An Electrochemical and XPS Study of Reduction of Nitrophenyl Films Covalently Grafted to Planar Carbon Surfaces

Nitrophenyl (NP) films were grafted to glassy carbon and pyrolyzed photoresist films by electroreduction of the corresponding diazonium salt. The as-prepared, multilayered films were examined using electrochemistry and X-ray photoelectron spectroscopy (XPS). Electrochemical analysis confirmed the absence of electrooxidizable groups whereas XPS showed approximately 35% of N was present in a reduced form. The reduced N is assigned to azo groups, which are known to be electroinactive in the film environment. NP films were reduced electrochemically in three media and also by chemical reduction in ethanolic disodium sulfide. The concentrations of aminophenyl and hydroxylaminophenyl groups produced by each method were estimated electrochemically, and the relative amounts of unreacted NP groups were established from XPS measurements. Aminophenyl is the major product for all reduction methods, and Na2S gives the cleanest and most complete conversion to aminophenyl groups, with less than 5% residual NP. Reduced NP films were reacted with carboxylic acid and acid chloride derivatives; the highest yield of electroactive-coupled product was obtained for a film electroreduced in H2SO4 and reacted with acid chloride. The detailed electrochemical and XPS analysis reveals the limitations of electrochemistry for determining the composition of these films.