Insight into Subsurface Adsorption Derived from a Lattice-Gas Model and Monte Carlo Simulations

Theoretical
gas-surface models that describe adsorption over a
broad range of adsorbate concentrations can provide qualitative insight
into chemical phenomena, such as subsurface adsorption, surface reconstruction,
and industrial heterogeneous catalysis. However, most atomistic, quantum-mechanical
models of gas-surface adsorption are limited to low adsorbate coverage
due to the large computational cost of models built using many surface
atoms and adsorbates. To investigate adsorption in the subsurface
of a crystalline solid with increasing coverage, we present a lattice-gas
adsorption model that includes surface and subsurface sites of the
solid and is fully parametrized using density functional theory. We
apply the model to study the competition between the surface and subsurface
adsorption of atomic oxygen on the Ag(111) surface. Oxygen population
distributions calculated using the model in combination with Monte
Carlo simulations show the onset of subsurface adsorption above a
total coverage of 0.375 monolayer and a greater accumulation of oxygen
in the second than in the first subsurface at total coverages between
0.5 and 1 monolayer. Our simulations also show that oxygen atoms do
not percolate into the bulk region of silver for total coverages of
up to 1 monolayer, indicating that oxygen adsorbed in the subsurface
is distinct from oxygen absorbed in the bulk in this coverage range.
Computations of core–electron binding energies and projected
density of states for the equilibrium oxygen distribution at 0.5 monolayer
reveal qualitative differences in the oxygen–silver bonding
at the surface and subsurface, suggesting that oxygen adsorbed in
the two regions could play distinct roles in the surface chemistry.