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

TL;DR

This study uses density-functional theory and thermodynamics to analyze the stability of various oxygen-terminated ZnO surfaces under different environmental conditions, revealing hydrogen adsorption preferences and surface defect formations.

Contribution

It provides a comprehensive phase diagram of ZnO surface structures considering oxygen vacancies and hydrogen adatoms, highlighting the influence of temperature and pressure.

Findings

Hydrogen prefers to adsorb at the surface with 1/2 monolayer coverage.

High temperature and low pressure favor oxygen-deficient surfaces.

Clean, defect-free surfaces are unlikely due to hydrogen and water adsorption.

Abstract

Using density-functional theory in combination with a thermodynamic formalism we calculate the relative stability of various structural models of the polar O-terminated (000-1)-O surface of ZnO. Model surfaces with different concentrations of oxygen vacancies and hydrogen adatoms are considered. Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2 gas 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 be adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At high temperatures and low pressures the hydrogen can be removed and a structure with 1/4 of the surface oxygen atoms missing becomes the most stable one. The clean, defect-free surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, since we find that the dissociative adsorption of molecular hydrogen and water (if also the Zn-terminated surface is present) is energetically very preferable, it is very unlikely that a clean, defect-free (000-1)-O surface can be observed in experiment.