Comprehensive Analysis of the Chemical and Structural Transformations of Mg-Al-CO3 Layered Double Hydroxides with Different Mg/Al Ratios at Elevated Temperatures.

Mg-Al layered double hydroxides (LDHs) with CO32- interlayer anions are promising CO2 adsorbents. Here, we analyzed the quantitative gas evolution behaviors of Mg-Al LDH particles with different Mg/Al ratios during the multistep chemical/structural transformations at elevated temperatures. The Mg/Al molar ratio strongly affects the behavior: the transformation changes from two apparent steps to three steps depending on the Mg/Al ratio. The transformation occurs in essentially the same way as that observed for large Mg-Al LDH crystals: (1) release of the interlayer water, (2) partial dehydroxylation of the hydroxyl layers followed by coordination of carbonate ions to the metals, and (3) collapse of the layered structure. We provide a molecular/atomic level picture of the structure in each step of the transformation by first-principles density functional theory (DFT) calculation. The structurally optimized model and reexamination of experimental data showed that step (1) results in a large decrease in the interlayer distance of the LDH from ∼7.6 to ∼6.7 Å (a decrease of ∼0.9 Å) and the possible explanation is the waving of the metal hydroxide layers. This study provides a comprehensive understanding of the structural changes of LDHs with various Mg/Al ratios to resolve the various interpretations in the literature.

Thermal self-reduction of metal hydroxide acrylate monolayer nanoparticles leads formation of nanoparticulate and porous structured alloys.

Chemical and physical designs of alloy nanomaterials have attracted considerable attention for the development of highly functional materials. Although polyol processes using ionic precursors are widely used to synthesise alloy nanoparticles, the reduction potential of polyols limits their chemical composition, making it difficult to obtain 3d transition metals. In this study, we employed pre-synthesized metal hydroxide salt monolayer nanoparticles as precursors to obtain alloy nanoparticles. Simultaneous dehydroxylation of the hydroxide moiety and decomposition of the organic moiety allowed the formation of stable face-centred cubic metals passing through the metal carbide and metastable hexagonal close-packed metal phases. This self-reduction process enabled the formation of nanoparticulate bimetallic alloys and macroporous/mesoporous-structured bimetallic alloys by compositing hard/soft templates with pre-synthesized metal hydroxide salt nanoparticles. We believe that the strategy presented in this study can be used to design nanostructures and chemical compositions of multimetallic alloy nanoparticles as well as bimetallic systems.

Thermal Conversion of Nanocrystalline Metal Hydroxide Salts to Metal Carbides, Pnictides, Chalcogenides, and Halides.

A general procedure for synthesizing various inorganic compounds in a similar manner is required in the field of material chemistry. The use of solid-state reactive agents with inorganic precursors is a successful approach in this direction. In this study, organic-inorganic hybrid metal hydroxide salts (MHSs) were utilized to synthesize various inorganic compounds by a simple heat treatment method because they can be assumed to be "premixed" inorganic precursors and solid-state reactive agents. Comparative studies revealed that the nanocrystalline characteristics and coordination of the carboxylate of the synthesized MHSs enabled simultaneous dehydration of hydroxides and decomposition of carboxylates and subsequent formation of metals and metal sulfides. Manganese, iron, cobalt, nickel, and zinc sulfides, as well as nickel carbides, pnictides, chalcogenides, and halides were obtained using the same procedure. We believe that using nanocrystalline organic-inorganic hybrid MHSs as both inorganic precursors and organic reactive agents will be a simple and versatile way to prepare a wide variety of inorganic complex compounds.

High Heat Resistance of the Structural Coloration of Colloidal Arrays with Inorganic Black Additives.

Structurally colored materials consisting of arrays of submicrometer-sized particles have drawn a great deal of attention because of their advantages, including low cost, low impact on human health as well as the environment, and resistance to fading. However, their low thermal stability is considered to be a critical issue for their practical use as colorants. Black-colored substances that can absorb the white color are added to colloidal array-type structurally colored materials to enhance their chromaticity. The poor thermal stability of commonly used black coloring additives, carbon black and Fe3O4 nanoparticles, is a main factor that reduces the heat resistance of structural coloration. Here, we demonstrate the preparation of structurally colored materials with extraordinarily high heat resistance of coloration, up to 900 °C. Several metal oxides, i.e., calcium manganese-based oxide (CCMO), chromium-iron-cobalt-nickel oxide (CFCNO), and lanthanum manganite (LMO), are synthesized and employed as black additives for structurally colored coatings prepared by the electrophoretic deposition of spherical silica particles. When CCMO is used as a black additive, the coloration heat resistance of the film is stable up to 700 °C. On the other hand, the films maintain vivid structural colors after exposure to 900 °C temperatures when CFCNO and LMO are employed as black additives. On the basis of this finding, high heat resistance of structural colors requires both heat resistance of the black additives and nonreactivity with the components of the spherical particles used for colloidal arrays.

