The Role of Surface Potential in Formation of Branched Cubic Pt Mesocrystals by Oriented Attachment

Crystals grow through a variety of pathways, such as monomer-by-monomer addition or attachment of higher species, including amorphous or crystalline particles and ionic or molecular clusters. Oriented attachment (OA) is a common pathway of crystal growth in which primary nanoparticles become crystallographically aligned before they make contact. The process of particle approach and attachment is determined by the interplay between interparticle forces, including vdW attraction, repulsion associated with the liquid-solid interfacial structures at particle surfaces, steric hindrance due to ligand-ligand interactions, and electrostatic repulsion between like-charged particle surfaces. The latter depends strongly on the surface potential, which can be affected by changes in the solvent composition and pH as the system evolves through repeated attachment events that eliminate particle surface area. In this study, we explore the formation of branched cubic Pt mesocrystals via the oriented aggregation of nanoparticles, employing cryo TEM, liquid-phase TEM, and DFT calculations. We find that the mesocrystals form via aggregation of particles into a disorganized cluster and subsequent OA events. The nanoparticles in the cluster are initially spatially separated and become oriented before attachment. The inner particles in the cluster start attaching on (100) faces, forming a cube-shaped core. As time progresses, nanoparticle attachment switches to occurring on the (111) faces, causing branched rods to grow on the faces of the cubes. Combining measurements of facet-specific zeta potential as a function of total particle surface area with DFT calculations, we demonstrate that the direction of OA in this system is altered from [100] to [111] due to the competitive adsorption of chloride ions and formate on Pt surfaces, which evolves as the total Pt surface area decreases through the OA process, altering the resulting electrostatic forces. The insights gained from this work have the potential to enable predictive control over the morphology of crystals formed through OA processes in order to produce tailored materials’ properties.