Noriko Yoshizawa

Structural changes in carbon aerogels with high temperature treatment

The structural change of carbon aerogels at high temperatures up to 2800°C has been investigated. Change in microtexture of fine particles, which constitute carbon aerogels derived from phenolic resin, was of a typical non-graphitized carbon. The microporosity decreased with an increase of heat-treatment temperature, and disappeared at 2000°C. The mesoporosity still remained even after heat-treatment up to 2800°C, though 50% of mesopore volume was lost because of the fusion of the particles with the change of carbon microtexture.

Amorphous diamond from C60 fullerene

Amorphous diamond was successfully quenched from shock‐compressed C60 fullerene by rapid cooling technique. This material is transparent and homogeneous glassy chips, and exists stably at ambient conditions. X‐ray diffractometry showed a halo, and electron diffractometry showed a diffuse pattern similar to those of common amorphous carbon materials. Electron energy loss spectroscopy represented evidently σ‐electron state in sp3 hybridization completely equivalent to that of a typical diamond. Therefore, this material is defined in long range order to be amorphous and in short range order to be diamond, namely it can be said as amorphous diamond.

Highly (111)-oriented Ge thin films on insulators formed by Al-induced crystallization

(111)-oriented Ge thin films on insulators are essential for advanced electronics and photovoltaic applications. We investigate Al-induced crystallization of amorphous-Ge films (50-nm thickness) on insulators focusing on the annealing temperature and the diffusion controlling process between Ge and Al. The (111)-orientation fraction of the grown Ge layer reaches as high as 99% by combining the low-temperature annealing (325 °C) and the native-oxidized Al (AlOx) diffusion-control layer. Moreover, the transmission electron microscopy reveals the absence of defects on the Ge surface. This (111)-oriented Ge on insulators promises to be the high-quality epitaxial template for various functional materials to achieve next-generation devices.

The Pore Structure Determination of Carbon Aerogels

The detailed adsorption isotherms of nitrogen on carbon aerogels at 77 K were measured. The N2 adsorption isotherm had a marked hysteresis. The adsorption isotherms were analyzed by high resolution αs-plots to evaluate their porosity. The αs-plots showed an explicit upward deviation from the linearity below αs = 0.5, suggesting the presence of micropores. The mesoporosity and microporosity were separately determined from the αs-plot. The predominant pores in carbon aerogels were mesopores and the percentage of micropores was in the range of 5 to 10% of the total pore volume. The N2 adsorption hysteresis was analyzed with the Saam-Cole theory under the assumption of the cylindrical pore shape. The parameters determined from the Saam-Cole method were associated with the carbon aerogel structure.

Synthesis and Characteristics of Graphene Oxide-Derived Carbon Nanosheet−Pd Nanosized Particle Composites

Carbon nanosheet (CNS)-Pd nanosized particle (NP) composites were synthesized by using graphite oxide (GO) and bis(ethylenediamine)palladium(II) (Pd(en)(2)(2+)) as the precursors, and their structure and adsorption properties were examined. It was found that the Pd(en)(2)(2+) complex ions can be intercalated into GO layers highly efficiently to form a layered structure containing a large amount of Pd (approximately 12 wt %). By the subsequent chemical reduction, Pd NPs (2-6 nm in size) are well dispersed between CNS to form a CNS-Pd NP composite and serve as spacers to increase the porosity of the composite. Hydrogen adsorption results demonstrate that both Pd NPs and CNS play important roles in hydrogen adsorption, particularly at a lower temperature and for CNS with deficient sites, which bring about a H(2) adsorption greater than those on other Pd-loaded nanocarbon materials reported so far. The unique composite nanostructure having large contents of Pd NPs (20-25 wt %) stabilized by CNSs is hopeful to be applied to the fields of H(2)-related catalysis, sensing, and so forth.

XRD evaluation of CO2 activation process of coal- and coconut shell-based carbons

Abstract Microporous carbons were prepared by CO2 activation of coal and coconut-shell chars. Specific surface area of the samples was determined by N2 adsorption as a parameter for surface structure, and was compared with some crystallographic parameters evaluated with X-ray diffraction (XRD) pattern. Number of aromatic layers increased linearly with increase of specific surface area in both series of samples, while interlayer spacing and fraction of stacking structure showed more complicated variation. This result was explained in terms of the earlier gasification of carbon atoms out of the stacking structure of aromatic layers. Results by TEM observation also supported this interpretation.

The modification of pore size in activated carbon fibers by chemical vapor deposition and its effects on molecular sieve selectivity

Abstract Pore size control of a series of activated carbon fibers was attempted by chemical vapor deposition (CVD) of benzene to clarify the influence of the pore distribution on the development of molecular sieving ability. Weight increase by CVD was found to saturate at a certain level respective to the fiber, reflecting their surface areas. However, when saturation was obtained, the molecular sieving selectivity between CO2 and CH4 was induced only in the smaller surface area fibers, whereas the fibers with larger surface areas lost the adsorption activity for both gases. Straight micropores developed from the surface can be controlled in their slit size, by carbon deposition selectively onto their wall, if benzene molecules can get into the pore. In contrast, the size of micropores developed on the walls of mesopores in the fibers with large surface area are difficult to control since the carbon deposition continues until the whole wall of the mesopore is covered by the deposited carbon, which plugs the micropores.

70 °C synthesis of high-Sn content (25%) GeSn on insulator by Sn-induced crystallization of amorphous Ge

Polycrystalline GeSn thin films are fabricated on insulating substrates at low temperatures by using Sn-induced crystallization of amorphous Ge (a-Ge). The Sn layer stacked on the a-Ge layer (100-nm thickness each) had two roles: lowering the crystallization temperature of a-Ge and composing GeSn. Slow annealing at an extremely low temperature of 70 °C allowed for a large-grained (350 nm) GeSn layer with a lattice constant of 0.590 nm, corresponding to a Sn composition exceeding 25%. The present investigation paves the way for advanced electronic optical devices integrated on a flexible plastic substrate as well as on a Si platform.

Enhanced hydrogen adsorptivity of single-wall carbon nanotube bundles by one-step c60-pillaring method.

Single-wall carbon nanotube (SWCNT) bundles were pillared by fullerene (C60) by the cosonication of C60 and SWCNT in toluene to utilize the interstitial pores for hydrogen storage. C60-pillared SWCNTs were confirmed by the shift in the X-ray diffraction peak and the expanded hexagonal and distorted tetragonal bundles revealed by high-resolution transmission electron microscopy. The H2 adsorptivity of the C60-pillared SWCNT bundles was twice that of the original SWCNT bundles, indicating a design route for SWCNT hydrogen storage.

Selective formation of large-grained, (100)- or (111)-oriented Si on glass by Al-induced layer exchange

By controlling the Si thickness and the annealing temperature used for Al-induced crystallization, we controlled the fraction of (100) and (111) orientations of polycrystalline Si (poly-Si) grains grown on glass. Changing the proportions of crystal orientation strongly influenced the average grain size of the poly-Si layer. By growing a 99% (111)-oriented poly-Si layer, formed with a 50-nm-thick Si layer at 375 °C, we produced a Si layer with grains nearly 40 μm in size. We discuss the growth mechanism from the perspective of competition between (100)- and (111)-oriented nuclei. This achievement holds promise for fabricating high-efficiency thin-film solar cells on inexpensive glass substrates.