Equilibrium oxygen storage capacity of ultrathin CeO2-\u03b4 depends non-monotonically on large biaxial strain

Chirranjeevi Balaji Gopal,Matteo Monti,Robert Sinclair,Sang Chul Lee,Zixuan Guan,William C Chueh,Max García-Melchor,Hendrik Bluhm,Andrey Shavorskiy,Aleksandra Vojvodic,Yezhou Shi

doi:10.1038/ncomms15360

Abstract

Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion–electron conducting oxides. However, its effect on oxygen storage capacity is not well established. Here, we fabricate ultrathin, coherently strained films of CeO2-δ between 5.6% biaxial compression and 2.1% tension. In situ ambient pressure X-ray photoelectron spectroscopy reveals up to a fourfold enhancement in equilibrium oxygen storage capacity under both compression and tension. This non-monotonic variation with strain departs from the conventional wisdom based on a chemical expansion dominated behaviour. Through depth profiling, film thickness variations and a coupled photoemission–thermodynamic analysis of space-charge effects, we show that the enhanced reducibility is not dominated by interfacial effects. On the basis of ab initio calculations of oxygen vacancy formation incorporating defect interactions and vibrational contributions, we suggest that the non-monotonicity arises from the tetragonal distortion under large biaxial strain. These results may guide the rational engineering of multilayer and core–shell oxide nanomaterials.

Highlights

Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion–electron conducting oxides
Donner et al.[40] showed that strained La0.5Sr0.5CoO3–d thin films undergo cation ordering, facilitated by lower oxygen vacancy formation energy and enhanced cation mobility under biaxial strain. Another recent ex situ study of strained SrCoO3-d observed an increase in oxygen nonstoichiometry with tensile strain[41], though the non-equilibrium nature of the experiment convolutes the kinetics and thermodynamics of oxygen vacancy formation
Using depth profiled ambient pressure X-ray photoelectron spectroscopy (APXPS) and by varying the thickness of strained films, we show that the buried interface does not contribute significantly to the observed strain effects

Summary

Introduction

Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion–electron conducting oxides. The fourfold increase in the surface [Ce0Ce] of ceria films upon biaxial straining translates to a decrease in the oxygen vacancy formation energy by B0.4 eV per O (see Supplementary Note 4).

Results

Conclusion