Takayoshi Sasaki

A Superlattice of Alternately Stacked Ni–Fe Hydroxide Nanosheets and Graphene for Efficient Splitting of Water

Cost-effective electrocatalysts based on nonprecious metals for efficient water splitting are crucial for various technological applications represented by fuel cell. Here, 3d transition metal layered double hydroxides (LDHs) with varied contents of Ni and Fe were successfully synthesized through a homogeneous precipitation. The exfoliated Ni-Fe LDH nanosheets were heteroassembled with graphene oxide (GO) as well as reduced graphene oxide (rGO) into superlattice-like hybrids, in which two kinds of oppositely charged nanosheets are stacked face-to-face in alternating sequence. Heterostructured composites of Ni2/3Fe1/3 LDH nanosheets and GO (Ni2/3Fe1/3-GO) exhibited an excellent oxygen evolution reaction (OER) efficiency with a small overpotential of about 0.23 V and Tafel slope of 42 mV/decade. The activity was further improved via the combination of Ni2/3Fe1/3 LDH nanosheets with more conductive rGO (Ni2/3Fe1/3-rGO) to achieve an overpotential as low as 0.21 V and Tafel plot of 40 mV/decade. The catalytic activity was enhanced with an increased Fe content in the bimetallic Ni-Fe system. Moreover, the composite catalysts were found to be effective for hydrogen evolution reaction. An electrolyzer cell powered by a single AA battery of 1.5 V was demonstrated by using the bifunctional catalysts.

New Layered Rare‐Earth Hydroxides with Anion‐Exchange Properties

We report the synthesis of a new series of layered hydroxides based on rare-earth elements with a composition of RE(OH)2.5Cl(0.5).0.8 H2O (RE: Eu, Tb, etc.) through the homogeneous precipitation of RECl3.x H2O with hexamethylenetetramine (HMT). Rietveld analysis combined with direct methods revealed an orthorhombic layered structure comprising a positively charged layer of [RE(OH)2.5-(H2O)0.8]0.5+ and interlayer Cl- ions. The Cl- ions were readily exchangeable for various anions (NO3-, SO4(2-), dodecylsulfonate, etc.) at ambient temperature. Photoluminescence studies showed that the compounds display typical RE3+ emission. With rare-earth-based host layers and tunable interlayer guests, the new compounds may be of interest for optoelectronic, magnetic, catalytic, and biomedical materials.

Oriented Monolayer Film of Gd2O3:0.05 Eu Crystallites: Quasi‐Topotactic Transformation of the Hydroxide Film and Drastic Enhancement of Photoluminescence Properties

Caught on film: A semitransparent and intensely luminescent monolayer film of oriented Gd(2)O(3):0.05 Eu platelet crystallites is fabricated by annealing the precursor hydroxide film (see scheme). The photoluminescence properties of the as-transformed film are greatly improved over those of the hydroxide film, and are much more pronounced than those of the corresponding Gd(2)O(3):0.05 Eu powder.

General insights into structural evolution of layered double hydroxide: underlying aspects in topochemical transformation from brucite to layered double hydroxide.

The topochemical transformation from transition-metal brucite hydroxide (Co(1-x)Fe(x)(OH)(2), Co(OH)(2), Co(1-x)Ni(x)(OH)(2)) to corresponding (Co(2+)-(Co(3+))-Fe(3+), Co(2+)-(Ni(2+))-Co(3+)) LDH under oxidizing halogen agents (iodine, bromine) exhibits different staging phenomena depending on the metallic composition/ratio in starting brucite. A plausible charge hopping mechanism based on valence interchange between redoxable charge center (Fe(3+)/Co(3+)) and neighboring divalent sites in the host sheet is proposed to understand the restoration of electron donor sites at the interface between brucite crystallites and halogen agents, which ensures a continual oxidative reaction, and a staged intercalation/diffusion of in situ reduced halide anions into the interlayer gallery commensurate with the host charge propagation. The discussion on the correlation between staging product and metallic composition/ratio offers a general perspective and new insights into M(2+)/M(3+) ratio and cation ordering, host layer charge, and phase evolution in LDH structure.

Gigantic Swelling of Inorganic Layered Materials: A Bridge to Molecularly Thin Two-Dimensional Nanosheets

Platy microcrystals of a typical layered material, protonated titanate, have been shown to undergo an enormous degree of swelling in aqueous solutions of various amines, including tertiary amines, quaternary ammonium hydroxides, and primary amines. Introducing these solutions expanded the crystal gallery height by up to ~100-fold. Through systematic analysis, we determined that ammonium ion intercalation is predominantly affected by the acid-base equilibrium and that the degree of swelling or inflow of H2O is controlled by the osmotic pressure balance between the gallery and the solution environment, both of which are relatively independent of electrolyte identity but substantially dependent on molarity. In solutions of tertiary amines and quaternary ammonium hydroxides, the uptake of ammonium ions increases nearly linearly with increasing external concentration before reaching a saturation plateau, i.e., ~40% relative to the cation-exchange capacity of the crystals used. The only exception is tetrabutylammonium ions, which yield a lower saturation value, ~30%, owing to steric effects. The swelling behaviors in some primary amine solutions differ as a result of the effect of attractive forces between amine solute molecules on the solution osmotic pressure. Although the swelling is essentially colligative in nature, the stability of the resultant swollen structure is heavily dependent on the chemical nature of the guest ions. Intercalated ions of higher polarity and smaller size help stabilize the swollen structure, whereas ions of lower polarity and larger size lead readily to exfoliation. The insight gained from this study sheds new light on both the incorporation of guest molecules into a gallery of layered structures in general and the exfoliation of materials into elementary single-layer nanosheets.

