Defect chemistry of oxide nanomaterials with high surface area: Ordered mesoporous thin films of the oxygen storage catalyst CeO_2-ZrO _2

摘要

Herein we report the electrical transport properties of a series of ordered mesoporous ceria-zirconia (Ce_xZr_(1-x)O_2, referred to as mp-CZO) thin films with both a cubic structure of (17 ± 2) nm diameter pores and nanocrystalline walls. Samples over the whole range of composition, including bare CeO_2 and ZrO_2, were fabricated by templating strategies using the large diblock copolymer KLE as the structure-directing agent. Both the nanoscale structure and the chemical composition of the mesoporous materials were analyzed by a combination of scanning and transmission electron microscopy, grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The total conductivity as a function of the film composition, temperature, and oxygen partial pressure was measured using impedance spectroscopy. The mesoporous solid solutions of CeO _2-ZrO_2 prepared in this work showed a higher stability against thermal ripening than both binary oxides, making them ideal model systems to study both the charge transport properties and the oxygen storage at elevated temperatures. We find that the redox properties of nanocrystalline mp-CZO thin films differ significantly from those of bulk CZO materials reported in the literature and, therefore, propose a defect chemical model of surface regions.