Surface structure of nickel oxide layers on a Rh(111) surface

Abstract

The formation of nickel oxide nanolayers by oxidizing Ni overlayers on Rh(111) has been investigated and their structures are reported as a function of the nickel coverage and oxygen pressure. Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and diffraction (XPD), and high-resolution electron energy loss spectroscopy (HREELS) have been applied to characterize the structure and stoichiometry of the nickel oxide nanolayers. Several different phases have been observed depending on the strain state of the metallic Ni overlayers. For the pseudomorphic Ni monolayer, two distinctly different oxide phases with (6×1)-Ni5O5 and (2√3×2)-Ni8O10 structures have been identified at oxygen-poor (p=5×10−8mbar) and oxygen-rich (p≥1×10−6mbar) conditions, respectively. Above one monolayer, where the Ni layers are relaxed, bulk-like NiO(100) films form at the O-rich conditions, whereas chemisorbed-type p(2×2)ONi(111) layers develop in the O-poor regime. X-ray photoelectron diffraction analysis has provided additional insight into the relaxation mechanism and the detailed atomic structure of the Ni-oxide nanolayers.