Water oxidation catalysis: Effects of nickel incorporation on the structural and chemical properties of the α-Fe₂O₃(0001) surface

Photoelectrochemical solar fuel synthesis devices based on photoactive hematite (α-Fe₂O₃) anodes have been extensively investigated, yet a fundamental understanding regarding its associated water oxidation surface reaction mechanism is still lacking. To help elucidate detailed reaction mechanisms, we studied water chemisorption and reaction as well as structural changes induced by Ni incorporation into the α-Fe₂O₃(0001) surface. Investigation by scanning probe and electron diffraction techniques show that vapor deposition of Ni and subsequent annealing to 700 K leads to the interdiffusion and incorporation of Ni into the near-surface region of hematite and changes the structure of the (0001) surface by the formation of FeO-like domains on the topmost layer. These results are discussed in the context of a proposed water oxidation mechanism on this surface in which Ni doping facilitates water oxidation by increasing O hole concentrations and forms less negatively charged O anions (∗O) and∗O···OH species [ Liao, P. L.; Keith, J. A.; Carter, E. A. J. Am. Chem. Soc. 2012, 134, 13296-13309. ]. Consistent with predictions from this theory, electrochemical measurements using cyclic voltammetry carried out on the ultrahigh vacuum-prepared surfaces demonstrated that Ni incorporation leads to higher current density and lower onset potential than the unmodified α-Fe₂O₃ surface. Our work utilizing a surface science approach helps to connect such theoretical predictions of reaction thermodynamics on well-defined structures and the performance of modified hematite model electrocatalysts for water oxidation.