ABSTRACT
Herein, we present an atomic in-situ investigation of Cu oxidation along different orientations stimulated by high-energy electron beams (E-Beam) in transmission electron microscopy (TEM). By following the microstructural evolution of the Cu substrate in real time, high-resolution TEM (HRTEM) images reveal an orientation-dependent oxidation mechanism, whereby Cu along [110] zone axis migrates onto the surface and be oxidized while Cu along [100] zone axis is oxidized completely both in bulk and at the surface. The different oxidation mechanisms can be attributed to the differing diffusion rates of oxygen in Cu structures along directions. Moreover, the growth of Cu oxides is found to follow a layer-by-layer mechanism, where Cu mostly migrates onto nanocrystal {110} planes. This behavior would lead to the oxides wider in geometric shape and therefore promote the aggregation of adjacent oxides. These findings have important implications for the practical use of copper-based materials in oxidizing environments.
ABSTRACT
Probing porosity evolution is essential to understand the degradation mechanism of electrocatalytic activity. However, spatially dependent degradation pathways for porous catalysts remain elusive. Here, we reveal the multiple degradation behaviors of individual PtCu3 nanocatalysts spatially by three-dimensional (3D) electron tomography. We demonstrate that the surface area-volume ratio (SVR) of cycled porous particles decreases linearly rather than reciprocally with particle size. Additionally, an improved SVR (about 3-fold enhancement) results in increased oxygen reduction reaction (ORR) efficiency at the early stage. However, in the subsequent cycles, the degradation of catalytic activity is due to the excessive growth of pores, the reduction of reaction sites, and the chemical segregation of Cu atoms. The spatial porosity evolution model of nanocatalysts is applicable for a wide range of catalytic reactions, providing a critical insight into the degradation of catalyst activity.
