Electron Tomography Analysis on the Structure and Chemical Composition of Nanoporous Metal Surfaces

Nanoporous metals have a porous structure with bicontinuous nanoscale voids and ligaments. Thus, nanoporous metals differ from their bulk counterparts in mechanical, physical, and chemical characteristics due to their unique ligament structure and high surface-to-volume ratio. The surface structure and chemistry of nanoporous metals play critical roles in their applications in catalysis, sensing, and other fields. The surfaces of nanoporous metals contain a substantial number of low-coordination sites, which are vital for improving their catalytic performance. Moreover, the addition of platinum to nanoporous gold has a massive impact on its catalytic and mechanical characteristics. High-resolution transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (STEM) are commonly used to study the atomic structure of crystals. However, since these techniques only provides two-dimensional projection images, it is usually hard or even impossible to directly and quantitatively resolve the three-dimensional (3D) structure of nanocrystals, especially their surface crystallography and coordination information. Compared to traditional TEM and STEM imaging technologies, electron tomography with atomic resolution provides a powerful means to resolve 3D atomic-resolution information of materials. In this work, the surface structure and chemical composition of nanoporous gold and nanoporous gold-platinum were analyzed using STEM, electron tomography, and three-dimensional reconstruction of energy dispersive spectroscopy (EDS) results. The atomic structure of the ligament surface was examined using electron tomography with atomic resolution. It was observed that, surface defects can be separated into two categories: kinks and steps on the {111} terrace, and dents and pits. Surface dents and pits introduce a greater number of low-coordination sites than kinks and steps. Furthermore, the segregation of Pt on the ligament surface was discovered by combining the atomic-resolution electron tomography with the 3D reconstruction of EDS results.