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.

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.

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.

Photoluminescence spectral change in layered titanate oxide intercalated with hydrated Eu3+.

A number of interesting photoluminescence properties of titanate layered oxide intercalated with hydrated Eu3+ have been demonstrated. Photoluminescence intensity of Eu3+ decreased rapidly with time during irradiation by UV light having energy higher than the band gap energy of the host TiO (Ti(1.81)O4) layer. This is presumably due to the decrease in energy transfer from the host TiO layer to Eu3+ as a result of the change in the hydration state of water molecules surrounding Eu3+, which is caused by the hole produced in the TiO valence band. When irradiation was discontinued, the emission intensity gradually recovered. The recovery time increased when the water in the interlayer is removed by heat treatment. This indicates that the state of interlayer water changes during irradiation and returns to its initial state after discontinuation of irradiation. The excitation spectra changed drastically at any given wavelength upon irradiation with UV light. A comparison of the excitation spectra before and after irradiation reveals that only the excitation peak at around the irradiation wavelength decreased upon irradiation, as in the case of spectral hole burning. The hydration state of water molecules surrounding Eu3+ presumably changes depending on the irradiation wavelength, leading to the above spectral change because the Eu/TiO film has a superlattice structure producing holes with different energies.

Photoluminescence properties of multilayer oxide films intercalated with rare earth ions by the layer-by-layer technique.

Multilayer oxide films consisting of a TiO-Eu3+-TiO-Tb3+-NbO-Tb3+-NbO-Eu3+ unit which was prepared by the layer-by-layer technique, showed photoluminescence with a high intensity containing both red and green lights.

All-graphene oxide device with tunable supercapacitor and battery behaviour by the working voltage.

We propose a new type of all-graphene oxide device. Reduced graphene oxide (rGO)/graphene oxide (GO)/rGO functions as both a supercapacitor and a battery, depending on the working voltage. The rGO/GO/rGO operates as a supercapacitor until 1.2 V. At greater than 1.5 V, it behaves as a battery using redox reaction.

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.

Optimization of proton conductivity in graphene oxide by filling sulfate ions.

Graphene oxide (GO) walled channels filled by sulfate ions exhibit an optimized proton conductivity, which is higher than the proton conductivity of all other forms of GO. The sulphate ion increases the water absorbing capacity and hydrogen bond reformation process in GO.

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.