Pressure-dependent Ni–O phase transitions and Ni oxide formation on Pt(111): An in situSTM study at elevated temperatures

Abstract

Growth, atomic structure and O₂ partial pressure dependent phase transitions of Ni–O structures and thin NiO films on Pt(111) have been studied using scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). In situ STM experiments were performed during film growth by reactive metal deposition at elevated temperatures (400–550 K) and variable O₂ pressure. Depending on the substrate temperature, one-dimensional network-like Ni–O structures and islands with (7 × 1) and (4 × 2) reconstructions are formed during the initial stages of growth. These structures transform reversibly to a (2 × 2) reconstruction in a narrow O₂ pressure range of 1.5–2 × 10⁻⁶ mbar and can be monitored by in situ STM. Upon reduction of the O₂ pressure to <10⁻¹⁰ mbar pseudomorphic Ni monolayers are obtained. The defect-free ordering of Ni atoms on Pt(111) in a single stacking domain indicates an O-surfactant induced growth mode. The structural properties of the O₂ pressure-dependent Ni–O phases are discussed in a simple model assuming NiO(001)-like atomic arrangements in the adsorbate overlayer. At higher coverage stable (111)-oriented NiO islands grow in a three-dimensional mode.