Tomonori Ohba

Bulk Production of a New Form of sp2 Carbon : Crystalline Graphene Nanoribbons

We report the use of chemical vapor deposition (CVD) for the bulk production (grams per day) of long, thin, and highly crystalline graphene ribbons (<20-30 microm in length) exhibiting widths of 20-300 nm and small thicknesses (2-40 layers). These layers usually exhibit perfect ABAB... stacking as in graphite crystals. The structure of the ribbons has been carefully characterized by several techniques and the electronic transport and gas adsorption properties have been measured. With this material available to researchers, it should be possible to develop new applications and physicochemical phenomena associated with layered graphene.

Cadmium(II) adsorption using functional mesoporous silica and activated carbon.

The role of surface functionality on silica and carbonaceous materials for adsorption of cadmium(II) was examined using various mesoporous silica and activated carbon. Silica surfaces were principally functionalized by mono-amino- and mercapto-groups, while carboxylic group was introduced to the activated carbons by oxidation. Functional groups on silica surface were formed using grafting and co-condensation techniques in their preparation. Mono-amino group was found more effective than di- and tri-amino groups for cadmium(II) adsorption on the grafted silica. Mono-amino groups prepared by co-condensation adsorbed cadmium(II) as much as 0.25mmol/g compared to mercapto- and carboxyl-groups which adsorbed around 0.12mmol/g, whereas Langmuir adsorption affinities were as strong as 50-60L/mmol for all of the three functions. The working pH range was wider for mercapto- and carboxyl-functions than for amino-group. Basic site could be an adsorption center for amino-functional groups while ion exchange sites were found to work for the mercapto- and carboxyl-functions to adsorb cadmium(II) from aqueous phase. Based on the experimental results, surface functional groups rather than structure of silica and carbon seemed to play a decisive role for cadmium(II) adsorption.

Elastic layer-structured metal organic frameworks (ELMs)

Elastic layer-structured metal organic frameworks (ELMs) having flexible two-dimensional structure show a gate phenomenon in sorption/desorption of simple gas molecules. The gate phenomenon is accompanied by expansion/shrinkage of the layers. The gas sorption/desorption is not based on a physical adsorption, but on a chemical reaction, which includes high cooperativity. The cooperative reaction could be analyzed thermodynamically. The gate phenomenon showed advantages in separation of CO2 from mixed gases and in storage of CH4 owing to easy release of absorbed molecules.

Control over Hierarchy Levels in the Self-Assembly of Stackable Nanotoroids

We report a precise control over the hierarchy levels in the outstanding self-organization process shown by chiral azobenzene dimer 1. This compound forms uniform toroidal nanostructures that can hierarchically organize into chiral nanotubes under the control by temperature, concentration, or light. The nanotubes further organized into supercoiled fibrils, which finally intertwined to form double helices with one-handed helical sense.

Synthesis, Structural Transformation, Thermal Stability, Valence State, and Magnetic and Electronic Properties of PbNiO3 with Perovskite- and LiNbO3-Type Structures

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.

Confinement in Carbon Nanospace-Induced Production of KI Nanocrystals of High-Pressure Phase

An outstanding compression function for materials preparation exhibited by nanospaces of single-walled carbon nanohorns (SWCNHs) was studied using the B1-to-B2 solid phase transition of KI crystals at 1.9 GPa. High-resolution transmission electron microscopy and synchrotron X-ray diffraction examinations provided evidence that KI nanocrystals doped in the nanotube spaces of SWCNHs at pressures below 0.1 MPa had the super-high-pressure B2 phase structure, which is induced at pressures above 1.9 GPa in bulk KI crystals. This finding of the supercompression function of the carbon nanotubular spaces can lead to the development of a new compression-free route to precious materials whose syntheses require the application of high pressure.

