Atom-Selective Analyses Reveal the Structure-Directing Effect of Cs Cation on the Synthesis of Zeolites.

To understand the crystallization mechanism of zeolites, it is important to clarify the detailed role of the structure-directing agent, which is essential for the crystallization of zeolite, interacting with an amorphous aluminosilicate matrix. In this study, to reveal the structure-directing effect, the evolution of the aluminosilicate precursor which causes the nucleation of zeolite is analyzed by the comprehensive approach including atom-selective methods. The results of total and atom-selective pair distribution function analyses and X-ray absorption spectroscopy indicate that a crystalline-like coordination environment gradually forms around Cs cations. This corresponds to the fact that Cs is located at the center of the d8r units in the RHO structure whose unit is unique in this zeolite, and a similar tendency is also confirmed in the ANA system. The results collectively support the conventional hypothesis that the formation of the crystalline-like structure before the apparent nucleation of the zeolite.

Symmetric Breakage-Induced Semimetallic State: Polymorphism in Ruthenate Nanosheets.

Atomic arrangements and their symmetries govern the physical properties of materials, including nanosheets that are low-dimensional nanomaterials. Although they have the same composition, symmetric changes associated with atomic displacements sometimes induce unexpected physical properties. Herein, we report that symmetric breakage induces a semimetallic state in chemically exfoliated ruthenate nanosheets. The atomic arrangements and symmetries are determined by a pair distribution function (PDF); subsequently, the physical properties are discussed using ab initio calculations and resistivity measurements. Ruthenate nanosheets can adopt an electronic structure similar to that of graphene owing to symmetric breakage. We experimentally confirmed the polymorphism in ruthenate nanosheets that highlights the importance of symmetric analysis, even in low-dimensional materials.

Structures and Role of the Intermediate Phases on the Crystallization of BaTiO3 from an Aqueous Synthesis Route.

Carbonate formation is a prevailing challenge in synthesis of BaTiO3, especially through wet chemical synthesis routes. In this work, we report the phase evolution during thermal annealing of an aqueous BaTiO3 precursor solution, with a particular focus on the structures and role of intermediate phases forming prior to BaTiO3 nucleation. In situ infrared spectroscopy, in situ X-ray total scattering, and transmission electron microscopy were used to reveal the decomposition, pyrolysis, and crystallization reactions occurring during thermal processing. Our results show that the intermediate phases consist of nanosized calcite-like BaCO3 and BaTi4O9 phases and that the intimate mixing of these along with their metastability ensures complete decomposition to form BaTiO3 above 600 °C. We demonstrate that the stability of the intermediate phases is dependent on the processing atmosphere, where especially enhanced CO2 levels is detrimental for the formation of phase pure BaTiO3.

Temperature-dependent electronic structure of bixbyite α-Mn2O3 and the importance of a subtle structural change on oxygen electrocatalysis.

Bixbyite α -Mn2O3 is an inexpensive Earth-abundant mineral that can be used to drive both oxygen evolution (OER) and oxygen reduction reactions (ORR) in alkaline conditions. It possesses a subtle orthorhombic → cubic phase change near room temperature that suppresses Jahn-Teller distortions and presents a unique opportunity to study how atomic structure affects the electronic structure and catalytic activity at a temperature range that is easily accessible in OER/ORR experiments. Previously, we observed that heat-treated α -Mn2O3 had a better performance as a bifunctional catalyst in the oxygen evolution (OER) and oxygen reduction reactions (ORR) (Dalton Trans. 2016, 45, 18,494-18,501). We hypothesized that heat-treatment pinned the material into a more electrochemically active cubic phase. In this manuscript, we use high-resolution X-ray diffraction to collect the temperature-dependent structures of α -Mn2O3, and then input them into ab initio calculations. The electronic structure calculations indicate that the orthorhombic → cubic phase transition causes the Mn 3d and O 2p bands to overlap and mix covalently, transforming α -Mn2O3 from a semiconductor to a semimetal. This subtle change in structure also modifies Mn-O-Mn bond distances, which may improve the activity of the material in oxygen electrochemistry. OER and ORR experiments were performed using the same electrode at various temperatures. They show a jump in the exchange current density near the phase change temperature, demonstrating the higher activity of the cubic phase.

