Water agglomerates on Fe3O4(001)

Abstract

<p id="d4289640e257">Determining the structure of water on metal oxide surfaces is a key step toward a molecular-level understanding of dissolution, corrosion, geochemistry, and catalysis, but hydrogen bonding and large, complex unit cells present a major challenge to modern theory. Here, we utilize state-of-the-art experimental techniques to guide a density functional theory (DFT)-based search for the minimum-energy configurations of water on Fe ₃O ₄(001). A subsurface reconstruction dominates adsorption at all coverages. An ordered array of partially dissociated water agglomerates form at low coverage, and these serve to anchor a hydrogen-bonded network. We argue that similar behavior will occur whenever a surface presents a well-spaced array of active sites for dissociation. Given the propensity of metal oxides to undergo surface reconstructions, this is likely often. </p><p class="first" id="d4289640e266">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 Fe ₃O ₄(001); a complex mineral surface relevant in both modern technology and the natural environment. Strong OH–H ₂O 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. </p>