Surface-induced Phase Transition Research Articles

Suppressing the voltage decay of Li1.2Mn0.6Ni0.2O2 cathode materials enabled by LiMn0.8Fe0.2PO4/C for long-cycling lithium-ion batteries

Li-rich layered oxides can provide high energy density, but their further application is hindered by capacity decrease and voltage decay in the course of cycling. Herein, we blended layered oxide Li1.2Mn0.6Ni0.2O2 (LL) with olivine structured LiMn0.8Fe0.2PO4/C (LFMP) cathode material via a simple method, with improved results of excellent cycle stability, low voltage decay and good thermal stability. The result of TEM and ICP reveals that the LL enabled by LFMP can mitigate the surface-induced phase transition (layered to spinel) and suppress Mn dissolution of LL. As a consequence of CV and GITT test and analysis, the buffer effect of LFMP for LL is benefit to support the layered structure in subsequent cycles, which can provide the long cycle capacity and lower voltage decay for the hybrid materials.

Surface-induced phase transitions in thin films of dendrimer block copolymers

Abstract The phase morphologies and phase transitions of dendrimer block copolymer thin films confined between two homogeneous, planar hard substrates had been investigated by a three-dimensional real space self-consistent field theory (SCFT). From the perspectives of property and strength of the preferential substrate, when the film system confined within neutral substrates, the thinner film was easier to take the undulated and perpendicular cylinder phases. For the attractive preference of the substrate on block segment A, the polymer films tended to take the surface-wetting structures that was composed by block segment A. On the contrary, for the repulsive preference of the substrate on block segment A, a phase transition of cylinder-lamellae could be observed increasing with the relative surface strength of the preferential substrate.

Nanometric Surface Oscillation Spectroscopy of Water-Poor Microemulsions

Selectively exchanging metal complexes between emulsified water-poor microemulsions and concentrated solutions of mixed electrolytes is the core technology for strategic metal recycling. Nanostructuration triggered by solutes present in the organic phase is understood, but little is known about fluctuations of the microemulsion-water interface. We use here a modified version of an optoelectric device initially designed for air bubbles, in order to evidence resonant electrically induced surface waves of an oily droplet suspended in an aqueous phase. Resonant waves of nanometer amplitude of a millimeter-sized microemulsion droplet containing a common ion-specific extractant diluted by dodecane and suspended in a solution of rare earth nitrate are evidenced for the first time with low excitation fields (5 V/cm). From variation of the surface wave spectrum with rare earth concentration, we evidence uptake of rare-earth ions at the interface and at higher concentration the formation of a thin "crust" of liquid crystal forming at unusually low concentration, indicative of a surface induced phase transition. The effect of the liquid crystal structure on the resonance spectrum is backed up by a model, which is used to estimate crust thickness.

Dynamic manipulation of the local pH within a nanopore triggered by surface-induced phase transition.

Manipulating the local pH within nanoconfinement is essential in nanofluidics technology and its applications. Since the conventional strategy utilizes the overlapping of an electric double layer formed for charge compensation by protons near a negatively charged pore-wall surface, pH variation within a pore is limited to the acidic side. To achieve the variation at the alkaline side, we developed a system comprising a hydrophobic pore-wall surface and aqueous solution containing hydrophobic cations. Beyond a threshold cation concentration, a nanopore is filled with the second phase where the cations are remarkably enriched due to surface-induced phase transition (SIFT) originating from the hydrophobic effect. It is accompanied by the enrichment of coexisting anions. We experimentally show that pH in the second phase is much higher than in the bulk solution. Electrochemical measurements strongly suggest that the pH value can be increased from 4.8 to 10.7 within a 10 nm nanopore in the most significant case. This is ascribed to the enrichment of hydroxide anions. We argue that the extent and rate of pH variation are controlled as desired.