Marked Adsorption Irreversibility of Graphitic Nanoribbons for CO2 and H2O

Graphene and graphitic nanoribbons possess different types of carbon hybridizations exhibiting different chemical activity. In particular, the basal plane of the honeycomb lattice of nanoribbons consisting of sp(2)-hybridized carbon atoms is chemically inert. Interestingly, their bare edges could be more reactive as a result of the presence of extra unpaired electrons, and for multilayer graphene nanoribbons, the presence of terraces and ripples could introduce additional chemical activity. In this study, a remarkable irreversibility in adsorption of CO(2) and H(2)O on graphitic nanoribbons was observed at ambient temperature, which is distinctly different from the behavior of nanoporous carbon and carbon blacks. We also noted that N(2) molecules strongly interact with the basal planes at 77 K in comparison with edges. The irreversible adsorptions of both CO(2) and H(2)O are due to the large number of sp(3)-hybridized carbon atoms located at the edges. The observed irreversible adsorptivity of the edge surfaces of graphitic nanoribbons for CO(2) and H(2)O indicates a high potential in the fabrication of novel types of catalysts and highly selective gas sensors.

Tuning of Gate Opening of an Elastic Layered Structure MOF in CO2 Sorption with a Trace of Alcohol Molecules

It is important to tune the sorption behavior of metal-organic framework (MOF) materials. Ethanol treatment on the hydrated form of [Cu(bpy)(2)(BF(4))(2)], which is a representative flexible MOF showing the fascinating gate phenomenon on CO(2) sorption, induces an easier dehydration and a significant decrease in the CO(2) gate pressure. The results of IR, X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) measurements indicated that water molecules in the lattice of the hydrated form can be removed even at room temperature after the ethanol treatment and the basic 2D layered structure remains with a slight interlayer expansion. The results of thermogravimetric (TG) and gas chromatograph/mass spectrometry (GC/MS) analyses and of CO(2) sorptions indicated that the change of the gate phenomenon was caused by a trace of residual ethanol molecules included in the structure. Similar phenomena were observed on alcohols with different polarity and molecular size.

Effect of a quaternary ammonium salt on propylene carbonate structure in slit-shape carbon nanopores.

The effect of addition of tetraethylammonium tetrafluoroborate (Et(4)NBF(4)) on the structure of propylene carbonate (PC) confined in slit-shaped carbon nanopores of activated carbon fiber (pore width = 1.0 nm) was studied by synchrotron X-ray diffraction and reverse Monte Carlo simulation. PC molecules are randomly packed in the slit carbon nanopores of 1 nm in the absence of Et(4)NBF(4). Addition of Et(4)N(+) and BF(4)(-) ions promotes formation of considerably ordered double layers of PC molecules even in the highly restricted slit pore space. PC molecules can accept these ions efficiently. This structural modulation function of PC molecular assemblies should contribute to the evolution of supercapacitance in carbon nanopores.

Cluster-associated filling of water molecules in slit-shaped graphitic nanopores

Water adsorption isotherms of hydrophobic activated carbon fibres (ACFs) having average pore widths, w, of 0.7 and 1.1 nm at 303 K were greatly different from each other; the adsorption isotherm of ACF with w = 0.7 nm had no clear hysteresis loop, while that of ACF with w = 1.1 nm had an explicit hysteresis loop. The structures of water adsorbed in both nanopores were studied using adsorption measurement, grand canonical Monte Carlo (GCMC) simulation, and in situ small angle X-ray scattering (SAXS). The in situ SAXS study of water in the 1.1 nm pores with the aid of GCMC simulation showed that adsorption proceeds through isolated cluster formation, whereas a mixed structure of a partial monolayer array and isolated clusters are formed on the desorption near the fractional filling, φ = 0.4. This adsorbed structure difference is the origin of the adsorption hysteresis. In contrast, both adsorption and desorption processes have the isolated cluster structure near φ = 0.4 in the case of 0.7 nm pores, agreeing with the absence of a clear hysteresis loop.