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

Abstract

International audienceThe 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 <2<(11)over bar>0> and <10<(1)over bar>0> planes show a higher grade of conversion. Both experimental and DFT data validate the use of TiOxCy as an innovative support for electrocatalysis

Application of Polarization Modulation Infrared Reflection Absorption Spectrosocpy Under Electrochemical Control for Structural Studies of Biomimetic Assemblies

“…This material has been used as a new mirror in IRRAS for quantitative analysis of the molecular structure and orientation of arachidic acid moleecules in Langmuir-Blodgett monolayers [103]. At the micrscopic level the compact hybrid TiO x C y material is not homogenous [104,105]. On its surface TiO 2 , TiC, C (graphite) and substoichimetric titanium oxides (TiO 2−x ) have been identified.…”

Effect of Air‐Aging on the Electrochemical Characteristics of TiOxCy Films for Electrocatalysis Applications

“…In TiOC 550 , the fraction of TiO 2 is the same for GE and NE, but the TiC (TiO) content is diminished and the fraction of TiO x slightly enhanced at the surface. These observations suggest a reoxidation of the reduced titania species, in particular the TiO x C y compounds, in the TiOC films at the surface, which is known to happen during air exposure . The decreased fraction of C−Ti and higher relative amount of carbon (in particular C−C), observed at GE (Figure (b,d)) evidence that the carburized films are covered by a layer of carbon …”

“…Carburized compact anodic TiO 2 films on planar titanium substrates provide a flexible model system for the investigation of Ti−O−C materials in the context of electrocatalysis applications. The optimized carbothermal treatments of compact anodic TiO 2 with acetylene or ethylene as carbon source, can produce Ti−O−C films of different chemical composition and structure ,. Their simple morphology allows the use of scanning probe techniques as analysis tool, and facilitates the deposition of catalyst nanoparticles .…”

“…The titanium substrate stabilizes the Ti−O−C film and provides the electrical contact, which is essential for electrochemical investigations. However, the optimal film composition with regard to interfacial electrical conductivity and catalytic performance of deposited Pt has not been identified yet ,,. TiO x C y films are prone to surface reoxidation (to TiO 2 ) when exposed to air or during anodic polarization, which is due to the intrinsic thermodynamic instability of the oxycarbide phase ,.…”

“…However, the optimal film composition with regard to interfacial electrical conductivity and catalytic performance of deposited Pt has not been identified yet ,,. TiO x C y films are prone to surface reoxidation (to TiO 2 ) when exposed to air or during anodic polarization, which is due to the intrinsic thermodynamic instability of the oxycarbide phase ,. Moreover, and interestingly for the development of (intermediate temperature) direct ethanol fuel cell electrodes, it was shown that the addition of ethanol inhibits anodic oxidation of TiO x C y .…”