Hierarchical Heterogeneity at the CeOx–TiO2 Interface: Electronic and Geometric Structural Influence on the Photocatalytic Activity of Oxide on Oxide Nanostructures

Abstract

Mixed oxide interfaces are critical for delivering active components of demanding catalytic processes such as the photocatalytic splitting of water. We have studied CeOx–TiO2 catalysts with low ceria loadings of 1, 3, and 6 wt % that were prepared with wet impregnation methods to favor a strong interaction between CeOx and TiO2. In these materials the interfaces between CeOx–TiO2 have been sequentially loaded (1%, 3%, and 6%), with and without Pt (0.5 wt %). The structure and properties of the catalysts were characterized using several X-ray and electron based techniques including XRD, XPS, UPS, NEXAFS, UV–vis, and HR-STEM/STEM-EELS to unravel the local morphology, bulk structure, surface states, and electronic structure. The combination of all these techniques allows us to analyze in a systematic way the complete structural and electronic properties that prevail at the CeOx–TiO2 interface. Fluorite structured nanocrystallites of ceria on anatase-structured titania were identified by both XRD and NEXAFS. A sequential increase of the CeOx loading led to the formation of clusters, then plates, and finally nanoparticles in a hierarchical manner on the TiO2 support. The electronic structures of these catalysts indicate that the interaction between TiO2 and CeO2 is closely related to the local morphology of nanostructured CeO2. Ce3+ cations were detected at the surface of CeO2 and at the interface of the two oxides. In addition, the titania is perturbed by the interaction with ceria and also with Pt. The photocatalytic activity for the splitting of H2O using UV light was measured for these materials and correlated with our understanding of the electronic and structural properties. Optimal catalytic performance and photoresponse results were found for the 1 wt % CeOx–TiO2 catalyst where low dimensional geometry of the ceria provided ideal electronic and geometrical properties. The structural and electronic properties of the interface were critical for the photocatalytic performance of this mixed-oxide nanocatalyst system.