Effect of Surface Microstructure and Chemistry on Nanoparticle Exsolution in Perovskite Oxides—Optimizing Nanocatalyst Design

Catalysts are crucial in advancing energy conversion and storage systems, particularly in electrochemical reactions. A recent advance in this field has been the synthesis of self-assembled metal nanoparticles through a process known as “exsolution.” Exsolution involves the in-situ growth of metal nanoparticles anchored to a host oxide, with perovskite oxides typically serving as the parent oxide in this process. Perovskites are important as they provide electron and ion conductivity, which is key in energy-related applications like solid-oxide fuel and electrolysis cells (SOFC/SOEC). [1] To fundamentally understand exsolution mechanisms, epitaxial single-crystal films are used to eliminate the effects of polycrystalline structures and grain boundaries. [2] However, comparisons between thin films and bulk (e.g., pellet) samples are not always one-to-one, primarily because differences in stoichiometry influence exsolution.[3] Additionally, perovskite surfaces are central in the electrochemical reactions in SOFC/SOECs. [4] Therefore, it is important to thoroughly analyze and understand the structure and chemistry of these surfaces and their effect on exsolution to control nanoparticle formation. The detailed mechanisms underlying exsolution at the surface are not yet fully understood, and there is limited knowledge about the surface structure, which hinders the controlled development and design of these nanocatalysts. [1,5]<br/><br/>In this work, we use advanced surface-sensitive characterization tools to investigate how surface microstructure and chemistry influence the formation of nickel (Ni) nanoparticles from the exsolution of La_0.5Sr_0.5Ti_0.94Ni_0.06O₃ (LSTN) perovskite thin films. We analyze the microstructure of near-single-crystal films on a single-crystal strontium-tin oxide (STO) substrate and polycrystalline films on yttrium-doped zirconium oxide (YSZ) and silicon (Si) substrates, separately. By using grazing-incidence wide-angle scattering (GIWAXS) we analyze the crystal structure at the top surface with a penetration depth of approximately 10 nm. We observe that polycrystalline films on YSZ, which exhibit a preferred orientation of the (110) out-of-plane and high crystallinity, lead to an increased density and surface area of exsolved nanoparticles. In contrast, polycrystalline LSTN on Si shows lower crystallinity and higher disorder, resulting in significantly less exsolved Ni from the LSTN. We hypothesize that a high degree of preferred orientation and crystallinity facilitates cation diffusion, resulting in more Ni^0 nanoparticle formation. Further, we use grazing-incidence small-angle X-ray scattering (GISAXS) to observe and calculate the size of the exsolved nanoparticles. To study and quantify the surface chemistry, we employ in-situ near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). By analyzing the Ni 3p peak in XPS, we track the in-situ reduction of Ni from Ni^+2 into Ni^0. Establishing correlations between surface chemistry and nanoparticle properties is crucial for controlling exsolution surfaces tailored for specific electrochemical reactions in SOFC/SOECs.<br/><br/>[1] Neagu, D. <i>et al.</i> <i>J. Phys. Energy</i> 5, 031501 (2023)<br/>[2] Weber, M. et al. <i>Nat. Materials</i> 23, 406-412 (2024)<br/>[3] Neagu, D. <i>et al. Nat. Chemistry </i>5, 916-923 (2013)<br/>[4] Kubicek, M. <i>et al. J. Mater. Chem. A. </i>5, 119-12000 (2017)<br/>[5] Kersell, H., Nemsak, S. et al. <i>Faraday Discuss. </i>236, 141-156 (2022)