Intervalence Charge Transfer of Ruthenium-Nitrogen Moieties Embedded within Nitrogen-Doped Graphene Quantum Dots

Nitrogen-doped graphene quantum dots (NGQDs) were prepared by a facile hydrothermal method and functionalized with ruthenium metal ions by exploiting the unique complexation of selected nitrogen dopants with ruthenium ions. The NGQDs were 2.0 +/- 0.4 nm in diameter with clearly defined lattice fringes and consisted of three to four graphene sheets, as revealed in transmission electron microscopic and atomic force microscopic measurements. Complexation of NGQDs with ruthenium ions likely occurred through the pyridinic nitrogen dopants, leading to the incorporation of multiple metal centers within the conjugated graphitic C sp(2) scaffolds (Ru-NGQDs), which was confirmed by UV-vis absorption, photoluminescence, and X-ray photoelectron spectroscopic measurements. In comparison with the pristine NGQDs, Ru-NGQDs exhibited a red-shift of the absorption band in UV-vis measurements, along with significantly reduced intensity of the photoluminescence emissions, due to quenching by the metal-nitrogen moieties. Electrochemically, the Ru-NGQD compounds exhibited two pairs of voltammetric waves, with a peak spacing of 150 mV, suggesting Class II intervalence charge transfer. This was further confirmed in near-infrared spectroscopic measurements where an absorption band emerged at ca. 1450 nm at mixed valence by using Ce(SO4)(2) as the oxidizing reagent. The results highlight the unique applications of graphene scaffolds in facilitating nanoscale charge transfer.