  The nitrogen substitution into the oxygen sites of several oxide materials
leads to a reduction of the band gap to the visible light energy range, which
makes these oxynitride semiconductors potential photocatalysts for efficient
solar water splitting. Oxynitrides typically show a different crystal structure
compare to the pristine oxide material. Since the band gap is correlated to
both the chemical composition and the crystal structure, it is not trivial to
distinguish what modifications of the electronic structure induced by the
nitrogen substitution are related to compositional and/or structural effects.
Here, X-ray emission and absorption spectroscopy is used to investigate the
electronic structures of orthorhombic perovskite LaTiOxNy thin films in
comparison with films of the pristine oxide LaTiOx with similar orthorhombic
structure and cationic oxidation state. Experiment and theory show the expected
upward shift in energy of the valence band maximum that reduces the band gap as
a consequence of the nitrogen incorporation. But this study also shows that the
conduction band minimum, typically considered almost unaffected by the nitrogen
substitution, undergoes a significant downward shift in energy. For a rational
design of oxynitride photocatalysts the observed changes of both the unoccupied
and occupied electronic states have to be taken into account to justify the
total band gap narrowing induced by the nitrogen incorporation.
