Real-time scanning tunneling microscopy observations of the oxidation of a Ti/Pt(111)-(2 × 2) surface alloy using O₂ and NO₂

The authors have used scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) to study the nascent oxidation of an ordered TiPt (111) - (2×2) surface alloy exposed to oxygen (O2) or nitrogen dioxide (N O2) under ultrahigh vacuum conditions. The TiPt (111) - (2×2) surface alloy was formed by depositing an ultrathin Ti film on Pt(111) and annealing to 1050 K. This produces an alloy film in which the surface layer is pure Pt and the second layer contains Ti atoms in a (2×2) structure, which causes the pattern observed by STM and LEED. Real-time imaging of the surface at 300 K was carried out by continuously scanning with the STM while either O2 or N O2 was introduced into the chamber. O2 exposures did not cause any gross structural changes; however oxygen was detected on the surface afterward using AES. Annealing this surface to 950 K resulted in the formation of an ordered Ti Ox overlayer as characterized by both LEED and STM. In contrast, N O2 exposures caused definite changes in the surface morphology at 300 K, and the root-mean-square roughness increased from 3.5 to 7.1 Å after a large N O2 exposure. No ordered structures were produced by this treatment, but annealing the surface to 950 K formed an ordered pattern in LEED and corresponding clear, well-resolved structures in STM images. We account for these observations on the disruption or reconstruction of the TiPt (111) - (2×2) surface alloy by arguments recalling that Ti oxidation is an activated process. The energetic barrier to Ti Ox formation cannot be surmounted at room temperature at low oxygen coverages, and annealing the surface was necessary to initiate this reaction. However, the higher oxygen coverages obtained using the more reactive oxidant N O2 lowered the chemical potential in the system sufficiently to overcome the activation barrier to extract Ti from the alloy at room temperature and form a disordered Ti Ox film. These results illustrate the importance of the surface oxygen coverage in nucleating the room temperature oxidation of the Pt-Ti surface alloys and further show the ability of N O2 in ultrahigh vacuum studies for probing the chemistry that will occur at higher O2 pressure.