(715a) A Theoretical Study of the Origins of Ru Nanocrystal Phase Control Directed By Solvent

The properties of noble-metal nanocrystals for specific applications have been traditionally tuned by tailoring parameters such as size, shape, and internal structure (stacking faults or twins). Exploring the polymorphism of nanocrystals, specifically controlling their phases, represents a nascent research area with many mechanistic details yet to be elucidated. We studied the mechanistic origins of phase-controlled synthesis of Ru nanocrystals, which can affect the activity and selectivity of Ru-catalyzed reactions. In experiments, Ru nanocrystals were synthesized from Ru(acac)₃ salt in two different polyol solvents, beginning with hexagonal close packed (hcp) seeds. In ethylene glycol (EG) the seeds continued to grow to hcp crystals and in triethylene glycol (TEG), a face-centered cubic (fcc) shell grew on top of the hcp seeds. We determined origins of these differences theoretically using Ab Initio Molecular Dynamics (AIMD) and Density Functional Theory (DFT) total energy calculations. Based on our theoretical findings, the origin of the solvent-dependence of crystal phase is the reduction kinetics, which occur in two different ways in the two different solvents. We studied the reduction of RuO₂, a likely intermediate in the decomposition of Ru(acac)₃ to Ru. We find that RuO₂ is completely reduced to Ru atoms in EG solution and that Ru atoms preferentially bind to hcp sites on Ru(0001). In TEG solution, RuO₂ remains intact and is preferentially deposited onto fcc sites on Ru(0001). Adsorbed RuO₂ is subsequently reduced by TEG to generate fcc-Ru layers. Our study gives insights to understand underlying mechanisms of reduction kinetics supported by solvent in metal nanocrystal synthesis.