Tuning Point Defects by Elastic Strain Modulates Nanoparticle Exsolution on Perovskite Oxides

Abstract

Exsolution generates stable and catalytically active metal nanoparticles via phase precipitation out of a host oxide. An ability to control the size and dispersion of the exsolution particles is desirable for design of nanostructured (electro)catalysts. Here, we demonstrate that tuning point defects by lattice strain affects both the thermodynamics and the kinetics of iron (Fe 0 ) exsolution on La 0.6 Sr 0.4 FeO 3 (LSF) thin film model. By combining in situ surface characterization and ab initio defect modeling, we show oxygen vacancy and Schottky defects to be the primary point defects formed upon Fe 0 exsolution. Lattice strain tunes the formation energy, and thus the abundance of these defects, and alters the amount and size of the resulting exsolution particles. In addition, we find that the density of exsolved nanoparticles matches the concentration of oxygen vacancy pairs, thus pointing to the surface oxygen vacancy pairs as preferential nucleation sites for exsolution. The tensile-strained LSF with a facile formation of these critical point defects results in a higher Fe 0 metal concentration, a larger density of nanoparticles, and a reduced particle size at its surfaces. These results provide important mechanistic insights and highlight the role of point-defect engineering in designing nanostructured catalysts in energy and fuel conversion technologies.

Exsolution Synthesis of Nanocomposite Perovskites with Tunable Electrical and Magnetic Properties

“…[ 43 ] Moreover, thin‐film LSF has also been employed as model systems to investigate surface exsolution in previous studies. [ 30,37,44 ] We showed that exsolution in such oxide thin films generates not only metal nanoparticles (as in prior work), but also more complex nanostructures with implications for the thin‐film properties; including crystalline Ruddlesden‐Popper (RP) domains and percolated Fe‐deficient nm‐scale channels. These phenomena, to the best of our knowledge, are reported for the first time in LSF.…”

“…During the heating step, the normalized O 1s intensity gradually decreased (Figure S15, Supporting Information), which indicates oxygen vacancy formation (i.e., lattice reduction). [ 37 ] The Fe 0 species appeared on the surface at 450 °C, which marked the onset of Fe 0 exsolution. Upon further heating in 0.5 Torr H 2 , the surface concentrations of Fe 0 and O* increased synergistically with temperature, indicating an increasing extent of Fe 0 exsolution in the LSF film.…”

“…[27,28] Herein, we propose that exsolution can provide new opportunities in synthesizing functional nanocomposite oxides with customizable structures and functionalities. First, since exsolution is a partial decomposition process, it can generate multiple phases [34] and different types of lattice defects [35][36][37] in the host oxides. Such defects are paramount for a wide range of electrical, optical, and magnetic properties, [38][39][40] which provides a tuning knob to tailor materials' functionality by controlling the extent of exsolution.…”

“…This task, however, is not trivial. In our previous study, [ 37 ] we have shown that the surface exsolved metal nanoparticle can be quickly re‐oxidized into metal oxides upon air exposure. Moreover, we have shown that the nanoparticle exsolution in thin‐film samples would not induce noticeable changes in the ex situ XRD diffraction.…”

“…Moreover, we have shown that the nanoparticle exsolution in thin‐film samples would not induce noticeable changes in the ex situ XRD diffraction. [ 37 ] Therefore, it is very challenging, if not impossible, to accurately measure the exsolution states and the corresponding materials properties’ in the exsolved nanocomposites using ex situ characterizations. To address this challenge, here we employed state‐of‐the‐art near ambient pressure X‐ray photoelectron spectroscopy (NAP‐XPS) measurements to synthesize and characterize exsolved nanocomposite at different exsolution extents and at different stages of re‐oxidation.…”