Interaction of highly charged ions with carbon-based materials using Kobe EBIS

Interaction of highly charged ions (HCIs) with surfaces produce various specific phenomena as a consequence of the potential energy that HCI possesses. In the present study, we have observed photon emission, structural, magnetic, and electronic modification on various carbon-based materials such as carbon nanotube by the impact of HCIs using an electron beam ion source named Kobe EBIS installed at the Kobe University. In order to study the potential effect, HCIs of Ar^q+ (q = 6–16) with the intensity of 0.1–1 nA are projected on the surface with a constant kinetic energy (16 keV). For photon emission measurements, we observed spatial and spectral distribution of visible light emission from the surface during irradiation with HCIs. On the other hand, the structural modification of multi-walled carbon nanotubes (MWCNTs) irradiated with HCIs has been analyzed using a transmission electron microscopy and Raman spectroscopy. Irradiation effects on the resistivity of single MWCNT supported on micrometer scale bridge pattern were also measured. We have also measured magnetic structure of highly oriented pyrolytic graphite irradiated with HCIs using electron spin resonance at low temperature. At the present paper, we will review our recent experimental results on the interaction of HCI with various carbon-based materials.     

Gold/Palladium Alloy for Carbon–Halogen Bond Activation: An Unprecedented Halide Dependence

New catalytic activity of gold/palladium alloy nanoclusters (NCs) for carbon–halogen bond activation is demonstrated. In the case of an aryl chloride, the inclusion of gold in a bimetallic catalyst is indispensable to achieve the coupling reactions. Gold has the unique effect of stabilizing palladium, such that Pd²⁺ leached from clusters by means of spillover of chloride during oxidative addition. The thus‐formed spillover intermediate further reacts heterogeneously in both Ullmann and Suzuki‐type coupling reactions through a new type of mechanism. In the case of an aryl bromide, Ullmann coupling occurs through the spillover of bromide, similar to that of aryl chloride. However, a significant fraction of palladium also leached, which diminished the Ullmann coupling activity of the aryl bromide and, as a result, the order of reactivity was ArCl>ArBr. With regard to the activation of the C?Br bond towards a Suzuki‐type reaction, the inclusion of a higher gold content in gold/palladium clusters stabilized palladium to prevent the leaching of Pd²⁺ from the clusters by means of spillover of bromide. The spillover intermediate reacts heterogeneously with PhB(OH)₂, palladium‐rich gold/palladium, or pure palladium clusters; the oxidative addition of ArBr favors the extraction of palladium from the clusters, yielding Pd²⁺ intermediates. The extracted intermediates react homogenously (Pd^2+/Pd⁰ catalysis) with PhB(OH)₂, which results in the higher selectivity of the cross‐coupling product. An initial step to observe such unprecedented halide dependency, together with the dynamic behavior of palladium on the surface of gold is the oxidative addition of Ar?X. A detailed insight into the first oxidative addition process was also examined by quantum chemical calculations.     

Mechanism of the Aerobic Homocoupling of Phenylboronic Acid on Au20−: A DFT Study

The mechanism of the gold nanocluster‐catalyzed aerobic homocoupling of arylboronic acids has been elucidated by means of DFT calculations with Au₂₀^? as a model cluster for the Au:[poly(N‐vinylpyrrolidin‐2‐one)] catalyst. We found that oxygen affects the adsorption of phenylboronic acid and, by lowering the energy barrier, a water molecule enhances dissociation of the C?B bond, which is probably the rate‐determining step. The key role of oxygen is in activating the surface of the gold cluster by generating Lewis acidic sites for adsorption and activation of the phenylboronic acid, leading to the formation of biphenyl through a superoxo‐like species. Moreover, the oxygen adsorbed on the Au nanocluster can act as an oxidant for phenylboronic acid, giving phenol as a byproduct. As shown by NBO analysis, the basic aqueous reaction medium facilitates the reductive elimination process by weakening the Au?C bond, thereby enhancing the formation of biphenyl. The coupling of phenyl and reductive elimination of biphenyl occur at the top or facet site with low‐energy‐barrier through spillover of phenyl group on Au NC. The present findings are useful for the interpretation or design of other coupling reactions with Au NC.     

Synthesis and Pyrolysis of Fullerenol-stabilized Pt Nanocolloids as a unique Approach to Pt-doped Carbon

An aqueous colloidal dispersion of Pt nanoparticles (NPs) stabilized by fullerenol C₆₀(OH)₁₂ (Pt:C₆₀(OH)₁₂) was successfully synthesized via liquid-phase chemical reduction. The subsequent pyrolysis of Pt:C₆₀(OH)₁₂ at different temperatures was conducted to afford Pt-doped carbon with different chemical compositions (Pt:C60ⁿ). X-ray absorption spectroscopy (XAS) and Infrared (IR) absorption spectroscopy and thermogravimetric measurements revealed that the thus-prepared nanocomposite consists of Pt NPs and high valent Pt-C₆₀(OH)₁₂ complex. One distinct feature of C₆₀(OH)₁₂ matrix as catalyst support is the suppression of size growth of Pt NPs during the pyrolysis up to 300 °C. Electrochemical experiments using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed to find that Pt:C60³⁰⁰ (pyrolyzed at 300 °C) exhibited higher activity than others, that was attributed to the π-extended feature of the as-obtained carbon.     

Preparation of Ring and Chain Condensed Oligophosphates

Preparations of small-ring and short-chain condensed phosphates were made by dry and wet processes, respectively. The crystallization of tetra- hexa-, and octaphosphates from the phosphate solution was not easy, and metal salts of these oligophosphates were amorphous and unstable at normal temperature. Guanidine tetraphosphate and ammonium hexaphosphate were crystalline and stable at normal temperature.     

Heterogeneous distribution of radiocesium in aerosols, soil and particulate matters emitted by the Fukushima Daiichi Nuclear Power Plant accident: retention of micro-scale heterogeneity during the migration of radiocesium from the air into ground and river systems

We analyzed ¹³⁷Cs in aerosols, rock, soil and river suspended sediment collected after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Based on the results, we discuss the post-event behavior and transportation of radiocesium in the environment from the air into ground and river systems. First, radionuclides were emitted from the FDNPP as airborne ‘hot’ particles, which contained water-soluble fractions of radiocesium. Radiocesium was still present in a water-soluble fraction after deposition on the ground. Subsequent interaction of the ‘hot’ particles with water (e.g. rainfall) dissolved and strongly fixed the radiocesium on rock and soil particles, thus changing the radiocesium into insoluble forms. The distribution of ‘hot spots’ was possibly controlled by the initial position of deposition on the ground. Consequently, ‘hot spots’ were studded on the rock surface rather than being uniformly distributed. The distribution of radiocesium in river suspended particles was not homogeneous during water transportation, reflecting the heterogeneity of radiocesium in rock and soil. Leaching experiments demonstrated that radiocesium in rock, soil and river suspended sediment was fairly insoluble, showing that the adsorption reaction is irreversible. The micro-scale heterogeneous distribution of radiocesium in aerosols, soil and suspended particles was due to the presence of ‘hot’ particles in aerosols. Dissolution of radiocesium in the ‘hot’ particles in the aerosols and subsequent irreversible adsorption onto the soil particle complex are responsible for the preservation of the heterogeneity both in soil and in river suspended particles.