Phase-Controlled Ruthenium Nanocrystals on Colloidal Polydopamine Supports and Their Catalytic Behaviors in Aerobic Oxidation Reactions

The past decade has witnessed rapidly growing interestin noblemetal nanostructures adopting unconventional metastable crystal phases.In the case of Ru, chemically synthesized nanocrystals typically form thermodynamically favored hexagonal close-packed (hcp) crystal lattices,whereas it remains significantly more challenging to synthesize Runanocrystals in the metastable face-centered cubic (fcc) phase. Inthis work, we have synthesized polydopamine (PDA)-supported hcp andfcc Ru nanocrystals in a phase-selective manner through one-pot thermalreduction of appropriate Ru(III) precursors in a polyol solvent. Benefitingfrom the unique surface-adhesion function of PDA, we have been ableto grow phase-controlled sub-5 nm Ru nanocrystals directly on colloidalPDA supports without prefunctionalizing the particle surfaces withany molecular linkers or surface-capping ligands. Success in phase-controlledsynthesis of capping ligand-free Ru nanocrystals dispersed on thesame support material enables us to systematically compare the intrinsicmass-specific and surface-specific activities of fcc and hcp Ru nanocatalyststoward the aerobic oxidation of a chromogenic molecular substrate,3,3 & PRIME;,5,5 & PRIME;-tetramethylbenzidine (TMB), under a broadrange of reaction conditions. We use UV-vis absorption spectroscopyto monitor the conversion of the reactant molecules into the one-electronand two-electron oxidation products in real time during Ru-catalyzedoxidation of TMB, which is found to be a mechanistically complex molecule-transformingprocess involving multiple elementary steps. The apparent reactionrates and detailed kinetic features are observed to be not only intimatelyrelated to the crystalline structures of the Ru nanocatalysts butalso profoundly influenced by several other critical factors, suchas the pH of the reaction medium, the initial concentration of TMB,Ru coverage on the PDA supports, and degree of nanoparticle aggregation.