Spontaneous Grafting of Nitrophenyl Groups to Planar Glassy Carbon Substrates: Evidence for Two Mechanisms

The covalent grafting of nitrophenyl functionalities to planar carbon substrates by reaction with 4-nitrobenezene diazonium salt at open circuit potential has been studied in aqueous acid and acetonitrile solutions. Atomic force microscopy and electrochemical measurements reveal that the reaction proceeds through two distinct mechanisms. Rapid film growth occurs via reduction of the 4-nitrobenezene diazonium cation by the substrate, giving an aryl radical that couples to the surface. Film growth by this mechanism ceases once the film has reached a thickness at which electron transfer through the passivating film is no longer possible. Slow film growth via a secondary, potential-independent mechanism continues even after the substrate-dependent reaction has ceased. We tentatively propose that slow film growth involves grafting of an aryl cation originating from thermal heterolytic decomposition of the diazonium cation.