Understanding the Electrophoretic Deposition Accompanied by Electrochemical Reactions Toward Structurally Colored Bilayer Films.

Safe, low-cost structurally colored materials are alternative colorants to toxic inorganic pigments and organic dyes. Colloidal amorphous arrays are promising structurally colored materials because of their angle-independent colors. In this study, we focused on precise tuning of the chromaticity by preparing bilayer colloidal amorphous arrays through electrophoretic deposition (EPD). Systematic investigations with various EPD conditions clarified the contributions of each condition to the EPD process and the competing electrochemical reactions, which enabled us to prepare well-colored coatings. EPD films composed of colloidal amorphous array bilayers were successfully synthesized with controlled film thickness. Chromaticity of the films was found to be precisely controlled by the EPD duration. We believe that this understanding of the EPD process and its application to synthesis of structurally colored bilayer films will bring structurally colored materials closer to practical industrial use.

Synthetic Route of Layered Titanium Nitride Chloride TiNCl Using Sodium Amide.

A synthetic route in a closed system for layered titanium nitride chloride TiNCl has been developed using sodium amide NaNH2 as a nitrogen source. A highly crystalline sample is obtained by an appropriate thermal decomposition of aminated titanium chloride. The obtained TiNCl was also characterized using electronic resistivity measurement and photoemission spectroscopy. TiNCl showed hopping conduction compatible with an in-gap state revealed by photoelectron spectroscopy. However, it appeared highly electron-doped, albeit without showing superconductivity. Comparison with the spectrum of superconducting sodium-doped samples suggests the presence of the microstructure required to exhibit superconductivity.

Interconnection of organic-inorganic hybrid nano-building blocks towards thermally robust mesoporous structures.

The use of organic-inorganic hybrid nanoparticles will enable a control of the characteristics of both the nanoparticles and constructed fine structures. In this study, we report the synthesis of acrylate-intercalated layered manganese, cobalt, and nickel hydroxide nanoparticles and their assembly into ordered mesoporous structures. Polymerization of the intercalated acrylates takes place by means of a radical initiator. The formed organic network improved the thermal stability of the layered hydroxides, which results in thermally robust mesoporous structures. Additionally, it is found that the polymerization can be initiated and progressed at 200 °C without any initiators for the layered nickel hydroxide system. This allows for the scalable solid-state thermal polymerization of intercalated acrylates and the formation of thermally robust hierarchically ordered meso/macroporous powders as well as mesoporous films. The electrochemical characterization reveals that the thermally robust mesoporous films having regulated mesopores allow for the effective diffusion of molecules/solvent compared with the films having collapsed mesoporous structures.

Size Effect of Hydroxide Nanobuilding Blocks and Nonionic Block Copolymer Templates on the Formation of Ordered Mesoporous Structures.

The use of precrystallized nanoparticles as nanobuilding blocks (NBBs) is a promising way to obtain mesoporous materials with crystalline walls. In this study, the size effects of both hydroxide NBBs and nonionic block copolymer (BCP) templates on the formation of ordered mesostructures are investigated. The diameter of layered nickel hydroxide NBBs was controlled at the sub-2 nm scale by an epoxide-mediated alkalinization process. Commercially available nonionic BCPs (gyration radii in the range of 11.9-43.9 Å) were used. Mesoperiodic structures were formed by the evaporation-induced self-assembly process. A proper size combination of hydroxide NBBs, smaller than 12.5 Å, and BCPs, larger than 19.9 Å, is shown to be necessary to form ordered mesostructures.

Environmentally Benign Synthesis and Color Tuning of Strontium-Tantalum Perovskite Oxynitride and Its Solid Solutions.

