Deformed Surface Oxides: Uncommon Structure of a (6 × 1) NiO Surface Oxide on Rh(111)

We have investigated the formation of a nickel oxide monolayer on Rh(111) by scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations. This monolayer displays a corrugated (6 × 1) superstructure, with a formal Ni5O5 stoichiometry. The interplay between polarity and interface energy leads to the formation of a structure with pronounced troughs, which may be regarded as a building deformation, containing building blocks derived from nonpolar (100) and polar (111) NiO surface terminations. The calculations also show that the (6 × 1) Ni5O5 phase has a higher thermodynamic stability than the related octopolar NiO bulk terminations. SECTION: Surfaces, Interfaces, Catalysis I recent years, the surface and interface properties of 3d oxide materials have received considerable interest due to their wide range of technological applications, ranging from magnetic storage media to catalytic materials. Yet, even the structure of an oxide surface, such as NiO, can be rather complex. Due to the rocksalt (NaCl) bulk structure of nickel oxide, the crystal can display both nonpolar surfaces such as NiO(100), with no macroscopic dipole moment, and also polar surfaces such as NiO(111). In the case of a polar surface, the resulting polar instability has to be compensated for by either structural or electronic modifications. An overview of the dominant compensating mechanisms can be found in a recent review by Goniakowsky. In the case of the polar NiO(111) surface, surface X-ray diffraction experiments indicate a compensation by the formation of a p(2 × 2) octopolar reconstruction. On the other hand, if the oxide surface is not the termination of the NiO bulk structure but is present as an ultrathin layer (with a thickness of only a few nanometers) supported on a single-crystal metal substrate, the structural and electronic flexibility at the interface allows for the presence of new phases that would be unstable in the thick-film limit. Moreover, such surface oxide structures are determined not only by polarity considerations but also by the strain induced by the lattice mismatch between the metallic substrate material and the (bulk) nickel oxide. In particular, fcc (111) surfaces offer an interesting playground as the lattice symmetry of the substrate favors a polar (111) termination while the polarity considerations usually predict a lower surface energy for the nonpolar (100) bulk terminations. In addition, low dimensionality aspects of the supported Ni oxide structures have to be considered and may give rise to different physical behavior. Ni oxide films with a thickness of several atomic layers have been investigated with scanning tunneling microscopy (STM) and X-ray photon spectroscopy (XPS) on Ni(111), Cu(111), and Au(111). Yet, the knowledge about the structural properties at the onset of the growth of the NiO layers is rather sparse. In this Letter, we report on the formation of a (6 × 1) NiO monolayer supported on a Rh(111) single-crystal surface and analyze the influence of the polarity and strain on the structure of this surface oxide phase. Oxidation of submonolayer Ni films in 5 × 10−8 mbar of oxygen causes the formation of twodimensional (2-D) islands exhibiting a (6 × 1) structure (Figure 1a), which grow in size with increasing Ni coverage and eventually form a wetting layer at 1 ML (Figure 1b). The atomically resolved STM image in Figure 1c shows that the (6 × 1) structure displays zigzag ridges aligned pseudomorphically along the ⟨110⟩ substrate directions, which are separated by 6 Rh lattice constants, yielding the (6 × 1) periodicity (the unit cell is indicated on the image). Between the ridges, hexagonally ordered areas are visible. The atomic corrugations along and across the ridges are 0.1 and 0.3 Å, respectively. Occasionally, other ridge periodicities like (4 × 1) and (2 × 1) have been also Received: November 18, 2011 Accepted: December 27, 2011 Published: December 27, 2011 Letter