Studies of the adsorption of tetrasulfonated phthalocyanines on graphite substrate

The adsorption of iron and cobalt terasulfonated phthalocyanines (TsPcs) on ordinary pyrolytic graphite has been investigated as a function of pH and ionic strength of the adsorption solution as well as the potential. The charge corresponding to the voltammetric redox peaks of adsorbed complexes was used as a measure of the surface concentration. Adsorption of CoTsPc occurs readily from its freshly prepared aqueous solutions and is generally independent of pH. For FeTsPc, however, adsorption does strongly depend on pH. High surface coverage is achieved only from acid solutions rather than from pure water and alkaline solutions. This can be explained in terms of the form(s) of the complexes existing in the solution phase in the presence of air. uv-Visible spectroscopic studies coupled with the addition of CN^? to the macrocycle solutions provide evidence that in pure water and alkaline solutions FeTsPc exists predominantly in the μ-oxo form (FeTsPc)₂O, which seems not to favor the adsorption process. No evidence of the μ-oxo complex was found for FeTsPc in acid solutions and CoTsPc in aqueous solutions over the pH range examined (1–13). The adsorption of FeTsPc was at maximum when the potential was held at ?0.55 V vs sce in 0.1 M NaOH.