Crystal Phase Selection Research Articles

Biomineralization at the Molecular Level: Amino Acid-Assisted Crystallization of Zeotype AlPO4·1.5H2O–H3

Three aminoacids: proline, histidine, and lysine are used as organic additives in the hydrothermal synthesis of the zeotype material AlPO 4 •1.5H 2 O-H3 (framework topology code APC). A detailed analysis of the factors controlling the formation of AlPO 4 •1.5H 2 O-H3 is performed. The role of aminoacids in the formation process of the frameworks is addressed based on comparisons of the crystal phase selection of different synthetic recipes and the insights provided by the FTIR analysis of the synthetic gels and the crystalline products. The phase transition of AlPO 4 •1.5H 2 O-H3 to AlPO 4-C (APC), and subsequently to AlPO 4-D (APD) upon heating to 1000 o C was followed by in situ XRD analysis. The present work has been the first progress to understanding the role of the bioactive molecules in the crystallization of inorganic open-framework microporous materials. An active, non-classical structuredirecting role that aminoacids play during the zeotype crystallization is proposed, i.e., positively charged aminoacids cap the growing surface of microporous AlPO 4 , and attract aluminate and phosphate anions from the solution phase for further growth.

Zeolite synthesis by the excess fluoride approach in the presence of piperidinium-based structure-directing agents

Here, we investigate the effects of fluoride concentration on the phase selectivity of crystallization in (alumino)silicate synthesis mixtures containing a series of N -alkyl- N -methylpiperidinium ions, where alkyl is methyl to hexyl, as organic structure-directing agents (OSDAs). Considerable diversity in the phase selectivity was observed when the HF/OSDA ratio in synthesis mixtures is 2.0 or higher. Using this excess fluoride approach, we synthesized five different aluminosilicate zeolites (i.e., EMM-10P, EU-1, MCM-22P, ZSM-12, and ZSM-58) that did not crystallize under the normal F − concentration conditions (HF/OSDA = 1.0). Our synthesis results also reveal that the same zeolite phase (i.e., levyne, nonasil, octadecasil, or ZSM-5) can crystallize over a wide range (0.5–2.0) of HF/OSDA ratios. More interestingly, F − concentration in the synthesis mixture was found to influence the crystal size and framework Al distribution of levyne. • Zeolite syntheses using piperidinium-based OSDAs in fluoride media are performed. • Notable changes in the zeolite product are observed at HF/OSDA ratios ≥ 2.0. • Zeolite crystal size varies with the F − concentration in synthesis mixtures. • F − concentration in the synthesis mixture is a zeolite framework Al distribution factor.

3D-Assembled rutile TiO2spheres withc-channels for efficient lithium-ion storage

Three-dimensional (3D) TiO2 architectures have attracted significant attention recently as they can improve the electrochemical stability and realize the full potential of TiO2-based anodes in lithium ion batteries. Here, flower-like rutile TiO2 spheres with radially assembled nanorods (c-channels) were fabricated via a simple hydrothermal method. The 3D radial architecture affords massive active sites to fortify the lithium storage. Moreover, the presence of c-channels facilitates electrolyte infiltration and offers facile pathways for efficient Li+ transport. As a result, this flower-like rutile TiO2 anode gives significantly enhanced specific capacities (615 mA h g-1 at 1 C and 386 mA h g-1 at 2 C after 400 cycles) and a superior long-term cyclability (up to 10 000 cycles with a specific capacity of 67 mA h g-1 at 100 C). Kinetic analysis reveals that the enhanced diffusion-controlled and surface capacitive storage leads to the excellent electrochemical behavior. This work not only exhibits the enormous advantages of 3D architectures with c-channels, but also provides access to structural design and crystal phase selection for TiO2-based anode materials.

Simulation of GaAs Nanowire Growth and Crystal Structure.

