Chikako Ogata

Photoluminescence of perovskite nanosheets prepared by exfoliation of layered oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion).

Luminescent perovskite nanosheets were prepared by exfoliation of single- or double-layered perovskite oxides, K2Ln2Ti3O10, KLnNb2O7, and RbLnTa2O7 (Ln: lanthanide ion). The thickness of the individual nanosheets corresponded to those of the perovskite block in the parent layered compounds. Intense red and green emissions were observed in aqueous solutions with Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets, respectively, under UV illumination with energies greater than the corresponding host oxide band gap. The coincidence of the excitation spectrum and the band gap absorbance indicates that the visible emission results from energy transfer within the nanosheet. The red emission intensity of the Gd1.4Eu0.6Ti3O10-nanosheets was much stronger than that of the La0.90Eu0.05Nb2O7-nanosheets reported previously. The strong emission intensity is a result of a two-step energy transfer cascade within the nanosheet from the Ti-O network to Gd(3+) and then to Eu(3+). The emission intensities of the Gd1.4Eu0.6Ti3O10- and La0.7Tb0.3Ta2O7-nanosheets can be modulated by applying a magnetic field (1.3-1.4 T), which brings about a change in orientation of the nanosheets in solution. The emission intensities increased when the excitation light and the magnetic field directions were perpendicular to each other, and they decreased when the excitation and magnetic field were collinear and mutually perpendicular to the direction of detection of the emitted light.

Proton Conductivities of Graphene Oxide Nanosheets: Single, Multilayer, and Modified Nanosheets

Proton conductivities of layered solid electrolytes can be improved by minimizing strain along the conduction path. It is shown that the conductivities (σ) of multilayer graphene oxide (GO) films (assembled by the drop-cast method) are larger than those of single-layer GO (prepared by either the drop-cast or the Langmuir-Blodgett (LB) method). At 60% relative humidity (RH), the σ value increases from 1×10(-6) S cm(-1) in single-layer GO to 1×10(-4) and 4×10(-4) S cm(-1) for 60 and 200 nm thick multilayer films, respectively. A sudden decrease in conductivity was observed for with ethylenediamine (EDA) modified GO (enGO), which is due to the blocking of epoxy groups. This experiment confirmed that the epoxide groups are the major contributor to the efficient proton transport. Because of a gradual improvement of the conduction path and an increase in the water content, σ values increase with the thickness of the multilayer films. The reported methods might be applicable to the optimization of the proton conductivity in other layered solid electrolytes.

Dynamic Control of Photoluminescence for Self‐assembled Nanosheet Films Intercalated with Lanthanide Ions by Using a Photoelectrochemical Reaction

Semiconductor oxide nanosheets synthesized by exfoliation of layered oxides are two-dimensional crystals with a thickness of about 1 nm. New layered materials and their films can be reassembled by electrostatic self-assembly deposition (ESD) and by layer-by-layer (LBL) techniques, respectively. Since the nanosheets have a negative charge in aqueous solution they can be used with various cationic species as the starting materials. Layered materials prepared from nanosheets and lanthanide (Ln) ions are promising as new functional materials because Ln ions have unique properties, such as luminescence and magnetic properties, that are attributable to the 4f electron orbital. For example, the titanate layered oxide intercalated with Eu ions prepared from titanate nanosheets and Eu ions has unique luminescence properties. The layered oxide gives a red emission from the Eu ions which is induced by energy transfer through excitation of the bandgap of the titanate nanosheet, and the emission from the Eu ions is promoted by intercalated water molecules. Furthermore, spectral hole burning caused by the intercalated water molecules was observed in the excitation spectra at room temperature. Nanosheets of TiOx, NbOx, and TaOx give a high photocurrent during the photoelectrochemical reaction under UV illumination with an energy higher than that of the bandgap. This finding indicates that a large charge separation is produced between the holes in the valence band and the electrons in the conduction band during excitation of the bandgap. Consequently, layered oxide materials intercalated with Ln ions simultaneously exhibit both photoluminescence and a photoelectrochemical reaction during excitation of the bandgap on illumination with UV light. The study reported herein demonstrates a new form of dynamic control over the photoluminescence of Ln ions intercalated in self-assembled nanosheet films of TiOx and NbOx. The photoluminescence properties of Ln ions are changed by factors such as a change in the pH value and the addition of anionic species. However, it is difficult to dynamically control the photoluminescence properties of Ln ions. In the present system, the emission intensities of the intercalated Eu and Tb ions can be readily controlled by varying the applied potential. The nanosheet/Ln (Ti1.81O4 nanosheet/Eu 3+ (TiO/Eu) and Nb6O17 nanosheet/Tb 3+ (NbO/Tb)) films were prepared and fixed on boron-doped diamond electrodes by the LBL technique. The chemical compositions of the TiO/Eu and NbO/Tb films were EuxTi1.81O4 (x = 0.20–0.30) and TbyNb6O17 (y= 1.30–1.50), respectively. These are close to the theoretical neutral compositions (Eu0.25Ti1.81O4 and Tb1.33Nb6O17). The Ln ions were sandwiched between nanosheets (see Figure S-1 in the Supporting Information). Figure 1 shows a sche-

