Asami Funatsu

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.

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.

Reversibly tunable upconversion luminescence by host-guest chemistry.

Tuning upconversion (UPC) luminescence using external stimuli and fields, as well as chemical reactions, is expected to lead to novel and efficient optical sensors. Herein, highly tunable UPC luminescence was achieved through a host-guest chemistry approach. Specifically, interlayer ion exchange reactions reversibly tuned the emission intensity and green-red color of Er/Yb-codoped A2La2Ti3O10 layered perovskite, where A corresponds to proton and alkali metal ions, enabling the visualization of host-guest interactions and reactions.

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.

Mass production of titanium oxide (Ti2O52−) nanosheets using a soft, solution process

Ti2O52− nanosheets were successfully synthesized using a solution process under mild conditions. Concentrated nanosheet solutions (mass production) were easily obtained by the addition of sodium dodecyl sulfate (SDS). A drastic and specific enhancement of the photoluminescence of Eu3+ by energy transfer from the nanosheets occurred in the solution state.