Effect of Synthesis Conditions on the Physical and Electrocatalytic Properties of Ru@Pt Nanoparticles

A first-time comparison of carbon-supported Ru@Pt core@shell nanoparticles (NPs) with varying Pt shell coverages (0–3 monolayers, ML), synthesized by two different methods, was carried out. This includes the polyol method (“Method I”), where a suspension of NPs (plus stabilizer) is exposed to vulcan carbon (VC) powder particles to attach them to the surface, as well as a “stabilizer-free” method (“Method II”), involving the reduction (using ethanol) and direct nucleation of Ru core NPs on the carbon surface, followed by thermal annealing and then Pt shell deposition. It is shown that for both Methods I and II, the Ru/Pt ratios in the NPs could be controlled by the ratio of metal precursors in the synthesis solution. NPs produced using either Method I or Method II exhibited Ru@Pt core@shell structures, with a controllable Pt shell coverage, as confirmed by scanning transmission electron microscopy and electron energy loss spectroscopy analyses. NPs produced by Method I were smaller (3–4 nm), more spherical in shape, contained a residual stabilizer, and were more agglomerated than Method II NPs (4–6 nm). X-ray diffraction analysis showed that Method I NPs were also more amorphous, while the thermal annealing step in Method II was critical to obtaining highly crystalline Ru cores when compared to their nonannealed analogues. By tuning the composition, size, and Pt shell coverage of both types of Ru@Pt NPs, it was shown that Method II NPs with annealed Ru cores were more electrocatalytically active than their nonannealed analogues and Method I NPs.