First‐principles study of the reconstruction and hydroxylation of the polar NiO(111) surface

Density‐functional theory (DFT) calculations have been combined with a thermodynamic formalism to determine a phase diagram of lowest‐energy structures and compositions of the polar NiO(111) surface in equilibirum with oxygen, hydrogen, and water reservoirs at finite temperatures and pressures. Consistent with experiment we find that for a wide range of oxygen and hydrogen chemical potentials the surface is fully hydroxylated and shows a (1 × 1) periodicity. At higher temperatures and H‐poor conditions water can be removed from the surface and the two (2 × 2) octopolar reconstructions become the thermodynamically most stable configurations. Other structures, which have been proposed on the basis of experimental data after high‐temperature annealing, have to be considered to be kinetically limited metastable phases. In O‐poor conditions no reduced surface structures are found to be thermodynamically stable. However, in O‐rich environments and at low hydrogen chemical potential the surface can be oxidized by a partial removal of hydrogen or incorporation of additional oxygen. The structural motifs are closely related to the cadmium iodide structure of Ni(OH)2 and NiO2.