Synthesis of Hydrophilic Poly(vinylpyrrolidone)/CuS Free-Standing Thin Films Exhibiting Photothermal Conversion.

Free-standing films without the need for any support materials attract attention because of their excellent flexibility in use and ability to be transferred to various substrates. However, free-standing films containing large amounts of inorganic crystalline particles are hard to achieve due to their low strength. In this study, we found the possibility of preparing a free-standing composite film of CuS/polyvinylpyrrolidone (PVP) at a large loading of CuS (>50%) from a concentrated colloidal dispersion of CuS nanoparticles modified with PVP. Despite the large amount of inorganic crystals contained in the free-standing film, the film was strong enough to be handled without any support materials. As a proof-of-concept application of the free-standing film, a solar water evaporation experiment was performed. The CuS/PVP free-standing film exhibited photothermal conversion under light illumination to generate heat and accelerate water evaporation, achieving an evaporation rate of 4.35 kg·m-2 h-1 and an evaporation efficiency of 96.3% at a power density of 3 suns. In addition, thanks to the free-standing feature, one side of the CuS/PVP film could be hydrophobized with polydimethylsiloxane to form a Janus thin film, allowing for floating on the water surface. As a result, effective water evaporation was achieved because of the selective evaporation of water from the air/water interface.

Suppression of Pressure-Induced Phase Transitions in a Monoclinically Distorted LiNbO3-Type CuNbO3 by Preference for a CuO3 Triangular Coordination Environment.

Pressure-induced phase transitions in a monoclinically distorted LiNbO3-type CuNbO3 with triangularly coordinated Cu and octahedrally coordinated Nb were experimentally and computationally investigated. Phase transitions into GdFeO3-type or NaIO3-type structures generally observed in LiNbO3-type compounds below 30 GPa were not detected in CuNbO3 even at the maximum experimental pressure, 32.4 GPa. Our density functional theory calculations revealed that the phase transition is suppressed by the preference for the CuO3 triangular coordination environment, which reduces the total internal energy. This study clarifies that the change in the coordination environment of given ions can affect the pressure-induced phase transition.

Tunability of the bandgap of SnS by variation of the cell volume by alloying with A.E. elements.

We clarified that the bandgap of inorganic materials is strongly correlated with their effective coordination number (ECoN) via first-principles calculations and experimental confirmations. Tin mono-sulphide (Pnma) and germanium mono-sulphide (Pnma) were selected as model cases since these materials successively alter the ECoN as the cell volume changes and show an uncommon relationship between cell volume and bandgap. Contrary to the common semiconductors, the bandgaps of SnS (Pnma) and GeS (Pnma) have a positive relationship with respect to cell volume. This unique phenomenon was explained by incorporating the concept of ECoN into the theoretical studies. The theory proposed in this study is widely applicable to semiconductors with low-symmetry structures. Further, we experimentally demonstrated that the bandgap of SnS (Pnma) can be broadly tuned by changing the unit cell volume via alloying with alkali-earth (A.E.) metals, which could allow SnS to be applied to Si-based tandem photovoltaics. Alloying with A.E. elements also stabilised Cl as an n-type donor, which enabled n-type conduction in the bandgap-widened SnS film in the SnS-based semiconductors.

Synthesis of CaSnN2 via a High-Pressure Metathesis Reaction and the Properties of II-Sn-N2 (II = Ca, Mg, Zn) Semiconductors.

