The two-dimensional sheet of sp2-hybridized carbon atoms in graphene helps grant the material unique and useful chemical, material, and electronic properties. Adding hydrogen atoms to the carbons can improve those properties, for example by turning graphene into a semiconductor.
New experimental and computational experiments reveal how energy dissipates through a graphene sheet when hydrogen atoms strike it, making C–H bond formation possible (Science 2019, DOI: 10.1126/science.aaw6378).
Alec. M. Wodtke of the University of Göttingen and the Max Planck Institute for Biophysical Chemistry was experimenting with hydrogen scattering from metal surfaces when his group observed that graphene grown on platinum behaved differently. Hydrogen and metal atoms collide like billiard balls, he says, but when hydrogen hits a graphene carbon it doesn’t bounce off, it forms a bond.
Wodtke’s collaborator Thomas F. Miller III of the California Institute of Technology used computational models to better understand what happens. When struck by a hydrogen atom, graphene dissipates energy in two ways: an up and down vibration of the sheet and a shock wave moving out across its surface from the point of impact.
The latter, Wodtke explains, is the response of neighboring carbons as the impacted carbon changes its electronic structure from sp2- to sp3-hybridization as it forms or attempts to form a bond with hydrogen. The researchers say the ripple, which only lasts about 10 femtoseconds, explains why hydrogen can stick even when it strikes the sheet at high energy, near 2 eV. Previous models predicted lower sticking probabilities. When the hydrogen does stick, according to Wodtke, it’s because the rehybridized carbon chases down the ricocheting hydrogen from behind and catches it.
This study is the first in which hydrogen “sticking probabilities can be measured for specific energies and incidence angles, which makes it a big deal,” says Bret Jackson, a chemist at the University of Massachusetts Amherst who has modeled this graphene system. Wodtke says this work is one step on a long path to better understanding how hydrogen interacts with graphene.