Impermeability of graphene denied by hydrogen

Physicists have demonstrated that the considered installed
proof of defect-free graphene for all liquids and gases, is performed
not for all substances. In the new, much more precise experiments, the scientists
recorded the seepage of hydrogen and confirmed the absence of transmission of other
gases, including helium, atoms of which is substantially smaller than the molecules
hydrogen. Apparently, discovered the process associated with the interaction of hydrogen
with folds and waves on the surface of a substance, write the authors in the journal Nature.

Graphene is one of the modifications of pure graphene, which is
is sheets of single-atom thickness with a hexagonal structure of relations.
Graphene has a huge list of exclusive and unusual properties, which
make it interesting both from the point of view of fundamental science, and in terms
of potential applications. However, scientists have not yet invented a cheap
a scalable method for the synthesis of high quality graphene, so it remains a matter of curiosity of physicists and materials scientists.

One of the important properties of graphene is extremely high
the energy barrier of the transmission of atoms and molecules. Calculations based on the theory of
density functional predicted values are not less than a few electron volts,
that should completely prevent the penetration of any liquids or gases at
normal conditions. These results were confirmed by experiments in which
exactly enough to register flow through the graphene tens of thousands of atoms in
second.

Physics from the University of Manchester, University of Nijmegen
and Wuhan University under the leadership of Nobel laureate Andrei
Geim (Andre Geim) launched a new series of experiments in which were able to achieve
increase the sensitivity by 8-9 orders of magnitude compared to the previous
experiments. It turned out that graphene with a precision of a few atoms per hour
indeed impermeable to helium, neon, nitrogen, oxygen, argon, krypton
and xenon. However, this was not so in the case of hydrogen, which required
a separate theoretical explanation.

For the experiment, physicists have done in the single-crystal graphite or nitride
boron wells of micrometer diameter and a depth of about 50 nanometers, and then
tightly covered them on top of single-layer graphene membrane. Received
purpose micro containers were placed in an atmosphere of various gases, and for possible
the penetration of substances inside of it followed by the curvature of the membrane.

Since the inside of microcontainer was a mixture of different
gases (air), and outside of it — pure substance, the partial pressure at the
different sides of the membrane differ. If graphene were permeable to this
the composition of the surrounding gas, he would gradually penetrate into the container,
there is increased pressure and lead to swelling of the membrane. To determine the effect
the authors used atomic force microscope.

The experiments involved over a dozen containers, and various
the atmospheres they were up to a month. The result is no visible flow through
the membrane of all investigated gases except hydrogen, were found.
The upper estimate of the rate of penetration of one billion atoms per second
square meter in the case of helium, which has the smallest atoms and
considered the most “crafty” stuff, makes a single layer of graphene less
permeable than kilometer layer of quartz glass. This corresponds to
the energy barrier of more than 1.2 electron volts.

The only exception was hydrogen which
penetrated in significant quantities inside the purpose micro containers, although the size of it
much more helium atoms. Theoretical estimates show that
the energy barrier for molecular hydrogen exceeds ten
electron volts, and atomic is in the range from 2.6 to 4.6 electron volts, but for the collapse
one molecule requires additional energy of about 4.5 electron volts. Measured
the permeability was equal to 2 × 1010 particles per second per square meter and
despite a convincing registration in this work, otherwise so
small streams not lock out.

Experiments with hydrogen was repeated for different values of
temperature. It turned out that the flux is changing exponentially in accordance with the law
Of Arrhenius, allowing to experimentally determine the energy barrier, it
was equal to 1.0 ± 0.1 electron volts. This is a relatively small value is much lower
than its theoretical evaluation.

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