Investigating oxides by electrochemical projection of the oxygen off-stoichiometry diagram onto a single sample

TL;DR

This paper introduces a novel electrochemical method to project oxygen off-stoichiometry diagrams onto a single sample, enabling detailed, continuous study of oxide materials' properties as a function of oxygen content with reduced experimental complexity.

Contribution

A new electrochemical approach creates in-plane oxygen gradients in thin films, allowing comprehensive, spatially resolved analysis of oxide properties without sample variability or multi-step treatments.

Findings

Successful creation of in-plane oxygen gradients in thin films

Demonstrated spatially resolved analysis using XANES, diffraction, and electrical measurements

Applicable to various oxide materials for fundamental and applied research

Abstract

The oxygen stoichiometry is an essential key to tune functional properties of advanced oxide materials and thus has motivated numerous studies of the oxygen off-stoichiometry diagram, with the aim to determine and control structural, electronic, ionic, electrochemical and optical properties, as well as thermodynamic quantities such as the oxygen storage capacity, among others. Here, a novel approach is developed, which allows to project a broad range of oxygen chemical potentials onto a single thin film sample with unprecedented control via electrochemical polarization. Therefore, a specifically designed electrochemical cell geometry is deployed, resulting in a well-defined, linear, 1D in-plane oxygen concentration gradient, independent of variations in the materials electrical resistivity, whose endpoints can be flexibly controlled via the external pO2 and applied overpotential. This allows for an unparalleled study of materials properties as a continuous function of the oxygen content using spatially resolved tools (spectroscopic, diffraction, microscopy, local electrical probes, etc.) and thereby greatly reduces experimental efforts while also avoiding sample-to-sample variability, multi-step treatments, sample evolution effects, etc. This work presents the proof-of-concept of in-plane oxygen gradients, based on spatially resolved ex/in situ and novel fixed-energy X-ray absorption near edge spectroscopy (XANES), X-ray diffraction, ellipsometry and electrical resistivity measurements in hyper-stoichiometric La2NiO4+{\delta} and sub-stoichiometric (La,Sr)FeO3-{\delta} thin films. It thereby demonstrates the readiness and wide applicability of this innovative approach, which can be highly relevant for fundamental as well as applied research.