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.

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.

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.

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.

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.

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.

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.

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.

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.

Two-dimensional cyano-bridged coordination polymer of Mn(H2O)2[Ni(CN)4]: structural analysis and proton conductivity measurements upon dehydration and rehydration

The interlayer spaces in the two-dimensional cyano-bridged coordination polymer were found to be non-ionic and not proton conductive.

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.

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.

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 CH₃NH₃PbBr_xI_x-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.

Bimetallic Metal-Organic Frameworks for Controlled Catalytic Graphitization of Nanoporous Carbons

Single metal-organic frameworks (MOFs), constructed from the coordination between one-fold metal ions and organic linkers, show limited functionalities when used as precursors for nanoporous carbon materials. Herein, we propose to merge the advantages of zinc and cobalt metals ions into one single MOF crystal (i.e., bimetallic MOFs). The organic linkers that coordinate with cobalt ions tend to yield graphitic carbons after carbonization, unlike those bridging with zinc ions, due to the controlled catalytic graphitization by the cobalt nanoparticles. In this work, we demonstrate a feasible method to achieve nanoporous carbon materials with tailored properties, including specific surface area, pore size distribution, degree of graphitization, and content of heteroatoms. The bimetallic-MOF-derived nanoporous carbon are systematically characterized, highlighting the importance of precisely controlling the properties of the carbon materials. This can be done by finely tuning the components in the bimetallic MOF precursors, and thus designing optimal carbon materials for specific applications.

Ultrahigh performance supercapacitors utilizing core-shell nanoarchitectures from a metal-organic framework-derived nanoporous carbon and a conducting polymer

Nanoarchitectured nanoporous carbon/conducting polymer core–shell nanocomposites (carbon–PANI) are prepared from metal–organic framework-derived carbon and the controlled polymerization of polyaniline nanorods.

Hitherto, many reports on composite materials for electrochemical applications are based on one-dimensional carbon nanotubes or two-dimensional graphene materials. However, these composite materials usually suffer from a stacking problem during electrochemical cycling. A smart nanoarchitectural design is needed for composite materials in order to overcome this problem. Recent research on electrochemical energy storage (EES) applications has focused on the development of three-dimensional (3-D) core–shell structures. The basis for high performance electrochemical energy storage is to control the efficient intercalation of ions in such a 3-D structure. Here, we demonstrate controlled synergy between the physicochemical properties of nanoporous carbon and conducting polyaniline polymer (carbon–PANI), which leads to some new interesting electrochemical properties. The time-dependent controlled optimization of the core–shell nanocomposites consisting of nanoporous carbon with a thin layer of PANI nanorod arrays gives useful control over supercapacitor performance. Furthermore, these carbon–PANI nanocomposites can electrochemically access ions with remarkable efficiency to achieve a capacitance value in the range of 300–1100 F g^–1. When assembled in a two electrode cell configuration, the symmetric supercapacitor (SSC) based on carbon–PANI//carbon–PANI shows the highest specific energy of 21 W h kg^–1 and the highest specific power of 12 kW kg^–1. More interestingly, the SSC shows capacitance retention of 86% after 20 000 cycles, which is highly superior compared to previous research reports.

Coordination environments and π-conjugation in dense lithium coordination polymers

A weak relationship between the longest C–O bond in an anionic organic ligand and the shortest Li–O bond was found.

Transition metal coordination complexes of chrysazin

Eleven novel coordination compounds, composed of chrysazin (1,8-dihydroxyanthraquinone) and different first-row transition metals (Fe, Co, Ni, Cu), were synthesised and the structures determined by single-crystal X-ray diffraction.

Mesoporous Pt nanospheres with designed pore surface as highly active electrocatalyst

A novel strategy for large-scale synthesis of shape- and size-controlled mesoporous Pt nanospheres (MPNs) through a slow reduction reaction in the presence of surfactant is reported here for the first time.

A novel strategy for large-scale synthesis of shape- and size-controlled mesoporous Pt nanospheres (MPNs) through a slow reduction reaction in the presence of surfactant is reported here for the first time. The slow reduction reaction exclusively results in well-defined mesoporous architectures distinctly different from the dendritic constructions reported previously. More importantly, abundant catalytically active sites are created on the highly accessible mesoporous surfaces by the selective adsorption of bromide ions. The MPNs prepared by using the new synthetic route not only show superior electrochemical performance toward methanol oxidation reaction and oxygen reduction reaction, but also exhibit extremely high structural thermostability, which makes them promising catalysts for industrial applications.

Topochemical conversion of a dense metal-organic framework from a crystalline insulator to an amorphous semiconductor

A dense, insulating metal–organic framework (MOF), is successfully converted into a semiconducting amorphous MOF via a topochemical route.

The topochemical conversion of a dense, insulating metal–organic framework (MOF) into a semiconducting amorphous MOF is described. Treatment of single crystals of copper(i) chloride trithiocyanurate, Cu^ICl(ttcH₃) (ttcH₃ = trithiocyanuric acid), 1, in aqueous ammonia solution yields monoliths of amorphous Cu^I_1.8(ttc)_0.6(ttcH₃)_0.4, 3. The treatment changes the transparent orange crystals of 1 into shiny black monoliths of 3 with retention of morphology, and moreover increases the electrical conductivity from insulating to semiconducting (conductivity of 3 ranges from 4.2 × 10^–11 S cm^–1 at 20 °C to 7.6 × 10^–9 S cm^–1 at 140 °C; activation energy = 0.59 eV; optical band gap = 0.6 eV). The structure and properties of the amorphous conductor are fully characterized by AC impedance spectroscopy, X-ray photoelectron spectroscopy, X-ray pair distribution function analysis, infrared spectroscopy, diffuse reflectance spectroscopy, electron spin resonance spectroscopy, elemental analysis, thermogravimetric analysis, and theoretical calculations.

Intrinsic and extrinsic proton conductivity in metal-organic frameworks

Hydrated interparticle phases are considered to make a considerable contribution to proton conduction in many metal-organic frameworks.

Layered structures and nanosheets of pyrimidinethiolate coordination polymers

A lamellar pyrimidinethiolate coordination polymer has been exfoliated to form nanosheets and found to be electronically insulating.

Mesoporous metallic cells: design of uniformly sized hollow mesoporous Pt-Ru particles with tunable shell thicknesses

A new class of hollow mesoporous Pt-Ru and Pt particles with uniform size, named 'mesoporous metallic cells', are synthesized through a dual-templating approach using colloidal silica particles and non-ionic surfactants. To realize the full potential of mesoporous metals as electrocatalysts, the shell thicknesses, compositions, and hollow cavity sizes are precisely controlled.