Substrate Grain-Dependent Chemistry of Carburized Planar Anodic TiO₂ on Polycrystalline Ti

Abstract

Mixtures or composites of titania and carbon have gained considerable research interest as innovative catalyst supports for low-and intermediate-temperature proton-exchange membrane fuel cells. For applications in electro-catalysis, variations in the local physicochemical properties of the employed materials can have significant effects on their behavior as catalyst supports. To assess microscopic hetero-geneities in composition, structure, and morphology, a microscopic multitechnique approach is required. In this work, compact anodic TiO2 films on planar polycrystalline Ti substrates are converted into carbon/titania composites or multiphase titanium oxycarbides through carbothermal treatment in an acetylene/argon atmosphere in a flow reactor. The local chemical composition, structure, and morphology of the converted films are studied with scanning photoelectron microscopy, micro-Raman spectroscopy, and scanning electron microscopy and are related with the crystallographic orientations of the Ti substrate grains by means of electron backscatter diffraction. Different annealing temperatures, ranging from 550 to 850 degrees C, are found to yield different substrate grain-dependent chemical compositions, structures, and morphologies. The present study reveals individual time scales for the carbothermal conversion and subsequent surface re-oxidation on substrate grains of a given orientation. Furthermore, it demonstrates the power of a microscopic multitechnique approach for studying polycrystalline heterogeneous materials for electrocatalytic applications

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

“…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 …”

“…This statement could explain why no TiO x C y compound has been observed with XPS in Refs . and. Carburization at temperatures of 650 °C and higher promotes the conversion of the entire anodic TiO 2 film to TiO x C y , whose average stoichiometry is affected by the annealing temperature ,.…”

“…In contrast, a strong correlation between graphite‐like carbon species and the annealing temperature is observed, which is responsible for the trend of the overall surface C concentration on the TiOC films. The initial increase of the C=C concentration from 550 °C to 650 °C can be ascribed to an improving carbon deposition efficiency, and the decrease with further increasing annealing temperature can be explained by thermally activated diffusion of carbon into the bulk of the film . Figure depicts Ti 2 p and C 1 s core level spectra recorded at NE of pristine TiOC and after air exposure of the films for two months, and the respective fractions of Ti and C species.…”

“…This chemical analysis and previously published results reveal carburization and reoxidation pathways of TiOC films. The chemical composition of pristine TiOC depends on the annealing temperature due to a temperature‐dependent carbon deposition and film conversion efficiency ,. Apparently, the carbothermal treatment at 550 °C yields a superficial conversion of the compact anodic oxide film.…”

“…Apparently, the carbothermal treatment at 550 °C yields a superficial conversion of the compact anodic oxide film. While TiO x C y compounds are detected with XPS at the surface of pristine TiOC 550 (Figure ), the bulk of the TiOC 550 film is reported to consist of some reduced TiO x species in a mixed phase (anatase and rutile) crystalline TiO 2 matrix, which brings about a bluish optical appearance of TiOC 550 ,. When exposed to air, the carburized layer at the surface of TiOC 550 , and in particular the TiC species, are oxidized within a short time (Figure ), resulting in a C/TiO 2 composite film, with a thin layer of ( sp 2 and sp 3 ) carbon on a still slightly reduced, crystalline oxide.…”