An incredibly fast bonding between graphene and hydrogen

An article published in the journal “Science” reports the discovery of the possibility of generating a transient chemical bond between hydrogen atoms and a graphene sheet. A team of researchers coordinated by the Max Planck Institute (MPI) for Biophysical Chemistry in Göttingen, Germany and the California Institute of Technology (CalTech) in Pasadena, USA, bombarded graphene sheets with hydrogen atoms generating one of the fastest reactions ever studied since it occurs in only 10 femtoseconds in which hydrogen yields most of its energy.

This result offers the possibility of producing a band gap, which can make graphene a semiconductor with new applications in the field of electronics. Liv Hornekær of the Interdisciplinary Nanoscience Center-iNANO of the Aarhus University, Denmark, published another article in the same issue of “Science” that describes the work done by the authors of the research.

Graphene is made up of a carbon layer with a thickness of just one atom. It has a structure in which the atoms form hexagons with 120° angles. This material has an extraordinary strength, greater than steel, and has excellent electrical properties. Although it has already been celebrated for some time, graphene also has some not-so-little disadvantages that go beyond the problems of producing quality sheets. Many great promises are connected to applications in the field of electronics but graphene can’t be used as a semiconductor, since it has no band gap. This new study could change the situation showing the possibilities emerged from an experiment that provided surprising results.

When the researchers bombarded a graphene sheet laid on a platinum plate they expected them to fly away immediately, insteadi they formed a bond with carbon atoms for an extraordinarily short time of only 10 femtoseconds, that is ten millionths of a billionth of a second, then bounced off. Hydrogen atoms had a high speed and therefore a lot of energy so another surprise came from the fact that very little remained when they flew away.

To understand why hydrogen atoms lost most of their energy and where it went, the researchers developed theoretical methods and computer simulations they later compared with experiments. What happens during those 10 femtoseconds of chemical bonding is that hydrogen atoms transfer almost all their energy to the graphene’s carbon atoms and that triggers a sound wave that propagates outwards, a bit like the effect of a stone thrown into water. According to the researchers that sound wave helps to form a hydrogen-carbon bond more easily than scientists expected and from what was predicted by previous models.

The image (Courtesy Oliver Bünermann / Max Planck Institute for Biophysical Chemistry & University of Göttingen. All rights reserved) shows a hydrogen atom (blue) that strikes the surface of graphene (black) and forms an extremely fast bond with a carbon atom (red). The high energy of the hydrogen atom is absorbed first by neighboring carbon atoms (orange and yellow) and then passed on to the gaphene surface in the form of a sound wave.

Oliver Bünermann of the University of Göttingen explained that he and his colleagues had to conduct the experiments in an ultra-high vacuum to keep the graphene surface perfectly clean. A laser system was used to prepare the hydrogen atoms and to detect them after collisions. In short, the result is interesting because it improves our understanding of certain chemical, physical and electrical phenomena but these are lab experiments. Applying certain discoveries to industrial products is far more complex and that’s what holds back the spread of graphene. However there are discoveries that can produce initially unexpected applications and this research shows that our knowledge of graphene’s properties is still incomplete.

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