Oxygen-Pressure-Limited Recovery of the Hematite α‑Fe₂O₃(0001) Surface from a Reduced Fe₃O₄(111)-like Layer

The oxidation kinetics of hematite α-Fe₂O₃(0001) surfaces are vital for its applications in catalysis, environmental remediation, and industrial processes. Despite prior studies, the roles of temperature, oxygen partial pressure, and oxygen chemical potential in controlling nucleation and growth kinetics are not fully understood. Using real-time Low Energy Electron Microscopy/Diffraction (LEEM/LEED), we systematically investigate the oxidation of a reduced Fe₃O₄(111)-like surface layer to hematite under controlled conditions. We show that complete recovery of the hematite surface termination is closely linked to the nucleation and lateral growth of a two-dimensional honeycomb (H) phase. H-phase nucleation and growth do not follow standard Arrhenius kinetics. A set of experiments comprising oxidation at constant oxygen partial pressure, constant temperature, and constant chemical potential, while varying temperature, pressure, or both, indicates that oxygen supply limits the H-phase growth rate. Under constant oxygen partial pressure p _O2, increasing the temperature accelerates nucleation but decelerates the growth rate of the H-phase. The growth dramatically slows for p _O2 < ∼2 × 10^-6 mbar. Keeping the oxygen chemical potential constant, i.e., regulating both p _O2 and temperature to keep the thermodynamic driving force provides increasing growth rates with increasing temperature only if p _O2 > ∼2 × 10^-6 mbar. Our study thus elucidates the interplay between thermodynamics and kinetics in hematite surface oxidation, informing strategies to optimize surface properties for catalytic and industrial processes.