Oxygen Reduction Catalyzed by Platinum Nanoparticles Supported on Graphene Quantum Dots

Nanosized graphene quantum dots were prepared by acid etching of carbon fibers and used as effective substrate supports for platinum nanoparticles which were synthesized by thermolytic reduction of platinum(II) chloride in ethylene glycol. Transmission electron microscopic measurements showed that the resulting nanocomposite (Pt/G) particles exhibited an average diameter of 2.79 ± 0.38 nm, with clearly defined lattice fringes of 0.23 nm that might be assigned to the interlayer spacing of the (111) crystal planes of fcc Pt. In addition, the Pt nanoparticles were found to be wrapped with a low-contrast halo that likely arose from the poorly crystalline graphene quantum dots. X-ray diffraction studies confirmed the composite nature of the Pt/G nanoparticles, and the average size of the crystal domains of the Pt/G nanoparticles was found to be close to the nanoparticle physical dimensions, whereas for commercial Pt/C nanoparticles the size of the crystal domains was only half that of the nanoparticle diameter. XPS measurements showed the formation of metallic platinum along with sp2 and oxygenated carbons in the nanocomposite nanoparticles. Significantly, in comparison with commercial Pt/C catalysts, the Pt/G nanocomposites showed markedly enhanced catalytic activity in oxygen reduction reactions, with an onset potential of +1.05 V that was 70 mV more positive than that of Pt/C, and a specific activity that was almost nine times higher. These results were ascribed to the abundant structural defects of the nanosized graphene quantum dots that manipulated the dissociative adsorption of oxygen and the binding of reaction intermediates O* and HO* on platinum surfaces.