Growing GaAs nanowires with well-defined crystal structures is a challenging task, but may be required for the fabrication of future devices. In terms of crystal phase selection, the connection between theory and experiment is limited, leaving experimentalists with a trial and error approach to achieve the desired crystal structures. In this work, we present a modeling approach designed to provide the missing connection, combining classical nucleation theory, stochastic simulation, and mass transport through the seed particle. The main input parameters for the model are the flows of the growth species and the temperature of the process, giving the simulations the same flexibility as experimental growth. The output of the model can also be directly compared to experimental observables, such as crystal structure of each bilayer throughout the length of the nanowire and the composition of the seed particle. The model thus enables for observed experimental trends to be directly explored theoretically. Here, we use the model to simulate nanowire growth with varying As flows, and our results match experimental trends with a good agreement. By analyzing the data from our simulation, we find theoretical explanations for these experimental results, providing new insights into how the crystal structure is affected by the experimental parameters available for growth.

Engineering new defective phases of UiO family metal–organic frameworks with water

Tuning the water concentration in the synthesis of three UiO MOFs allows crystal phase selection via ordering of defects.

Effect of Zr doping and Li2O-2B2O3 layer on the structural electrochemical properties of LiNi0.5Co0.2Mn0.3O2 cathode material: experiments and first-principle calculations

In order to enhance the electrochemical performance of LiNi0.5Co0.2Mn0.3O2 (NCM), an in situ doping with zirconium (Zr) by wet grind-solid state method then coating with Li2O-2B2O3 (LBO) by crystal phase selection method is successfully developed. At the same time, based on the density functional theory (DFT) with the generalized gradient approximation (GGA), use of the first-principle calculation method theoretically proved the experimental results. The XRD results of experimental and the calculated both revealed the lattice parameters become larger after Zr doped. The thickness of the LBO layer uniformly deposited on the surface of the Zr-doped sample is 5 nm, which can scavenge and neutralize HF erosion. Particularly, the Zr-doped and LBO-coated NCM sample exhibited excellent cycling performance with 90.5% capacity retention after 100 cycles at 1 C, which are apparently higher than 76.97% of NCM. The initial discharge capacity of double optimized sample was 170.4 mAh g−1 at 1 C which is higher than 157.1 mAh g−1 of NCM. Zr and LBO layers can effectively reduce the energy of the system and the band gap. Obtained results were consistent with the experimental results. Therefore, co-modification method plays an import role to optimize the material from the structure to the surface and hence greatly improved the battery performance.

Charge density mismatch synthesis of zeolite beta in the presence of tetraethylammonium, tetramethylammonium, and sodium ions: Influence of tetraethylammonium decomposition

The effects of a temperature change on the phase selectivity of crystallization in a tetraethylammonium (TEA+)-tetramethylammonium (TMA+)-Na+ mixed structure-directing agent (SDA) system, which had been previously used in the synthesis of UZM-9 and UZM-5 at 100 and 150 °C, respectively, have been further investigated in order to shed more light on the course of charge density mismatch (CDM) syntheses of aluminosilicate zeolites. Nanocrystalline zeolite beta with a Si/Al ratio of 8.1, essentially the same ratio as that (8.0) of its initial synthesis mixture, was the phase crystallized at 155 °C, whereas heating at higher temperatures ranging from 160 to 200 °C for up to 21 days gave no crystalline products. The course of chemistry at these temperatures is highly influenced by the increasing instability of the TEA+ SDA. The overall results of this study demonstrate that the TMA+, as well as the TEA+ and Na+ ions, plays an important role in the nucleation step of zeolite beta prepared here, providing another example of SDA cooperation in a CDM zeolite synthesis. An idealized charge density disproportionation model is employed to track the evolving chemistry of the aluminosilicate solids and solutions. The discrepancy between the observed solution chemistry and that expected by the model suggests the presence of undesirable reactions in the system, namely TEA+ decomposition.