High-Rate Charging of Zinc Anodes Achieved by Tuning Hydration Properties of Zinc Complexes in Water Confined within Nanopores

Rechargeable batteries constructed with high-energy-density metal anodes, such as zinc and lithium, often suffer from dendrite formation during high-rate charging, which can lead to short circuits and reduced battery life. Here we report a novel method of realizing high-rate charging of zinc anodes together with the suppression of zinc dendrite formation by remarkably accelerating the electrodeposition within the nanopores of a porous electrode. By tuning not only the affinity between the wall-surface of nanopores and water but also that between the zinc complex with polyvalent carboxylates and water, we can establish a condition under which a surface-induced phase transition (SIFT) occurs. With the occurrence of SIFT, the penetration of zinc complexes into the nanopores becomes quite fast toward the formation of a second phase within the nanopore where the concentration of zinc complexes is orders of magnitude higher than in the bulk solution. More specifically, we use a hydrophobic nanoporous electrode,...

Surface-induced phase transitions of wormlike chains in slit confinement

On the basis of a self-consistent field theory treatment of semi-flexible polymer chains, we analyze the effects of the flexibility on the structure of polymers sterically confined between two parallel, structureless walls separated by a distance. The model is built from a wormlike chain formalism which crosses over from the rod limit to the flexible limit, and the Onsager-type interaction which describes the orientation-dependent excluded-volume interaction. Three surface states were obtained from the numerical solution to the theory: uniaxial, biaxial, and condensed. As the overall density increases in such a lyotropic system, first order phase transitions between uniaxial and biaxial states, biaxial and condensed states can occur.

Penetration of Platinum Complex Anions into Porous Silicon: Anomalous Behavior Caused by Surface-Induced Phase Transition

We investigate the dynamics of the penetration of platinum complex anions into nanopores during platinum deposition within a nanoporous silicon electrode. The pore-wall surface is hydrophobic and the anions are large enough to behave like hydrophobic solutes. Some of the observations are anomalous in the sense that they cannot be understood in terms of the phenomenological theory for diffusion based on Fick’s law. For example, the penetration is faster when the pore diameter is smaller and the anion size is larger. The penetration rate remains unexpectedly fast even when the pores become deeper. The penetration can be made faster using large coexisting cations with sufficiently high hydrophobicity. We show that these results can be interpreted only by statistical mechanics of confined molecular liquids. When the manipulated variables (pore diameter, anion concentration, sizes of platinum complex anions and coexisting cations, etc.) are chosen so that the surface-induced phase transition (SIFT) can take place, the penetration is drastically accelerated. Under the condition with the SIFT occurrence, a strongly attractive, long-ranged effective surface–anion interaction comes into play, leading to the anomalous behavior. The experimental result is in qualitatively good accord with the theoretical argument. The outcome is of vital importance in controlling the mass transfer within nanoporous media and designing next-generation electrochemical devices.

Effect of Displacement Deposition on Platinum Deposition within Nanoporous Silicon

In the use of nanoporous electrode, the control of mass transfer within nanopores is crucially important to elicit their best performance. However, mass-transfer rate for ionic species through electrolyte solution within nanopores tends to be very low, so that only a limited proportion of the whole specific surface area contributes to electrochemical reactions. In the case of the metal filling within the porous silicon by electrodeposition, poor supply of metal precursor into the nanopores also causes a problem to induce a plugging at the pore opening. We have investigated the pore filling with platinum in an aqueous solution with tuning the hydration property of the surface of pore wall and platinum complex anions [1-3]. The important insight we have found is the surface-induced phase transition (SIFT: the concentration of the hydrophobic solutes near the hydrophobic surface becomes orders of magnitude higher than that in the bulk) in the nanopores plays an important role in electrodeposition of platinum within the nanoporous silicon. Our previous reports, however, focused only on the presence of platinum deposits in the porous layer corresponding to the various experimental conditions and didn’t elucidate the dynamic aspects of the penetration of platinum complex anions into the nanopores. We adopted the time developments of the mixed-potential measured during the displacement deposition of platinum within the porous silicon to find the peculiar penetration dynamics that could not be understood within the framework of simple diffusion. One of the typical observations shows the penetration is faster when the pore diameter is smaller and the anion size is larger. In this paper, we also discuss the effect of the pore depth on the penetration dynamics. [1] K. Fukami, R. Koda, T. Sakka, T. Urata, K. Amano, H. Takaya, M. Nakamura, Y. Ogata, and M. Kinoshita, Chem. Phys. Lett., 2012, 542, 99-105. [2] K. Fukami, R. Koda, T. Sakka, Y. Ogata, and M. Kinoshita, J. Chem. Phys., 2013, 138, 094702. [3] R. Koda, A. Koyama, K. Fukami, N. Nishi, T. Sakka, T. Abe, A. Kitada, K. Murase, and M. Kinoshita, J. Chem. Phys., 2014, 141, 074701.