A facile method was successfully developed to prepare strontium-tantalum perovskite oxynitride, SrTaO2N, and its solid solutions. Urea was employed as a solid nitriding agent to eliminate the use of toxic NH3 gas. In addition, utilization of sol-gel-derived Ta2O5 gel as a Ta precursor allowed for completion of nitridation within a shorter period and at a lower calcination temperature compared with the conventional ammonolysis process. Optimization of the reaction conditions, such as the urea content, allowed for the production of solid solutions of SrTaO2N and Sr1.4Ta0.6O2.9. The products exhibited optical absorption and chromatic colors because of the narrower band gaps of oxynitrides compared with those of oxides. The O/N ratios of the solid solutions were easily adjusted by varying the amount of urea in the mixture of precursors. As a result, the colors of the products ranged from yellow to brown. The nitridation process and products developed in this study are interesting environmentally benign alternatives to conventional inorganic pigments.

High-Pressure Synthesis and Crystal Structure of the Sulfur-Richest Chromium Sulfide CrS3 Composed of Cr(III) and Disulfide Ions.

The new binary chromium sulfide CrS3 has been synthesized by reaction of Cr3S4 and sulfur mixtures at 800 °C and 13 GPa. CrS3 crystallizes in an orthorhombic unit cell with a = 4.6742(7) Å, b = 5.7315(8) Å, c = 10.603(2) Å, and V = 284.873(4) Å3. It has a novel structure composed of Cr2S10 edge-shared octahedral dimers, which share all of their corners to form a three-dimensional structure. All of the sulfur atoms form S22- disulfide ions with a S-S distance of 2.063(5) or 2.068(8) Å. The structure of CrS3 is a derivative of the crystal structure of marcasite FeS2, in which one in three metal sites of the marcasite structure is vacant in the CrS3 structure.

Robust Structurally Colored Coatings Composed of Colloidal Arrays Prepared by the Cathodic Electrophoretic Deposition Method with Metal Cation Additives.

Structurally colored coatings composed of colloidal arrays of monodisperse spherical particles have attracted great attention owing to their versatile advantages, such as low cost, resistance to fading, and low impacts on the environment and human health. However, the weak mechanical stability is considered to be a major obstacle for their practical applications as colorants. Although several approaches based on the addition of polymer additives to enhance the adhesion of particles have been reported, the challenge remains to develop a strategy for the preparation of structurally colored coatings with extremely high robustness using a simple process. Here, we have developed a novel approach to fabricate robust structurally colored coatings by cathodic electrophoretic deposition. The addition of a metal salt, i.e., Mg(NO3)2, to the coating dispersion allows SiO2 particles to have a positive charge, which enables the electrophoresis of SiO2 particles toward the cathode. At the cathode, Mg(OH)2 codeposits with SiO2 particles because OH- ions are generated by the decomposition of dissolved oxygen and NO3- ions. The mechanical stability of the colloidal arrays obtained by this process is remarkably improved because Mg(OH)2 facilitates the adhesion of the particles and substrates. The brilliant structural color is maintained even after several cycles of the sandpaper abrasion test. We have also demonstrated the coating on a stainless steel fork. This demonstration reveals that our approach enables a homogeneous coating on a complicated surface. Furthermore, the high durability of the coating is clarified because the coating did not peel off even when the fork was stuck into a plastic eraser. Therefore, the coating technique developed here will provide an effective method for the pervasive application of the structural color as a colorant.

Mechanistic Insight on the Formation of GaN:ZnO Solid Solution from Zn-Ga Layered Double Hydroxide Using Urea as the Nitriding Agent.

A solid solution of GaN and ZnO (GaN:ZnO) is promising as a photocatalyst for visible-light-driven overall water splitting to produce H2. However, several obstacles still exist in the conventional preparation procedure of GaN:ZnO. For example, the atomic distributions of Zn and Ga are nonuniform in GaN:ZnO when a mixture of the metal oxides, i.e. Ga2O3 and ZnO, is used as a precursor. In addition, GaN:ZnO is generally prepared under a harmful NH3 flow for long durations at high temperatures. Here, a facile synthesis of GaN:ZnO with homogeneous atomic composition via a simple and safe procedure is reported. A layered double hydroxide (LDH) containing Zn2+ and Ga3+ was used to increase the uniformity of the atomic distributions of Zn and Ga in GaN:ZnO. We employed urea as a nitriding agent instead of gaseous NH3 to increase the safety of the reaction. Through the optimization of reaction conditions such as heat treatment temperature and content of urea, single-phase GaN:ZnO was successfully obtained. In addition, the nitridation mechanism using urea was investigated in detail. NH3 released from the thermal decomposition of urea did not directly nitride the LDH precursor. X-ray absorption and infrared  spectroscopies revealed that Zn(CN2)-like intermediate species were generated at the middle temperature range and Ga-N bonds formed at high temperature along with dissociation of CO and CO2.

