This is because the friction counterbalances the particles’ Brownian motion, which would otherwise disturb the ordered structure. Furthermore, through a detailed examination of the distribution of the Brownian motion, it is demonstrated that an increase in the friction has a similar effect as a decrease in temperature.
    Advanced Powder Technology - ADVANCED POWDER TECHNOL. 01/2010; 21(1):57-63.
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    ABSTRACT: We have studied the formation of stripe patterned films of ordered particle arrays on completely solvophilic substrates by using a self-organization technique. In this method, a substrate immersed in a suspension is withdrawn vertically at a controlled temperature. We have also systematically examined the effects of several experimental parameters. Well-defined stripes spontaneously form at the air-solvent-substrate contact line because of a very dilute suspension in a quasi-static process. The stripe width depends on particle concentration, withdrawal rate, and surface tension, while the stripe spacing depends on the thickness of stripes, surface tension, and type of substrate. A stripe width and the adjacent spacing show a clear correlation, strongly indicating the synchronized formation of a stripe and the next spacing. The evaporation rate does not affect stripe width and spacing but determines the growth rate of stripe patterned films. Based on these results, we propose a new mechanism for stripe formation, which is neither a stick-slip motion of the contact line nor dewetting but a negative feedback of particle concentration caused by a concavely curved shape of the meniscus, demonstrating not only its qualitative but also its quantitative validity.
    Langmuir 07/2009; 25(13):7287-95. · 4.38 Impact Factor
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    ABSTRACT: We conduct grand canonical Monte Carlo simulations and free energy analysis for a gate adsorption phenomenon, which is experimentally observed in flexible frameworks of porous coordination polymers. Our calculations demonstrate that the stabilization provided by the guest adsorption drives the structural transition, surmounting the energy cost in creating the adsorption space due to the movement of the host framework. Furthermore, the existence of an energy barrier between two local minima in the free-energy landscape is found to result in hysteretic adsorption.
    The Journal of Chemical Physics 05/2009; 130(16):164707. · 3.12 Impact Factor

Publication Stats

449 Citations
142.82 Total Impact Points

Institutions

  • 1997–2014
    • Kyoto University
      • Department of Chemical Engineering
      Kioto, Kyōto, Japan
    • Cornell University
      Ithaca, New York, United States
  • 2004–2007
    • Central Research Institute of Electric Power Industry
      • Energy Engineering Research Laboratory
      Tokyo, Tokyo-to, Japan
  • 2006
    • University of Texas at Austin
      • Department of Chemical Engineering
      Austin, TX, United States
  • 1993
    • Lappeenranta University of Technology
      Villmanstrand, Southern Finland Province, Finland