One-step Polyol Synthesis and Langmuir−Blodgett Monolayer Formation of Size-tunable Monodisperse Rhodium Nanocrystals with Catalytically Active (111) Surface Structures

Size-tunable monodisperse Rh nanocrystals can offer unique properties for many heterogeneous catalytic reactions (such as hydrogenation, hydroformylation, and hydrocarbonylation) of both scientific and technological interest. In this article, we report the synthesis of monodisperse, well-shaped Rh nanocrystals in a range of 5−15 nm by a one-step polyol reduction at temperatures of 170−230 °C under Ar, using rhodium(III) acetylacetonate [Rh(acac)3] as the source of metal ions, 1,4-butanediol as the reducing solvent, and poly(vinylpyrrolidone) as the capping agent. Two-dimensional projects of the nanocrystals are polygons, dominated by hexagons, pentagons, and triangles with catalytically active (111) surfaces (>65% yield). Over 45% of the polygons are multiple (111) twinned particles (hexagons and pentagons), favored by thermodynamics. To achieve size uniformity, adjustment of the reduction kinetics of Rh(acac)3 in the nucleation and crystal growth stages has been shown to depend upon several synthetic parameters including an Ar or air atmosphere, reaction temperature and time, and Rh(acac)3 concentration. Due to the present well-controlled polyol reduction kinetics, the size of the Rh nanocrystals can be tuned by changing the Rh(acac)3 concentration in a proper range. Monolayer films of the Rh polygons have been formed on silicon wafers by the Langmuir−Blodgett method and have been used as model heterogeneous catalysts for the study of ethylene hydrogenation.