Investigating the Electronic Structure of Prospective Water-splitting Oxide BaCe$_{0.25}$Mn$_{0.75}$O$_{3-\delta}$ Before and After Thermal Reduction

TL;DR

This study combines first-principles calculations and X-ray absorption spectroscopy to analyze the electronic structure of BaCe$_{0.25}$Mn$_{0.75}$O$_{3- extdelta}$ before and after reduction, aiding the design of water-splitting oxides.

Contribution

It provides a detailed electronic structure analysis of BCM using combined computational and experimental methods, revealing the effects of reduction on electronic properties.

Findings

Reduction localizes electron density around oxygen vacancies.

First O K-edge peak decreases upon reduction, indicating diminished O-2p contribution.

The study offers insights into the electronic changes relevant for hydrogen generation.

Abstract

BaCe$_{0.25}$Mn$_{0.75}$O$_{3-\delta}$ (BCM), a non-stoichiometric oxide closely resembling a perovskite crystal structure, has recently emerged as a prospective contender for application in renewable energy harvesting by solar thermochemical hydrogen generation. Using solar energy, oxygen-vacancies can be created in BCM and the reduced crystal so obtained can, in turn, produce H2 by stripping oxygen from H2O. Therefore, a first step toward understanding the working mechanism and optimizing the performance of BCM, is a thorough and comparative analysis of the electronic structure of the pristine and the reduced material. In this paper, we probe the electronic structure of BCM using the combined effort of first-principles calculations and experimental O K-edge x-ray absorption spectroscopy (XAS). The computed projected density-of-states (PDOS) and orbital-plots are used to propose a simplified model for orbital-mixing between the oxygen and the ligand atoms. With the help of state-of-the-art simulations, we are able to find the origins of the XAS peaks and to categorize them on the basis of contribution from Ce and Mn. For the reduced crystal, the calculations show that, as a consequence of dielectric screening, the change in electron-density resulting from the reduction is strongly localized around the oxygen vacancy. Our experimental studies reveal a marked lowering of the first O K-edge peak in the reduced crystal which is shown to result from a diminished O-2p contribution to the frontier unoccupied orbitals, in accordance with the tight-binding scheme. Our study paves the way for investigation of the working-mechanism of BCM and for computational and experimental efforts aimed at design and discovery of efficient water-splitting oxides.