Water Oxidation by Size-Selected Co27 Clusters Supported on Fe2 O3

Abstract The complexity of the water oxidation reaction makes understanding the role of individual catalytic sites critical to improving the process. Here, size‐selected 27‐atom cobalt clusters (Co 27 ) deposited on hematite (Fe 2 O 3 ) anodes were tested for water oxidation activity. The uniformity of these anodes allows measurement of the activity of catalytic sites of well‐defined nuclearity and known density. Grazing incidence X‐ray absorption near‐edge spectroscopy (GIXANES) characterization of the anodes before and after electrochemical cycling demonstrates that these Co 27 clusters are stable to dissolution even in the harsh water oxidation electrochemical environment. They are also stable under illumination at the equivalent of 0.4 suns irradiation. The clusters show turnover rates for water oxidation that are comparable or higher than those reported for Pd‐ and Co‐based materials or for hematite. The support for the Co 27 clusters is Fe 2 O 3 grown by atomic layer deposition on a Si chip. We have chosen to deposit a Fe 2 O 3 layer that is only a few unit cells thick (2 nm), to remove complications related to exciton diffusion. We find that the electrocatalytic and the photoelectrocatalytic activity of the Co 27 /Fe 2 O 3 material is significantly improved when the samples are annealed (with the clusters already deposited). Given that the support is thin and that the cluster deposition density is equivalent to approximately 5 % of an atomic monolayer, we suggest that annealing may significantly improve the exciton diffusion from the support to the catalytic moiety.