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In materials confined within nanometer channels in single-walled carbon nanotube (SWNT) bundles, interesting properties which are not observed in bulk materials are expected. In the present paper, we report an X-ray diffraction (XRD) study on water adsorption in SWNT bundles. It was found that a substantial amount of water is absorbed inside SWNTs at room temperature (RT). The desorptionadsorption of water molecules occurred reversibly above RT. We found that the liquid-like water is transformed into a new solid form, i.e., ice nanotubes, at 235 K.

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Recently published research from the National Renewable Energy Laboratory (NREL) reports that biohybrid hydrogen electrodes comprising metallic single-walled carbon nanotube (SWNT) networks and the hydrogenase from Clostridium acetobutylicum achieved a new activity record for hydrogenase-based electrode electrocatalysis. These results demonstrate that hydrogenase/ SWNT electrodes have the potential to provide a cheaper but equally efficient alternative to the precious metal catalysts, such as platinum, for application in photoelectrochemical or fuel cells. The high-performance hydrogen electrodes are based on the [FeFe]-hydroge-nase from C. acetobutylicum immobilized onto SWNT networks. The researchers prepared the electrodes with varying ratios of metallic (m-) and semiconducting (s-) SWNTs to explore the role of SWNT electronic structure in the biohybrid electrodes. Although most hydrogenase/SWNT electrodes showed improved performance in comparison to the hydrogenase immobilized directly to bulk carbon, high current densities up to 12 mA cm-2 (at-1 V vs. SHE) were achieved with hydrogenase immobilized on SWNT networks with high m-SWNT content. Using electrochemical methods, NREL researchers showed that m-SWNTs contribute to increased electrode electroactive surface available for hydrog-enase binding and improve electronic coupling between the electrode and the hydrogenase redox sites.

Studies on confined water are important not only from the viewpoint of scientific interest but also for the development of new nanoscale devices. In this work, we aimed to clarify the properties of confined water in the cylindrical pores of single-walled carbon nanotubes (SWCNTs) that had diameters in the range of 1.46 to 2.40 nm. A combination of x-ray diffraction (XRD), nuclear magnetic resonance, and electrical resistance measurements revealed that water inside SWCNTs with diameters between 1.68 and 2.40 nm undergoes a wet-dry type transition with the lowering of temperature; below the transition temperature T(wd), water was ejected from the SWCNTs. T(wd) increased with increasing SWCNT diameter D. For the SWCNTs with D = 1.68, 2.00, 2.18, and 2.40 nm, T(wd) obtained by the XRD measurements were 218, 225, 236, and 237 K, respectively. We performed a systematic study on finite length SWCNT systems using classical molecular dynamics calculations to clarify the effect of open ends of the SWCNTs and water content on the water structure. It was found that ice structures that were formed at low temperatures were strongly affected by the bore diameter, a = D - σ(OC), where σ(OC) is gap distance between the SWCNT and oxygen atom in water, and the number of water molecules in the system. In small pores (a < 1.02 nm), tubule ices or the so-called ice nanotubes (ice NTs) were formed irrespective of the water content. On the other hand, in larger pores (a > 1.10 nm) with small water content, filled water clusters were formed leaving some empty space in the SWCNT pore, which grew to fill the pore with increasing water content. For pores with sizes in between these two regimes (1.02 < a < 1.10 nm), tubule ice also appeared with small water content and grew with increasing water content. However, once the tubule ice filled the entire SWCNT pore, further increase in the water content resulted in encapsulation of the additional water molecules inside the tubule ice. Corresponding XRD measurements on SWCNTs with a mean diameter of 1.46 nm strongly suggested the presence of such a filled structure.

It is known that at low temperature, water inside single-wall carbon nanotubes (water-SWNTs) undergoes a structural transition to form tube-like solid structures. The resulting ice NTs are hollow cylinders with diameters comparable to those of typical gas molecules. Hence, the gas-adsorption properties of ice- and water-SWNTs are of interest. Here, we carry out the first systematic investigation into the stability of water-SWNTs in various gas atmospheres below 0.1 MPa by means of electrical resistance, X-ray diffraction, NMR measurements and molecular dynamics calculations. It is found that the resistivity of water-SWNTs exhibits a significant increase in gas atmospheres below a critical temperature Tc, at which a particular type of atmospheric gas molecule enters the SWNTs in an on-off fashion. On the basis of this phenomenon, it is proposed that water-SWNTs can be used to fabricate a new type of molecular nanovalve.

From 29Si NMR study, we present evidence for spatially inhomogeneous development of antiferromagnetic (AF) ordering below T(o) = 17.5 K in URu2Si2. In the pressure range between 3.0 and 8.3 kbar, we have observed the 29Si NMR lines arising from the AF region as well as the previously observed 29Si NMR line which correspond to the nonmagnetic region in the sample. The AF volume fraction is enhanced by applied pressure, whereas the magnitude of internal field at the Si site remains constant (910 Oe) up to 8.3 kbar. In the AF region, the ordered moment is about an order of magnitude larger than 0.03 mu(B)/U.

In this paper, we report novel ferroelectric properties of a new form of ice inside single-walled carbon nanotubes (SWCNTs). These are called "ice nanotubes" (ice NTs) and they consist of polygonal water rings stacked one-dimensionally along the SWCNT axis. We performed molecular dynamics (MD) calculations for the ice NTs under an external electric field and in a temperature range between 100 and 350 K. It is revealed that ice NTs show stepwise polarization with a significant hysteresis loop as a function of the external field strength. In particular, pentagonal and heptagonal ice NTs are found to be the world's smallest ferroelectrics with spontaneous polarization of around 1 microC/cm(2). The n-gonal ice NT, where n = 5, 6, or 7, has (n + 1)-polarized structures with different polarizations. These findings suggest potential applications of SWCNTs encapsulating dielectric materials for the fabrication of the smallest ferroelectric devices. Experimental evidence for the presence of ice NTs inside SWCNTs is also discussed in great detail.