Carbothermal Transformation of TiO2 into TiOxCy in UHV: Tracking Intrinsic Chemical Stabilities

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• 作者：Laura Calvillo ; Diego Fittipaldi ; Celine R眉diger ; Stefano Agnoli ; Marco Favaro ; Carlos Valero-Vidal ; Cristiana Di Valentin ; Andrea Vittadini ; Nathalie Bozzolo ; Suzane Jacomet ; Luca Gregoratti ; Julia Kunze-Liebh盲user ; Gianfranco Pacchioni ; Gaetano Granozzi
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：October 2, 2014
• 年：2014
• 卷：118
• 期：39
• 页码：22601-22610
• 全文大小：671K
• ISSN：1932-7455

文摘

The conversion of anodic TiO₂ films into TiO_xC_y 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 TiO_xC_y 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 TiO_x species. Above ca. 750 K, the formation of TiO_xC_y 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 TiO_xC_y 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 TiO_xC_y starts), the TiO_xC_y 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 <21虆1虆0> and <101虆0> planes show a higher grade of conversion. Both experimental and DFT data validate the use of TiO_xC_y as an innovative support for electrocatalysis.