Understanding the Hydrothermal Formation of NaNbO3: Its Full Reaction Scheme and Kinetics.

Sodium niobate (NaNbO3) attracts attention for its great potential in a variety of applications, for instance, due to its unique optical properties. Still, optimization of its synthetic procedures is hard due to the lack of understanding of the formation mechanism under hydrothermal conditions. Through in situ X-ray diffraction, hydrothermal synthesis of NaNbO3 was observed in real time, enabling the investigation of the reaction kinetics and mechanisms with respect to temperature and NaOH concentration and the resulting effect on the product crystallite size and structure. Several intermediate phases were observed, and the relationship between them, depending on temperature, time, and NaOH concentration, was established. The reaction mechanism involved a gradual change of the local structure of the solid Nb2O5 precursor upon suspending it in NaOH solutions. Heating gave a full transformation of the precursor to HNa7Nb6O19·15H2O, which destabilized before new polyoxoniobates appeared, whose structure depended on the NaOH concentration. Following these polyoxoniobates, Na2Nb2O6·H2O formed, which dehydrated at temperatures ≥285 °C, before converting to the final phase, NaNbO3. The total reaction rate increased with decreasing NaOH concentration and increasing temperature. Two distinctly different growth regimes for NaNbO3 were observed, depending on the observed phase evolution, for temperatures below and above ≈285 °C. Below this temperature, the growth of NaNbO3 was independent of the reaction temperature and the NaOH concentration, while for temperatures ≥285 °C, the temperature-dependent crystallite size showed the characteristics of a typical dissolution-precipitation mechanism.

Controlled Growth of Srx Ba1-x Nb2 O6 Hopper- and Cube-Shaped Nanostructures by Hydrothermal Synthesis.

Controlling the shape and size of nanostructured materials has been a topic of interest in the field of material science for decades. In this work, the ferroelectric material Srx Ba1-x Nb2 O6 (x=0.32-0.82, SBN) was prepared by hydrothermal synthesis, and the morphology is controllably changed from cube-shaped to hollow-ended structures based on a fundamental understanding of the precursor chemistry. Synchrotron X-ray total scattering and PDF analysis was used to reveal the structure of the Nb-acid precursor, showing Lindqvist-like motifs. The changing growth mechanism, from layer-by-layer growth forming cubes to hopper-growth giving hollow-ended structures, is attributed to differences in supersaturation. Transmission electron microscopy revealed an inhomogeneous composition along the length of the hollow-ended particles, which is explained by preferential formation of the high entropy composition, SBN33, at the initial stages of particle nucleation and growth.

Condensed ferric dimers for green photocatalytic synthesis of nylon precursors.

Although iron oxides have been extensively studied as photocatalysts because of their abundance and environmental compatibility, their performance is notoriously low due to factors such as low photoinduced charge-separation efficiency. Iron oxides, thus, must be modified with expensive and/or toxic materials to attain higher performances, which devalues their appeal as sustainable materials. Here, we design an iron oxide exhibiting an unprecedentedly high photocatalytic performance unrealized by previous photocatalysts such as TiO2 for reactions including the selective oxidation of cyclohexane to industrial nylon precursors. The iron oxide photocatalyst consists of ferric dimers, otherwise extremely unstable, formed via etching of Fe and O sites from ferric oxide nanoparticles immobilized within porous silica. We demonstrate a remarkably high photoinduced charge-separation efficiency (long lifetime of active species) of the ferric dimers due to their electronic structure and the potential of this supported photocatalyst for many more reactions.

Conductive Nanosized Magnéli-Phase Ti4O7 with a Core@Shell Structure.

Magnéli-phase Ti4O7, known for its high electrical conductivity and corrosion resistance, is typically prepared by hydrogen reduction at high temperatures (∼1000 °C), leading to large particles. Nanosized Ti4O7 have been explored for application toward high specific surface area electrode materials and electrocatalyst supports; nonetheless, the particle size of Ti4O7 is still insufficient for utilization as a support. In this study, we have pursued a novel synthetic approach for nanosized Ti4O7 platelets with a length of 10-80 nm and thickness of 3-10 nm even under high-temperature conditions. We herein describe the use of SiO2 beads as a core to obtain a SiO2 core coated with multilayers of TiO2 nanosheets exfoliated from layered H2Ti4O7 which is subsequently subjected to high-temperature reduction to prepare a SiO2-core@Ti4O7-shell structure. The pair distribution function technique has proven that the shell is transformed to single-phase Ti4O7. The electrical double layer capacitance of SiO2-core@Ti4O7-shell was much larger than that of conventionally synthesized Ti4O7 particles with a micrometer size. The results show the beneficial effects of the SiO2-core@Ti4O7-shell structure, and it is the first example of the synthesis for conductive Ti4O7 with a high specific surface area even under conditions of high-temperature synthesis.

