Serhiy M. Luzan

Hydrogenation, purification, and unzipping of carbon nanotubes by reaction with molecular hydrogen: road to graphane nanoribbons.

Reaction of single-walled carbon nanotubes (SWNTs) with hydrogen gas was studied in a temperature interval of 400-550 °C and at hydrogen pressure of 50 bar. Hydrogenation of nanotubes was observed for samples treated at 400-450 °C with about 1/3 of carbon atoms forming covalent C-H bonds, whereas hydrogen treatment at higher temperatures (550 °C) occurs as an etching. Unzipping of some SWNTs into graphene nanoribbons is observed as a result of hydrogenation at 400-550 °C. Annealing in hydrogen gas at elevated conditions for prolonged periods of time (72 h) is demonstrated to result also in nanotube opening, purification of nanotubes from amorphous carbon, and removal of carbon coatings from Fe catalyst particles, which allows their complete elimination by acid treatment.

Phase Transitions in Graphite Oxide Solvates at Temperatures Near Ambient

It is demonstrated that solvent-saturated graphite oxide can be considered to be solid solvate, and two phases with distinctly different solvent composition are found near room temperature. Phase transitions between these two solvated phases were observed using synchrotron powder X-ray diffraction and DSC for methanol, ethanol, acetone, and dimethylformamide (DMF) solvents. Solvate A, formed at room temperature, undergoes a reversible phase transition into expanded Solvate L at temperatures slightly below ambient due to insertion of one monolayer of solvent molecules between the GO planes. The phase transition is reversible upon heating, whereas the low-temperature expanded phase L can be quenched to room temperature for ethanol and DMF solvates.

Hydrogen-Driven Cage Unzipping of C60 into Nano-Graphenes

Annealing of C60 in hydrogen at temperatures above the stability limit of C–H bonds in C60Hx (500–550 °C) is found to result in direct collapse of the cage structure, evaporation of light hydrocarbons, and formation of solid mixture composed of larger hydrocarbons and few-layered graphene sheets. Only a minor part of this mixture is soluble; this was analyzed using matrix-assisted laser desorption/ionization MS, Fourier transform infrared (FTIR), and nuclear magnetic resonance spectroscopy and found to be a rather complex mixture of hydrocarbon molecules composed of at least tens of different compounds. The sequence of most abundant peaks observed in MS, which corresponds to C2H2 mass difference, suggests a stepwise breakup of the fullerene cage into progressively smaller molecular fragments edge-terminated by hydrogen. A simple model of hydrogen-driven C60 unzipping is proposed to explain the observed sequence of fragmentation products. The insoluble part of the product mixture consists of large planar polycyclic aromatic hydrocarbons, as evidenced by FTIR and Raman spectroscopy, and some larger sheets composed of few-layered graphene, as observed by transmission electron microscopy. Hydrogen annealing of C60 thin films showed a thickness-dependent results with reaction products significantly different for the thinnest films compared to bulk powders. Hydrogen annealing of C60 films with the thickness below 10 nm was found to result in formation of nanosized islands with Raman spectra very similar to the spectra of coronene oligomers and conductivity typical for graphene.

Hydrogen-Driven Collapse of C60 Inside Single-Walled Carbon Nanotubes†

The search for new materials for hydrogen storage is important for the development of future hydrogen energy applications. In this Thesis, it is shown that new materials with interesting properties can be synthesized by the reaction of hydrogen with various nanocarbon precursors. The thesis consists of two parts. The first part is devoted to studies of hydrogen storage in some metal-organic frameworks (MOFs) and nanostructured carbon materials, while the second part describes synthesis of new materials by the reaction of hydrogen gas with various carbon materials (i.e. fullerene C60, single-walled carbon nanotubes (SWCNTs), and fullerene C60 encapsulated inside SWCNTs (C60@SWCNTs)).Hydrogen adsorption was measured for a set of Zn- and Co-based MOFs at near ambient temperatures. MOFs synthesized using different metal clusters and organic connecting ligands allowed to study effects of different surface area, pore volume, and pore shapes on hydrogen storage parameters. Hydrogen adsorption values in the studied MOFs correlated well with surface area and pore volume but did not exceed 0,75wt.%. Therefore, new methods to improve the hydrogen storage capacity in MOFs were investigated. The addition of metal catalysts was previously reported to improve significantly hydrogen storage in MOFs. In this thesis the effect of Pt catalyst addition on hydrogen adsorption in MOF-5 was not confirmed. Contrary to previous reports, hydrogen adsorption in MOF-5 mixed/modified with Pt catalysts had fast kinetics, correlated well with surface area, and was on the same level as for unmodified MOF-5. New nanostructured carbon materials were synthesized by the reaction between fullerene C60 and coronene/anthracene. Despite negligible surface area these materials adsorbed up to 0,45wt.% of hydrogen at ambient temperatures.The reaction of fullerene C60 with hydrogen gas was studied at elevated temperatures and hydrogen pressures. In situ gravimetric monitoring of the reaction was performed in a broad temperature interval with/without addition of metal catalysts (i.e. Pt and Ni). The reaction resulted in synthesis of hydrogenated fullerenes C60Hx (with x≤56) followed by fullerene cage fragmentation and collapse upon prolonged duration of hydrogen treatment. Possible mechanisms of C60 hydrogenation and fragmentation were discussed. It is demonstrated that reaction of SWCNTs with hydrogen gas at elevated temperatures and hydrogen pressures can be used for nanotube opening, purification from amorphous carbon, side-wall hydrogenation, and partial unzipping of SWCNTs. Some graphene nanoribbons (GNRs) were synthesized as the result of SWCNTs unzipping. A surprising ability of hydrogen to penetrate inside SWNTs and to react with encapsulated fullerene C60 was demonstrated.

Effect of catalysts on the reaction of C60 with hydrogen

The reaction of C60/catalyst with hydrogen gas was studied at 400°C and 50 bar of H2 pressure. The addition of Pt- or Ni-catalysts significantly accelerated kinetics of the hydrogenation reaction and resulted in a dramatic change of the C60Hx crystal structure. Samples reacted without catalyst preserved the fcc structure typical for pristine C60 but with expanded unit cells. Fulleranes C60Hx obtained using catalytic hydrogenation exhibited not only the fcc structure (at relatively low hydrogenation degree) but also the bcc structure of C60Hx (with x > 18). The bcc structure corresponds to highly hydrogenated material with an average volume per C60 molecule of 817–849 Å3.