Oxygen-Pressure-Limited Recovery of the Hematite α-Fe$_2$O$_3$(0001) Surface from a Reduced Fe$_3$O$_4$(111)-Like Layer

Abstract

The oxidation kinetics of hematite α-Fe$_2$O$_3$(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 Fe$_3$O$_4$(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. While higher temperatures accelerate nucleation, they slow lateral growth at constant oxygen pressure, indicating that oxygen supply limits the oxidation rate. Below an oxygen partial pressure threshold (~2$\times$10$^{-6}$ mbar), growth dramatically slows, underscoring the critical role of oxygen availability. Below a certain oxygen pressure threshold, the growth time rapidly increases. Our study elucidates the interplay between thermodynamics and kinetics in hematite surface oxidation, informing strategies to optimize surface properties for catalytic and industrial processes.