Work-function and structures of (100), (111) and (101) Au surfaces with/without oxygen

The Work function (f)is fundamental for chemistry and electronics. Additionally, f can be used to examine the validity of the theoretical surfaces by comparing it with experimental f, even in the absence of long-range orders. In the reported and present experiments, the difference in f between pristine and oxygen-covered Au surfaces (df) is <1 eV at =<1 ML (1 ML: one full-monolayer). Contrarily, the available density functional theory (DFT) reports df ~ 3 eV for Au(111) surfaces at 1 ML. Hence, we study structures of O-atom-covered Au(100), Au(110), and Au(111) surfaces using DFT. The calculated most stable structures show df <1.1 eV at =<1 ML and a nearly constant df at > 1 ML, which match experiments and are confirmed using hybrid functional. These agreements result from the stability-criteria transition between low and high O-coverages, driven by the O-induced displacements of Au-atoms and the new surface structures at high O-coverages. The most stable structures exhibit molecule-like O arrangements at Au(111) surfaces at 1 ML and all surfaces at 2 ML; the former is considered chemisorption. At Au(111) surfaces, some structures containing O-atoms in subsurfaces have formation energies that approach those of the most stable structures, while the variation of these structures increases with surface size. Hence, mixing these structures with the most stable structures is believed to destroy long-range orders, which agrees with the experiments. The density of states at the surfaces calculated using the hybrid functional exhibit small bandgaps at the Au(100) and Au(110) surfaces at 1 ML.