Formation of Highly Branched Nanocubes through Multi-Stage Oriented Attachment of Pt Nanoparticles

Crystals grow through various pathways, such as monomer-by-monomer addition and attachment processes among ions, amorphous or crystalline particles, and clusters. Oriented attachment is a common path for crystal growth. The process of particle attachment is determined by the interplay between particles, i.e. interparticle forces, including vdW attraction, hydration repulsion, steric hindrance, and electrostatic interactions, which depend on the liquid-solid interfacial structures at particle surfaces. Herein, we investigate the formation process of branched Pt nanocubes via oriented aggregation of nanoparticles using a combination of cryo TEM, liquid-phase TEM, and simulations. Initially, primary nanoparticles of 2 to 3 nm in size are generated and then agglomerated to form a cluster. This cluster will further grow through nanoparticle and cluster aggregation. The nanoparticles within the cluster are spatially separated at early stage and continuously rearrange their alignment via oriented attachments, leading to clusters with core/shell structures. The inner particles in the cluster begin to attach on the (100) plane forming the cube-shaped core. At later growth stage, nanoparticles attach along the (111) plane, forming branched rods on top of the cube as the shell. The core/shell structure finally evolves into a branched cubic mesocrystal through the gradual ordering. Low pH and high chloride ions can induce changes in the surface structure and charge between clusters and nanoparticles, leading to the nanoparticles preferentially attaching along (100) at the early stage and (111) at the later stage. Simulations based on molecular dynamics and density functional theory will be also performed to examine the influence of HCl and formates on Pt (100) and (111) surfaces, elucidating their relationship with (100) and (111) attachments at different chloride ions.