Molecularly flat films of the redox [Os(bpy)2(PVP)5Cl]Cl complex obtained by spontaneous adsorption at HOPG electrodes

The morphology and electrochemical properties of redox active films obtained by spontaneous adsorption of [Os(bpy)2(PVP)5Cl]Cl, bpy = 2,2′-bipyridyl and PVP = poly(4-vinylpyridine), at highly oriented pyrolytic graphite electrodes (HOPG) are reported. Two growth regimes with distinctive topographies are observed as a function of the adsorption time from diluted aqueous solution of the complex. [Os(bpy)2(PVP)5Cl]Cl exhibits a strong affinity to the clean HOPG surface leading to the formation of compact layers with well defined electrochemical properties. Cyclic voltammograms at the initial stages of the first growth regime exhibited the characteristic features of weakly interacting surface confined redox species. Ex situ AFM analysis revealed that the freshly prepared films under these conditions are composed of individual molecules as well as nanometer sized aggregates. The aggregates undergo structural changes over time leading to smoother surfaces. Towards the final stages of the first growth regime, the topography of the films is remarkably smooth featuring height fluctuation of about 1 nm over regions as large as 4 μm. These molecularly flat films exhibit rather narrow voltammetric waves indicating significant attractive interactions between the adsorbed complex units. A second growth regime is developed after 1 h of adsorption time, characterised by rougher outer layers. The voltammetric responses of these films are more complex, probably due to heterogeneities in the electron/ion transport rates across the film. It is demonstrated that spontaneous adsorption of the complex allows the formation of redox polymer phases with some of the smoothest surfaces ever reported from solution phase deposition process.