Ruthenium nanocrystals with a face-centered cubic structure and well-controlled facets

Abstract

Ruthenium (Ru) is an intriguing catalytic material for a variety of reactions. However, its extremely low abundance in the earth crust and ever-increasing price have created a barrier to the large-scale use of this metal. One solution to this issue is to engineer the shape or surface structure, as well as the crystal phase of Ru nanocrystals, in an attempt to optimize their catalytic properties. This dissertation is focused on the development of synthetic strategies for the facile synthesis of Ru nanocrystals with a face-centered cubic (fcc) phase and well-defined surface structures. Based on the integration of seed-mediated growth and wet chemical etching, I was were able to fabricate Ru nanocages with a cubic, octahedral, or icosahedral shape. The as-synthesized nanocages were characterized by ultrathin and porous walls (< 1.2 nm) as well as well-controlled surface structures. Most interestingly, the Ru nanocages adopted an fcc phase rather than the conventional hexagonal close-packed (hcp) structure typical of bulk Ru. Both the fcc phase and surface structures of the nanocages could be well preserved up to 300 °C. The facet-dependent properties of the fcc-Ru nanocages were evaluated toward the reduction of 4-nitrophenol and hydrazine decomposition. Density functional theory calculations suggested the enhanced catalytic performance of the fcc-Ru nanocages toward N2 dissociation for ammonia synthesis relative to hcp-Ru nanoparticles. I also developed a hydrothermal approach to the synthesis of fcc-Ru octahedra with enhanced thermal stability using Rh nanocubes as the seeds. In particular, the fcc-Ru octahedra could retain both the crystal phase and the octahedral shape up to 400 °C. When benchmarked against the hcp-Ru nanoparticles, the octahedral nanocrystals exhibited enhancement in terms of specific activity toward oxygen evolution. In the last project, I developed a facile method for the synthesis of fcc-Ru cuboctahedral nanoframes via the galvanic replacement between Pd seeds and a Ru(III) precursor. The nanoframes could largely retain the fcc phase and frame structure up to 350 °C. When used as catalysts for hydrazine decomposition, both the activity and selectivity of the fcc-Ru nanoframes were substantially improved relative to hcp-Ru nanoparticles.