First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen

摘要

Using density-functional theory in combination with a thermodynamic formalismwe calculate the relative stability of various structural models of the polarO-terminated (000-1)-O surface of ZnO. Model surfaces with differentconcentrations of oxygen vacancies and hydrogen adatoms are considered.Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2gas phase we determine a phase diagram of the lowest-energy surface structures. For a wide range of temperatures and pressures we find that hydrogen will beadsorbed at the surface, preferentially with a coverage of 1/2 monolayer. Athigh temperatures and low pressures the hydrogen can be removed and a structurewith 1/4 of the surface oxygen atoms missing becomes the most stable one. Theclean, defect-free surface can only exist in an oxygen-rich environment with avery low hydrogen partial pressure. However, since we find that thedissociative adsorption of molecular hydrogen and water (if also theZn-terminated surface is present) is energetically very preferable, it is veryunlikely that a clean, defect-free (000-1)-O surface can be observed inexperiment.