Researchers from the University of Maryland have used molecular dynamics simulation to demonstrate graphene nano-cages which will open and close in response to an electric charge using a technique they call hydrogenation-assisted graphene origami (HAGO). The cages can stably store hydrogen molecules at a density of 9.7 wt % hydrogen—significantly above the US Department of Energy (DOE) target of 5.5 wt % by 2017 and 7.5 wt % by 2020.
The team has also demonstrated the potential to reach an even higher density and doing so is a future research goal. A paper on their work is published in the journal ACS Nano.
… self-folding of graphene, or graphene origami, has been subjected to intensive study due to the need to fabricate unconventional nanostructures via approaches beyond conventional material preparation techniques. Progresses in patterning graphene with atomic-scale precision have further paved the way toward achieving graphene origami in a programmable fashion. … The unique feature of such an unconventional bottom-up nanomanufacture technique is that a material building block can self-assemble into a final folded structure, which is typically energetically more favorable and thus more stable than the original building block.
… The two-dimensional nature of graphene makes the chemical functionalization of graphene a promising approach to modulating the graphene properties. For example, hydrogenation of graphene involves bonding atomic hydrogen to the carbon atoms in graphene. Such a reaction changes the hybridization of graphene from sp2 into sp3. As a result, the two-dimensional atomic structure of pristine graphene is distorted. … advances on programmable bonding of atomic hydrogen to carbon atoms in pristine graphene open up new avenues for manipulating the morphology of graphene and therefore exploring graphene-based novel nanomaterials.
… In this letter, we use molecular dynamics (MD) simulations to demonstrate the hydrogenation-assisted graphene origami (HAGO), in which initially planar, suitably patterned graphene can self-assemble into three-dimensional nanoscale objects of desirable geometric shapes. We further demonstrate that the HAGO process can be modulated by an external electric field, enabling programmable opening and closing of the resulting three-dimensional nano-objects, a desirable feature to achieve molecular mass manipulation, storage, and delivery.—Zhu and Li (2014)
Hydrogenation of a carbon atom in pristine graphene induces a local structural change around that carbon atom; the chemically adsorbed hydrogen atom attracts its bonded carbon atom while it repels other neighboring carbon atoms. In this case, the three initially planar carboncarbon (C-C) bonds associated with the hydrogenated carbon atom would locally bend away from the hydrogen atom. If the graphene is hydrogenated on both sides, the resulting hydrogenated graphene (termed as graphane) would overall retain a planar morphology because the local distortions of the C-C bonds neutralize each other.
However, if the graphene is single-sided hydrogenated, the local distortion at each hydrogenated carbon atom is accumulated. For example, the authors explained, if hydrogenation lines are introduced in one side of a graphene, the accumulated distortion can effectively fold the graphene along the hydrogenation lines to a certain angle.
The authors, Shuze Zhu and Teng Li from the Department of Mechanical Engineering and Maryland NanoCenter, University of Maryland, demonstrated an array of unconventional nanostructures enabled by HAGO, including hexahedral nanocage, octahedral nanocage, and nanobasket.
In the HAGO-enabled graphene nanocage, the interwall vdW interaction energy plays a key role in maintaining the nanocage structural stability. Therefore, manipulation of such an interaction energy potentially offers the morphological tunability of the graphene nanocage (e.g., controllable opening and closing). We next show that an external electric field can effectively reduce the interwall adhesion. Therefore, by tuning the external electric field, facile control of the morphology of graphene nanocage can be achieved. … Inspired by the above feature, we next demonstrate controlled opening and closing of HAGO-enabled graphene nanocage, an otherwise hard to achieve but highly desirable mechanism for molecular mass manipulation.—Zhu and Li (2014)
They showed the use of the HAGO-enabled nanocages for controllable uptake and release of fullerenes and nanoparticles as well as the ultrahigh density of hydrogen storage. The hydrogen storage density using graphene nanocages depends on the nanocage size, which can be estimated by a simple model.
The US National Science Foundation supported the team’s research.
Shuze Zhu and Teng Li (2014) “Hydrogenation-Assisted Graphene Origami and Its Application in Programmable Molecular Mass Uptake, Storage, and Release,” ACS Nano doi: 10.1021/nn500025t