Oriented films of layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface.

Layered rare-earth hydroxide crystallites self-assembled at the hexane/water interface were transferred to various substrates to form a monolayer film, which exhibited photoluminescence properties and ion-exchange ability.

Titanoniobate and niobate nanosheet photocatalysts: superior photoinduced hydrophilicity and enhanced thermal stability of unilamellar Nb3O8 nanosheet

Unique photocatalysts based on novel two-dimensional (2D) oxide nanosheets have been synthesized and their photochemical activity has been examined. Monolayer films of titanoniobate and niobate (TiNbO5, Ti2NbO7, Ti5NbO14, and Nb3O8) nanosheets, synthesized by exfoliating layered oxide precursors through a soft-chemical procedure, were fabricated on quartz glass substrate via a sequential adsorption method. All the nanosheet films exhibited good photoinduced hydrophilicity, while their oxidation activity was very low. This behavior can be regarded as inherent features of nanosheet-type photocatalysts having molecularly thin 2D anisotropy. Such ultrathin flexible structures are advantageous for facilitating photo-driven surface wettability change. Especially, the hydrophilic conversion property of Nb3O8 nanosheet was highly efficient, showing activity that was at least one order of magnitude superior to that of the widely used photocatalyst film of polycrystalline anatase TiO2. Moreover, the monolayer film of Nb3O8 nanosheet was found to have enhanced thermal stability and chemical resistance, particularly against diffusion of sodium ions at elevated temperature: Nb3O8 nanosheet film heated on sodium-rich glass (soda-lime glass) substrate maintained excellent hydrophilic conversion activity, whereas Ti0.87O2 nanosheet as well as anatase (TiO2) based photocatalysts was virtually deactivated. These features are a great advantage of Nb3O8 nanosheet photocatalysts for developing the practical super-hydrophilic applications where post-annealing is indispensable.

All-Nanosheet Ultrathin Capacitors Assembled Layer-by-Layer via Solution-Based Processes

All-nanosheet ultrathin capacitors of Ru0.95O20.2-/Ca2Nb3O10-/Ru0.95O20.2- were successfully assembled through facile room-temperature solution-based processes. As a bottom electrode, conductive Ru0.95O20.2- nanosheets were first assembled on a quartz glass substrate through a sequential adsorption process with polycations. On top of the Ru0.95O20.2- nanosheet film, Ca2Nb3O10- nanosheets were deposited by the Langmuir-Blodgett technique to serve as a dielectric layer. Deposition parameters were optimized for each process to construct a densely packed multilayer structure. The multilayer buildup process was monitored by various characterizations such as atomic force microscopy (AFM), ultraviolet-visible absorption spectra, and X-ray diffraction data, which provided compelling evidence for regular growth of Ru0.95O20.2- and Ca2Nb3O10- nanosheet films with the designed multilayer structures. Finally, an array of circular films (50 μm ϕ) of Ru0.95O20.2- nanosheets was fabricated as top electrodes on the as-deposited nanosheet films by combining the standard photolithography and sequential adsorption processes. Microscopic observations by AFM and cross-sectional transmission electron microscopy, as well as nanoscopic elemental analysis, visualized the sandwich metal-insulator-metal structure of Ru0.95O20.2-/Ca2Nb3O10-/Ru0.95O20.2- with a total thickness less than 30 nm. Electrical measurements indicate that the system really works as an ultrathin capacitor, achieving a capacitance density of ∼27.5 μF cm(-2), which is far superior to currently available commercial capacitor devices. This work demonstrates the great potential of functional oxide nanosheets as components for nanoelectronics, thus contributing to the development of next-generation high-performance electronic devices.

Analysis of the structure and degree of crystallisation of 70Li2S–30P2S5 glass ceramic

Heat treatment of Li2S–P2S5 glasses to transform the glass to a glass ceramic enhances the ionic conductivity by precipitating a Li7P3S11 metastable crystalline phase with high ionic conductivity. Although the precipitated phase is identical, the conductivity achieved varies depending on the heat treatment conditions. Solid-state 31P MAS NMR spectroscopy to investigate the structure of the glass ceramic in this study revealed a strong correlation between the ionic conductivity and the degree of crystallisation, in which the conductivity of the glass ceramic increased with increasing proportion of metastable Li7P3S11 phase.

Tuning the surface charge of 2D oxide nanosheets and the bulk-scale production of superlatticelike composites.

The surface charge of various anionic unilamellar nanosheets, such as graphene oxide (GO), Ti0.87O2(0.52-), and Ca2Nb3O10(-) nanosheets, has been successfully modified to be positive by interaction with polycations while maintaining a monodispersed state. A dilute anionic nanosheet suspension was slowly added dropwise into an aqueous solution of high molecular weight polycations, which attach on the surface of the anionic nanosheets via electrostatic interaction. Surface modification and transformation to positively charged nanosheets were confirmed by various characterizations including atomic force microscopy and zeta potential measurements. Because the sizes of the polycations used are much larger than the nanosheets, the polymer chains may run off the nanosheet edges and fold to the fronts of the nanosheets, which could be a reason for the continued dispersion of the modified nanosheets in the suspension. By slowly adding a suspension of polycation-modified nanosheets and pristine anionic nanosheet dropwise into water under suitable conditions, a superlatticelike heteroassembly can be readily produced. Characterizations including transmission electron microscopy and X-ray diffraction measurements provide evidence for the formation of the alternately stacked structures. This approach enables the combination of various pairs of anionic nanosheets with different functionalities, providing a new opportunity for the creation of unique bulk-scale functional materials and their applications.