Hex Phases Research Articles

Constituent Isomerism-Induced Quasicrystal and Frank–Kasper σ Superlattices Based on Nanosized Shape Amphiphiles

Naturally, subtle variations in the chemical structures of constituent molecules may significantly affect their multiscale spatial arrangements, properties, and functions. Deceptively simple spheri...

SERS active self-assembled diphenylalanine micro/nanostructures: A combined experimental and theoretical investigation.

Enhancing Raman signatures of molecules by self-assembled metal nanoparticles, nanolithography patterning, or by designing plasmonic nanostructures is widely used for detection of low abundance biological systems. Self-assembled peptide nanostructures provide a natural template for tethering Au and Ag nanoparticles due to its fractal surface. Here, we show the use of L,L-diphenylalanine micro-nanostructures (FF-MNSs) for the organization of Ag and Au nanoparticles (Nps) and its potential as surface-enhanced Raman scattering (SERS)-active substrates. The FF-MNSs undergo an irreversible phase transition from hexagonally packed (hex) micro-nanotubes to an orthorhombic (ort) structure at ∼150 °C. The metal Nps form chains on hex FF-MNSs as inferred from transmission electron microscopy images and a uniform non-aggregated distribution in the ort phase. The high luminescence from the ort FF-MNS phase precludes SERS measurements with AgNps. The calculated Raman spectra using density-functional theory shows a higher intensity from rhodamine 6G (R6G) molecule in the presence of an Ag atom bound to ort FF compared with hex FF. The SERS spectra obtained from R6G bound to FF-MNSs with AuNps clearly show a higher enhancement for the ort phase compared with hex FF, corroborating our theoretical calculations. Our results indicate that FF-MNSs both in the hex and ort phases can be used as substrates for the SERS analysis with different metal nanoparticles, opening up a novel class of optically active bio-based substrates.

Ion Transport in Nanostructured Phosphonated Block Copolymers Containing Ionic Liquids.

The morphology and conductivity of poly(styrenephosphonate-b-methylbutylene) block copolymers containing ionic liquids are investigated. The block copolymers display a series of well-defined self-assembled morphologies, i.e., lamellae, gyroid, hexagonal cylinder (HEX), body-centered cubic, and A15 lattice, in the absence and presence of ionic liquids. The observation of an equilibrium A15 lattice for linear diblock copolymers, in contrast to a number of theoretical studies, is ascribed to the packing frustration accompanied by prevailing electrostatic interactions. The samples having the A15 lattice show a substantially higher morphology factors (0.83-0.96) than those with the HEX phases (0.42-0.69). This indicates the structural advantage of the A15 lattice with a well-defined 3D symmetry over the 2D HEX structure for constructing less tortuous ion-conduction pathways. This unprecedented study portends the rational design of nanostructured phosphonated polymer membranes with improved conductivity.

ChemInform Abstract: Adsorption of NO and CO and Specific Features of Their Interaction on the Pt(100) Surface

AbstractReview: 127 refs.

Adsorption of NO and CO and specific features of their interaction on the Pt(100) surface

This article presents an analytical review of the author’s results and the literature concerning the nature of species resulting from NO and CO adsorption on the unreconstructed (1 × 1) and reconstructed hexagonal (hex) Pt(100) surfaces, including specific features of the reactions between these species. At 300 K, both surfaces adsorb NO and CO mainly in their molecular states. When adsorbed on Pt(100)-1 × 1, the NOads and COads molecules are uniformly distributed on the surface. Under the same conditions, the hexagonal surface undergoes adsorption-induced reconstruction with the formation of NOads/1 × 1 and COads/1 × 1 islands, which are areas of the unreconstructed phase saturated with adsorbed molecules and surrounded with the adsorbate-free hex phase. In adsorption on structurally heterogeneous surfaces containing both hex and 1 × 1 areas, the 1 × 1 and hex phases are occupied in succession, the latter undergoing reconstruction into the 1 × 1 phase. The reaction between NO and CO on the unreconstructed surfaces occurs even at room temperature and results in the formation of N2 and CO2 in quantitative yield. On the hexagonal surface, a stable layer of adsorbed molecules as (NOads + COads)/1 × 1 mixed islands forms under these conditions. Above 350 K, the reaction in the mixed islands is initiated by the desorption of small amounts of the initial compounds, and this is followed by rapid self-acceleration leading to a surface explosion yielding N2, CO2, and N2O (minor product). These products show themselves as very narrow desorption peaks in the temperature-programmed reaction spectrum.

