Origin for the chloride and citrate ions tuned morphology of Pd particles

Abstract

Capping agents such as halide and citrate (CA) are crucial in the colloidal synthesis of noble metal nanoparticles (NPs) with desirable shape for specific catalytic reaction. Halide ions have been shown to stabilize the cubic NP while CA ions favor the octahedral NP regardless of the noble metals, but the mechanism remains unclear. We combine DFT calculations and Wulff construction to understand how the morphology of Pd particle changes with Cl and CA chemical potential (μ). Three main factors are identified to affect the morphology of Pd particle: μ of adsorbate, its number per unit area (NAS/S), and rigidity or flexibility. Increasing μ of adsorbate or its NAS/S causes decreased surface free energy and increased exposed surface area. Moreover, rigid adsorbates (PdClx) occupy a larger surface area for surfaces with high atomic density such as Pd(111), leading to cubic Pd particle at high μCl. Conversely, flexible adsorbates (CA) that can adjust their structure based on the surface occupy smaller surface area on Pd(111), favoring octahedral Pd particle. This work reveals the origin for capping agent tuned morphology of Pd NP, and the insights can be used to guide metal NP synthesis with desired morphology for catalytic reactions of technological interest.