Kazuto Hatakeyama

Simple photoreduction of graphene oxide nanosheet under mild conditions

Graphene oxide (GO) nanosheets were reduced by UV irradiation in H2 or N2 under mild conditions (at room temperature) without a photocatalyst. Photoreduction proceeded even in an aqueous suspension of nanosheets. The GO nanosheets reduced by this method were analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. It was found that epoxy groups attached to the interiors of aromatic domains of the GO nanosheet were destroyed during UV irradiation to form relatively large sp2 islands resulting in a high conductivity. I-V curves were measured by conductive atomic force microscopy (AFM; perpendicular to a single nanosheet) and a two-electrode system (parallel to the nanosheet). They revealed that photoreduced GO nanosheets have high conductivities, whereas nonreduced GO nanosheets are nearly insulating. Ag+ adsorbed on GO nanosheets promoted the photoreduction. This photoreduction method was very useful for photopatterning a conducting section of micrometer size on insulating GO. The developed photoreduction process based on a photoreaction will extend the applications of GO to many fields because it can be performed in mild conditions without a photocatalyst.

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.

DNA analysis based on toehold-mediated strand displacement on graphene oxide

Fluorescent dye-labeled probe DNA was immobilized on fluorescence-quenching graphene oxide (GO) through a capture DNA. When targets were added, the probes were released from the GO through toehold-mediated strand exchange. Higher emission recovery and more signal contrast were achieved relative to conventional methods that are based on direct adsorption of probes.

Intense photoluminescence from ceria-based nanoscale lamellar hybrid.

Nanosheets, which are ultrathin inorganic crystals, have the potential to exhibit unique surface states and quantum effects. These nanosheets can be further manipulated to form lamellar structures for the fabrication of advanced hybrid nanomaterials. Here we report that conventionally nonluminescent ceria yields intense UV photoluminescence with an internal quantum yield (QY) of 59% when self-organized into a nanosheet lamellar architecture with dodecyl sulfate (DS) bilayers. The origin of luminescence exist at the organic/inorganic interfaces, where surface Ce(3+) ions of ceria nanosheet layers graft with DS anions to activate radiative 5d → 4f transition.

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.

Proton conductors produced by chemical modifications of carbon allotropes, perovskites and metal organic frameworks

Proton conductors are distinct from electronic conductors by virtue of their low conductivity values and the necessity for the presence of a third material as a carrier to transport protons. Proton conductors have applications in fuel cells, hydrogen separating phases, steam electrolysis, sensors and biological transport systems. Though naturally occurring proton conductors show very poor conductance, designing superionic conductors is possible nowadays due to the improvements in porosity and interlayer oriented proton conduction tracks, alignment of particle conduction sites, hydration dynamics, water uptake capacity and stability. Insight into all the possible ways for improving the proton conductivity values of materials needs to be considered for designing new types of superionic conductors. Based on this necessity, herein, we have reviewed the gradual trend in proton conductivities in ancient type perovskites and ceramics, metal organic frameworks and the most recently developed oxidized carbon allotropes. As the strategies for improving the conduction tracks and proton migration behaviours of these three classes are not the same, they are discussed in different sections, in light of their intrinsic properties, scope of modification and structures. By consideration of a detailed list of updated articles, reports associated with strategies for obtaining gradually increased proton conductivities, including the improvement of conduction tracks, assembly of particles and increase in water content, have been focused on for detailed discussion. Furthermore, an in-depth discussion on the function of proton conductors in energy applications and biological transport systems has been reviewed concisely.

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.

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.

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.