Narihito Tatsuda

Self-assembly of monodisperse starburst carbon spheres into hierarchically organized nanostructured supercapacitor electrodes

We report a three-dimensional (3D) porous carbon electrode containing both nanoscale and microscale porosity, which has been hierarchically organized to provide efficient ion and electron transport. The electrode organization is provided via the colloidal self-assembly of monodisperse starburst carbon spheres (MSCSs). The periodic close-packing of the MSCSs provides continuous pores inside the 3D structure that facilitate ion and electron transport (electrode electrical conductivity ∼0.35 S m(-1)), and the internal meso- and micropores of the MSCS provide a good specific capacitance. The capacitance of the 3D-ordered porous MSCS electrode is ∼58 F g(-1) at 0.58 A g(-1), 48% larger than that of disordered MSCS electrode at the same rate. At 1 A g(-1) the capacitance of the ordered electrode is 57 F g(-1) (95% of the 0.24 A g(-1) value), which is 64% greater than the capacitance of the disordered electrode at the same rate. The ordered electrode preserves 95% of its initial capacitance after 4000 charging/discharging cycles.

Application of laser Raman spectroscopy to the analysis of stress distribution of fibres in composites

In this study the measurement was carried out for two types of composites (unidirectional fibre composite and randomly dispersed short fibre composite) to evaluate the stress concentration caused by fibre breakage in the former composite and to evaluate the stress levels of individual short fibres in the latter composite

Monodispersed mesoporous silica spheres with various mesopore symmetries.

Monodispersed mesoporous silica spheres (MMSS) with different mesopore symmetries, such as hexagonal, cubic, or the mixture of hexagonal/cubic, are synthesized changing synthesis conditions. It seems that the direction of mesopores is retained through the particle in MMSS with cubic symmetry. In the case of hexagonal/cubic mixed symmetry, cubic structure is observed at the center of the particle, while hexagonal structure is observed near the surface. It is assumed that cubic structure forms at early stage of the particle growth and hexagonal symmetry forms at the later stage, leading to the formation of cubic core/hexagonal shell structure.

Preparation of titanium dioxide in the nano-spaces of activated carbon using carbon dioxide supercritical fluid as a solvent

The penetration of titanium tetraisopropoxide (TTIP) dissolved in supercritical CO2 into the nano-spaces of activated carbon was studied in terms of coating TiO2 on a carbon surface. The amount of TiO2, which was derived from TTIP through thermal decomposition, depended on the density of the solvent and the content of an entrainer, 2-propanol (IPA), in the CO2. In the system without the entrainer, the amount of TiO2 in the activated carbon pores increased with the solvent density. The addition of the entrainer to the solvent fluid also influenced the TTIP adsorption level. The molar fraction of CO2 in the solvent (XCO2) should be more than 0.9 at 393 K for the penetration of TTIP, above which only the supercritical phase existed in the system without any other liquid phases. These results suggested that a supercritical fluid is useful, and that control of the solvent content, density and treatment time should be important for the preparation of activated carbon/metal oxide composites.

Investigation of the stability of Platinum nanoparticles incorporated in mesoporous silica with different pore sizes.

The effect of the pore size of mesoporous silica on the stability of Pt nanoparticles (NPs) has been investigated. TEM observation and XRD measurement were conducted in situ for Pt loaded mesoporous silica with different mesopore sizes. It turns out that smaller pores are more effective to stabilize Pt NPs below 600 °C. However, aggregation of Pt NPs on the surface of particles is not fully suppressed more than 1000 °C in ambient atmosphere even though smaller mesopore size is applied. The type of precursor does not affect the stability of Pt NPs.

Novel method to incorporate Si into monodispersed mesoporous carbon spheres.

Liquid silicon precursor is used as a silicon source and very simple and easy method for the incorporation of Si into mesoporous carbon spheres is presented. By using capillary condensation, the liquid precursor, Cyclopentasilane, penetrates into mesopores of carbon spheres homogeneously and subsequent heating brings the decomposition of the precursor and the formation of silicon inside meso-channels of carbon even though the decomposition is done much higher than the boiling point of the precursor. The homogeneous distribution of silicon is verified by EDX mapping of the composite as well as SEM observation of the calcined one. More than 45wt% of Si can be incorporated into mesopores by just one operation. The Si@mesoporous carbon composite works as an anode for a Lithium ion battery.

Adsorption of carbon dioxide on mesoporous silicas near the critical temperature

The adsorption of carbon dioxide on mesoporous silicas was studied near the critical temperature (Tc) of CO2, i.e. 304.2 K. The critical temperature in the mesopores at which the first-order phase transition (capillary condensation) was observed (Tcp) was estimated from the inverse slope of the adsorption isotherms and the behaviour of the adsorption isotherms per unit surface area. The values of Tcp for CO2 in mesopores whose radii, rp, were 3.02 and 2.14 nm were higher than those estimated using a slab model in which the adsorbing parts of layer were clearly separated from the non-adsorbing regions. The differential heat of adsorption decreased with increasing pore size of the mesoporous adsorbent. This result also suggested a deviation from the slab model.

Silicon deposition in nanopores using a liquid precursor

Techniques for depositing silicon into nanosized spaces are vital for the further scaling down of next-generation devices in the semiconductor industry. In this study, we filled silicon into 3.5-nm-diameter nanopores with an aspect ratio of 70 by exploiting thermodynamic behaviour based on the van der Waals energy of vaporized cyclopentasilane (CPS). We originally synthesized CPS as a liquid precursor for semiconducting silicon. Here we used CPS as a gas source in thermal chemical vapour deposition under atmospheric pressure because vaporized CPS can fill nanopores spontaneously. Our estimation of the free energy of CPS based on Lifshitz van der Waals theory clarified the filling mechanism, where CPS vapour in the nanopores readily undergoes capillary condensation because of its large molar volume compared to those of other vapours such as water, toluene, silane, and disilane. Consequently, a liquid-specific feature was observed during the deposition process; specifically, condensed CPS penetrated into the nanopores spontaneously via capillary force. The CPS that filled the nanopores was then transformed into solid silicon by thermal decomposition at 400 °C. The developed method is expected to be used as a nanoscale silicon filling technology, which is critical for the fabrication of future quantum scale silicon devices.