Time-resolved pair distribution function analysis of disordered materials on beamlines BL04B2 and BL08W at SPring-8.

A dedicated apparatus has been developed for studying structural changes in amorphous and disordered crystalline materials substantially in real time. The apparatus, which can be set up on beamlines BL04B2 and BL08W at SPring-8, mainly consists of a large two-dimensional flat-panel detector and high-energy X-rays, enabling total scattering measurements to be carried out for time-resolved pair distribution function (PDF) analysis in the temperature range from room temperature to 873 K at pressures of up to 20 bar. For successful time-resolved analysis, a newly developed program was used that can monitor and process two-dimensional image data simultaneously with the data collection. The use of time-resolved hardware and software is of great importance for obtaining a detailed understanding of the structural changes in disordered materials, as exemplified by the results of commissioned measurements carried out on both beamlines. Benchmark results obtained using amorphous silica and demonstration results for the observation of sulfide glass crystallization upon annealing are introduced.

Curved Fragmented Graphenic Hierarchical Architectures for Extraordinary Charging Capacities.

An approach to assemble hierarchically ordered 3D arrangements of curved graphenic nanofragments for energy storage devices is described. Assembling them into well-defined interconnected macroporous networks, followed by removal of the template, results in spherical macroporous, mesoporous, and microporous carbon microball (3MCM) architectures with controllable features spanning nanometer to micrometer length scales. These structures are ideal porous electrodes and can serve as lithium-ion battery (LIB) anodes as well as capacitive deionization (CDI) devices. The LIBs exhibit high reversible capacity (up to 1335 mAh g-1 ), with great rate capability (248 mAh g-1 at 20 C) and a long cycle life (60 cycles). For CDI, the curved graphenic networks have superior electrosorption capacity (i.e., 5.17 mg g-1 in 0.5 × 10-3 m NaCl) over conventional carbon materials. The performance of these materials is attributed to the hierarchical structure of the graphenic electrode, which enables faster ion diffusion and low transport resistance.

Lepidocrocite-Type Titanate Formation from Isostructural Prestructures under Hydrothermal Reactions: Observation by Synchrotron X-ray Total Scattering Analyses.

The formation of titanium dioxides, such as rutile and anatase, is known to proceed through the formation of a lepidocrocite-type layered structure under hydrothermal conditions, but the nucleation of this intermediate is still not understood well. Here, the nucleation of lepidocrocite-type layered titanates under hydrothermal conditions is observed by tracking the structural changes by in situ time-resolved pair distribution function analyses. We found that titanate clusters or corrugated layered prestructures having <1 nm domains with lepidocrocite-type connectivity were formed even before thermal treatment in alkaline aqueous solution. Upon thermal treatment, a two-dimensional layered structure grew directly from the prestructure, not from the amorphous polymeric hydroxide dissolved in the solution. Thus, we conclude that the formation of the lepidocrocite-like prestructure is the key for forming a layered titanate under hydrothermal conditions.

Zeolitic intralayer microchannels of magadiite, a natural layered silicate, to boost green organic synthesis.

Despite the considerable attention given to the applications of magadiite in previous research, the properties of this natural layered silicate have remained mysterious due to the lack of crystal structure information. On the other hand, no one has doubted the intercalation capability between the layers. Here we succeed in determining the structure of magadiite using X-ray pair distribution functions and synchrotron powder diffractometry. We discover unexpected zeolitic microchannels within the layers. We describe efficient synthesis of 100% pure benzoic acid from toluene by using magadiite as an additive in a TiO2 photocatalytic system oxidizing toluene. Based on the uncovered structure of magadiite, we clarify the mechanism of this unique photocatalytic system: the microchannels of magadiite not only separate/accommodate the desired partially oxidized product formed on TiO2 but also prevent the accumulation of the overoxidized products on the TiO2 surface that deactivates the photocatalytic activity.

