Adsorption and incorporation of transition metals at the magnetite ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001) surface

The adsorption of Ni, Co, Mn, Ti, and Zr at the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$-reconstructed ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ surface was studied by scanning tunneling microscopy, x-ray and ultraviolet photoelectron spectroscopy, low-energy electron diffraction (LEED), and density functional theory (DFT). Following deposition at room temperature, metals are either adsorbed as isolated adatoms or fill the subsurface cation vacancy sites responsible for the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ reconstruction. Both configurations coexist, but the ratio of adatoms to incorporated atoms depends on the metal; Ni prefers the adatom configuration, Co and Mn form adatoms and incorporated atoms in similar numbers, and Ti and Zr are almost fully incorporated. With mild annealing, all adatoms transition to the incorporated cation configuration. At high coverage, the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ reconstruction is lifted because all subsurface cation vacancies become occupied with metal atoms, and a $\left(1\times 1\right)$ LEED pattern is observed. DFT+U calculations for the extreme cases, Ni and Ti, confirm the energetic preference for incorporation, with calculated oxidation states in good agreement with photoemission experiments. Because the site preference is analogous to bulk ferrite $\left({\mathrm{XFe}}_2{\mathrm{O}}_4\right)$ compounds, similar behavior is likely to be typical for elements forming a solid solution with ${\mathrm{Fe}}_3{\mathrm{O}}_4$.

Figure 1

(a) DFT+U-optimized structural model of the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }\text{−}{\mathrm{Fe}}_3{\mathrm{O}}_4$(001) surface. The outermost layer consists of a bulklike layer containing Fe in octahedral sites (${\mathrm{Fe}}_{\mathrm{oct}}$, dark blue) and O (red). The first subsurface layer consists of tetrahedrally coordinated Fe (light blue): two bulklike ${\mathrm{Fe}}_{\mathrm{tet}}$ and one additional ${\mathrm{Fe}}_{\mathrm{int}}$ per $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ unit cell. The black triangle indicates the region where the subsurface differs from the regular spinel structure. Two octahedral Fe atoms are missing from the subsurface ${\mathrm{Fe}}_{\mathrm{oct}}$-O layer (S-2) beneath ${\mathrm{Fe}}_{\mathrm{int}}$ (dashed blue circles). (b) STM image of a clean ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001) surface, in which the yellow square illustrates the $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ unit cell. The bright double protrusions on the Fe rows (example highlighted by the cyan arrow) are due to surface hydroxyl groups, a common adsorbate at this surface. The inset shows a LEED pattern of the reconstructed ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001) surface. The red and yellow squares highlight the $\left(1\times 1\right)$ and $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ periodicities, respectively.

Figure 2

Overview of the adsorption of different metals at the ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001) surface at room temperature. STM images show that the deposited metals either adsorb as adatoms (red circles) or form bright surface features on the ${\mathrm{Fe}}_{\mathrm{oct}}$ rows (yellow arrows). The ratio of these two configurations varies for the different metals. (a) 0.2 ML Ni: almost exclusively adatoms. (b) 0.2 ML Mn: few adatoms, many on-the-row features (c) 0.4 ML Ti: hardly any adatoms, high coverage of bright on-the-row features. (d) 0.4 ML Zr: very few adatoms, mainly on-the-row features.

Figure 3

Co adatoms transition into on-the-row features at room temperature. STM image sequence (same area) of 0.4 ML $\text{Co}/{\mathrm{Fe}}_3{\mathrm{O}}_4$(001) taken at room temperature. Three Co adatoms (red circles) are transformed into on-the-row features (encompassed by the yellow rectangles).

Figure 4

Thermal stability study of $\text{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ (different area in each image): Examples of adatoms and on-the-row features are highlighted by red circles and yellow rectangles. (a) At room temperature almost exclusively adatoms are observed. (b) After annealing to 423 K the number of adatoms is diminished. They coexist with an increased number of on-the-row features. (c) Almost all adatoms are converted to on-the-row features.

Figure 5

High coverage of $\text{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ deposited room temperature weakens the surface reconstruction and results in a higher fraction of on-the-row features: (a) 1 ML Ni: High adatom coverage, mostly adatoms with $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ periodicity (yellow outline). A few $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ unit cells are occupied by two Ni adatoms indicating an unreconstructed area (red square). The inset shows weak $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ spots in the LEED pattern. (b) 2.15 ML Ni: Predominantly bright and straight Fe rows (surface filled by on-the-row features), larger patches of adatoms filling two sites per $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ cell (red frames), unreconstructed LEED pattern.

Figure 6

Normal-emission XPS spectra showing regions characteristic of the chemical state of the deposited metals. The red curves were acquired following deposition at room temperature. For Ni (a), additional spectra were acquired following deposition at low temperature (blue curve) and annealing to 465 K (black curve). Two different Ni species contribute to the 200 K and 300 K spectra. The peak positions in (b)–(d) indicate strongly oxidized metal species, in agreement with incorporation into the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ lattice.

Figure 7

Comparison of photoemission spectra of clean ${\mathrm{Fe}}_3{\mathrm{O}}_4$(001), 1 ML $\text{Ni}/{\mathrm{Fe}}_3{\mathrm{O}}_4$(001), and 0.4 ML $\text{Ti}/{\mathrm{Fe}}_3{\mathrm{O}}_4$(001), both as deposited at room temperature. (a) In the Fe $2p_{3/2}$ region, an increase of ${\mathrm{Fe}}^{2+}$ after metal deposition is observed. (b) In the valence band spectrum, the metal deposition causes a significant increase of intensity at $\approx 1$ eV.

Figure 8

Summary of DFT calculations. (a) DFT+U optimized structural model of the reconstructed ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ surface with a Ni adatom (side view). (b) Bulk-terminated structural model with a Ti atom substituting Fe in the energetically most favorable site (DFT+U optimized) for incorporated Ti and Ni atoms. [(c)–(f)] Density of states (DOS) of Ni/Ti and ${\mathrm{Fe}}_{\mathrm{oct}}$ in the surface and first two subsurface layers for the clean surface and the favored configurations of Ni and Ti. [(g)–(k)] Simulated STM images of the four configurations based on a $(\sqrt{2}\times \sqrt{2})\text{R}{45}^{\circ }$ unit cell for the pristine surface (g), the Ni adatom (h), and the ${\mathrm{Ni}}_{\mathrm{oct}}$(S-2) configuration (i), and a $\left(2\times 2\right)$ supercell for the ${\mathrm{Ti}}_{\mathrm{oct}}$(S-2) configuration (k).