A novel ternary nitride semiconductor, CaSnN2, with a layered rock-salt-type structure (R3̅m) was synthesized via a high-pressure metathesis reaction. The properties and structures of II-Sn-N2 (II = Ca, Mg, Zn) semiconductors were also systematically studied, and the differences among them were revealed by comparison. These semiconductor materials showed a rock-salt- or wurtzite-type structure depending on the combined effect of the synthetic conditions and the characteristics of the group II elements. Additionally, the rock-salt-type structures of CaSnN2 and MgSnN2 (i.e., the ambient-pressure phase) were different from those predicted using first-principles calculations. Further, on the basis of first-principles calculations and consideration of the pressure effect, the recovered CaSnN2 sample showed an R3̅m structure. CaSnN2 and MgSnN2 showed a band gap of 2.3-2.4 eV, which is suitable for overcoming the green-light-gap problem. These semiconductors also showed a strong cathode luminescence peak at room temperature, and generalized gradient approximation (GGA) calculations revealed that CaSnN2 has a direct band gap. These inexpensive and nontoxic semiconductors (II-Sn-N2 semiconductors (II = Ca, Mg, Zn)), with mid band gaps are required as pigments to replace cadmium-based materials. They can also be used in emitting devices and as photovoltaic absorbers, replacing InxGa1-xN semiconductors.

Synthesis of a Crystalline and Transparent Aerogel Composed of Ni-Al Layered Double Hydroxide Nanoparticles through Crystallization from Amorphous Hydrogel.

Enormous efforts have been devoted to the development of crystalline aerogels toward heterogeneous catalysis, energy storage, ion/molecular absorption, and luminescence. However, properties of aerogels are not fully exploited due to their low content of functional moieties embedded in their solid networks, low crystallinity, and limited chemical compositions. Herein, we develop a one-pot approach based on crystallization from amorphous metal hydroxides modified with a ß-diketone ligand, toward crystalline transition-metal hydroxide aerogels. Synthesis of monolithic and crystalline aerogels of layered double hydroxide (LDH) was performed in a Ni-Al system starting from aqueous ethanol solutions of NiCl2·6H2O and AlCl3·6H2O with acetylacetone (acac) as an organic ligand. Propylene oxide (PO) as an alkalization reagent was added into precursory solutions to yield monolithic wet gels. The successive pH increase induces the formation of a three-dimensional (3-D) solid framework composed of amorphous Al(OH)3. Then, amphoteric Al(OH)3 undergoes crystallization into Ni-Al LDH via an acetylacetone-driven dissolution-crystallization of metal hydroxides without destroying the preformed 3-D solid framework. The process allows us to obtain crystalline aerogel monoliths with high porosity and high transparency after supercritical CO2 drying of wet gels. The present scheme can be expected to synthesize functionalized aerogel composed of crystalline transition-metal oxide/hydroxide nanobuilding blocks (NBBs).

Imparting CO2 reduction selectivity to ZnGa2O4 photocatalysts by crystallization from hetero nano assembly of amorphous-like metal hydroxides.

Imparting an enhanced CO2 reduction selectivity to ZnGa2O4 photocatalysts has been demonstrated by controlled crystallization from interdispersed nanoparticles of zinc and gallium hydroxides. The hydroxide precursor in which Zn(ii) and Ga(iii) are homogeneously interdispersed was prepared through an epoxide-driven sol-gel reaction. ZnGa2O4 obtained by a heat-treatment exhibits a higher surface basicity and an enhanced affinity for CO2 molecules than previously-reported standard ZnGa2O4. The enhanced affinity for CO2 molecules of the resultant ZnGa2O4 leads to highly-selective CO evolution in CO2 photo-reduction with H2O reductants. The present scheme is promising to achieve desirable surface chemistry on metal oxide photocatalysts.

Fracture-induced amorphization of polycrystalline SiO2 stishovite: a potential platform for toughening in ceramics.

Silicon dioxide has eight stable crystalline phases at conditions of the Earth's rocky parts. Many metastable phases including amorphous phases have been known, which indicates the presence of large kinetic barriers. As a consequence, some crystalline silica phases transform to amorphous phases by bypassing the liquid via two different pathways. Here we show a new pathway, a fracture-induced amorphization of stishovite that is a high-pressure polymorph. The amorphization accompanies a huge volume expansion of ~100% and occurs in a thin layer whose thickness from the fracture surface is several tens of nanometers. Amorphous silica materials that look like strings or worms were observed on the fracture surfaces. The amount of amorphous silica near the fracture surfaces is positively correlated with indentation fracture toughness. This result indicates that the fracture-induced amorphization causes toughening of stishovite polycrystals. The fracture-induced solid-state amorphization may provide a potential platform for toughening in ceramics.

