Epitaxial strain-tuned oxygen vacancy formation, reduction behavior, and electronic structure of perovskite $\mathrm{SrIr}{\mathrm{O}}_3$

The formation and study of oxygen vacancies are critical for the development of enhanced functional oxides; in the oxygen evolution reaction (OER), oxygen vacancies are proposed to influence the activity and degradation of electrocatalysts. We use thin films of state-of-the-art OER catalyst $\mathrm{SrIr}{\mathrm{O}}_3$ deposited on crystal substrates with varied lattice parameters to demonstrate the effect of epitaxial strain on oxygen vacancy formation. Through in situ x-ray diffraction under elevated temperatures and reducing conditions of 3% ${\mathrm{H}}_2$/balance ${\mathrm{N}}_2$, we show that tensile epitaxial strain makes oxygen vacancy formation more favorable, whereas compressive epitaxial strain has no significant effect compared with an unstrained film. We further use in situ and ex situ x-ray absorption spectroscopy to reveal the effect of strain on the favorability of full reduction of Ir species within $\mathrm{SrIr}{\mathrm{O}}_3$ films from $\mathrm{I}{\mathrm{r}}^{4+}$ to $\mathrm{I}{\mathrm{r}}^0$ and on the energy levels of unoccupied electronic states in the out-of-plane direction, respectively. This study adds experimental evidence for the link between strain and oxygen vacancy formation in $5d$ thin film perovskites, for which the discussion has been dominated by theory-based approaches.

Figure 1

Characterization of as-deposited SIO thin films. (a) High-resolution XRD data and kinematical diffraction model for extracting lattice parameter and thickness from SIO films on STO, GSO, and KTO substrates. (b) AFM of SIO|STO film displaying atomic terraces with minimal surface particle formation.

Figure 2

In situ XRD of strained SIO thin films. (a) Temperature series scans of (002) diffraction peaks for SIO film and STO substrate under 100 SCCM 3% ${\mathrm{H}}_2$/balance ${\mathrm{N}}_2$. (b) Calculated out-of-plane elongation of $c_{\mathrm{film}}$ vs temperature as compared with $c_{\mathrm{film}}$ from $30{}^{\circ }\mathrm{C}$ for strained SIO films.

Figure 3

Calculated $c_{\mathrm{film}}$ elongation from in situ XRD measurements under gas environments: (a) 100 SCCM 3% ${\mathrm{H}}_2$/balance ${\mathrm{N}}_2$ until chemical expansion occurs, then 100 SCCM ${\mathrm{O}}_2$ during cooling, and (b) medium vacuum ($\le 0.5$ mbar).

Figure 4

In situ XANES at the Ir $L_2$ edge. (a) Absorption spectra of SIO|STO film heated under 100 SCCM 3% ${\mathrm{H}}_2$/balance He. The spectra are self-absorption corrected and free of nonlinear distortion. (b) Absorption spectra for Ir metal and ${\mathrm{IrO}}_2$ reference powders (solid lines) and calculated weighted sums of these signals to demonstrate convolution concept (dashed lines). (c) Calculated $\mathrm{I}{\mathrm{r}}^{4+}$ fraction vs temperature from linear combination fitting of XANES spectra for strained SIO films. Results from duplicate scans are represented with transparent points. Data points are fit to sigmoid functions displayed with dashed lines.

Figure 5

Total electron yield oxygen $K$-edge XAS. (a) Comparison of strained SIO films collected at ${20}^{\circ }$ incident beam angle. (b) Comparisons of spectra from ${20}^{\circ }$ and ${70}^{\circ }$ incident beam angles for SIO films on STO, GSO, and KTO, with difference spectra (in black) magnified by 2× for visual aid.