Metal Permeation into Multi-layered Graphene Oxide

Understanding the chemical and physical properties of metal/graphene oxide (M/GO) interfaces is important when GO is used in electronic and electrochemical devices because the metal layer must be firmly attached to GO. Here, permeation of metal from the surface into GO paper bulk at the M/GO interface was observed at room temperature for metals such as Cu, Ag, Ni, Au, and Pt. Cu, Ag, and Ni quickly permeated GO as ions into the bulk under humid conditions. At first, these metals changed to hydrated ions as a result of redox reactions (with reduction of GO) at the surface, and then permeated the interlayers. Au and Pt were observed to permeate GO as atoms into the GO bulk at room temperature, although the permeation rates were low. These surprising results are considered to be due to the presence of many defects and/or edges with oxygenated groups in the GO paper.

Super proton/electron mixed conduction in graphene oxide hybrids by intercalating sulfate ions

We successfully developed an efficient proton/electron mixed conductor composed of a single phase material that functions at room temperature by introducing sulfate ions into graphene oxide interlayers. The promising properties of this material would allow for its wide use in fuel cells, supercapacitors, and gas separation membranes.

All-Graphene Oxide Flexible Solid-State Supercapacitors with Enhanced Electrochemical Performance

The rapid development of flexible and wearable electronics has led to an increase in the demand for flexible supercapacitors with enhanced electrochemical performance. Graphene oxide (GO) and reduced GO (rGO) exhibit several key properties required for supercapacitor components. Although solid-state rGO/GO/rGO supercapacitors with unique structures are promising, their moderate capacitance is inadequate for practical applications. Herein, we report a flexible solid-state rGO/GO/rGO supercapacitor comprising H2SO4-intercalated GO electrolyte/separator and pseudocapacitive rGO electrodes, which demonstrate excellent electrochemical performance. The resulting supercapacitor delivered an areal capacitance of 14.5 mF cm-2, which is among the highest values achieved for any rGO/GO/rGO supercapacitor. High ionic concentration and fast ion conduction in the H2SO4-intercalated GO electrolyte/separator and abundant CH defects, which serve as pseudocapacitive sites on the rGO electrode, were responsible for the high capacitance of this device. The rGO electrode, well separated by the H2SO4 molecular spacer, supplied highly efficient ion transport channels, leading to excellent rate capability. The highly packed rGO electrode and high specific capacitance resulted in a high volumetric energy density (1.24 mWh cm-3) observed in this supercapacitor. The structure, without a clear interface between GO and rGO, provides extremely low resistance and flexibility for devices. Our device operated in air (25 °C 40%) without the use of external electrolytes, conductive additives, and binders. Furthermore, we demonstrate a simple and versatile technique for supercapacitor fabrication by combining photoreduction and electrochemical treatment. These advantages are attractive for developing novel carbon-based energy devices with high device performance and low fabrication costs.

