Hydrogen adsorption on graphene-supported metal clusters has brought much controversy due to the complex nature of the bonding between hydrogen and metal clusters. The bond types of hydrogen and graphenesupported Ti clusters are experimentally and theoretically investigated. Transmission electron microscopy shows that Ti clusters of nanometer size are formed on graphene. Thermal desorption spectroscopy captures three hydrogen desorption peaks from hydrogenated graphene-supported Ti clusters. First-principles calculations also found three types of interaction: two types of bonds with different partial ionic character and physisorption. The physical origin for this rests on the charge state of the Ti clusters: when Ti clusters are neutral, H 2 is dissociated, and H forms bonds with the Ti cluster. On the contrary, H 2 is adsorbed in molecular form on positively charged Ti clusters, resulting in physisorption. Thus, this work clarifies the bonding mechanisms of hydrogen on graphene-supported Ti clusters. ■ INTRODUCTIONThe unique nature of metal clusters and 2D materials has advanced the field of materials science; however, a fundamental understanding behind scientific phenomena related to such materials often leads to further questions. 1,2 Utilizing metal clusters and 2D materials results in functional materials that possess outstanding physical and chemical properties. 3−5 2D materials like graphene are found to be good substrates for supporting metal clusters due to their chemical stability and high surface to volume ratio. 6−8 Graphene-supported metal clusters show high reactivity, which leads toward applications in the fields of catalysis and energy storage. 9−11The chemistry of metal clusters on graphene involves complex bonding between metal clusters and gas molecules when gases such as hydrogen are introduced, resulting in difficulty of experimental measurements and analysis. In particular, hydrogen adsorption on graphene-supported metal clusters often leads to controversial arguments over how hydrogen is adsorbed, whether the hydrogen adsorption takes place on the metal clusters or on defect sites of graphene, whether spillover effects occur, and what types of bonds between hydrogen and metal are formed. Experimental studies report multiple hydrogen desorption peaks, and the hydrogen desorption properties are difficult to reproduce. 7,8,12−14 First-principles calculations and experimental measurements are performed to achieve a fundamental understanding of the interactions between hydrogen and graphene-supported metal clusters. In particular, Ti clusters when combined with graphene are predicted to be a particularly suitable material for hydrogen adsorption with range of 3.6 to 7.8 wt % hydrogen uptake. 15−19 Therefore, in this paper, the interactions between hydrogen and graphene-supported Ti clusters are theoretically and experimentally explored. ■ MATERIALS AND METHODSSiC and Cu substrates are implemented to support graphene because such substrates are reported to preserve the electronic structure of g...

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Fusion performance of reversed shear discharges with an L-mode edge has been significantly improved in a thermonuclear dominant regime with up to 2.8 MA of plasma current in the JT-60U tokamak. The core plasma energy is efficiently confined due to the existence of persistent internal transport barriers formed for both ions and electrons at a large minor radius of r͞a ϳ 0.7 near the boundary of the reversed shear region. In an assumed deuterium-tritium fuel, the peak fusion amplification factor defined for transient conditions involving the dW ͞dt term would be in excess of unity. [S0031-9007(97)04592-4] PACS numbers: 52.55.Fa, 52.55.PiThe reversed shear discharges are considered attractive for a steady state operation with a large bootstrap current fraction in tokamak reactors as proposed for SSTR [1] and ITER [2], since it would be possible to match the hollow current profile to a bootstrap current profile in a steady state. While the central magnetic shear in tokamak plasmas is naturally reversed during a sufficiently long discharge duration with a large bootstrap current fraction [3], the forced shear reversal operation by enhancing a skin current effect has become important for establishing a controlled approach to the steady state [4].In nuclear fusion research, critical conditions in which fusion power produced in plasmas is equal to loss power from the plasmas have been pursued as a crucial milestone ultimately towards the commercial use of thermonuclear fusion energy. In order to determine whether the reversed shear scenario is workable, it is crucially important to demonstrate the fusion-relevant performance, particularly in the thermonuclear fusion regime with the shear reversal operation. So far, however, most of the previous experiments addressing high fusion reactivity in tokamaks have been limited to a hot-ion regime with substantial beam-thermal reactions for deuterium plasmas in TFTR supershot [5], JET hot-ion H mode [6] and JT-60U high-b p H mode [7], and deuterium-tritium (D-T) plasmas in TFTR supershot [8]. Although fusion performance has been recently enhanced with strong profile and shaping control in deuterium reversed shear plasmas with an H-mode edge in DIII-D [9], the projected D-T fusion power is substantially below the loss power from the plasma. In the present paper, it is shown that fusion performance has been significantly improved in JT-60U for reversed shear discharges with an L-mode edge in a thermonuclear fusion regime, so that the transient fusion amplification factor defined as below would be in excess of unity.In JT-60U, the experimental campaign of the reversed shear discharges aiming at high fusion amplification factor ͑Q͒ has been intensively performed with D beams into D plasmas. The confinement properties for the reversed shear discharges created in JT-60U are characterized by (i) the significant reduction of heat and particle transport for electrons as well as ions around the internal transport barrier (ITB), (ii) a large extension of the enhanced confinement region up...

Group 5 elements (niobium and tantalum) and group 6 elements (molybdenum and tungsten) were exposed to helium plasma, and the resulting surface structures were observed by electron microscopy. Group 5 elements showed hole structures, where the size of the holes ranged from several tens of nm to a few hundred nm in diameter, while group 6 elements showed fiber-like structures. As a first step in understanding such differences, the difference in helium agglomeration energies and changes in the stress tensor as a function of the number of He atoms at interstitial sites were investigated for each element using density functional theory. The calculations revealed that helium atoms prefer to agglomerate in both of these groups. However, helium in group 6 elements can agglomerate more easily than group 5 elements due to higher binding energy. These results indicate a possible correlation between the shape of helium plasma induced surface nanostructures and the atomic level properties due to helium agglomeration.

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