Contributo HIGHLIGHT per Report Istituto 2012
Title: Hydrogen in Graphene
Author: Valentina Tozzini, v.tozzini@sns.it
Collaborators: Vittorio Pellegrini
Abstract
Graphene is currently considered one of the most promising materials for new
technologies, potentially as revolutionary as plastic was in the middle of past century.
Graphene is extremely robust and light, capable of being modeled in a number of
different shapes (fullerenes, nanotubes, nanoribbons, nanosponges) and has unique
electronic properties that makes it an extremely efficient heat and current conductor.
It was recently proposed that combining graphene with hydrogen could enhance its
versatility, especially after the theoretical/experimental proof that the completely
hydrogenated graphene equivalent (i.e. graphane) displays different electronic ad
structural properties. In order to further explore this possibility, we theoretically
studied two specific aspects of the hydrogenated graphene systems: the electronic and
structural properties of hybrid graphane/graphene systems and the possibility of using
graphene for chemisorption-based hydrogen storage systems. The main results are: (i)
the electronic properties of graphene/graphane hybrid systems can be finely tuned by
controlling the hydrogenation down to the single atom level; (ii) an efficient hydrogen
storage and release process can be achieved at room conditions by controlling the
curvature of graphene sheets.
Beyond clarifying some issues related to the possibility of using graphene in the two
different fields of nano-electronics and energy storage, these results point towards the
possibility of building all-graphene-based integrated self-powered electronic devices.
Text
Graphene is a two dimensional sp2 hybridized honeycomb lattice of carbon atoms. It can
be obtained from graphite by mechanical or chemical exfoliation, or by deposition of
carbon on substrates[1-2]. Due to its peculiar band structure with linear dispersion ad K
points, it is a potentially very fast and efficient conductor. Its fully hydrogen saturated
version, graphane, is an insulator[3-5]. These circumstances open the possibility of
building graphene/graphane based nanoelectronics by “sculpting” graphene-like
structures within a graphane matrix.
We studied graphene nanoribbons (GNR) sculpted in graphane by means of Density
Functional Theory (DFT) and Car-Parrinello molecular dynamics[6]. GNR in graphane
are very stable, with minimal structural distortion rapidly decreasing with the distance
from the edge (Figure 1 (a)). More interestingly, along the edge within the nanoribbon
we observe a Peierls distortion manifesting with a bond-length alternation (BLA). This
is a very subtle effect emerging only with very accurate calculation (with density
functionals including explicit exchange). The Peierls distortion turns out to be related to
opening of the band gap, being ultimately the cause of the semiconducting nature of the
zig-zag graphene nanoribbons (Figure 1(b)). In fact, the BLA and the gap tend to vanish
as the nanoribbons width increases and the graphene bulk behavior prevails. For
nanoribbons widths larger than ~1nm this effect is masked by the emergence of spin
polarization at the edge orbitals, that add a “magnetic” symmetry breaking to the
“structural” one, inducing an additional gap opening. Also this, however, tends to vanish
for large nanoribbons (Figure 1 (b), squares). The gap can be tuned between 1.5 and
~0.2 eV, by varying the nanoribbons width in the range 0.5–10nm, allowing engineering
non trivial nano-electronic devices by controlling at the atomic level the
dehydrogenation of a graphane matrix.
The versatility of graphene suggests exploring its properties when mechanically
deformed. Using DFT and Car-Parrinello, we produced a corrugated graphene
(multilayer) by laterally compressing an approximately squared ~2nm large supercell
(Figure 2, top)[7]. The rippled structure has variable local curvature (concave and
convex) exploring a wide range of values from nearly flat up to almost that of fullerene.
Hydrogen binding energy (evaluated on sampled sites) is linearly dependent on the
local curvature, spanning a range of ~2eV. This implies that hydrogen binds very
favorably to the convex sites, while it is weakly bound or unstable on concave sites. This
naturally suggests a scheme for a possible hydrogen storage device based on the control
of the graphene corrugation (Figure 2, bottom). In the injection phase the corrugation of
the graphene sheet must be created and maintained. Hydrogen will then bind to the
convex sites ensuring a stable storage. The release can be obtained inverting the local
curvature of the graphene sheet: the hydrogen will find itself in the concave regions and
spontaneously detach. Both the binding and release phases were observed in
simulations. In silico, the curvature inversion is forced by an external mechanical
displacement field simulating a transverse mechanical wave. Practical ways to create
and control ripples in the real system are currently under study, and include the
coupling to mechanical waves generators, and the functionalization with optically or
electro-magnetically controllable molecular switches.
Figures
Figure 1 Structural and electronic properties of graphene NR sculpted in graphane. (a) Top view
(top) and lateral view (bottom) of the graphene/graphane edge (in ball&sticks). The plots report
the C-C bond-length as a function of the distance from the edge (filled and empty symbols are for
graphene NR in graphane and for H-passivated GNR). (b) Band-gap as a function of the GNR
width (W, in log scale). Calculation with and without spin polarization, for NR in graphane and H
passivated (symbol as explained). In the inset, a pictorial view of a zig-zag nanoribbon within the
graphane matrix.
Figure 2 Hydrogen storage in rippled graphene. Top Panel: preparation of the system for H
binding. From left: graphene corrugation by lateral compression: the graphene supercell is
represented in ball&stick and colored according to the local curvature (red=convex, blue
concave); variation of the binding energy as a function of the curvature (red dots correspond to
the values for nanotubes and fullerenes); qualitative binding energy profiles are reported with
the same color code as the curvature (with black=flat); a view of the graphene supercell with H
bound on most favorable sites. Bottom panel: Scheme for the adsorption-desorption cycle.
During injection, hydrogen is cracked and chemisorbed on convex sites (storage). Then the
curvature of the sheets is inverted and the hydrogen detaches from concave sites in molecular
form.
References
[1] X. Li et al., Science 319, 1229 (2008)
[2] M. Y. Han et al., Phys. Rev. Lett. 98, 206805 (2007)
[3] J. O. Sofo, A. S. Chaudhari, and G. D. Barber, Phys. Rev. B 75, 153401 (2007)
[4] M. H. F. Sluiter and Y. Kawazoe, Phys. Rev. B 68, 085410 (2003)
[5] D. C. Elias et al., Science 323, 610 (2009)
[6] V Tozzini, V Pellegrini, Phys Rev B 81, 113404 (2010)
[7] V Tozzini, V Pellegrini J. Phys. Chem. C 115, 25523–25528 (2011)