Nanoscale understanding and control of metal exsolution in perovskite oxides

Weber, Moritz Lukas; Guillon, Olivier (Thesis advisor); Dittmann, Regina (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

The design of active and durable catalysts is a key requirement for the development of efficient energy conversion technologies urgently needed to meet the challenges of global warming. Metal exsolution has attracted much attention as an elegant synthesis route for nanostructured perovskite catalysts. Thermal reduction of the parent oxide induces the release of reducible cations from the host phase, that assemble in the form of metallic nanoparticles at the perovskite surface. To date the atomistic processes that govern the exsolution behavior in perovskites are inadequately understood. Here, the compositional variety and structural complexity of exsolution-active parent materials often complicate the systematic investigation and comparison of the exsolution response. This thesis investigates the exsolution of nickel nanoparticles in Nb- and Ni- co-doped strontium titanate on the basis of well-defined epitaxial thin film model systems. The main issues addressed are the principles underlying the accommodation of Ni dopants within the perovskite host lattice, the influence of defects and surface reconstructions on the exsolution behavior and most importantly, the dynamics of metal exsolution processes and the role of surface space charge regions for the exsolution kinetics. A comprehensive study of the synthesis and structural analysis of ceramic oxides and epitaxial thin films forms the basis of this work. The characterization of the exsolution behavior is based on the detailed analysis of the surface morphology evolution upon thermal annealing at low oxygen partial pressure, combined with methodologies for the control of the sample defect structure, surface chemistry and sample geometry. Furthermore, in-situ diffraction and in-situ spectroscopy techniques have been employed to study and disentangle the bulk and surface material response of the perovskite during metal exsolution. Collectively, the combined chemical, structural and morphological investigations under reducing conditions reveal strong surface limitations of metal exsolution. The surface and bulk properties of the material response shows widely different dynamics and appear to be mostly uncorrelated. In accordance, the exsolution volume, i.e. the volume of the self-assembled surface nanoparticles is restricted to a small fraction of the total amount of Ni present in the perovskite bulk. In accordance surface properties were found to govern the exsolution kinetics. In this context, space charge regions at the perovskite surface have emerged to play a key role for the process. The formation of space charge regions was probed under oxidizing and reducing conditions by in-situ spectroscopy, and found to be interrelated with the exsolution dynamics. Based on the observations, a novel model of the exsolution process as well as strategies for the control of the metal exsolution behavior by surface engineering is presented. Achieving control over the surface properties of perovskites is pivotal for the rational design of high-performance energy materials, where the concept of metal exsolution opens novel possibilities for the generation of catalytic centers of high stability.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Synthesis in Energy Technology [524510]