Preparation of pH-Responsive Hollow Capsules via Layer-by-Layer Assembly of Exfoliated Layered Double Hydroxide Nanosheets and Polyelectrolytes.

pH-Responsive smart capsules were developed by the layer-by-layer assembly with a colloidtemplating technique. Polystyrene (PS) particles were employed as core templates. Acid-soluble inorganic nanosheets were prepared from Mg-Al layered double hydroxide (LDH) by an exfoliation technique. LDH nanosheets and anionic polyelectrolytes were alternatively deposited on PS core particles by the layer-by-layer assembly using electrostatic interaction. Hollow capsules were obtained by the removal of the PS core particles. The hollow capsules obtained thus were collapsed at acidic conditions by dissolution of LDH nanosheets in the hollow shells. The dissolution rate, i.e., the responsiveness of capsule, is tunable according to the strength of acids.

Structurally colored coating films with tunable iridescence fabricated via cathodic electrophoretic deposition of silica particles.

In recent years, colloidal arrays of submicrometer-sized monodisperse particles used as structurally colored coatings have drawn great attention due to their non-bleaching properties and low impact on human health and the environment. In this paper, structurally colored coating films were fabricated using monodisperse SiO2 particles via the cathodic electrophoretic deposition (EPD) technique. The addition of a strong polycation, poly(diallyldimethylammonium chloride) (PDDA), enables the cathodic EPD of SiO2 particles and carbon black (CB) additives. Optimizing the quantities of PDDA and CB results in the appearance of vivid structural color from the coating films. The arrangement of the particle array is controllable by varying the pH of the water added to the coating sols for EPD. Structurally colored coating films with and without iridescence, i.e., angular dependence, can be fabricated on demand by a simple operation of the EPD process. In addition, the coating film prepared by cathodic EPD displayed high abrasion resistance because PDDA acts not only as a charge control agent but also as a binder.

ZrO2 Nanocrystals As Catalyst for Synthesis of Dimethylcarbonate from Methanol and Carbon Dioxide: Catalytic Activity and Elucidation of Active Sites.

The catalytic activity of zirconium oxide (ZrO2) nanocrystals for the reaction of carbon dioxide (CO2) with methanol to form dimethylcarbonate (DMC) was investigated. ZrO2 nanocrystals prepared by hydrothermal synthesis at various temperatures were compared. The size of the ZrO2 nanocrystals monotonically increased with the hydrothermal temperature, according to specific surface area, transmission electron microscope measurements, and their X-ray diffraction peak widths. The ZrO2 nanocrystals prepared by hydrothermal synthesis were found to exhibit high catalytic activity owing to their high surface area and catalytically active surfaces arising from their high crystallinity. Next, adsorbed species generated from CO2 on the ZrO2 surfaces were measured using CO2 temperature-programmed desorption (TPD) and in situ FT-IR spectroscopy. The results confirmed the presence of several kinds of adsorbed species including bidentate bicarbonate (b-HCO3-), bidentate carbonate (b-CO32-), and monodentate carbonate (m-CO32-). The relationship between the amounts of these surface species and the catalytic activity of the ZrO2 was investigated for the first time. The amount of the bidentate species (b-HCO3- and b-CO32-) was found to correlate well with the catalytic activity, demonstrating that the surface sites that afford these species contribute to the catalytic activity for this reaction.

High-Pressure Synthesis and Electronic Structure of a New Superconducting Strontium Germanide (SrGe3) Containing Ge2 Dumbbells.

We obtained a new strontium germanide (SrGe3) by high-pressure and high-temperature synthesis. It was prepared at 13 GPa and 1100 °C. The space group and cell constants are I4/mmm (No. 139), a = 7.7800(8) Å, c = 12.0561(13) Å, and V = 729.74(17) Å(3). SrGe3 crystallizes in the CaSi3 structure composed of Ge-Ge dumbbells and Sr(2+) ions. SrGe3 is a type II superconductor with a transition temperature of 6.0 K.

High-pressure synthesis and structural characterization of the type II clathrate compound Na(30.5)Si(136) encapsulating two sodium atoms in the same silicon polyhedral cages.

