Tracking the Hydrogen Motion in Defective Graphene

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Bulk defective graphene produced by thermal exfoliation of graphite oxide was treated under H2 and investigated with X-ray photoemission spectroscopy, neutron spectroscopy, and solid state nuclear magnetic resonance. Graphene defects appear effective in dissociating H2 molecule and in promoting H … Continue reading

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Decoration of graphene with nickel nanoparticles: study of the interaction with hydrogen

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Graphene obtained from thermal exfoliation of graphite oxide was chemically functionalized with nickel nanoparticles (NPs) without exposing the system to oxidizing agents. Its structural, physical and chemical properties have been studied by means of TEM, X-ray photoelectron and Raman spectroscopies, … Continue reading

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Ionic conductivity in the Mg intercalated fullerene polymer Mg2C60

We report the identification of the new intercalated fullerene polymer Mg2C60. Mg intercalation was obtained either by solid state reaction between C60 and Mg, or by thermal decomposition of the metallorganic precursor Mg–Anthracene–(THF)3. High resolution powder synchrotron and neutron diffraction data have clearly shown that Mg2C60 is isostructural to the superionic conductor Li4C60, where fullerenes form a two-dimensional network connected either by four-membered carbon rings, or single C–C bonds. Because of its peculiar structural arrangement Mg2C60 behaves as a good ionic conductor by means of uncorrelated ionic hopping across very small energy barriers (ΔE less than 100 meV), as found from DC and AC conductivity measurements, thus suggesting its possible use in future Mg-ion batteries.

Reproduced with permission. Copyright 2012, Elsevier

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Reversible hydrogen absorption in sodium intercalated fullerenes

The hydrogen absorption of sodium intercalated fullerenes was determined and
compared to pure fullerenes C60. Up to 3.5 mass% hydrogen can reversibly be absorbed in NaxC60 at 200 C and a hydrogen pressure of 200 bar. The absorbed amount of hydrogen is significantly higher than for the case when only the sodium would be hydrogenated. At 200 bar the onset of hydrogen absorption is observed at 150 C. At a pressure of 1 bar hydrogen the major desorption starts at 250 C and is completed at 300 C. This absorption and desorption temperatures are significantly reduced compared to pure C60, either due to a catalytic reaction of hydrogen on sodium or due to the negatively charged C60. The hydrogen ab/desorption is accompanied by a partial de/reintercalation of sodium. A minor part of the hydrogen is ionically bonded in NaH and the major part is covalently bonded in C60Hx. The sample can be fully dehydrogenated and no NaH is left after desorption. In contrast to C60, where the fullerene cages for high hydrogen loadings are destroyed during the sorption process, the NaxC60 sample stays intact.

Reproduced with permission. Copyright 2012, Elsevier


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Muons Probe Strong Hydrogen Interactions with Defective Graphene

We present the first muon spectroscopy investigation of graphene, focused on chemically produced, gram-scale samples, appropriate to the large muon penetration depth. We have observed an evident muon spin precession, usually the fingerprint of magnetic order, but here demonstrated to originate from muon–hydrogen nuclear dipolar interactions. This is attributed to the formation of CHMu (analogous to CH2) groups, stable up to 1250 K where the signal still persists. The relatively large signal amplitude demonstrates an extraordinary hydrogen capture cross section of CH units. These results also rule out the formation of ferromagnetic or antiferromagnetic order in chemically synthesized graphene samples.

Reproduced with permission. Copyright 2011, American Chemical Society

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NMR Study of the Mott Transitions to Superconductivity in the Two Cs3C60 Phases

We report a NMR and magnetometry study on the expanded intercalated fulleride Cs3C60 in both its A15 and face centered cubic structures. NMR allowed us to evidence that both exhibit a first-order Mott transition to a superconducting state, occurring at distinct critical pressures pc and temperatures Tc. Though the ground state magnetism of the Mott phases differs, their high T paramagnetic and superconducting properties are found similar, and the phase diagrams versus unit volume per C60 are superimposed. Thus, as expected for a strongly correlated system, the interball distance is the relevant parameter driving the electronic behavior and quantum transitions of these systems.

Reproduced with permission. Copyright 2010, American Physical Society

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Fullerenium Salts: A New Class of C60-Based Compounds

We report on the preparation and characterization of a fullerenium salt in the solid state, where the fullerene is in the 2+ oxidized state. To succeed in this long-standing challenge, we exploit the oxidizing power of one of the strongest Lewis acids, AsF5. The weak nucleophilic character of its conjugate base is essential in stabilizing the fullerene dication in a crystal lattice. High-resolution structural analysis of this compound, with the formula C60(AsF6)2, indicates that the highly reactive C602+ units are arranged according to a novel 1D “zigzag” polymer structure. The molecules are connected by an alternating sequence of four-membered carbon rings ([2 + 2] cycloaddition) and single C−C bonds. The long awaited high-Tc superconductivity and magnetism, expected in a hole-doped C60 compound, are replaced instead by a semiconducting behavior, quite probably originating from the reduced crystal and molecular symmetry upon polymerization. The small value of the energy gap (approximately 70 meV) suggests, nevertheless, the proximity of a metallic phase.

Reproduced with permission. Copyright 2010, American Chemical Society

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Superionic Conductivity in the Li4C60 Fulleride Polymer

We report on the extraordinary superionic conductivity in the fulleride polymer Li4C60 , a crystalline material with no disorder. 7Li , NMR, and dc/ac conductivity show uncorrelated ionic hopping across small energy barriers (ΔEa200meV ) and an ionic conductivity of 102S/cm at room temperature, higher than in “standard” ionic conductors. Ab initio calculations of the molecular structure find intrinsic unoccupied interstitial sites that can be filled by Li+ cations in stoichiometric Li4C60 even at low temperatures. The low energy required for the occupation of these sites allows a sizable Li+ diffusion above 130 K. The results suggest novel application of lithium intercalated fullerides as electrodes in Li ions batteries.

Reproduced with permission. Copyright 2009, American Physical Society

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