Hierarchical exsolution in vertically aligned heterostructures

Abstract

Metal nanoparticle exsolution from metal oxide hosts has recently garnered great attention to improve the performance of energy conversion and storage devices. In this study, the nickel exsolution mechanisms in a vertically aligned nanostructure (VAN) thin film of heteroepitaxial (Sr_0.9Pr_0.1)_0.9Ti_0.9Ni_0.1O_3-δ-Ce_0.9Gd_0.1O_1.95 with a columnar architecture was investigated for the first time. Experimental results and Density Functional Theory (DFT) calculations reveal that the multiple vertical interphases in a VAN with a hierarchical arrangement provide faster and more selective Ni diffusion pathways to the surface than traditional bulk diffusion in epitaxial films. Kinetic studies conducted at different temperatures and times indicate that the nucleation process of the exsolved metal nanoparticles primarily takes place at the surface through the phase boundaries of the columns. The vertical strain is crucial in preserving the film's microstructure, yielding a robust heteroepitaxial architecture after reduction. This innovative heteromaterial opens up new possibilities for designing efficient devices through advanced structural engineering to achieve controlled nanoparticle formation.

Fig. 1. Microstructure characterisation of vertically aligned nanostructure (VAN) films.

a Cross-sectional HAADF-STEM image of the as-deposited VAN film and (b) EDS mapping revealing the column composition. c HAADF-STEM image of the VAN after the exsolution process in reducing conditions at 650 °C for 1 min and (d) EDS mapping showing the presence of Ni nanoparticles. e Schematic representation of the heteroepitaxial growth and atomic arrangement of (f) CGO and (g) SPTNO phases on STO (001). h Electrical conductivity of epitaxial SPTNO and VAN in air and 5% H₂-Ar at different temperatures. The inset in (h) shows an illustration of the setup employed for the electrical measurements by Van der Pauw method.

Fig. 2. Nucleation process in vertically aligned nanostructures (VAN) at different temperatures and times.

SEM images of the VAN reduced at different temperatures for 6 h, a 550 °C, b 600 °C and c 650 °C. Red arrows show the preferred orientation for nucleation. White arrows show the secondary phase merging into the Ni particle. SEM images of the VAN reduced at 650 °C during different times, d 1 min, e 1 h and f 12 h. g Low-magnification SEM image showing the homogeneous nanoparticle distribution. Population density and particle size distribution as a function of the (h) reduction time (i) and temperature.

Fig. 3. Comparison between the Ni exsolution process and particle size in traditional epitaxial films and vertically aligned nanostructures (VAN).

Top-view SEM images at different magnifications of a SPTNO and b VAN after reduction at 650 °C for 12 h. Orange arrows show the presence of trapped Ni nanoparticle within the bulk film. Cross-sectional HAADF-STEM images and EDS mappings of (c, d) SPTNO and (e, f) VAN. Illustrations of the nanoparticle nucleation processes under reducing conditions in (g) epitaxial films, where Ni nanoparticles are trapped within the film, and (h) VAN, which promotes fast Ni migration towards the surface.

Fig. 4. DFT calculation for Ni migration within the bulk and interface.

Schematics of Ni migration pathways across the a perovskite bulk and b perovskite-fluorite interface. The green, light blue, pink and yellow spheres denote Sr, Ti, Ni and Ce cations, respectively. The orange spheres denote O ions. The pink arrows show the Ni migration pathways in both structures. c Calculated energy barriers for Ni migration for both scenarios, showing a lower migration energy for the diffusion through the interface.