Interfacial Effects on the Band Edges of Functionalized Si Surfaces in Liquid Water

By
combining ab initio molecular dynamics simulations
and many-body perturbation theory calculations of electronic energy
levels, we determined the band edge positions of functionalized Si(111)
surfaces in the presence of liquid water, with respect to vacuum and
to water redox potentials. We considered surface terminations commonly
used for Si photoelectrodes in water splitting experiments. We found
that, when exposed to water, the semiconductor band edges were shifted
by approximately 0.5 eV in the case of hydrophobic surfaces, irrespective
of the termination. The effect of the liquid on band edge positions
of hydrophilic surfaces was much more significant and determined by
a complex combination of structural and electronic effects. These
include structural rearrangements of the semiconductor surfaces in
the presence of water, changes in the orientation of interfacial water
molecules with respect to the bulk liquid, and charge transfer at
the interfaces, between the solid and the liquid. Our results showed
that the use of many-body perturbation theory is key to obtain results
in agreement with experiments; they also showed that the use of simple
computational schemes that neglect the detailed microscopic structure
of the solid–liquid interface may lead to substantial errors
in predicting the alignment between the solid band edges and water
redox potentials.