Kinetics of Interfacial Electron Transfer at Single-Layer Graphene Electrodes in Aqueous and Nonaqueous Solutions

Abstract

We present a catalog of electron transfer mediators for investigating the heterogeneous electron transfer kinetics of large-area, single-layer graphene electrodes. Scanning electrochemical microscopy (SECM) was used to probe the apparent standard electron transfer rate constant of mediators in aqueous solutions and in acetonitrile and dimethylformamide, allowing for studies of graphene electroactivity at different potentials and in both aqueous and nonaqueous media. In aqueous solution, iron(III) ethylenediaminetetraacetic acid, hexacyanoruthenate(II), hexacyanoferrate(II), hexacyanoferrate(III), octacyanomalybdate(IV), cobalt(III) sepulchrate, and hydroxymethylferrocene exhibited quasi-reversible electron transfer behavior. The electron transfer kinetics of hexaammineruthenium(III), methyl viologen, and tris(2,2'-bipyridyl)ruthenium(II) were found to be reversible in these studies. The electron transfer rate constant of hydroxymethylferrocene and ferrocene, in organic media, was similar to that for hydroxymethylferrocene in water, which, although fast, shows clear kinetic complications that we believe are inherent to graphene. A series of viologens, known to be reversible at metal electrodes, exhibited quasi-reversible electron transfer. For [Co(dapa)(2)](2+), where dapa is 2,6-bis[1-(phenylimino)ethyl]pyridine, in dimethylformamide, the oxidation state of the redox pair investigated affected the observed kinetics. Under similar experimental conditions, the Co(I/II) couple exhibited nearly reversible behavior whereas Co(II/III) had finite kinetics. This behavior was ascribed to the large difference in self-exchange rates for these two processes. To demonstrate the utility of using these mediators for examining graphene electrodes, the kinetics of two mediators with quasi-reversible electron transfer behavior, iron ethylenediaminetetraacetic acid and hexacyanoruthenate, were measured in the presence of a redox-active species [Os(bpy)(2)(dipy)Cl]PF(6), where bpy is 2,2'-bipyridine and dipy is 4,4'-trimethylenedipyridine, adsorbed onto the graphene surface. The kinetics of both mediators were enhanced in the presence of one-hundredth of a monolayer of the osmium complex, showing that even small amounts of impurities on the graphene surface are capable of enhancing the observed kinetics.

Enhanced electron transfer in composite films of reduced graphene oxide and poly(N-methylaniline)

“…4b (curve 4) is very similar to the CV of GC showing almost reversible redox behavior and distinct oxidation and reduction peaks for the Ru(NH 3 ) 6 2+/3+ redox couple (E p,a =-0.07 V, E p,c =-0.20 V, ∆E p =0.13 V). This is in good accordance with the nearly reversible electron transfer kinetics for Ru(NH 3 ) 6 3+ on single-layer graphene electrodes in aqueous solutions [10]. We assume that the PNMA-ClO 4 -rGO and PNMA-ClO 4 -GO films have equal surface areas.…”

Localized electrochemistry for the investigation and the modification of 2D materials

“…Different redox mediators are considered, and small kinetic constants for Fe(CN)6 3-and higher constants for ferrocene are observed. 19 These observations are in line with the previously mentioned studies dealing with HOPG. The importance of folding effects at graphene edges is also investigated by electrochemistry.…”

Electrochemistry of Graphene

“…The same group reported HET kinetics on a monolayer CVD graphene for an extensive library of redox mediators in both aqueous and organic (acetonitrile and N,N-dimethylformamide) media, ranging from quasi-reversible to near-reversible kinetics, although the data were interpreted in terms of the Butler-Volmer formalism, the transfer coefficient, , values fell outside the usual range expected (0.3 < < 0.7). Significantly, an enhancement in kinetics was observed upon intentional coverage of a fraction of the surface (1/100th) with osmium complex "impurities," demonstrating that even the smallest amount of impurities can alter the apparent kinetics on a graphene surface [115]. Brownson et al studied mono-and multilayer polycrystalline CVD graphene, which was left on the Ni-film growth substrate.…”