(Invited) Electrodeposition in Microporous Silicon from the Viewpoint of Hydration Property: Effect of Coexisting Ions in Zinc Electrodeposition

A chemical reaction in a nanospace is decelerated once a diffusion-limited condition is reached due to the difficulty in supply of reactants from the bulk. We illustrate how to overcome this problem for platinum electrodeposition within nanoporous silicon electrodes. The surface-induced hydration structure of reactants is essential. We make nanopore surfaces hydrophobic by covering them with organic molecules and adopt platinum complex ions with sufficiently large sizes as reactants. Such ions, which are only weakly hydrated, are excluded from the bulk aqueous electrolyte solution to the surface, and thus they are hydrophobic in this sense. When the ion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. Using statistical-mechanical theory for confined molecular liquids, we show that this change originates from a surface-induced phase transition. When the affinity of the surface with water was gradually reduced with fixing the ion concentration, qualitatively the same transition phenomenon was observed. We examine how the platinum electrodeposition behavior is affected by the cation species coexisting with the anions. We compare the experimental results obtained using three different cation species: K+, (CH3)4N+, and (C2H5)4N+. It is shown that the threshold concentration, beyond which the electrochemical deposition within nanopores is drastically accelerated, is considerably dependent on the cation species. The threshold concentration becomes lower as the cation size increases. Finally, we extend the concept to electrodeposition of other metals such as zinc. Zinc is normally deposited as dendritic crystals under a high current density. However, the dendritic deposition is suppressed by using microporous silicon electrodes. In addition, by choosing the coexisting ions, the dendritic growth of zinc is strongly suppressed. This strategy is crucial for the development of next-generation rechargeable batteries using a metal anode.

Effect of surface texture on freezing in nanopores: surface-induced versus homogeneous crystallization.

Freezing of argon in ordered and disordered carbon pores of a similar diameter D ∼ 2.4 nm is investigated using extensive molecular simulations with large system sizes up to 10(4) atoms. While crystallization in the atomistically smooth pore consists in a surface-induced phase transition occurring at a temperature larger than the bulk, crystallization in the disordered pores, which is only partial as it is spatially restricted to the pore center, occurs through homogeneous crystallization. These results shed light on solidification in pores by showing that there is a crossover between surface-induced and homogeneous crystallization upon increasing the surface disorder of the host material. In the latter case, the Gibbs-Thomson equation, in which crystallization is assumed to occur when the crystal size equals the pore size corrected for the thickness of the unfreezable layer at the pore surface, is in reasonable agreement with the observed freezing temperature.

Effect of cation species on surface-induced phase transition observed for platinum complex anions in platinum electrodeposition using nanoporous silicon.

In an earlier work [K. Fukami et al., J. Chem. Phys. 138, 094702 (2013)], we reported a transition phenomenon observed for platinum complex anions in our platinum electrodeposition experiment using nanoporous silicon. The pore wall surface of the silicon electrode was made hydrophobic by covering it with organic molecules. The anions are only weakly hydrated due to their large size and excluded from the bulk aqueous solution to the hydrophobic surface. When the anion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. It was shown that this change originates from a surface-induced phase transition: The space within a nanopore is abruptly filled with the second phase in which the anion concentration is orders of magnitude higher than that in the bulk. Here we examine how the platinum electrodeposition behavior is affected by the cation species coexisting with the anions. We compare the experimental results obtained using three different cation species: K(+), (CH3)4N(+), and (C2H5)4N(+). One of the cation species coexists with platinum complex anions [PtCl4](2-). It is shown that the threshold concentration, beyond which the electrochemical deposition within nanopores is drastically accelerated, is considerably dependent on the cation species. The threshold concentration becomes lower as the cation size increases. Our theoretical analysis suggests that not only the anions but also the cations are remarkably enriched in the second phase. The remarkable enrichment of the anions alone would give rise to the energetic instability due to electrostatic repulsive interactions among the anions. We argue that the result obtained cannot be elucidated by the prevailing view based on classical electrochemistry. It is necessitated to consult a statistical-mechanical theory of confined aqueous solutions using a molecular model for water.

