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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. · 3.09 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. · 3.09 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. · 4.19 Impact Factor
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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; · 4.53 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. · 4.19 Impact Factor
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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+.
05/2011;
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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. · 4.19 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. · 7.40 Impact Factor
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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. · 4.19 Impact Factor
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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. · 4.19 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. · 3.09 Impact Factor
