  In the search for new renewable energy to replace fossil fuels, Hydrogen is
one of the most promising candidates for clean energy production. But cheap
Hydrogen separation and storage is still a big challenge. Photoelectrochemical
devices look promising for the decomposition of the water molecule into
$2H_2+O_2$. Every day new materials and combinations are discovered or invented
to improve the efficiency of the complex total process. A necessary condition
for the photoelectrochemical process to work without a bias voltage is that the
minimum of the semiconductor conduction band (CBM) must be more positive than
the reduction potential $H^{+}$ to $H_2$, whereas the highest value of the
semiconductor valence band (VBM) must be more negative than the oxidation
potential of $H_2O$ to $O_2$. Thus, band alignment studies in interfaces of
semiconductors with water become of vital importance. In this work, first
principles calculations based on density functional theory (DFT) in the all
electron and the pseudo potential approaches have been performed for the
analysis of the band alignment in $SrTaO_2N/H_2O$ interfaces. Different surface
terminations were analyzed, together with the dependence of the gap and band
alignment with lattice constants for systems grown on mismatched substrates.
Water structures were built from classical molecular dynamics and its
electronic structure calculated using DFT. The calculations show that the
$SrTaO_2N$ (001) is suitable for photoelectrochemical applications on a wide
range of lattice constant a, except for a compression/elongation of -2%, -1%
and 3%, while STN (110) results suitable for photoelectrochemical devices over
a wider range of lattice constants from -1% to 3%.
