Single Rh adatoms stabilized on {α}-Fe2O3(1-102) by co-adsorbed water

Oxide-supported single-atom catalysts are commonly modelled as a metal atom substituting surface cation sites in a low-index surface. Adatoms with dangling bonds will inevitably coordinate molecules from the gas phase, and adsorbates such as water can affect both stability and catalytic activity. Here, we use scanning tunneling microscopy (STM), noncontact atomic force microscopy (ncAFM), and x-ray photoelectron spectroscopy (XPS) to show that high densities of single Rh adatoms are stabilized on alpha-Fe2O3(1-102) in the presence of 2x10^(-8) mbar of water at room temperature, in marked contrast to the rapid sintering observed under UHV conditions. Annealing to 50 {\deg}C in UHV desorbs all water from the substrate leaving only the OH groups coordinated to Rh, and high-resolution ncAFM images provide a direct view into the internal structure. We provide direct evidence of the importance of OH ligands in the stability of single atoms, and argue that their presence should be assumed when modelling SAC systems.