  Semiconductor compounds are widely used for water splitting applications,
where photo-generated electron-hole pairs are exploited to induce catalysis.
Recently, powders of a metallic oxide (Sr$_{1-x}$NbO$_3$, 0.03 < x < 0.20) have
shown competitive photocatalytic efficiency, opening up the material space
available for finding optimizing performance in water-splitting applications.
The origin of the visible light absorption in these powders was reported to be
due to an interband transition and the charge carrier separation was proposed
to be due to the high carrier mobility of this material. In the current work we
have prepared epitaxial thin films of Sr$_{0.94}$NbO$_{3+{\delta}}$ and found
that the bandgap of this material is ~4.1 eV, which is very large. Surprisingly
the carrier density of the conducting phase reaches 10$^{22}$ cm$^{-3}$, which
is only one order smaller than that of elemental metals and the carrier
mobility is only 2.47 cm$^2$/(V$\cdot$s). Contrary to earlier reports, the
visible light absorption at 1.8 eV (~688 nm) is due to the bulk plasmon
resonance, arising from the large carrier density, instead of an interband
transition. Excitation of the plasmonic resonance results in a multifold
enhancement of the lifetime of charge carriers. Thus we propose that the hot
charge carriers generated from decay of plasmons produced by optical absorption
is responsible for the water splitting efficiency of this material under
visible light irradiation.
