Mattia Gaboardi

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

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, and SQuID magnetometry. The formation of 17 nm super-paramagnetic (SPM) monodispersed Ni NPs was observed. Nitrogen sorption experiments at 77 K yield a Brunauer–Emmet–Teller specific surface area (BET-SSA) of 505 m2 g−1 and helium adsorption at room temperature gives a skeletal density of 2.1 g cm−3. The interaction with atomic hydrogen was investigated by means of Muon Spin Relaxation (μSR) showing a considerable fraction of captured muonium (∼38%), indicative of strong hydrogen–graphene interactions. Hydrogen adsorption has been measured via pressure concentration isotherms demonstrating a maximum of 1.1 mass% of adsorbed hydrogen at 77 K and thus a 51% increased hydrogen adsorption compared to other common carbon based materials.

Non-interacting hard ferromagnetic L10 FePt nanoparticles embedded in a carbon matrix

Monodispersed ferromagnetic FePt nanoparticles, partially ordered in the L10 phase, were directly prepared without further annealing by high temperature synthesis (≈300 °C) involving poly(N-vinyl-2-pyrrolidone) and Triton X-100 as protective agent and reaction solvent respectively. Depending on the synthesis conditions, nanoparticles with average sizes ranging from 5 to 7 nm and coercive fields reaching 0.1 T at 300 K were obtained, but they invariably aggregate by magnetic dipolar interaction. By increasing the solvent viscosity (using PEG 600), 5 nm superparamagnetic nanoparticles are embedded in an amorphous matrix derived from solvent condensation/decomposition, thus avoiding aggregation. Nanoparticles are then completely converted to the hard tetragonal L10 phase, preserving the original size, by annealing in a vacuum at higher temperatures that, at the same time, transform the matrix into amorphous carbon. Annealing at 650 °C for 3 h leads to coercive fields of about 0.25 T at RT and 1.3 T at 5 K (without reaching the saturation magnetization) and to a peculiar squeezing of the hysteresis loops. Subsequent treatments at higher temperatures induce a further shrinking of the loop and a reduction of the coercive field. The possible explanation takes into account that, by raising the annealing temperature, an increasing number of nanoparticles becomes free to rotate inside the matrix, aligning like “nano-compasses” with the applied magnetic field. However a fraction of nanoparticles remains still locked to the matrix, generating a superimposed magnetically hard contribution.

Hydrogen motions in defective graphene: the role of surface defects

Understanding the mobility of H at the surface of carbon nanostructures is one of the essential ingredients for a deep comprehension of the catalytic formation of H2 in interstellar clouds. In this paper, we combine neutron vibrational spectroscopy with DFT molecular dynamics simulations to study the local environment of H structures chemisorbed at the surface of disordered graphene sheets. At 5 K, the ground state is composed of large clusters of hydrogen chemisorbed at sp2 carbon sites, on the edges and in voids of the graphene sheets. At temperatures of ∼300 K, a high degree of dispersion of the clusters is observed, involving the breaking and reforming of covalent bonds which, at low temperatures, is mediated by incoherent tunnelling of hydrogen.

Ammonia-storage in lithium intercalated fullerides

Ammonia has been proposed as an indirect hydrogen carrier, as solid-state ammonia-storage could be easier than directly absorbing hydrogen in materials. Here we investigate the structural evolution of hyper-ammoniated lithium fullerides (ND3)yLi6C60 during ammonia desorption, using in-situ high intensity neutron powder diffraction. In (ND3)yLi6C60, ammonia molecules are stored in their neutral state inside the inter-fullerene interstices and are coordinated to the intercalated Li ions, forming Li–ND3 clusters. Li6C60 is found to absorb up to 36.8 wt% ND3, which corresponds to approximately 14 ammonia molecules per C60. The ammonia release, studied either in-situ or ex-situ by means of manometric analyses and differential scanning calorimetry, takes place in two main steps, at 350–410 K and 500–540 K, respectively. This corresponds to two clear 1st order structural phase transitions and the absorption process is partially reversible. These findings suggest that the system could be a good candidate for ammonia-storage applications.

Muons probe magnetism and hydrogen interaction in graphene

Muon spin resonance (μSR) is a powerful technique for investigating the local magnetic fields in materials through implanted muons. Here we report a μSR study of chemically produced thermally exfoliated graphene. Our results provide an experimental answer to the many theoretical investigations of magnetic properties of graphene. The observed muon spin precession is attributed to a localized muon–hydrogen nuclear dipolar interaction rather than to a hyperfine interaction with magnetic electrons. This proves the absence of magnetism in chemically produced thermally exfoliated graphene.

Decorated and Modified Graphenes as Electrodes in Na and Li-Ion Batteries

Nowadays, rechargeable Li-ion batteries represent the state of the art for the power supply in technological devices. However, the wide-scale implementation of this technology, for example in the automotive field or for large stationary applications, could raise issues, i.e. concerning the limited lithium mineral reserves. The investigation of alternatives to lithium is hence highly desirable, although it requires the identification of new materials suitable as components for new batteries, displaying similar or possibly even better performances with respect to the current systems. Here we show that electrodes based on graphene derivatives are able not only to support the insertion of Li+, but also of Na+ ions, with high capacity and stability upon cycling, leading to the development of novel Na-ion batteries.