STM and AFM characterization of thin metal oxide films electrodeposited from [P2Mo18O62]6-

Two groups of techniques have been devised for the electrodeposition of new electroactive oxide films from [P2Mo18O62](6-). In the first group, two adsorption procedures were used: simple immersion of the electrode in a solution containing 10(-4) M [P2Mo18O62](6-) in a pH 3.50 medium or cycling of the electrode in this solution in the potential domain of the first three two-electron waves of the heteropolyanion results in surfaces which retain the oxometalate by mere adsorption. Strikingly, during the cycling, it was found that a fourth wave appears in the potential domain of the first three two-electron waves of [P2Mo18O62](6-), indicating an evolution of the heteropolyanion in the solution. Such an evolution was also observed with aged solutions. Then, the potential program for the actual modification step was run by cycling either of these electrodes from -0.2 V to -0.87 V vs. SCE in pure supporting electrolyte. Analysis of the STM images of the surfaces show essentially monomers 1.2-1.5 nm in diameter just after adsorption and a sizeable increase of the dimensions of the patterns after modification. The predominant sizes of these aggregates after modification remain in the range 10-12 nm. The second group of techniques consists in a modification of the electrode surface directly in the solution containing the heteropolyanion. A fixed potential as well as cycling prove efficient. Thick films are obtained readily, which are better imaged by tapping mode AFM. An increase of the pH to 4.50, in appropriate conditions, seems to be favourable to the deposition kinetics. The aggregates in the topmost layers are up to 40 nm in diameter and are assembled in interconnected islands. As a whole, these two groups of techniques appear to exert an important influence on the aggregate sizes. The paper demonstrates that these sizes might be relatively well controlled by the choice of experimental conditions.