Surface-induced liquid-gas transition in salt-free solutions of model charged colloids

We report a novel phenomenon of a surface-induced phase transition in salt-free solutions of charged colloids. We develop a theory of this effect and confirm it by Molecular Dynamics simulations. To describe the colloidal solution we apply a primitive model of electrolyte with a strong asymmetry of charge and size of the constituent particles - macroions and counterions. To quantify interactions of the colloidal particles with the neutral substrate we use a short-range potential which models dispersion van der Waals forces. These forces cause the attraction of colloids to the surface. We show that for high temperatures and weak attraction, only gradual increase of the macroion concentration in the near-surface layer is observed with increase of interaction strength. If however temperature drops below some threshold value, a new dense (liquid) phase is formed in the near-surface layer. It can be interpreted as a surface-induced first-order phase transition with a critical point. Using an appropriately adopted Maxwell construction, we find the binodal. Interestingly, the observed near-surface phase transition can occur at the absence of the bulk phase transition and may be seemingly classified as prewetting transition. The reported effect could be important for various technological applications where formation of colloidal particle layers with the desired properties is needed.

Electrochemical deposition of platinum within nanopores on silicon: Drastic acceleration originating from surface-induced phase transition

An electrochemical reaction within nanopores is remarkably decelerated once a diffusion-limited condition is reached due to the difficulty in supply of reactants from the bulk. Here, we report a powerful method of overcoming this problem for electrochemical deposition of platinum within nanopores formed on silicon. We made the pore wall surface of the silicon electrode hydrophobic by covering it with organic molecules and adopted platinum complex ions with sufficiently large sizes. Such ions, which are only weakly hydrated, are excluded from the bulk aqueous electrolyte solution to the surface and rather hydrophobic in this sense. When the ion concentration in the bulk was gradually increased, at a threshold the deposition behavior exhibited a sudden change, leading to drastic acceleration of the electrochemical deposition. Using our statistical-mechanical theory for confined molecular liquids, we show that this change originates from a surface-induced phase transition: The space within nanopores is abruptly filled with the second phase within which the ion concentration is orders of magnitude higher. When the affinity of the surface with water was gradually reduced with fixing the ion concentration, qualitatively the same transition phenomenon was observed, which can also be elucidated by our theory. The utilization of the surface-induced phase transition sheds new light on the design and control of a chemical reaction in nanospace.

Ternary Fluid Mixture Confined between Surfaces: Surface-induced Phase Transition and Long-range Oscillatory Forces

Abstract Surface forces between a soft cellulose surface and a hard silica particle were measured in wet hexane with or without the addition of a surfactant. In the absence of a surfactant, the adhesion force was enhanced as a result of capillary condensation of water. The effect of the surfactant in reducing the adhesion force with increasing water content in a ternary fluid mixture of a surfactant, hexane, and water was systematically studied. Long-range oscillations of the force–distance curves are discussed in the light of various mesophase compositions in the bulk.

Surface-induced phase transitions in dense nanoparticle arrays of lamella-forming diblock copolymers

The surface-induced phase transitions in dense nanoparticle arrays of lamella-forming diblock copolymers are investigated by using the real-space self-consistent field theory. The dense nanoparticle array provides a distinct and strong confinement environment where there are incomplete confinements in three spatial directions. Several complicated phases with cubic symmetries, such as the monocontinuous and bicontinuous phases, are identified in the dense nanoparticle arrays. Through adjusting the strength of surface preference, the disorder phases are induced into the complicated order phases in the nanoparticle arrays with small periods while the order–order transitions occurs in those with large periods. Investigations on the free and entropic energies indicate that these surface-induced phase transitions are of first-order and the stabilities of order phases are intimately correlated to the strength of surface preference. Our results enrich the knowledge about the phase behaviors of macromolecules in confined systems, which may be helpful to fabricate the novel nanomaterials based on the block copolymers.

