Determination of conduction and valence band electronic structure of LaTiOxNy thin film

TL;DR

This study investigates how nitrogen substitution affects the electronic structure of LaTiOxNy thin films, revealing shifts in both valence and conduction bands that contribute to band gap reduction, crucial for designing efficient photocatalysts.

Contribution

It provides a detailed analysis of the electronic structure modifications in LaTiOxNy due to nitrogen incorporation, highlighting changes in both valence and conduction bands.

Findings

Nitrogen incorporation causes an upward shift in valence band maximum.

Conduction band minimum shifts downward significantly.

Total band gap narrowing involves changes in both occupied and unoccupied states.

Abstract

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.