Hydrogen Sticks to Graphene More Than Expected and Now We Know Why

For all its complexity, chemistry amounts to just two things: making and breaking bonds between atoms. Understanding how those bonds are formed and destroyed has been a running goal for many chemists, including Caltech's Tom Miller.

In a new paper published in the journal Science, Miller, professor of chemistry, reveals findings that he says fundamentally change our understanding of what is going on behind the scenes in some chemical reactions.

The work focuses on what is known as surface chemistry—the chemical reactions that occur at the boundary between two phases of matter: either a gas reacting with a solid, a solid reacting to a liquid, or a liquid reacting with a gas.

In this latest research, Miller and his collaborators looked at how graphene, a sheet-like form of carbon, interacts with hydrogen atoms.

To picture their experimental setup, imagine propping a trampoline up against the wall of your house. Now, start throwing softballs at the trampoline as hard as you can. That is essentially what Miller and his team were doing, except instead of a trampoline, they had graphene, and instead of softballs, they had hydrogen atoms.

If graphene indeed behaved like a trampoline, as previously thought, then the vast majority of balls would come bouncing back at you without sticking, and pretty fast. It would be like playing a solo game of dodgeball.

However, this is not what Miller's team observed when they collided hydrogen atoms into graphene. While some bounced back at high speed, others bounced back only weakly, and a large fraction did not come back at all. This discrepancy suggested that something was amiss in the understanding of how the hydrogen and graphene were interacting. The weakly bouncing hydrogen atoms had a lot of energy before they hit the graphene but not much afterward. That energy had to be going somewhere.

"This unexpected stickiness and this unexpected population of slow bouncers was intriguing and surprising," Miller says. "The hydrogen atoms were doing something that was dumping a lot of kinetic energy and allowing them to stick to the surface. That's very different than what we expected to happen."

Miller wanted to know where all of that energy was going. To find out, he developed quantum simulation methods that revealed the mechanism by which the hydrogen atoms were interacting with graphene during the collisions.

Miller draws an analogy between the insights from the current work and that of the late Caltech professor Ahmed Zewail, who used ultrafast laser pulses to characterize the motion of the atoms during chemical bond breaking and formation. Zewail won the Nobel Prize in Chemistry in 1999 for pioneering this field of research.

"Zewail did that using femtosecond—one quadrillionth of a second—lasers," Miller says. "Our current work uses a combination of scattering experiments and quantum simulations to learn about the same femtoscecond timescales but in a surface-chemistry context."

In observing the collisions between hydrogen atoms and the graphene in this way, Miller's team was able to see why graphene was stickier than expected and why many of the hydrogens atoms were bouncing off with so little energy.

Their assumption that the graphene would act like a trampoline was wrong, Miller says.

When hydrogen atoms are shot at graphene, most bounce off, but some actually form a covalent bond with one of the carbon atoms. When they do that, the hydrogen atoms change the arrangement of bonds in the graphene surface. By causing this widespread change in the graphene bonding pattern, the hydrogen atoms efficiently transfer much of their kinetic energy to the graphene. Sometimes, the hydrogen stays stuck to the graphene, but at other times, it only forms a temporary bond. Those temporary bonds account for the slow bouncers. The slow bouncers make a bond with the graphene, but before it can break, most of their energy has dissipated through the graphene's structure.

Rather than a trampoline, the graphene is acting more like a pane of safety glass being cracked by rocks thrown at it, Miller says. The glass absorbs the energy of the rocks, and the rocks either get embedded in it or bounce off weakly.

Miller says that this finding has potential implications in many fields of study, including interstellar chemistry, atmospheric chemistry, and the development of catalysts and sensors.

"It's a very simple and fundamental collision process," he says. "And it's beautiful to see transient chemical bond formation giving rise to such a pronounced effect in this completely unexpected scenario."

The paper describing Miller's findings, titled, "Imaging covalent bond formation by H-atom scattering from Graphene," appears in the April 25 issue of Science. Miller's co-authors include Hongyan Jiang, Yvonne Dorenkamp, and Marvin Kammler and Alec Wodtke of the Georg-August University of Göttingen and the Max Planck Institute for Biophysical Chemistry; Feizhi Ding of Caltech; Frederick R. Manby of the University of Bristol; Alexander Kandratsenka of the Max Planck Institute for Biophysical Chemistry; and Oliver Bünermann of the Georg-August University of Göttingen.

Funding was provided by the German Research Foundation, the Ministry of Culture and Science of North Rhine-Westphalia, and the Volkswagen Foundation. Miller was supported by the Department of Energy, including support from the Caltech Joint Center for Artificial Photosynthesis program.