SCIENCE sciencemag.orgsynthesize the aerogel, they fabricated gra-
phene aerogels using a previously reported
technique ( 6 ) for use as sacrificial templates
upon which they grew hBN layers. Because
hBN is highly resistant to oxidation and its
thermal stability in air is superior to that
of graphene, the graphene template can be
easily removed by oxidation to leave behind
pure hBN aerogel.
For atomically thin walls, the aerogel
density can be as low as 0. 1 mg cm–3, lower
than most known solids. The aerogel also
exhibits ultrahigh elastic deformation (up to
9 5%) and specific surface area (> 1080 m^2 g–1).
Most important, it provides ultralow thermal
conductivity (see the figure) concurrently
with thermal shock resistance for several
hundred cycles, making it highly attractive
for extreme applications such as thermal
shields of space vehicles. Xu et al. show thatthe hyperbolic double-paned aerogel cell
walls allow realization of a negative coeffi-
cient of thermal expansion and a negative
Poisson’s ratio. Both of these properties are
opposite to conventional materials and are
only achieved via careful engineering of the
material and its structure ( 7 , 8 ).
In addition to its extreme thermal shield-
ing, high surface area, and refractory nature,
the atomically thin–walled aerogel realized
by Xu et al. serves as a starting point for
a new class of 3 D structures realized from
2 D materials for applications requiring ar-
chitectures with high surface-to-volume ra-
tios, including catalysis and electrochemical
energy storage. Further, the optical proper-
ties of these and aerogels made from other
2D semiconducting materials remain unex-
plored; if they can be engineered to have
sufficiently low absorption, they become
suitable structural material candidates for
laser sails and directed-photon propulsion
systems, such as those being proposed for
interstellar probes ( 9 ). jREFERENCES
1. X. Xu et al., Science 363 , 723 (2 019 ).
2. H. D. Gesser, P. C. Goswami, Chem. Rev. 89 , 765 (1 989 ).
3. J. P. Randall, M. A. B. Meador, S. C. Jana, ACS Appl. Mater.
Interfaces 3 , 613 (2 011 ).
4. G. George, G. Costas, 2D Mater. 4 , 032001 (2 017 ).
5. H. Sun, Z. Xu, C. Gao, Adv. Mater. 25 , 2554 ( 201 3).
6. X. Xu et al., Adv. Mater. 28 , 9223 (2 016 ).
7. A. Yeganeh-Haeri, D. J. Weidner, J. B. Parise, Science 257 ,
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8. M. Eidini, G. H. Paulino, Sci. Adv. 1 , e 1500224 (2 015 ).
9. H. A. Atwater et al., Nat. Mater. 17 , 861 ( 201 8).10 .1 126 /science.aaw 5670SURFACE CHEMISTRYMoving through the crowd
Atoms or molecules can diffuse rapidly on surfaces
covered by other adsorbants
By Olaf M. MagnussenC
hemical processes generally involve
the transport of atoms and molecules,
which on the nanoscale is dominated
by diffusion. In interface chemistry,
diffusion along surfaces and interfaces
is an elementary and often determin-
ing step. For example, it plays a central role
in crystal growth and dissolution, the depo-
sition of thin films and nanostructures, and
the self-assembly of two-dimensional organic
layers. It also governs the transport of the re-
acting species on the surfaces of catalysts and
thus the speed by which these species inter-
mix or reach specific sites, where the catalytic
reaction occurs. On page 71 5 of this issue,
Henß et al. show that this transport can be
rapid even in the presence of a layer of other
adsorbants on the surface ( 1 , 2 ).
Surface diffusion has been extensively
studied for chemically bound atoms and
small molecules that cover a small frac-
tion of an otherwise clean solid surface ( 3 ).
These adsorbates typically move on the sur-
face by jumping between neighboring bind-
ing sites (see the figure, top left). During a
jump, they must transiently occupy posi-
tions in which they are bound less strongly.
This leads to an energy barrier, which the
adsorbates overcome through thermal exci-
tation. At a given temperature, the rate by
which jumps between different positions
on the surface lattice occur thus depends
on the spatial variation of the binding en-
ergy; that is, it is exclusively determined by
the interactions of the adsorbate with the
atoms of the underlying solid.
This simple picture is, however, usually in-
sufficient to fully describe surface diffusion
in the real world. Even if only one species is
present on the surface, the adsorbates may
encounter other adsorbates during their ran-
dom walk on the surface and interact with
those. In the simplest case, the only effect
of this is that positions already occupied
by adsorbates are not available to others,
a concept called “site blocking.” In reality,
however, the adsorbates additionally attract
or repulse each other, resulting in modified
energy barriers for diffusion in the vicinity ofother adsorbates. If the surface fraction cov-
ered by adsorbates is high, these effects can
lead to changes in the adsorbates mobility by
orders of magnitude ( 4 ).
Similar effects may be expected for the
motion of adsorbates on a surface covered by
another atomic or molecular species, which
Henß et al. address in their report. The dif-
fusion of adsorbates on surfaces crowded
by other coadsorbed species is the normal
case for interface processes in ambient or
high-pressure gas and liquid phases. How-
ever, despite its prevalence in technological
systems and natural environments, it has
been studied only sparsely and is not well
understood. For this reason, the influence of
coadsorbates on surface diffusion has mostly
been either ignored or treated by simplified
concepts such as site blocking.
In the past decade, the complexity of this
situation has been revealed in some in-depth
studies of adsorbate systems with well-de-
fined composition and geometry ( 5 , 6 ). These
studies combine experimental data on sur-
face diffusion, obtained from video sequences
of atomic-resolution scanning tunneling mi-
croscopy images, with density functional the-
ory calculations of the pathways by which the
adsorbates move between neighboring sites.
Hsieh et al. have investigated the diffusion of
hydrogen on a chlorine-covered silicon sur-
face and observed a direct exchange of the
hydrogen atoms with neighboring chlorine
atoms ( 5 ). They attributed the surprisingly
low energy barrier for this exchange diffusion
to the transient formation of a more weakly
bound HCl adsorbate. Rahn et al. studied
the diffusion of sulfur atoms on copper elec-
trodes that were immersed in aqueous solu-
tion and covered by dense halide adlayers ( 6 ).
They found an opposite dependence of the
sulfur diffusion rates on the electrode voltage
for sulfur coadsorbed with chloride and bro-
mide, respectively, indicating different diffu-
sion mechanisms for the two coadsorbates.
In both studies, the diffusing adsorbates
and the surrounding coadsorbate were
bound in an identical geometry to the sur-
face and had the same distances to neighbor-
ing adsorbates (see the figure, top right). The
diffusion processes can then be conveniently
described via motions on one lattice that is
common to both species. However, adsor-
bate and coadsorbate often bind differently
to the surface and thus do not occupy iden-Interface Physics Group, Institute for Experimental and
Applied Physics, University of Kiel, 24098 Kiel, Germany.
Email: [email protected]“Xu et al. have in fact
rationally designed
hyperbolic aerogels with
hBN, a 2 D ceramic.”
15 FEBRUARY 2019 • VOL 363 ISSUE 6428 695
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