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

Abstract

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 Ti∕Pt(111)-(2×2) surface alloy exposed to oxygen (O2) or nitrogen dioxide (NO2) under ultrahigh vacuum conditions. The Ti∕Pt(111)-(2×2) surface alloy was formed by depositing an ultrathin Ti film on Pt(111) and annealing to 1050K. 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 300K was carried out by continuously scanning with the STM while either O2 or NO2 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 950K resulted in the formation of an ordered TiOx overlayer as characterized by both LEED and STM. In contrast, NO2 exposures caused definite changes in the surface morphology at 300K, and the root-mean-square roughness increased from 3.5to7.1Å after a large NO2 exposure. No ordered structures were produced by this treatment, but annealing the surface to 950K 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 Ti∕Pt(111)-(2×2) surface alloy by arguments recalling that Ti oxidation is an activated process. The energetic barrier to TiOx 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 NO2 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 TiOx 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 NO2 in ultrahigh vacuum studies for probing the chemistry that will occur at higher O2 pressure.