Amorphous nickel boride membrane on a platinum–nickel alloy surface for enhanced oxygen reduction reaction

Results

Synthesis and characterization

The scheme summarizing the structural transformation of Pt–Ni alloy to Pt–Ni-based composite is illustrated in Fig. 1a. Figure 1b shows the transmission electron microscopy (TEM) image of PtNi3 octahedrons (12.5±1.5 nm) prepared by our previously reported method19. The hydrophilic surfactant of poly(vinylpyrrolidone) (PVP) ensured the favourable interaction of these monodispersed NPs with polar solvents, which is benefitial for the subsequent systematic investigation in aqueous solution. The parent PtNi3 polyhedrons were first dispersed in water and kept stirring for 5 min, followed by introducing a fresh sodium borohydride solution. After stirring for another 30 min, the as-prepared products were collected after washing and centrifugation. It was speculated that Pt–Ni alloy would undergo a chemical etching process, which was induced by the active BH4−. Figure 1c,d is the TEM and high-angle annular dark-field scanning transmission electron microscope images of the as-obtained hybrid Pt–Ni/Ni–B structures, respectively. The dealloyed Pt–Ni NPs were covered by an atomically thick membrane with lower contrast, accompanied by the transformation of the Pt–Ni octahedrons to concave octahedrons.

Figure 1: Scheme and TEM images of Ni–B membrane growing process.

(a) Scheme illustrating the structural transformation from PtNi3 octahedron to Pt-Ni/Ni–B composite. TEM images of (b) initial PtNi3 octahedrons and (c) Pt-Ni/Ni–B composite. (d) High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of Pt-Ni/Ni–B composite. Scale bars, 50 nm.