Strain Quantification in Au-Pd Core-Shell Nanorods

Bimetallic core-shell catalysts represent a new pathway to create highly selective and highly active catalysts. This can be done by using a relatively inactive metal as the core material and a more active metal as the shell material. The composition of both the core and shell structure can then be altered in order to tune the selectivity of the nanocatalyst. The synergistic effects of using bimetallic core-shell catalysts arise in part from the misfit strain that is encountered as a result of the difference in lattice spacings between the core and shell materials. The catalysts investigated here consist of an Au core and a Pd shell. Particles with four different Pd shell thicknesses were synthesized and the corresponding strain was measured. There is a 5.07% difference in the lattice spacings between Au and Pd, we therefore expect strain values to be near this amount. In this work, we directly measured the displacement fields that arise due to lattice mismatch in Au-Pd nanorods using High Resolution Scanning Transmission Electron Microscopy (HRSTEM) and 4D Scanning Transmission Electron Microscopy (4D-STEM). The strain was then calculated using three different analytical methods: Geometric Phase Analysis (GPA), Gaussian Peak Fitting, and nanodiffraction. These methods all measure the variations in local lattice parameters and plot these values for every pixel in the original STEM image, this results in a 2D strain map. These maps were then compared to see which produced the highest quality strain quantification