Water agglomerates on Fe3O4(001)

Abstract

Determining the structure of water adsorbed on solid surfaces is a notoriously difficult task and pushes the limits of experimental and theoretical techniques. Here, we follow the evolution of water agglomerates on Fe₃O₄(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH-H₂O bonds drive the formation of partially dissociated water dimers at low coverage, but a surface reconstruction restricts the density of such species to one per unit cell. The dimers act as an anchor for further water molecules as the coverage increases, leading first to partially dissociated water trimers, and then to a ring-like, hydrogen-bonded network that covers the entire surface. Unraveling this complexity requires the concerted application of several state-of-the-art methods. Quantitative temperature-programmed desorption (TPD) reveals the coverage of stable structures, monochromatic X-ray photoelectron spectroscopy (XPS) shows the extent of partial dissociation, and noncontact atomic force microscopy (AFM) using a CO-functionalized tip provides a direct view of the agglomerate structure. Together, these data provide a stringent test of the minimum-energy configurations determined via a van der Waals density functional theory (DFT)-based genetic search.

Keywords: Fe3O4; H-bond network; cooperativity; magnetite; water.

Fig. 1.

Quantification of water adsorbed on Fe₃O₄(001) by TPD. (A) Experimental TPD spectra obtained for initial D₂O coverages ranging from 0 to 14 molecules per Fe₃O₄(001)–(√2 × √2)R45° unit cell (Inset: higher temperature range showing desorption peaks ε and φ, which originate from surface defects). The colored curves indicate the coverages for which a particular desorption feature (labeled α′, β, γ, and δ) saturates, α marks the multilayer desorption peak. (B) Plot of the integrated TPD peak areas as a function of beam exposure. The colored data points correspond to the colored curves in A. Based on these data, we conclude the β, γ, and δ peaks saturate at coverages of eight, six, and three molecules per (√2 × √2)R45° unit cell, respectively. (C) Inversion analysis of the TPD data for D₂O on Fe₃O₄(001) for the different peaks. The filled area marks the uncertainty range of the coverage-dependent desorption energies for each peak.

Fig. 2.

Water monomers, dimers, and multiple neighboring protrusions on the Fe₃O₄(001) surface imaged by low-temperature (78 K) STM. (A) The as-prepared Fe₃O₄(001) surface. The (√2 × √2)R45° periodicity is indicated by the white square, and the white arrow highlights an O*H group. (Inset) Top and side views of the Fe₃O₄(001)–(√2 × √2)R45° surface structure with the SCV structure (the top view is aligned with the STM image, and the gray vector indicates the viewing direction to locate the side view). Only the Fe_oct atoms are imaged in STM. (B) STM image acquired after 0.05 L of water was adsorbed and heated to 255 K. The surface is clean, except for protrusions located at surface defects including antiphase domain boundaries in the (√2 × √2)R45° reconstruction (cyan arrow). (C) STM image following adsorption of 0.1 L of water at 120 K. Isolated single protrusions (yellow arrow), double protrusions (red arrow), and multiple neighboring protrusions (green arrow) are due to water molecules adsorbed on the Fe_oct rows.

Fig. 3.

Imaging water agglomerates on Fe₃O₄(001) with nc-AFM using a CO-functionalized tip. nc-AFM images obtained after exposing the as-prepared Fe₃O₄(001) surface to (A) 2.5 ± 0.5 H₂O/u.c., (B) ∼6 H₂O/u.c., and (C) ∼8 H₂O/u.c. The Q+ oscillation amplitudes were (A) 45 pm, (B) 110 pm, and (C) 65 pm, and the bias was set to 0 V in all images. In each case, water was dosed at 105 K, and the sample preheated to ∼155 K before imaging at 78 K. The coverages in A–C correspond roughly to the partial populations of the δ, γ, and β peaks in TPD, respectively. Partially dissociated water dimers and trimers on the Fe_oct rows are indicated by red and cyan arrows in A, respectively, and yellow arrows highlight protrusions bridging the Fe_oct rows in B. Additional water deposited on the surface appears as bright protrusions (yellow star), suggesting it protrudes significantly above the surface and the previously adsorbed molecules (C). The (√2 × √2)R45° surface unit cell is shown by a white square. Representative STM images of the same structures are shown in SI Appendix, Fig. S4.

Fig. 4.

O 1s XPS data showing that the water agglomerates formed on Fe₃O₄(001) are partially dissociated. The as-prepared surface exhibits a single peak at 530.1 eV due to the lattice oxygen atoms. The 2.6 D₂O/u.c. data should be compared with the surface shown in Fig. 3A and show roughly equal contributions from OD and D₂O, consistent with one dissociated molecule per water dimer/trimer. Most of the additional water adsorbed at a coverage of 7.7 H₂O/u.c. is molecular. Data were measured at 95 K, with monochromatic Al Kα radiation and at a grazing exit of 80° for the emitted photoelectrons.

Fig. 5.

Top view of the minimum-energy structures determined by DFT for water coverages of 1, 2, 3, 6, and 8 H₂O/u.c. Fe atoms are blue, O are red, and H are white. (A) An isolated molecule adsorbs intact, but partially dissociated water dimers and trimers are energetically preferred. Two partially dissociated trimer structures are calculated to be energetically degenerate. The (√2 × √2)R45° unit cell and both O* are highlighted. (B) DFT-based model at 6 H₂O/u.c. showing a ring-like structure based on full occupation of the Fe_oct rows with OH or H₂O, and water molecules bridging the O* sites. These bridging molecules are adsorbed partly through H bonds to surface O*H groups. The O*H groups beneath the adsorbed molecules are shown in the topmost white circle. Alternatively, the structure can be viewed as based on a pair of H₂O–OH–H₂O trimers (labeled 1 and 2). (C) DFT-based model at 8 H₂O/u.c. showing a complex structure utilizing dangling bonds in the 6 H₂O/u.c. structure to form a second bridge in the region of the yellow star. All adsorption energies are given in electronvolts.

Fig. 6.

The geometry of partially dissociated water dimers and trimers reveals a cooperative binding effect. (A) A molecular water dimer exhibits a relatively long intermolecular H bond, and the H-bond acceptor has a weakened interaction with the surface compared with an isolated molecule. (B) The partially dissociated water dimer exhibits a strong intermolecular H bond, and the H-bond–donating water molecule binds more strongly to the substrate. (C) In the partially dissociated water trimer, the second water molecule donates an H bond to the OH group, further weakening its bond to the substrate. All bond lengths are given in angstroms, and energies are in electronvolts.