Density functional theory for predicting polymeric forces against surface fouling

Polymer-based “non-stick” surfaces have been proposed as the next generation of effective and environmentally-friendly coating materials for protecting implanted biomedical devices and for marine antifouling. However, identification of polymeric systems for universal fouling control is often impeded by the poor knowledge of interactions between biological substances and polymeric substrates in diverse aqueous environments. In this article, we review predictions of the polymer density functional theory (DFT) on the structural and surface properties of polymer brushes and polymer nanocomposites that are potentially useful for antifouling applications. In comparison to alternative theoretical methods, DFT exhibits versatile features that are ideal for investigating various polymer-mediated interactions, self-organization of nanoparticles, and surface-induced phase transitions. It is capable of explicit description of important microscopic details of polymeric systems including the molecular excluded-volume effects, associating interactions, van der Waals attraction, Coulomb forces, and inter- and intra- molecular correlations. The theoretical descriptions of surface forces may provide helpful guidelines in the design and development of polymeric materials for preventing non-specific adsorption of biological substances.

Control of the entropic interactions and phase behavior of athermal nanoparticle/homopolymer thin film mixtures

It is shown that the phase behavior of an athermal thin film nanoparticle/polymer hybrid material of polystyrene (PS) with PS-grafted gold nanoparticles is readily tailored through control of the degrees of polymerization, N and P, of the grafted and the free chains, respectively, and the relative size of the nanoparticle to the average dimensions of both the grafted and the host chains. Complete miscibility or a surface-induced phase transition, leading to segregation of the particles to an interface, is readily achieved in this system.

Exactly solvable model for the QCD tricritical endpoint

An inclusion of temperature and chemical-potential-dependent surface-tension in the gas of quark-gluon bags model resolves a long-standing problem of a unified description of the first-and second-order phase transition with the crossover. The suggested model has an exact analytical solution and allows one to rigorously study the vicinity of the critical endpoint of the deconfinement phase transition. It is found that, at the curve of a zero surface-tension coefficient, there must exist the surface-induced phase transition of the seond or higher order. The present model predicts that the critical endpoint of quantum chromodynamics is the tricritical endpoint.

Quark-gluon bags with surface tension

The temperature and chemical potential dependent surface tension of bags is introduced into the gas of a quark-gluon bags model. This resolves a long standing problem of a unified description of the first- and second-order phase transition with the crossover. Such an approach is necessary to model the complicated properties of quark-gluon plasma and hadronic matter from the first principles of statistical mechanics. The suggested model has an exact analytical solution and allows one to rigorously study the vicinity of the critical endpoint of the deconfinement phase transition. The existence of higher order phase transitions at the critical endpoint is discussed. In addition, we found that at the curve of a zero surface tension coefficient there must exist the surface induced phase transition of the second or higher order, which separates the pure quark-gluon plasma (QGP) from the crossover states, which are the mixed states of hadrons and QGP bags. Thus, the present model predicts that the critical endpoint of quantum chromodynamics is the tricritical endpoint.

Surface-Induced Phase Transition of Asymmetric Diblock Copolymer in Selective Solvents

Surface-induced phase transition of asymmetric diblock copolymer in selective solvents is first theoretically investigated by using the real-space version of self-consistent field theory (SCFT). By varying the distance between two parallel hard surfaces (or the film thickness) W and the block copolymer concentration f(p), several morphologies are predicted and the phase diagram is constructed. Self-assembly morphologies of the diblock copolymer in dilute solution are found to change significantly with different film thickness. In confined systems, stable morphologies found in the bulk solution become unstable due to the loss of polymer conformation entropy. We find that in a very dilute block copolymer solution, phase separation can be induced through polymer depletion as the solution becomes more confined. Our findings provide an interesting starting point for a renewed effort in both experimental and theoretical investigations of confined block copolymer solutions.