We describe the production of size-selected manganese nanoclusters using a magnetron sputtering/aggregation source. Since nanoparticle production is sensitive to a range of overlapping operating parameters (in particular, the sputtering discharge power, the inert gas flow rates, and the aggregation length), we focus on a detailed map of the influence of each parameter on the average nanocluster size. In this way, it is possible to identify the main contribution of each parameter to the physical processes taking place within the source. The discharge power and argon flow supply the metal vapor, and argon also plays a crucial role in the formation of condensation nuclei via three-body collisions. However, the argon flow and the discharge power have a relatively weak effect on the average nanocluster size in the exiting beam. Here the defining role is played by the source residence time, governed by the helium supply (which raises the pressure and density of the gas column inside the source, resulting in more efficient transport of nanoparticles to the exit) and by the aggregation path length.
Emulating water oxidation catalyzed by the oxomanganese clusters in the photosynthetic apparatus of plants has been a long-standing scientific challenge. The use of manganese oxide films has been explored, but while they may be catalytically active on the surface, their poor conductivity hinders their overall performance. We have approached this problem by using manganese oxide nanoparticles with sizes of 4, 6 and 8 nm, produced in a sputter-gas-aggregation source and soft-landed onto conducting electrodes. The mass loading of these catalytic particles was kept constant and corresponded to 45%-80% of a monolayer coverage. Measurements of the water oxidation threshold revealed that the onset potential decreases significantly with decreasing particle size. The final stoichiometry of the catalytically active nanoparticles, after exposure to air, was identified as predominantly MnO. The ability of such a sub-monolayer film to lower the reaction threshold implies that the key role is played by intrinsic size effects, i.e., by changes in the electronic properties and surface fields of the nanoparticles with decreasing size. We anticipate that this work will serve to bridge the knowledge gap between bulk thick film electrocatalysts and natural photosynthetic molecular-cluster complexes.
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