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Carbothermal Transformation of TiO 2 into TiO x C y in UHV: Tracking Intrinsic Chemical Stabilities

2014; American Chemical Society; Volume: 118; Issue: 39 Linguagem: Inglês

10.1021/jp506728w

1932-7455

Laura Calvillo, Diego Fittipaldi, Celine Rüdiger, Stefano Agnoli, Marco Favaro, Carlos Valero‐Vidal, Cristiana Di Valentin, Andrea Vittadini, Nathalie Bozzolo, S. Jacomet, Luca Gregoratti, Julia Kunze‐Liebhäuser, Gianfranco Pacchioni, Gaetano Granozzi,

Copper-based nanomaterials and applications

The conversion of anodic TiO2 films into TiOxCy in ultrahigh-vacuum (UHV) has been traced by photoemission spectroscopy in order to optimize the process parameters and study the different phase stabilities. In addition, density functional theory (DFT) calculations have been performed in order to elucidate the main questions about TiOxCy composition and stability. The experimental data indicate that the anodic TiO2 film is stable both in UHV and ethylene background up to ca. 600 K, and at this temperature, it starts to reduce leading to suboxide TiOx species. Above ca. 750 K, the formation of TiOxCy starts, since the oxygen vacancies begin to be replaced by carbon atoms. A surface enrichment in TiO2 and elemental carbon has been detected on the converted TiOxCy film at room temperature. Real-time measurements have shown that this phenomenon takes place during the cool down process and DFT calculations suggest a possible explanation: as the temperature decreases below ca. 750 K (temperature at which the formation of TiOxCy starts), the TiOxCy phase is not thermodynamically stable, and it decomposes into TiO2 and elemental carbon. The comparison of the experimental valence band data with DFT results has also allowed to establish that the film surface is not homogeneous and that segregation of TiO and TiC systems may take place. On the other hand, the local compositional study carried out by scanning photoelectron microscopy has shown that the conversion of the film is not homogeneous but depends on the grain orientation, in particular crystallites with an orientation close to and planes show a higher grade of conversion. Both experimental and DFT data validate the use of TiOxCy as an innovative support for electrocatalysis.