Oxygen Electroreduction Catalyzed by Palladium Nanoparticles Supported on Nitrogen-Doped Graphene Quantum Dots: Impacts of Nitrogen Dopants

Abstract

Palladium nanoparticles supported on nitrogen-doped graphene quantum dots (NGQD) were synthesized by hydrothermal coreduction of palladium salts, citric acid, and urea at 160 °C for up to 12 h. Transmission electron microscopic studies showed that in the resulting PdNGQD nanocomposites, small palladium nanoparticles clustered into superstructures of 100 nm and larger. X-ray photoelectron spectroscopic studies showed that the NGQDs contained only p-type pyridinic and pyrrolic nitrogen centers, and although the total concentrations of nitrogen dopants were rather consistent (ca. 10 at. %) among the series of samples, the relative abundance of pyrrolic (pyridinic) nitrogens increased (decreased) with prolonging reaction duration, suggesting thermal conversion of pyridinic nitrogens into pyrrolic ones. The binding energy of the Pd 3d electrons was found to increase accordingly, probably due to enhanced electron withdrawing by the more acidic pyrrolic nitrogens. This suggests apparent interactions between pall...