In Situ Reduction, Oxygen Etching, and Reduction Using Formic Acid: An Effective Strategy for Controllable Growth of Monodisperse Palladium Nanoparticles on Graphene

Graphene-supported, monodisperse palladium nanoparticles (Pd NPs) are fabricated successfully by an effective strategy combining in situ reduction, O2 etching, and reduction using formic acid without the use of surfactants. By utilizing the reductive ability of graphene prepared previously by an in situ, self-generating template route, Pd2+ ions are first absorbed and partly in situ reduced to give Pd NPs on graphene (denoted as Pd/GN-SR). The Pd NPs in Pd/GN-SR exhibit poor monodispersion, large particle size, and low loading on graphene. Some unreduced Pd2+ ions still exist in the solution. Second, formic acid as the secondary reducing reagent is introduced into the same solution after the in situ reduction. Some Pd NPs on graphene are gradually etched to Pd2+ ions by oxygen dissolved in the solution, and therefore become smaller NPs. Meanwhile, with the formic acid further reduction, these Pd2+ ions and unreduced Pd2+ ions are together reduced as small-sized Pd NPs on graphene (denoted as Pd/GN-FA). X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy measurements show that the Pd NPs in Pd/GN-FA have smaller particle size and higher monodispersion on graphene compared with that in Pd/GN-SR. Furthermore, the electrooxidation of formic acid on the Pd/GN-FA catalyst has been investigated by cyclic voltammetry. The Pd/GN-FA catalyst exhibits enhanced electrocatalytic activity and stability towards formic acid oxidation owing to the small size, the high monodispersion of Pd NPs, and the stabilizing effect of graphene. The results presented herein indicate that the Pd/GN-FA might be a promising anode electrocatalyst in direct formic acid fuel cells.