Photon-induced Synthesis of Ultrafine Metal Nanoparticles on Graphene as Electrocatalysts: Impact of Functionalization and Doping

Utilizing reducing species generated by high-energy photons offers an alternative strategy to prepare metal nanoparticles (NPs) in the absence of foreign reductant. However, fine control of NP size and dispersity remains a big challenge. Herein, we report that by properly selecting the solvent, precursor concentration and carbon support, ultrafine palladium (Pd) NPs with an average size of 3.0 nm uniformly distributed on nitrogen-doped graphene (NG) are radiolytically prepared. Control experiments demonstrate that ethylene glycol with moderate reducibility is superior to water. Among four graphene-based supports with distinct functionalization or doping, nitrogen dopants outperform oxygen functional groups in anchoring the NPs and controlling the size and dispersity. Pd/NG also affords the lowest η10 (overpotential at a current density of 10 mA·cm−2) toward hydrogen evolution reaction (HER). Comparison of Pd/NG with varied Pd loadings indicates that the HER activity nearly reaches a plateau once the loading increases to 2.6 wt %, which is limited by the nitrogen content of NG. X-ray photoelectron spectroscopy unambiguously reveals the electron transfer from electron-rich Pd to electron-deficient nitrogen, especially the graphitic nitrogen, corroborating the decisive role of doped nitrogen in stabilizing the Pd NPs on NG. Extending the synthesis to platinum (Pt) yields Pt/NG that presents even lower η10 than the commercial 10 wt % Pt/carbon black due to the smaller NP size. These results collectively highlight the potential of high-energy photons for green, versatile and scalable synthesis of heterogeneous nanostructures.