Nanoscale Alloying, Phase-Segregation, and Core-Shell Evolution of Gold-Platinum Nanoparticles and Their Electrocatalytic Effect on Oxygen Reduction Reaction

The design of active and robust bimetallic nanoparticle catalysts requires the control of the nanoscale alloying and phase-segregation structures and the correlation between the nanoscale phase structures and the catalytic properties. Here we describe new findings of a detailed investigation of such nanoscale phase structures and their structure catalytic activity correlation for gold platinum nanoparticles prepared with controllable sizes and compositions. The nanoscale alloying and phase-segregation were probed as a function of composition, size, and thermal treatment conditions using X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, electrochemical characterization, and density functional theory modeling. The results have provided the experimental evidence in support of the theoretically simulated dependence of alloying and phase segregation on particle size and temperature. More importantly, new insights have been gained into the control of the nanoscale phase properties of this bimetallic system among alloyed, partially alloyed, or partially phase segregated structures. In contrast to the largely alloyed character for the catalysts treated at 300-400 degrees C, the higher-temperature treated catalysts (e.g., 800 degrees C) are shown to consist of a Pt-rich alloy core and a Au shell or a phase-segregated Au domains enriched on the surface. This conclusion is further supported by the electrochemical and electrocatalytic data revealing that the catalytic activity is highly dependent on the nanoscale evolution of alloying and phase segregation. The thermal control of the nanoscale alloying, phase-segregation, and core shell evolution of the nanoscale bimetallic catalysts provided the first example for establishing the correlation between the nanoscale phase structures and the electrocatalytic activity for oxygen reduction reaction correlation, which has profound implications to the design and nanoengineering of a wide variety of bimetallic or multimetallic nanostructures for advanced catalysts.