“X-Scheme” Charge Separation Induced by Asymmetrical Localized Electronic Band Structures at the Ceria Oxide Facet Junction

Photogenerated charge transfer can be tuned by crystal face controlling and heterostructure engineering. However, it is exceptionally challenging to understand the separation and transfer process of charge carriers, especially on the facet junction. Here, we show a separation pathway of the charge carriers in (111)/(100)CeO2–x nanomaterials as the "X-Scheme junction" using selected region electron energy-loss spectroscopies. The driving force for the "X-Scheme junction" can be attributed to the localized electronic band structures' asymmetry of the ceria, which gathers the photogenerated electrons on the surface of the (111) plane and accumulates the photogenerated holes on the (100) face. Surface oxygen vacancies channel the photogenerated electrons and holes further to the Ce3+ of the (111) plane and the Ce4+ of the (100) plane, respectively, resulting in an enhanced photo-oxidative nitric oxide removal efficiency under visible-light irradiation. Furthermore, the system is found to integrate metal nanoparticles to form Pd–CeO2–x, including Pd-(111)CeO2–x and Pd-(100)CeO2–x, which significantly restricts nitric dioxide production as a byproduct. This work provides a unique approach to understanding single-crystal photocatalysts based on the facet junction.