Enhancement of Water Oxidation Activity by Size-Selected Sub-10 nm Manganese Oxide Nanoparticles

Emulating photosynthetic water oxidation by using inorganic metal oxides has been a long-standing challenge. In plants photoexcitation is collected by chlorophylls and funneled into a cluster composed of manganese, oxygen, and calcium, known as the oxygen evolving complex. This natural complex provides motivation to explore artificial materials that use manganese oxide nanoparticles as electrocatalysts or photocatalysts. We approached this goal by soft-landing size-selected 4, 6, and 8 nm manganese nanoparticles, produced by a sputtering/aggregation nanocluster source, onto conducting electrodes. We then used these clusters as electrocatalysts for water oxidation. The water oxidation threshold revealed a significant decrease with decreasing particle size. XPS spectroscopy identified the final stoichiometry of the catalytically active nanoparticles, after exposure to air, as MnO. The ability of a sub-monolayer film of such sub-10 nm nanoparticles to effectively promote the catalytic process is noteworthy and demonstrates the influence of nanoscale size effects on their electronic structure.