  Catalytic hydrogenation of graphite has recently attracted renewed attention,
as a route for nano-patterning of graphene and to produce graphene
nano-ribbons. These reports show that metallic nanoparticles etch surface
layers of graphite, or graphene anisotropically along the crystallographic
zigzag <11-20> or armchair <1010> directions. On graphene the etching direction
can be influenced by external magnetic fields or the substrate. Here we report
the sub-surface etching of highly oriented pyrolytic graphite (HOPG) by Ni
nanoparticles, to form a network of tunnels, as seen by SEM and STM. In this
new nanoporous form of graphite, the top layers bend inward on top of the
tunnels, while their local density of states remains fundamentally unchanged.
Engineered nanoporous tunnel networks in graphite allow further chemical
modification and may find applications in storage or sensing.