Single crystals of sodium containing silicon clathrate compounds Na8Si46 (type I) and NaxSi136 (type II) were prepared from the mixtures of NaSi and Si under high-pressure and high-temperature conditions of 5 GPa at 600-1000 °C. The type II crystals were obtained at relatively low-temperature conditions of 700-800 °C, which were found to have a Na excess composition Na30.5Si136 in comparison with the compounds NaxSi136 (x ≤ 24) obtained by a thermal decomposition of NaSi under vacuum. The single crystal study revealed that the Na excess type II compound crystallizes in space group Fd3̅m with a lattice parameter of a = 14.796(1) Å, slightly larger than that of the ambient phase (Na24Si136), and the large silicon hexakaidecahedral cages (@Si28) are occupied by two sodium atoms disordered in the two 32e sites around the center of the @Si28 cages. At temperatures <90 K, the crystal symmetry of the compound changes from the face-centered to the primitive cell with space group P213, and the Na atoms in the @Si28 cages are aligned as Na2 pairs. The temperature dependence of the magnetic susceptibility of Na30.5Si136 suggests that the two Na ions (2 Na(+)) in the cage are changed to a Na2 molecule. The Na atoms of Na30.5Si136 can be deintercalated from the cages topochemically by evacuation at elevated temperatures. The single crystal study of the deintercalated phases NaxSi136 (x = 25.5 and 5.5) revealed that only excess Na atoms have disordered arrangements.

Robust infrared-shielding coating films prepared using perhydropolysilazane and hydrophobized indium tin oxide nanoparticles with tuned surface plasmon resonance.

Robust infrared (IR)-shielding coating films were prepared by dispersing indium tin oxide (ITO) nanoparticles (NPs) in a silica matrix. Hydrophobized ITO NPs were synthesized via a liquid phase process. The surface plasmon resonance (SPR) absorption of the ITO NPs could be tuned by varying the concentration of Sn doping from 3 to 30 mol %. The shortest SPR wavelength and strongest SPR absorption were obtained for the ITO NPs doped with 10% Sn because they possessed the highest electron carrier density. Coating films composed of a continuous silica matrix homogeneously dispersed with ITO NPs were obtained using perhydropolysilazane (PHPS) as a precursor. PHPS was completely converted to silica by exposure to the vapor from aqueous ammonia at 50 °C. The prepared coating films can efficiently shield IR radiation even though they are more than 80% transparent in the visible range. The coating film with the greatest IR-shielding ability completely blocked IR light at wavelengths longer than 1400 nm. The pencil hardness of this coating film was 9H at a load of 750 g, which is sufficiently robust for applications such as automotive glass.

Topochemical cross-linking of the [Hf2N2] layers with sulfur in α-HfNBr.

α-HfNBr (space group Pmmn) isostructural with FeOCl consists of orthorhombic [Hf2N2] layers sandwiched by bromide layers. By the reaction with Na2S at 750 °C in a sealed glass tube, the α-type [Hf2N2] layers are topochemically cross-linked with sulfur to form α-Hf2N2S with a mutual shift of the layers in the ab plane by (a/2 + b/2) to crystallize in space group Immm and the lattice parameters a = 4.1422(1), b = 3.5058(1), and c = 11.4043(3) Å. At a higher reaction temperature of 850 °C, the ß-type layered variant ß-Hf2N2S with a double honeycomb network is obtained, which adopts the La2O2S structure with space group P3m1 and the lattice parameters a = 3.5805(1) and c = 6.4063(1) Å.

New polymorph of Mo3S4 prepared using a high-pressure synthesis technique: crystal structure, electronic property, and band calculation.

A new polymorph of Mo3S4 was synthesized at 13 GPa and 1450 °C from a stoichiometric mixture of elements. It crystallizes in a triclinic unit cell (space group P1 (No. 2)) with cell constants of a = 6.364(2) Ǻ, b = 6.608(2) Ǻ, c = 6.809(2) Ǻ, α = 103.899(3) °, ß = 117.753(3) °, γ = 103.958(3) °, and V = 224.25(13) Ǻ(3). The structure of Mo3S4 is composed of edge- and face-sharing MoS6 octahedra. It was closely related to the structure of MMo2S4 type compounds (M = V, Cr, Fe, and Co). Mo3S4 can be regarded as a derivative with M = Mo. The calculated density of 6.160 g/cm(3) was much larger than 5.191 g/cm(3) of famous polymorphic Mo6S8 (Chevrel phase). Mo3S4 was metallic and did not show any superconducting transition down to 2 K. The bond-valence sums suggested that Mo3S4 can be classified in the class III-B of mixed-valence compounds; all Mo ions have a similar nonintegral valence. Electronic structure calculations revealed that the conduction band of Mo3S4 contains much contribution of the relatively narrow Mo 4d bands as well as the bands composed of hybridized Mo4d-S3d orbitals.