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

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.