Formation and Characterization of Hydrogen Boride Sheets Derived from MgB2 by Cation Exchange.

Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mechanical properties as well as hydrogen storage capacity. Here, we report the experimental realization of 2D hydrogen boride sheets with an empirical formula of H1B1, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB2) with an average yield of 42.3% at room temperature. The sheets feature an sp2-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray atomic pair distribution functions, electron diffraction, transmission electron microscopy, photo absorption, core-level binding energy data, infrared absorption, electron energy loss spectroscopy, and density functional theory calculations. The established cation-exchange method for metal diboride opens new avenues for the mass production of several types of boron-based 2D materials by countercation selection and functionalization.

Mesoporous Semimetallic Conductors: Structural and Electronic Properties of Cobalt Phosphide Systems.

Mesoporous cobalt phosphide (meso-CoP) was prepared by the phosphorization of ordered mesoporous cobalt oxide (meso-Co3 O4 ). The electrical conductivity of meso-CoP is 37 times higher than that of nonporous CoP, and it displays semimetallic behavior with a negligibly small activation energy of 26 meV at temperatures below 296 K. Above this temperature, only materials with mesopores underwent a change in conductivity from semimetallic to semiconducting behavior. These properties were attributed to the coexistence of nanocrystalline Co2 P phases. The poor crystallinity of mesoporous materials has often been considered to be a problem but this example clearly shows its positive aspects. The concept introduced here should thus lead to new routes for the synthesis of materials with high electronic conductivity.

Room-Temperature Rutile TiO2 Nanoparticle Formation on Protonated Layered Titanate for High-Performance Heterojunction Creation.

We report a methodology for creating protonated layered titanate-rutile heterojunctions on the outer particle surface of protonated layered titanate by treating layered potassium titanate (K0.8Ti1.73Li0.27O4) with dilute HCl and then drying it at room temperature under reduced pressure. After Pt co-catalyst loading, this protonated layered titanate/rutile composite with heterojunctions showed higher photocatalytic H2 evolution activity from water under simulated solar light compared to that of Pt-loaded P25, the standard photocatalyst for this reaction. The high photocatalytic activity was ascribable to enhanced photocatalytic activity of the protonated layered titanate based on an efficient charge separation at the protonated layered titanate-rutile heterojunction in addition to the sensitization effects of rutile, which absorbs light with longer wavelengths compared to those of protonated layered titanate.

Physical Stabilization of Pharmaceutical Glasses Based on Hydrogen Bond Reorganization under Sub-Tg Temperature.

Amorphous solid dispersions (ASDs) play a key role in the pharmaceutical industry through the use of high-energy amorphous state to improve solubility of pharmaceutical agents. Understanding the physical stability of pharmaceutical glasses is of great importance for their successful development. We focused on the anti-HIV agent, ritonavir (RTV), and investigated the influence of annealing at temperatures below the glass transition temperature (sub-Tg) on physical stability, and found that the sub-Tg annealing effectively stabilized RTV glasses. Through the atomic structure analyses using X-ray pair distribution functions and infrared spectroscopy, we ascertained that this fascinating effect of the sub-Tg annealing originated from strengthened hydrogen bonding between molecules and probably from a better local packing associated with the stronger hydrogen bonds. The sub-Tg annealing is effective as a physical stabilization strategy for some pharmaceutical molecules, which have relatively large energy barrier for nucleation.

Two-Dimensional Corrugated Porous Carbon-, Nitrogen-Framework/Metal Heterojunction for Efficient Multielectron Transfer Processes with Controlled Kinetics.

