Aqueous room-temperature synthesis of Au-Rh, Au-Pt, Pt-Rh, and Pd-Rh alloy nanoparticles: Fully tunable compositions within the miscibility gaps

Many binary late transition metal systems have large bulk miscibility gaps, and a variety of synthetic strategies have been developed to generate these non-equilibrium alloys as nanoparticles. While many of these methods strive to co-nucleate both elements by exploiting fast reduction kinetics or co-sequestration within a confined space, we show here that simple room-temperature borohydride co-reduction of appropriate aqueous metal salt solutions yields alloy nanoparticles in the bulk-immiscible Au-Rh, Au-Pt, Pt-Rh, and Pd-Rh systems. The compositions can be tuned across the entire Au _1-xRh_x, Au_1-xPt_x, Pt _1-xRh_x, and Pd_1-xRh_x solid solutions by varying the ratio of metal salt reagents, and they form in the presence of a variety of molecular and polymeric surface stabilizers. Reaction pathway studies on the model Au-Rh system suggest that the alloy nanoparticles form via a "conversion chemistry" mechanism: Au nanoparticle templates nucleate first, followed by diffusion of Rh to form homogeneous Au-Rh alloy nanoparticles. The alloy nanoparticles tend to be agglomerated, but this can be minimized by forming the nanoparticles directly on catalytically relevant high surface area carbon and biological supports, e.g. Vulcan carbon and wild-type M13 bacteriophage.