Density functional theory at the
level of (U)B3LYP has been used
to investigate the complete oxidation of the Si(111)-7 × 7 surface
by water. The results suggest that the initial water dissociation
readily occurs across an adjacent adatom–rest atom (Sia–Sir) pair, resulting in either Sia–OH + Sir–H (i.e., the Sia route)
or Sir–OH + Sia–H (i.e., the Sir route). Both routes are found to follow the precursor-mediated
pathway, while the Sir atom is concluded to be more reactive
than the Sia atom toward the initial decomposition of the
H2O molecule due to its higher binding affinity to the
incident water. With increasing water exposure and reaction temperature,
deep oxidation can be accomplished by steady oxygen atom insertion
into the Sia–Sis backbonds until the
Si4+ oxidation state is eventually developed. Our calculations
uncover that the most favorable pathway for deep oxidations begins
with the direct water dissociation over the Sia–Sis backbond, followed by H2 liberation. This differs
from the recent proposal via the OH insertion. Our deep oxidation
mechanism can also be applied to the oxidation of an isolated adatom,
which gives an explanation to the experimental observation where more
than 50% of Sia are involved in water oxidation. The present
work provides the detailed energetics that sheds light on the wet
oxidation mechanism of silicon surfaces at the molecular level.