Synthesis of Cu–Sb–S nanocrystals: insight into the mechanism of composition and crystal phase selection

In this work, Cu–Sb–S nanocrystals (NCs) with various compositions and crystal structures are realized by only tuning the amount of 1-dodecanethiol (DDT). When the DDT amount is 3.6, 5.5, 20.5, and 25.6 mmol, pure tetragonal Cu3SbS4, cubic Cu12Sb4S13, orthorhombic CuSbS2, and orthorhombic Cu3SbS3 NCs are respectively obtained. Systematic investigations demonstrate that different amounts of DDT are responsible for formation of the different compositions and crystal structures of the initial Cu2−xS seeds, which results into the formation of various Cu–Sb–S NCs. This reaction path is further verified by control experiments from successful preparation of Cu–Sb–S NCs comparable to that of the as-prepared ones directly from the reaction of Sb2S3 NCs with Cu2−xS seeds. The current study not only provides a facile and economical way to synthesize high-quality Cu–Sb–S NCs, but also opens a new route for preparation other I–V–VI multicomponent chalcogenides NCs, such as Cu–Bi–S systems.

Evolution of morphology and microstructure of GaAs/GaSb nanowire heterostructures.

In this paper, we successfully grow GaAs/GaSb core-shell heterostructure nanowires (NWs) by molecular beam epitaxy (MBE). The as-grown GaSb shell layer forms a wurtzite structure instead of the zinc blende structure that has been commonly reported. Meanwhile, a bulgy GaSb nanoplate also appears on top of GaAs/GaSb core-shell NWs and possesses a pure zinc blende phase. The growth mode for core-shell morphology and underlying mechanism for crystal phase selection of GaAs/GaSb nanowire heterostructures are discussed in detail.

Temperature dependence in density-fluctuation-induced crystallization in metallic glass by MeV electron irradiation

MeV electron irradiation can introduce Frenkel pairs, i.e., a vacancy-interstitial pair, in crystals. In metallic glass, density fluctuation, namely, a pair of free volume and anti-free volume, is introduced, resulting in the devitrification of the amorphous phase in some metallic glasses. In this study, we investigated the temperature dependence of MeV-irradiation-induced crystallization of metallic glasses. Phase selection in crystallization shows a significant temperature dependence in various metallic glasses at and under 298 K. The size of the crystalline precipitates changes with irradiation temperature in some metallic glasses.

Amino Acids in Iron Oxide Mineralization: (Incomplete) Crystal Phase Selection Is Achieved Even with Single Amino Acids

Iron oxides are important minerals in biology and materials science. Using biomimetic synthesis, a variety of iron oxides have been fabricated. However, it is still not clear how growth modifiers like amino acids and peptides select different crystal phases of a complex material like iron oxide. The current paper shows that already with single amino acids, (incomplete) crystal phase selection is achieved in vitro. In particular, l-histidine, l-threonine, and l-cysteine favor the formation of unstable crystal phases like ferrihydrite or lepidocrocite, although sometimes only at high amino acid concentrations. Other amino acids like l-valine have only minor effects when compared to control samples grown in the absence of amino acids. The effects of the amino acids can be rationalized via kinetic trapping and different interaction strengths of the amino acids with the growing iron oxide particles. The effects of the amino acids on the particle morphologies are less significant. The paper therefore shows that single amino acids can be a valuable tool for the materials chemist to fabricate and stabilize even unstable iron oxide crystal phases.

Phase Stability of Crystalline and Amorphous Phases and Formation of Nanostructure in Zr-Pd and Zr-Pt Alloys Under Electron Irradiation

AbstractElectron irradiation induced phase transition behavior of Zr-Pd and Zr-Pt alloys was investigated focusing on phase selection in crystallization by thermal annealing and electron irradiation. Nano quasi-crystalline (QC) phase was formed by thermal crystallization in Zr66.7Pd33.3 and Zr80Pt20 alloys, while nano two-type f.c.c. super-saturated solid solutions were formed by irradiation induced crystallization at 298K. In Zr66.7Pt33.3 alloy, polycrystalline Zr5Pt3 and Zr9Pt11 phases transformed to two-type f.c.c. nano-crystalline phase through an amorphous state by crystal-to-amorphous-to-crystal (C-A-C) transition during electron irradiation. Nano-composite phase composed of f.c.c. super-saturated solid solution and residual amorphous phase was stable rather than an amorphous single phase, thermal equilibrium crystalline phase and quasi-crystalline phase under 2.0MV electron irradiation at 298K, resulting in the formation of nano-composite structure by irradiation induced amorphization and crystallization.