An insight into the outer- and inner-sphere electrochemistry of oxygenated single-walled carbon nanohorns (o-SWCNHs)†
The extremely large surface areas as well as connecting nanopores of single-walled carbon nanohorn (SWCNH) aggregates have been tested, for the first time to the best of our knowledge, as metal-free, stable and cheap electrocatalysts for heterogeneous electron transfer reactions involving inorganic redox couples, including f-block elements such as actinides. From systematic studies of heterogeneous electron transfer reactions, the electrochemically reversible outer-sphere heterogeneous surface insensitive electron transfer reaction involving [Ru(NH3)6]3+/[Ru(NH3)6]2+ and the electrochemically near-to-reversible inner-sphere heterogeneous surface sensitive electron transfer reaction involving [Fe(CN)6]3−/[Fe(CN)6]4− on oxygenated SWCNHs (i.e., o-SWCNHs) have been compared to heterogeneous electron transfer involving the quasi-reversible [U(VI)O2(CO3)3]4−/[U(V)O2(CO3)3]5− redox reaction on o-SWCNHs. It was evident that the oxygen containing functional groups of o-SWCNH could catalyze the electron transfer process between uranium species in solution and the working electrode happened by following surface-sensitive inner-sphere electron transfer mechanism. Furthermore, the electrochemical stability, repeatability and reproducibility of the o-SWCNH modified glassy carbon electrode were found to be analytically acceptable for studying the electrochemistry of uranium in alkaline solutions with high ionic strength.