Helping Thy Neighbor: How Cobalt Doping Alters the Electrocatalytic Properties of Hematite.

We present analysis of the indirect role of Co doping on the electrocatalytic activity of α-Fe2O3. Upon substitution of one of the Fe atoms in the hematite surface, we observed a promoting effect of the substitution, upon which the overpotential required for the water-splitting reaction decreased in all substitution sites investigated. The cobalt site itself, however, does not exhibit the improved properties with respect to the undoped hematite. The promoting effect results purely from the altering of the properties of the nearest Fe atoms in the hematite surface. We conclude that the overpotential is reduced upon formation of the structure resembling the O2 molecule strongly interacting with the surface between the Co and Fe sites, and this is consistent with the catalytic activity of the surface vacancies of the hematite.