Perovskite, LiNbO3, Corundum, and Hexagonal Polymorphs of (In1–xMx)MO3

LiNbO(3) (LN), corundum (cor), and hexagonal (hex) phases of (In(1-x)M(x))MO(3) (x = 0.143; M = Fe(0.5)Mn(0.5)) were prepared. Their crystal structures were investigated with synchrotron X-ray powder diffraction, and their properties were studied by differential thermal analysis, magnetic measurements, and Mössbauer spectroscopy. The LN-phase was prepared at high pressure of 6 GPa and 1770 K; it crystallizes in space group R3c with a = 5.25054(7) Å, c = 13.96084(17) Å, and has a long-range antiferromagnetic ordering near T(N) = 270 K. The cor- and hex-phases were obtained at ambient pressure by heating the LN-phase in air up to 870 and 1220 K, respectively. The cor-phase crystallizes in space group R-3c with a = 5.25047(10) Å, c = 14.0750(2) Å, and the hex-phase in space group P6(3)/mmc with a = 3.34340(18) Å, c = 11.8734(5) Å. T(N) of the cor-phase is about 200 K, and T(N) of the hex-phase is about 140 K. During irreversible transformations of LN-(In(1-x)M(x))MO(3) with the (partial) cation ordering, the In(3+), Mn(3+), and Fe(3+) cations become completely disordered in one crystallographic site of the corundum structure, and then they are (partially) ordered again in the hex-phase. LN-(In(1-x)M(x))MO(3) exhibits a reversible transformation to a perovskite GdFeO(3)-type structure (space group Pnma; a = 5.2946(3) Å, b = 7.5339(4) Å, c = 5.0739(2) Å at 10.3 GPa) at room temperature and pressure of about 5 GPa.

Kinetics of hexagonal cylinders to face-centered cubic spheres transition of triblock copolymer in selective solvent: Brownian dynamics simulation

The kinetics of the transformation from the hexagonal packed cylinder (hex) phase to the face-centered-cubic (fcc) phase was simulated using Brownian dynamics for an ABA triblock copolymer in a selective solvent for the A block. The kinetics was obtained by instantaneously changing either the temperature of the system or the well-depth of the Lennard-Jones potential. Detailed analysis showed that the transformation occurred via a rippling mechanism. The simulation results indicated that the order-order transformation was a nucleation and growth process when the temperature of the system instantly jumped from 0.8 to 0.5. The time evolution of the structure factor obtained by Fourier transformation showed that the peak intensities of the hex and fcc phases could be fit well by an Avrami equation.

Effect of Added Homopolymer on the Hexagonal Phase Formed by Cylindrical Block Copolymer Micelles in a Selective Solvent

The effect of addition of A homopolymer chains to HEX phases formed by cylindrical micelles obtained by dissolving an AB diblock copolymers in an A selective solvent was investigated. The following general trends were observed: (i) order-order transitions from HEX → Gyroid (G) → Lamellae (LAM); (ii) the inter-cylinder spacing of the HEX phase increased; (iii) when the homopolymer molecular weight and concentration was high, macrophase separation of the system occurred. The A homopolymer chains do not mix significantly with the A domain of the diblock due to excluded volume interactions, causing the corona chain brush to contract. As a result, order-order transitions that reduce the interfacial curvature are observed.

Energetics of formation of alkali and ammonium cobalt and zinc phosphate frameworks

Alkali and ammonium cobalt and zinc phosphates show extensive polymorphism. Thermal behavior, relative stabilities, and enthalpies of formation of KCoPO 4, RbCoPO 4, NH 4CoPO 4, and NH 4ZnPO 4 polymorphs are studied by differential scanning calorimetry, high-temperature oxide melt solution calorimetry, and acid solution calorimetry. α-KCoPO 4 and γ-KCoPO 4 are very similar in enthalpy. γ-KCoPO 4 slowly transforms to α-KCoPO 4 near 673 K. The high-temperature phase, β-KCoPO 4, is 5–7 kJ mol −1 higher in enthalpy than α-KCoPO 4 and γ-KCoPO 4. HEX phases of NH 4CoPO 4 and NH 4ZnPO 4 are about 3 kJ mol −1 lower in enthalpy than the corresponding ABW phases. There is a strong relationship between enthalpy of formation from oxides and acid–base interaction for cobalt and zinc phosphates and also for aluminosilicates with related frameworks. Cobalt and zinc phosphates exhibit similar trends in enthalpies of formation from oxides as aluminosilicates, but their enthalpies of formation from oxides are more exothermic because of their stronger acid–base interactions. Enthalpies of formation from ammonia and oxides of NH 4CoPO 4 and NH 4ZnPO 4 are similar, reflecting the similar basicity of CoO and ZnO.

Comparison of crystallization kinetics in various nanoconfined geometries

Phase morphological effect on crystallization kinetics in various nanoconfined spaces in a polystyrene- block-poly(ethylene oxide) (PS- b-PEO) diblock copolymer with a PEO volume fraction of 37 vol% was investigated. The phase morphology was characterized by small-angle X-ray scattering and transmission electron microscopy techniques. When the sample was cast from chloroform solution and annealed at 150 °C, a double gyroid (DG) phase was obtained. After it was subjected to a large-amplitude reciprocating shear, the sample transformed to an oriented hexagonal cylinder (Hex) phase. To obtain a lamellar confined geometry, lamellar single crystals were grown from dilute solutions. The crystallization in the lamellar (Lam) phase was one-dimensionally (1D) confined, while it was two-dimensionally (2D) confined in the DG and Hex phases, although they had different structures. Differential scanning calorimetry (DSC) was employed to study the crystallization kinetics using the Avrami analysis for these three nanoconfined geometries. Heterogeneous nucleation was found in all three samples in the crystallization temperature ( T c) regions studied. DSC results indicated that the crystallization kinetics in the Lam phase was the fastest, and the PEO crystals possessed higher thermodynamic stability than in the DG and Hex phases. For the crystallization kinetics in two 2D-confined phases, at low T c (<35 °C) the PEO crystallization rates in the DG and Hex phases were similar, while at high T c (>35 °C) the PEO crystallization was slower in the DG phase than in the Hex phase. The Avrami exponent n-values for the DG and the Hex samples were similar (∼1.8), yet the values of ln K in the DG phase were smaller than those in the Hex phase. This suggested that the linear growth rate was slower in the DG phase than in the Hex phase due to continuous curved channels in the DG phase.

