Zinc depletion at the ZnFe2O4 (111) single crystal surface: X-ray spectroscopy experiments and computer simulations

We use experimental and computational techniques to investigate the zinc ferrite (ZnFe2O4) (111) single-crystal surface under different preparation methods. Surface-sensitive XPS and NEXAFS measurements show that upon annealing in ultra-high vacuum (UHV), Zn depletion occurs, leading to the formation of an iron-rich (111) surface, whereas annealing in the presence of O2 maintains a bulk-like ZnFe2O4 surface. After UHV annealing, a mixed Fe2+/Fe3+ state and a cation distribution like that of magnetite (Fe3O4) is observed, whereas after annealing in oxygen only Fe3+, mostly in octahedral coordination, is observed (as in ZnFe2O4). Temperature-dependent XPS confirms significant Zn depletion in the near-surface region above 500 °C under UHV, with almost no Zn remaining at 600 °C; under O2 atmosphere there is no zinc depletion up to 600 °C. A theoretical model illustrates how reduction from ZnFe2O4 to Fe3O4 with formation of O2 and Zn gas is thermodynamically feasible under UHV, whereas the same reaction is not favourable at higher oxygen pressures. Our findings demonstrate the impact of UHV treatment on ZnFe2O4 surfaces, and cautions that UHV environments, routinely employed for surface analysis, can themselves induce substantial modifications to the surface, thereby complicating the interpretation of measurements in the context of catalytic conditions.