Fundamental studies of titanium oxide-Pt(1 0 0) interfaces II. Influence of oxidation and reduction reactions on the surface structure of TiOx films on Pt(1 0 0)

Chemical reactions of titanium-oxide ultrathin films on Pt(1 0 0) and associated changes in the surface structure have been studied by STM (scanning tunneling microscopy), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and TPD (temperature programmed desorption). Such reactions and changes have important consequences for understanding and modeling of catalysis of related systems and the oxide structures formed have an additional significance in their subsequent use as masks or templates on Pt(1 0 0) in studies relevant to nanotechnology. In a previous paper [Matsumoto et al., Surf. Sci., submitted] we characterized a monolayer-oxide film exhibiting a (3 × 5) structure on Pt(1 0 0) as due to Ti2O3. Herein, we characterize changes that occur in this film during subsequent oxidation and reduction reactions, and propose models for the structures of TiOx films that are formed. O3 (ozone) or NO2 (nitrogen dioxide) oxidation of the (3 × 5)-Ti2O3 film at 600 K and subsequent annealing to 700–950 K in vacuum produced disordered oxide regions and domains of a (4 × 13) structure that we attribute to a TiO2 film with a square –Ti–O– net. This film is transformed further after annealing at 1000–1100 K to (3 × 5)-Ti2O3 domains and domains exhibiting a (2√2 × 2√2)R45° structure that we attribute to a Ti5O8 film. Additional heating of this film to 1200 K forms primarily a (3 × 5)-Ti2O3 oxide film. Three dimensional clusters of TiO2 are also produced by these oxidation and annealing procedures. Initial oxidation of a “flat” (3 × 5) oxide film at 600 K reconstructs this surface to form a multilayer, porous oxide film, and more extensive oxidation eventually forms a much less porous oxide film with pyramidal oxide crystallites. Oxidation of the (3 × 5) structure using NO (nitric oxide) differs from that above in that the (4 × 13) structure was not observed, but oxidation does produce the (2√2 × 2√2)R45° structure due to dissociation of NO at Pt sites. The titanium oxide films on Pt(1 0 0) described above block adsorption of CO at Pt sites at temperatures above 210 K, and also these surfaces do not oxidize CO. Exposure to a stronger reducing agent, HCOOH (formic acid), leads to molecular formic acid desorption at 371 K from a thick titanium oxide film containing (3 × 5) domains without a trace of surface reduction. However, formic acid exposures at low temperatures followed by heating in TPD reduce the (4 × 13) oxide structure, which is inert to reduction by CO, to form (3 × 5) and (2√2 × 2√2)R45° domains. Formic acid is also desorbed molecularly from clean Pt(1 0 0) at 330 K.