Surface hydroxylation and local structure of NiO thin films formed on Ni(111)

Abstract

X-ray photon spectroscopy (XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) have been combined to study the surface hydroxylation and the structure of NiO thin films formed on Ni(111) at 300 and 500 K. After oxidation at 300 K, the thin film consists of NiO(111) grains (3 ± 1 nm large) in parallel epitaxy with the substrate and homogeneously distributed on the substrate. The average thickness is three to four NiO(111) layers with local variations measured on different grains [ca 1 NiO(111) plane] and at boundaries between grains (up to 0.6 nm). The film is terminated by 0.85 ± 0.1 monolayer (ML) of hydroxyl groups (OH − according to XPS) stabilizing the unreconstructed (1 × 1) oxide surface structure. Exposing this film to 150 L (1 Langmuir = 10 −6 Torr s −1) of water leads to the formation of a second layer of hydroxyl groups with a total coverage of 1.5–2 ML with no significant variation of the structure of the film. The XPS data are well fitted with a terminal sequence (OHNiOH) equivalent to one layer of β-Ni(OH) 2 with partial filling of the topmost OH plane. After oxidation at 500 K, the surface is covered at 93 ± 3% by extended and atomically flat NiO(100) areas. Triangular NiO(111) grains (8 ± 2 nm large) in parallel epitaxy are present within the NiO(100) layer and cover 7 ± 3% of the surface. They are covered by a full monolayer of OH − groups also stabilizing the (1 × 1) structure. The NiO(100) layer is characterized by (2 × 2), (2 × 3), (3 × 2) and (3 × 3) local superstructures assigned to local deviations from the average NiO stoichiometry in the topmost plane. These areas do not adsorb hydroxyl groups on regular sites but possibly on defect sites (steps). Exposing this film to 150 L of water leads to the lateral extension of the triangular NiO(111) grains. A preferential reorientation of hydroxyl-free NiO(100) to OH −-covered NiO(111) upon water exposure is observed. This reorientation affects the NiO(100) matrix only at the perimeter of the NiO(111) grains, showing that the hydroxyl groups do not induce nucleation of NiO(111) in the NiO(100) layer but cause the growth of the existing NiO crystallites.