The properties of noble-metal nanocrystals have been traditionally tuned by tailoring parameters such as size, shape, and internal structure. Here, we consider tuning the phase of a metal crystal. Ru is inherently an hcp crystal - though fcc Ru can be grown. Experimentally, Ru nanocrystals (3-7 nm) were synthesized from Ru(acac)₃ salt and hcp Ru seeds using two different solvents: ethylene glycol (EG) and triethylene glycol (TEG). In EG, the Ru seeds continued to grow in the hcp phase, while with TEG solvent, the hcp Ru seeds grew an fcc shell of arbitrary thickness. To understand the origins of these trends, we used ab initio molecular dynamics (AIMD) and total energy calculations based on first-principles density functional theory (DFT). Inspired by experimental metal-organic chemical vapor deposition studies, which grow RuO₂ thin films from Ru(acac)₃, we assume that the large and unwieldy Ru salt decomposes to RuO₂ in solution. We then employ AIMD to study the interaction of RuO₂ with EG and TEG. In EG solution, RuO₂ was fully reduced to Ru atoms via donation of H atoms from EG. RuO₂ placed in TEG solution and simulated for the same duration as in EG remained as RuO₂. Our DFT total-energy calculations indicate Ru atoms in EG solution deposited on Ru(0001), prefer to bind to hcp sites, with no solvent influence, and would presumably grow an hcp crystal. In TEG solvent, RuO₂ is deposited onto the hcp Ru seed crystal. Our DFT total-energy calculations show that the Ru in RuO₂ prefers to bind to fcc sites of Ru(0001). We observed that TEG spontaneously loses its terminal protons (at zero K) to create adsorbed OH groups, which are then converted to water by partially oxidized solution-phase TEG species during structural optimization at zero K. A metastable fcc layer remains and subsequent calculations indicate this layer would continue to grow fcc - consistent with experiment. Overall, this study demonstrates the crucial role of reduction kinetics in phase-controlled synthesis of noble-metal nanocrystals and indicates potential extensions to other systems, where we envision the control of nanocrystal shape, internal structure, and phase.

Kristen Fichthorn

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