  The size of the band gap and the energy position of the band edges make
several oxynitride semiconductors promising candidates for efficient hydrogen
and oxygen production under solar light illumination. The intense research
efforts dedicated to oxynitride materials have unveiled the majority of their
most important properties. However, two crucial aspects have received much less
attention. One is the critical issue of the compositional/structural surface
modifications occurring during operation and how these affect the
photoelectrochemical performance. The second concerns the relation between the
electrochemical response and the crystallographic surface orientation of the
oxynitride semiconductor. These are indeed topics of fundamental importance
since it is exactly at the surface where the visible light-driven
electrochemical reaction takes place. In contrast to conventional powder
samples, thin films represent the best model system for these investigations.
This study reviews current state-of-the-art of oxynitride thin film fabrication
and characterization before focusing on LaTiO2N selected as representative
photocatalyst. We report the investigation of the initial physicochemical
evolution of the surface. Then we show that, after stabilization, the absorbed
photon-to-current conversion efficiency of epitaxial thin films can differ by
about 50% for different crystallographic surface orientations and be up to 5
times larger than for polycrystalline samples.
