The effect of a radical scavenger (DPPH: 2,2-diphenyl-1-picrylhydrazyl) on the spontaneous covalent grafting of nitrophenyl functionalities on a vitreous carbon substrate using the 4-nitrobenzene diazonium cation has been studied by electrochemical measurements and X-ray photoelectron spectroscopy. The addition of micromolar concentrations of DPPH to the diazonium solution efficiently limits the multilayer formation and leads to monolayer surface coverage. Control of polyaryl layer formation via the capture of the reactive nitrophenyl radical was also found to increase the proportion of nitrophenyl groups grafted to the surface via azo bridges. This work validates the recently reported strategy using a radical scavenger to prevent the formation of a polyaryl layer without interfering with direct surface grafting.