Determination of Conduction and Valence Band Electronic Structure of LaTiO_xN_yThin Film

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 compared to the pristine oxide material. As the band gap is correlated to both the chemical composition and the crystal structure, it is not trivial to distinguish which 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 are used to investigate the electronic structures of orthorhombic perovskite LaTiO N thin films in comparison with films of the pristine oxide LaTiO 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. This study also shows that the conduction band minimum, typically considered almost unaffected by 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.

Protagonists and spectators during photocatalytic solar water splitting with SrTaOxNy oxynitride

“…Both these effects can explain the downward shi in energy of the CBM. 48,49 The increase in intensities for the XANES spectra most likely result from the nitrogen substitution. In the cubic perovskite oxide KTaO 3 , the Ta-O-Ta angle is 180 .…”

Thin‐Film Oxynitride Photocatalysts for Solar Hydrogen Generation: Separating Surface and Bulk Effects Using Synchrotron X‐Ray and Neutron‐Based Techniques

“…upper occupied energy levels in the valence band, respectively. [70] These characterizations allow the measurement of the relative shifts in energy of the band edges between the two materials, as shown in Figure 8II. The bandgaps of the two materials were instead measured by near-IR-UV-vis spectroscopy.…”

“…II) a) CB and VB electronic structure determined from RIXS of LaTiO 3þz and LaTiO x N y thin films. II) b) Calculated density of states.Figure8(II) is reproduced with permission [70]. Copyright 2017, Wiley-VCH.…”

Energy Conversion Processes with Perovskite-type Materials

“…Both these effects have to be taken into consideration to explain the overall reduction of the band gap. [19] The specific chemical composition also affects the width of the band gap. Theoretical calculations of the band structures of ATaO 2 N with A = Ba, Sr, and Ca predict larger band gap with increasing the orthorhombic distortion (SrTaO 2 N and even more in CaTaO 2 N) with respect to the cubic structure of BaTaO 2 N. [16b] The B -O -B bond angle, which is 180º in the cubic structure, becomes smaller by decreasing the size of the A-site cation.…”