Designing Nanostructure Exsolution-Self-Assembly in a Complex Concentrated Oxide

Complex concentrated oxides (CCOs) are an emerging material class that includes high-entropy oxide (HEOs) and entropy-stabilized oxides (ESOs), whose unprecedented properties stem from disorder-induced distributions in electronic structure and chemistry caused by stabilizing many-cation (typically &gt; 5) solid solutions^1-3. Integrating these materials into composites with nanoscale tunability will enable tailored (multi)functionality beyond what is possible in a single phase^4-6. Here, we demonstrate a novel, highly extensible approach, <i>exsolution self-assembly</i> (ESA), to realize CCO-based nanocomposite thin films with intricate multi-element nanostructures. Using pulsed-laser deposition (PLD), we selectively reduce cations in a model perovskite CCO LaFe_0.7Ni_0.1Co_0.1Pd_0.05Ru_0.05O_3-δ, inducing defect-interaction-driven exsolution and simultaneous self-assembly of metal nanorods and metal-oxide core-shell nanoparticles, depending on oxygen partial pressure (P_O2). A correlated analysis using aberration-corrected scanning transmission electron microscopy (STEM) imaging, energy dispersive X-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS), geometric phase analysis (GPA) strain mapping, atom probe tomography (APT) with 3D mass spectrometry, and X-ray photoemission spectroscopy (XPS) was performed to characterize the ESA nanostructures and elucidate the nanostructure formation mechanisms underlying the highly tailorable synthesis approach.<br/><br/>With decreasing P_O2 from 3 mtorr, 0.15 mtorr, to 0.015 mtorr, concentration of oxygen vacancy increases, which tunes the extent of exsolution for different ESA nanostructures. At P_O2 of 3 mtorr, the LaFeO₃-based CCO thin film matrix shows uniform cation distribution. When P_O2 drops to 0.15 mtorr, ESA Pd nanorods grow from bottom of the thin film to top surface, with growth restricted by compressive stress exerted by the matrix in the in-plane direction and Pd availability in surroundings. When P_O2 further reduce by 10 times to 0.015 mtorr, Pd-Ni_xCo_1-xO metal-oxide core-shell nanoparticles embedded in the matrix form via seed growth effect triggered by growth of Pd followed by subsequent exsolution of Ni³⁺ and Co³⁺ in the CCO matrix. ESA is expected to synthesize complex and multi-dimensional nanostructures for electrochemical devices via integration of novel compositions and crystal structures of CCOs as well as PLD conditions.<br/><br/>References<br/><br/>1 Guo, H.<i> et al.</i> Designing nanostructure exsolution-self-assembly in a complex concentrated oxide. <i>In Revision</i> (2023). https://doi.org:http://dx.doi.org/10.2139/ssrn.4542882<br/>2 Guo, H., Wang, X., Dupuy, A. D., Schoenung, J. M. & Bowman, W. J. Growth of nanoporous high-entropy oxide thin films by pulsed laser deposition. <i>Journal of Materials Research</i> <b>37</b>, 124-135 (2022). https://doi.org:10.1557/s43578-021-00473-2<br/>3 Brahlek, M.<i> et al.</i> What is in a name: Defining “high entropy” oxides. <i>APL Materials</i> <b>10</b>, 110902 (2022). https://doi.org:10.1063/5.0122727<br/>4 Misra, S. & Wang, H. Review on the growth, properties and applications of self-assembled oxide–metal vertically aligned nanocomposite thin films—current and future perspectives. <i>Materials Horizons</i> <b>8</b>, 869-884 (2021). https://doi.org:10.1039/D0MH01111H<br/>5 Wang, J.<i> et al.</i> Exsolution Synthesis of Nanocomposite Perovskites with Tunable Electrical and Magnetic Properties. <i>Advanced Functional Materials</i> <b>32</b>, 2108005 (2022). https://doi.org:https://doi.org/10.1002/adfm.202108005<br/>6 Kawasaki, S.<i> et al.</i> Photoelectrochemical water splitting enhanced by self-assembled metal nanopillars embedded in an oxide semiconductor photoelectrode. <i>Nature Communications</i> <b>7</b>, 11818 (2016). https://doi.org:10.1038/ncomms11818