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 x 1) periodicity. At higher temperatures and H-poor conditions water can be removed from the surface and the two (2 x 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. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim