Structure and Reactivity of Pristine and Reduced Spinel CoFe₂O₄(001)/(100) Surfaces

Cobalt ferrite, CoFe2O4 (CFO), nanocrystals are efficient and competitive anode materials in the field of electrochemical water splitting. Using density functional theory with on-site Hubbard U corrections (DFT+U), we have investigated the structural, electronic, and magnetic properties of CFO (001)/(100) surfaces, as well as their reactivities toward water adsorption. Special attention has been focused on the formation of oxygen vacancies (VO), due to their key role in the oxidation activity of metal oxides, often based on the Mars-van Krevelen mechanism. Our results show that vacancy formation is easiest at oxygen sites that are not bound to tetrahedrally coordinated Fe. Water adsorbs mainly in molecular form on the Co/Fe metal cations, whereas it dissociates at defects. In comparison to other spinels, CFO is similar to NiFe2O4, exhibiting relatively low energy cost of VO formation and a strong affinity of the vacancies toward water. These findings suggest that CFO may be a more promising oxidation catalyst than NiCo2O4 and Co3O4.