Synthesis and Photoluminescence Properties of Niobate Layered Oxides Intercalated with Rare Earth Ions by Electrostatic Self-Assembly Methods

The niobate layered oxides intercalated with Tb3+ or Eu3+ ions were prepared by the electrostatic self-assembly deposition method. The Tb3+ ions in the interlayer exhibited green luminescence by energy transfer from the host NbO6 layer to the guest Tb3+ ions, which was based on the niobate nanosheet band gap excitation. The Tb3+ emission intensity decreased with decreasing interlayer water molecules, indicating that the presence of interlayer water molecules is inevitable for the Tb3+ emission assigned to the energy transfer. On the other hand, the emission intensity of Eu3+ in the interlayer was weaker than that of Tb3+.

Coal Oxide as a Thermally Robust Carbon-Based Proton Conductor

Inexpensive solid proton conducting materials with high proton conductivity and thermal stability are necessary for practical solid state electrochemical devices. Here we report that coal oxide (CO) is a promising carbon-based proton conductor with remarkable thermal robustness. The CO produced by simple liquid-phase oxidation of coal demonstrates excellent dispersibility in water owing to the surface carboxyl groups. The proton conductivity of CO, 3.9 × 10(-3) S cm(-1) at 90% relative humidity, is as high as that of graphene oxide (GO). Remarkably, CO exhibits much higher thermal stability than GO, with CO retaining the excellent proton conductivity as well as the capacitance performance even after thermal annealing at 200 °C. Our study demonstrates that the chemical modification of the abundant coal provides proton conductors that can be used in practical applications for a wide range of energy devices.

Ce–surfactant lamellar assemblies with strong UV/visible emission and controlled nanostructures

We describe self-organization–reorganization of a lanthanide (Ln)–surfactant lamellar hybrid with intense photoluminescence and controlled nanostructures. We reveal that UV luminescence exhibited by a Ce–dodecyl sulfate (DS) lamellar hybrid offers the maximum quantum yield (QY) of 90% among all types of Ce–organic hybrid materials. In addition, Ce–Tb dodecyl phosphate (DP) lamellar hybrids exhibit bright green emission due to the two-dimensional Ce3+–Tb3+ energy transfer (ET) process in Ln–PO4 layers. Furthermore, a Ce–DS film with a highly oriented nanostructure and Ce–Tb–DP colloidal nanosheets are fabricated through simple and effective reorganization routes. The concept of “reorganization routes” opens up new pathways to general- and large-scale fabrication of two-dimensional nanomaterials on the basis of metal–surfactant assemblies with a variety of physical and chemical properties.

Synthesis and photoluminescence properties of layered oxides intercalated with Eu3+ ions by electrostatic self-assembly method using oxide nanosheets

Layered oxides intercalated with Eu3+ ions were prepared by self-assembly deposition method based on the electrostatic interaction between negative charged oxide nanosheets and Eu3+ ions. The layered oxides were composed of two-dimensional host nanosheet layer with guest Eu3+ ions and gave Eu3+ emission based on an energy transfer from bandgap excitation of the host oxide nanosheets to the Eu3+ ions. The photoluminescence properties of the Eu3+ ions were influenced by the type of nanosheet and the intercalated water molecules. Relatively-strong emission of Eu3+ was observed from Eu3+-intercalated titanate layered oxide. The intensity of Eu3+ emission increased with increasing the amount of intercalated water molecules. This indicates that the energy transfer from TiO2−δ nanosheet to Eu3+ is promoted by the intercalated water molecules. In addition, the intensity of Eu3+ emission was stronger at high pH than at low pH. This emission change is presumably due to two phenomena. One is a fine hydration state change of Eu3+ in the interlayer, and the other is a change in energy transfer from the TiO2−δ nanosheet to Eu3+.