Shape Changes in AuPd Alloy Nanoparticles Controlled by Anisotropic Surface Stress Relaxation

Area of Science:

• Materials Science
• Nanotechnology
• Surface Science

Background:

• Nanoparticle shape significantly influences their properties and applications.
• Surface stress is a key factor in determining nanoparticle morphology.
• Alloying elements can be used to engineer nanoparticle structures.

Purpose of the Study:

• To investigate the effect of gold (Au) content on the shape of palladium-rich (Pd) nanoparticles.
• To understand the role of surface stress relaxation in shape transitions.
• To determine the equilibrium structures of AuPd nanoparticles.

Main Methods:

• Gas-phase synthesis of AuPd nanoparticles with varying compositions.
• Atomic-resolution characterization using High-Resolution Transmission Electron Microscopy (HRTEM)/Scanning Transmission Electron Microscopy (STEM) and Energy-Dispersive X-ray spectroscopy (EDX).
• Computational simulations including global optimization searches, free energy calculations, and atomic stress calculations.

Main Results:

• Pure Pd nanoparticles predominantly form FCC truncated octahedra (TO).
• Increasing Au content leads to a transition to icosahedral (Ih) structures, with Au atoms segregating to the surface.
• The TO to Ih transition is sharper under near-equilibrium growth conditions.
• Simulations confirm Ih as the equilibrium structure for higher Au content.
• Atomic stress calculations reveal that icosahedral structures better relax anisotropic surface stress, driving the shape change.

Conclusions:

• The shape of AuPd nanoparticles is controllable by tuning the Au content.
• Surface stress relaxation is the primary mechanism driving the transition from TO to Ih structures.
• Icosahedral structures are favored at higher Au concentrations due to superior surface stress relaxation.