Bimetallic nanoparticles (Pt/Co) have been proposed as promising alternative to pure Pt catalyst in the fuel cells. The composition distribution and lattice strain may be the two main factors for the enhanced catalytic activity, but measuring these within a catalytic nanoparticle presents a severe characterization challenge. Here we have used the atomic-resolution annular dark-field imaging (ADF) in the scanning transmission electron microscope (STEM) for the strain measurement and the simultaneous spectroscopy (EELS and EDS) to analyze the composition distribution. The composition variation, size and strain are then related together, and can be used as inputs to theoretical calculations to provide explanations for the enhanced catalytic activity.