Tetravalent dysprosium in a perovskite-type oxide.

The existence of tetravalent dysprosium in perovskite-type oxide barium zirconate is confirmed in this work. This discovery will stimulate many researchers in diverse fields such as catalysts, solid state ionics, sensors, and fluorescent materials.

Theoretical calculations of the thermodynamic stability of ionic substitutions in hydroxyapatite under an aqueous solution environment.

Defect formation energies in materials generally depend on chemical potentials determined by a chemical equilibrium condition. In particular, an aqueous solution environment is important for biomaterials such as hydroxyapatite studied here. Therefore, a methodology to obtain ionic chemical potentials under chemical equilibrium between solid and aqueous solution was introduced, and was applied to substitutional divalent cations formed via ion exchange with Ca(2+) in hydroxyapatite. The calculated ranking of the stability of substitutional cations in HAp was in good agreement with the experimentally observed trend. The present theoretical approach would be useful to explore the thermodynamic stability of defects in materials subjected to an aqueous solution environment.

First-principles calculations of Zn-K XANES in Ca-deficient hydroxyapatite.

The local environment of substitutional Zn(2+) in Ca-deficient hydroxyapatite (HAp) was investigated using experimental and theoretical analyses of the x-ray absorption near edge structure (XANES). For Zn-K XANES calculations, two situations of Zn(2+) were considered. One was Zn(2+) substituted for Ca sites in perfect HAp, and the other was a Ca-deficient HAp model of substitutional Zn(2+) associated with a Ca(2+) vacancy charge compensated by two protons. The model of Zn(2+) in perfect HAp did not reproduce the experimental Zn-K XANES spectrum. In contrast, the Ca-deficient HAp model agreed well with the experimental spectrum. This indicates that substitutional Zn(2+) in Ca-deficient HAp is associated with the Ca(2+) vacancy complex in HAp.

Mechanism of incorporation of zinc into hydroxyapatite.

The atomic level mechanism of incorporation of Zn(2+) into hydroxyapatite (HAp), which is a potential dopant to promote bone formation, was investigated, based on first principles total energy calculations and experimental X-ray absorption near edge structure (XANES) analyses. It was found that Zn(2+)-doped HAp tends to have a Ca-deficient chemical composition and substitutional Zn(2+) ions are associated with a defect complex with a Ca(2+) vacancy and two charge compensating protons. Moreover, first principles calculations demonstrated that Zn(2+) incorporation into HAp can take place by occupying the Ca(2+) vacancy of the defect complex. The Ca(2+) vacancy complex is not only the origin of the calcium deficiency in HAp, but also plays a key role in the uptake of trace elements during mineralization.

Strontium substitution in bioactive calcium phosphates: a first-principles study.

First-principles calculations are performed to investigate atomic and electronic structures of Sr(2+) ions substituting for Ca(2+) in octacalcium phosphate (OCP). The defect formation energies are evaluated from total energies of supercells and ionic chemical potentials of Sr(2+) and Ca(2+) determined under the chemical equilibrium with aqueous solution saturated with hydroxyapatite (HAp). The defect formation energy depends on the solution pH and the substitutional Ca sites in OCP, and the estimated equilibrium concentrations of Sr(2+) in OCP and HAp are in reasonable agreement with previous experimental results obtained in physiological conditions. It is also found that Sr(2+) ions can be more favorably substituted in OCP than in HAp. It is thought, therefore, that Sr(2+) plays its role to promote bone formation by being incorporated into the metastable OCP phase occurring during HAp nucleation.