Adiabatic reduction and hysteresis of the LFI model for NO + NH 3 on Pt{1 0 0}

The Lombardo–Fink–Imbihl (LFI) model for the NO + NH 3 reaction on Pt{1 0 0} consists of seven coupled ordinary differential equations (ODEs). Here, we present a numerical analysis for relaxation oscillations in the LFI and show that it cannot be adiabatically reduced to two coupled ODEs. We argue that this is because of the explicit consideration of the trapping processes of NO from 1 × 1 to hex phases in the kinetic mechanism of the reaction. Our analysis shows that an adiabatic reduction to three coupled ODEs can be achieved. Moreover, we examine the hysteretic behavior in detail.

Elucidation of Lean-NOxReduction Mechanism Using Kinetic Isotope Effect

The effect of an NO isotope on the kinetics of lean-NOx catalysis in (NO+C2H4+O2) reaction mixtures over Pt-ZSM-5 was theoretically analyzed and compared with experimental data obtained with a fixed-bed plug-flow reactor. The large kinetic isotope effect, as well as kinetic oscillations observed previously, was explained by NO decomposition kinetics on Pt surfaces which undergo phase transition between (1×1) and (hex) phases induced by adsorbed reactant species. Results indicate the importance of NO decomposition kinetics in lean-NOxcatalysis, lending further support to a lean-NOxreduction mechanism which involves NO decomposition accompanied by hydrocarbon oxidation.

An MeV ion scattering study of the Pt(100) surface during the catalytic oxidation of CO

The potential usefulness of work function measurements made simultaneously with Rutherford backscattering has been illustrated through the study of the temporal oscillations observed in the catalytic oxidation of CO over Pt(100). We emphasize the in situ character of the measurements and the capability of extending the pressure range to a few microns with no equipment modifications. For oscillations of amplitude less than 60 mV, a neglible fraction (i.e. < 8%) of the surface alternates between the (1 × 1) and reconstructed hex phases. A substantial number of surface atoms remain displaced throughout the oscillation period.

The interaction of CO and Pt(100). II. Energetic and kinetic parameters

We have investigated the mechanism and driving force of the CO-induced phase transition of the Pt(100) surface. In a preceding paper, we have concentrated on the mechanism by which CO removes the surface reconstruction. As discussed, the clean reconstructed (hex) surface of Pt(100) is more stable than the unreconstructed (1×1) phase, but if the CO coverage on hex exceeds a small critical value (θ≳0.05) nucleation of (1×1) patches occurs and proceeds until, at θ=0.5, the entire surface has been converted to the (1×1) phase. In this paper we present data which quantitatively describe the energetics of this system. Values for the heats of adsorption of CO on both the reconstructed and unreconstructed phases, as well as values for the preexponential factors for the desorption rates, are determined from quasiequilibrium LEED measurements at different coverages. The difference in the low coverage heats of adsorption of CO on the hex and (1×1) phases (27.5 vs 37.5 kcal/mol) is the driving force in the Pt phase transformation during adsorption. On the other hand, during desorption, the hex phase does not return until the coverage has decreased below 0.3. This measurement allows an estimation of the difference in stability between the clean (1×1) and hex phases: about 9–13 kcal per mole of Pt atoms. The transition between the two phases which are in equilibrium [COgas+hex-Pt(100) vs COads+(1×1)-Pt(100)] shows a hysteresis due to kinetic limitations. These kinetic effects are characteristic of a nucleation process involving a critical coverage.

Phase transitions in thallous nitrate. A reinvestigation

A detailed reinvestigation of the phase transitions in thallous nitrate using DSC, X-ray, IR and optical microscopy has been undertaken. The DSC measurements on anhydrous samples show that the orthorhombic [OR] → hexagonal [HEX] transition sets in at 349 ± 1 K and peaks around 353 K. However, its intensity depends upon several factors such as particle size, moisture content and thermal history of the sample. The HEX→cubic [C] transition sets in around 405 K and shows two peaks at ∼409 K and 413 K. Their relative intensities depend on the moisture content and thermal history of the sample. On cooling, the peaks show hysteresis and, by selective thermal cycling, the pairs of transitions, which correspond to the same process during heating and cooling, have been identified. IR spectra recorded in the OR and HEX phases at room temperature show that the symmetric stretching frequency (∼1040 cm −1) of the nitrate ion gets damped in the HEX phase. X-ray and optical microscopy data are in good agreement with the DSC observations.