The material choice for efficient electrocatalysts is limited because it is necessary to be highly active as well as highly stable. One direction to solve this issue is to understand elementary steps of electrode processes and build an unconventional strategy for a conversion of inert and, therefore, stable materials into efficient catalysts. Herein, we propose a simple concept for obtaining catalysts from inert and hence stable materials by forming their heterojunctions, namely, covering inert Au with corrugated carbon-nitrogen-based two-dimensional porous frameworks. It shows more than 10 times better activity for the hydrogen evolution reaction than for the pure Au surface, and it also demonstrates the high catalytic activity for the oxygen reduction reaction (ORR) via an effective four-electron reduction mechanism, which is different from the usual two-electron reduction typical for ORR on Au surfaces. This activity induced by formation of a heterojunction was analyzed by a conjugation of computational and experimental methods and found to originate from alternative efficient reaction pathways that emerged by the corrugated porous framework and the Au surface. This work provides not only the method for creating active surface but also the knowledge on elementary steps of such complicated multielectron transfer reactions, thereby leading to intriguing strategies for developing energy conversion reactions based on materials which had never been considered as catalysts before.

Noncrystalline Titanium Oxide Catalysts for Electrochemical Oxygen Reduction Reactions.

Titanium oxides crystals are widely used in a variety of fields, but little has been reported on the functionalities of noncrystalline intermediates formed in their structural transformation. We measured the oxygen reduction reaction activity of titanium oxide nanoparticles heat-treated for a different time and found that the activity abruptly increased at a certain time of the treatment. We analyzed their structures by using X-ray pair distribution functions with the help of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy and ascertained that the abrupt increase in the activity corresponded to a structural transformation from a reduced lepidocrocite-type layered titanate to a disordered structure consisting of domains of brookite-like TiO6 octahedral linkages. The further treatment transformed these brookite-like domains into another phase having more edge-sharing sites like the TiO-type cubic structure. This finding would position noncrystalline, disordered structure as a possible origin of the catalytic activity, though nanocrystalline rutile particles might be also considered as the origin.

Phase pure α-Mn2O3 prisms and their bifunctional electrocatalytic activity in oxygen evolution and reduction reactions.

Synthesizing manganese oxide materials with exact control of the nanoparticle shape and phase is difficult, making it challenging to understand the influence of the surface structure on electrocatalysis. Here we describe an inexpensive, low-temperature method to synthesize single-crystal orthorhombic phase α-Mn2O3 prisms bound by the {100} facets. The synthesis is the first method to use the cation bridging effect to assist in the creation of α-Mn2O3 prisms. According to structural analysis using X-ray diffraction, X-ray pair-distribution function (PDF) measurements and high resolution transmission electron microscopy, the material is composed exclusively of α-Mn2O3 prisms, and no additional amorphous or nanocrystalline phases are present. Heating the prisms transformed the material to a more symmetrical, cubic phase α-Mn2O3 that exhibited strong bifunctional electrocatalytic activity for the oxygen evolution (OER) and oxygen reduction (ORR) reactions. We compared the oxygen electrode activities (OEA) and found that the α-Mn2O3 prisms performed 79% better than commercially-produced α-Mn2O3 powders, indicating that these α-Mn2O3 prisms perform well as inexpensive, earth-abundant materials for reversible electrodes.

Observation of Quantum Confinement in Monodisperse Methylammonium Lead Halide Perovskite Nanocrystals Embedded in Mesoporous Silica.

Hybrid organic-inorganic metal halide perovskites have fascinating electronic properties and have already been implemented in various devices. Although the behavior of bulk metal halide perovskites has been widely studied, the properties of perovskite nanocrystals are less well-understood because synthesizing them is still very challenging, in part because of stability. Here we demonstrate a simple and versatile method to grow monodisperse CH3NH3PbBrxIx-3 perovskite nanocrystals inside mesoporous silica templates. The size of the nanocrystal is governed by the pore size of the templates (3.3, 3.7, 4.2, 6.2, and 7.1 nm). In-depth structural analysis shows that the nanocrystals maintain the perovskite crystal structure, but it is slightly distorted. Quantum confinement was observed by tuning the size of the particles via the template. This approach provides an additional route to tune the optical bandgap of the nanocrystal. The level of quantum confinement was modeled taking into account the dimensions of the rod-shaped nanocrystals and their close packing inside the channels of the template. Photoluminescence measurements on CH3NH3PbBr clearly show a shift from green to blue as the pore size is decreased. Synthesizing perovskite nanostructures in templates improves their stability and enables tunable electronic properties via quantum confinement. These structures may be useful as reference materials for comparison with other perovskites, or as functional materials in all solid-state light-emitting diodes.