Effect of Crystal Growth on the Thermodynamic Stability and Oxygen Reduction Reaction Activity of Cu-Pt Nanoparticles.

Abstract

Thermodynamically stable (ordered) platinum-based bimetallic nanoparticle (NP) catalysts are auspicious candidates for catalyzing the oxygen reduction reaction (ORR) in fuel cells. Although the cubic (L12) and tetragonal (L10) ordered phases have been extensively studied, very little is known about the cubic (D7) thermally stable/ordered CuPt7 with regard to its synthesis at room temperature and ORR activity. The typical synthetic approach to the ordered phase (L12 and L10) has been by thermal annealing of the disordered phase in an inert atmosphere. We demonstrate that by coordinating Cu2+ and Pt4+ ions to amino groups in aqueous polyethyleneimine (PEI) (precursor solution), slow crystal growth by a UV-light assisted photoreduction can be used to achieve ordered CuPt7 NPs at room temperature. Slow crystal growth produces a relatively expanded lattice (7.766 Å) of CuPt7 and a lesser ORR activity via a four-electron transfer pathway. Conversely, fast crystal growth through a NaBH4 assisted chemical reduction produces a disordered CuPt phase at room temperature and a contracted lattice (3.809 Å) that enhances the ORR activity of CuPt via a two-electron transfer pathway. Our comparative observations of CuPt and CuPt7 support the observation that lattice contraction is critical in the ORR activity of Cu-Pt nanoalloys.