Carbon Nanothreads from Benzene
We have found that benzene molecules can react in the solid state to form diamondoid nanothreads. These nanothreads differ from conventional carbon nanotubes in having tetrahedrally bonded carbon instead of trigonal carbon. See: Fitzgibbons, T.C., Guthrie, M., Xu, E., Crespi, V.H, Davidowski, S.K., Cody, G.D., Alem, N. and Badding, J.V., Benzene-derived carbon nanothreads, Nature Materials, doi: 10.1038/nmat4088.
Ambient Temperature Reversible Hydrogenation
Unsaturated carbon molecules and materials can change hybridization to sp3-bonded carbon upon compression (4). Hydrogen can affect this change in hybridization by bonding to compressed carbon. We are investigating the high pressure behavior of unsaturated carbon molecules as part of the Carnegie Institution of Washington DOE EFREE Energy Frontier Research Center. Applications of interest include hydrogen storage materials, luminescent materials, and high strength materials. Some solids can reversibly rehybridize from sp2 to sp3 bonding and back to the same sp2 bonding upon release of pressure (4). This behavior remains largely not understood and thus probing it is a key focus of our research.
Ambient temperature reversible hydrogenation of carbon is of particular interest because it places strict constraints on the C-H bond energetics: 2 C-H bonds must have an energy close to that of an H-H bond. Most C-H bonds in organic molecules and organic solids have an energy closer to that of H-H.
Our research collaboration with Angela Lueking and Vin Crespi at Penn State has shown that carbons doped with catalytic Pt nanoparticles can be hydrogenated reversibly at room temperature (28). A new mode at ~1180 cm-1 arises in the in Raman spectrum of oxidized and Pt doped activated carbon in the presence of hydrogen and disappears upon removal of hydrogen at ambient temperature. This mode also appears to arise in the Raman spectrum of hydrogen implanted graphene (28), but has not previously been recognized as such. Other carbons, such as graphene, also exhibit the mode, but it does not disappear upon removal of hydrogen. The bond can be convincingly attributed to H because it downshifts by the expected amount upon deuteration. Ambient temperature reversible hydrogenation of carbon is of particular interest because it places strict constraints on the C-H bond energetics: 2 C-H bonds must have an energy close to that of an H-H bond. Most C-H bonds in organic molecules and organic solids have an energy closer to that of H-H. Reversibility is a central issue in hydrogen storage, for example; chemical bonds are often too strong to allow for reversibility while physisorption does not allow for much storage at ambient temperature. Density functional calculations show that certain arrangements of paired H atoms bonded to a graphene sheet allow for bond strengths in the range needed for reversibility. Steric effects associated with the rehybridization of the sp2 graphene carbons to sp3 upon reaction with hydrogen weaken the bond. Unraveling the origin of the reversibility in certain carbons and the lack of it in other carbons is a central goal of this project. The experiments to date show that Raman spectroscopy can give new information at the microscopic level, i.e., the level of bonds, about the reversible and irreversible hydrogenation of carbon by catalytic Pt nanoparticles. Macroscopic measurements and inelastic neutron scattering measurements, for example, do not give such direct microsocopic level information about the C-H bonding.