Fabrication And Electrochemistry Of Graphene Nanoelectrodes

As a new class of two-dimensional carbon materials, graphene has attracted great attention in many fields due to the unique properties. Edges and defects in graphene sp2domains are crucial for its electronic structure and chemical properties. This paper obtains graphene flakes of different defect densities by using differently reduced graphene oxides (RGOs) and controlling the size of RGO flakes. Combining with nanoelectrode techniques, the fast heterogeneous ET kinetics and nanoscopic charge transport dynamics on individual graphene flake are investigated by using a number of inorganic and organic redox probes. The main contents and results are as follows:1. Fabrication of graphene nanoelectrodes and investigation of electric double layer effect at nanoscale electrochemical interfacesThrough high-speed centrifugation and size-selected ultrafiltration, flakes of RGOs of nanometer and submicrometer dimensions, respectively, are obtained. Using n-dodecanethiol-modified Au ultramicroelectrodes of appropriately small sizes, quick dipping in dilute suspensions of these small RGOs allows attachment of only a single flake on the thiol monolayer through hydrophobic interaction. The fabricated graphene ultramicroelectrodes can be used to study the fast heterogeneous ET kinetics at RGOs and the nanoscopic charge transport dynamics at electrochemical interfaces.The voltammetric enhancement and inhibition for the reduction of Ru(NH3)63+and Fe(CN)63-respectively upon removal of supporting electrolyte are found to significantly deviate in magnitude from those predicted by the electroneutrality-based electromigration theory at RGO flakes of submicrometer and nanometer dimensions, which may evidence the increased penetration of the diffuse double layer into the mass transport layer at nanoscopic electrochemical interfaces.2. Heterogeneous ET Kinetics of inorganic redox probes at N-RGO Flakes of high defect densityThrough high-temperature annealing of hydrazine-reduced graphene oxides, nitrogen doping N-RGOs are obtained. N-RGO flakes are used to prepared to differently sized graphene nanoelectrode using method described above and the heterogeneous electron transfer (ET) kinetics of Fe(CN)63-and Ru(NH3)63+at these electrodes are studied. Voltammetric analysis for the ET reactions on N-RGOs of nanometer sizes produces similarly high activity for electrochemical ET reactions to metal electrodes and obviously higher ET rate constants than that at large N-RGOs. Raman spectroscopic measurements showed the high defect density in the prepared N-RGO flakes and an increase in the defect density in the nanometer-sized flakes. The accordant increase in the Raman defect density and k0with the decreased N-RGO sizes suggests that the ET activity of RGO flakes is related to their local density of states (DOS) near the Fermi level, which increases with the defect density.3. Heterogeneous ET Kinetics of inorganic redox probes at I-RGO Flakes of low defect densityThrough high-temperature annealing of HI-reduced graphene oxides, iodine doped I-RGOs is obtained. The Raman spectroscopy shows the similar low defect density in the I-RGOs of nanometer sizes to those at large ones. The reason may be that iodide species not only can effectively reduce the various oxygen-containing functional groups on GOs, but also can adsorb at the defect sites to reduce the defect density. Voltammetric analysis for the Fe(CN)63-and Ru(NH3)63+reduction also produces lower ET rate constants at I-RGOs than that at N-RGOs, which do not change with the size of the I-RGO flakes. This provides further evidence to the correlation between the ET activity and the defect density at graphene surfaces.4. Heterogeneous ET Kinetics for quinone species at N-RGO electrodesBecause the electron transfer for quinones at electrodes is non-adiabatic in nature, the electron transfer rate constant may be affected by electronic density of states near Fermi level more significantly. We compare the rates of electron transfer for three kinds of quinone species at N-RGOs and gold electrodes. The k0values obtained at the small N-RGO flake are very close to those obtained on metal nanoelectrode, and much higher than those on at large N-RGO flake. The difference of k0value for quinones on the N-RGO flakes of different sizes is more significant than that for inorganic redox.