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

Experiments were conducted using a Specs FE-LEEM P90 instrument. Bright-field images were measured by capturing electrons collected from the central (0,0) diffracted beam. Diffraction patterns were acquired from a surface area of approximately 7 × 15 μm² without aperture confinement. The coverage of the H-phase was determined using ImageJ software as the difference between the areas of the R- and H-phase regions.

X-ray Photoelectron Spectroscopy (XPS) analysis was performed using a Specs system equipped with a Phoibos 150 spectrometer. Measurements were performed with a nonmonochromatized X-ray source emitting Mg Kα radiation (hν = 1253.6 eV), at 0° emission angle, utilizing the high-magnification mode with a 15 mm iris aperture. Core-level spectra were acquired with a pass energy of 20 eV, a step size of 0.1 eV, and a dwell time of 0.1 s. No charging effects were observed during the measurements.

To establish a stable, controlled starting surface for the oxidation studies, we used an α-Fe₂O₃(0001) hematite single crystal featuring the 2D honeycomb phase fully covering the surface. This surface is referred to as the H-phase in the following text. Samples featuring H-phase were subsequently reduced to magnetite Fe₃O₄(111) through four cycles of Ar⁺ sputtering and UHV annealing at 640 °C (see Methods section for details). Sputtering selectively removes surface oxygen atoms, reducing the near-surface volume to stoichiometric Fe₃O₄, forming a ∼20–25 nm thick layer that maintains the epitaxial relationship with the bulk Fe₂O₃ hematite. (6) We will refer to this surface as the R-phase in the following text.

The transition between the H- and R-phases was characterized using X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Microscopy/Diffraction (LEEM/LEED). In the XPS spectrum of the R-phase (Figure 1a), the Fe³⁺ satellite peaks are absent, whereas in the H-phase spectrum (Figure 1b), the satellite peaks are present, confirming the oxidation state. (29−31) LEEM/LEED provide complementary real and reciprocal-space views, elucidating the sample morphology at the mesoscale and its structure, respectively. Bright-field LEEM imaging revealed a uniform and well-prepared R-phase surface (Figure 1c). (7) The corresponding diffraction pattern in Figure 1d displays (2 × 2)-Fe₃O₄(111) diffraction spots (referred to the O planes (6)) characteristic of this surface. (6, 7, 32) The oxidation of the surface at temperatures of 660–700 °C and oxygen partial pressures of 6.3 × 10^–7–5.0 × 10^–6 mbar leads to full oxidation and to α-Fe₂O₃(0001) featuring the H-phase; depending on the oxidation conditions, it takes 55 to 450 min. The bright-field LEEM image of the H-phase (Figure 1e) shows a uniform surface, and the diffraction pattern (Figure 1f) exhibits the characteristic flowered pattern (30, 32, 33) around the bulk spots associated with the moiré due to the presence of the overlayer. (7) We note that the maghemite surface can exhibit the same (2 × 2) surface reconstruction as magnetite; (34) therefore, maghemite and magnetite cannot be distinguished by simple diffraction experiments alone.

J. Phys. Chem. C (2026) 130 (34): 12224–12231: Figure 1. Surface characterization of hematite (H-phase) and reduced magnetite (R-phase). (a) XPS spectrum of the R-phase, confirming the absence of Fe3+ satellite peaks and the successful reduction of α-Fe2O3 to Fe3O4. (b) XPS spectrum of the H-phase, highlighting the presence of Fe3+ satellite peaks characteristic of hematite. (c) Bright-field LEEM image at 46 eV of the magnetite (R-phase) surface, showing a uniform and well-prepared morphology. (d) (2 × 2) diffraction pattern at 70 eV characteristic for the R-phase, indicative of the well-ordered surface suitable for oxidation studies. (e) Bright-field LEEM image at 70 eV showing the H-phase surface. (f) Diffraction pattern at 70 eV characteristic of the H-phase. The (1 × 1) unit cell is marked by a rhombus in (d,f).

3.2. Time Evolution of Oxidation

To obtain quantitative insights into the R- to H-phase transformation, we performed three sets of real-time LEEM/LEED experiments, keeping constant either oxygen partial pressure p_O2, sample temperature T, or oxygen chemical potential μ, while varying either temperature or pressure, or in the last case, both parameters. By systematically separating the effects of these parameters, we revealed their impact on the R- to H-phase oxidation kinetics. An example of the oxidation kinetics at 660 °C and 6.3 × 10^–7 mbar (μ_O = −1.867 eV) using real-time LEEM/LEED is given in Figure 2. During the first stage of oxidation, the entire surface shows a uniform contrast and the diffraction pattern of the R-phase. At 75 min, moiré spots became visible, which indicates H-phase nucleation (Figure 2a). We refer to this time, t₀, as the nucleation time in the following. As oxidation progresses, the H-phase spots become more pronounced, while the R-phase spots progressively fade.

J. Phys. Chem. C (2026) 130 (34): 12224–12231: Figure 2. Surface evolution during the oxidation from magnetite to hematite at 660 °C and 6.3 × 10–7 mbar in real and reciprocal space. (a) Sequence of diffraction patterns taken at 70 eV showing the structural transformation from the R-phase to the H-phase. (b) Corresponding sequence of bright-field LEEM images capturing the transformation from the R-phase (brighter) to the oxidized H-phase (darker) over 355 min.

The bright-field LEEM image in Figure 2b illustrates the progressive transformation of the surface from the reduced R-phase to the H-phase. After nucleation, H-phase islands emerge and remain surrounded by the R-phase. As the oxidation advances, the R-phase regions on the surface gradually decrease, while the H-phase regions expand. By 355 min, the entire surface is covered by the H-phase, marking the completion of the oxidation process. The growth of the H-phase proceeds in a granular fashion, consistent with the rough growth front observed during oxidation of thin films. (24)