Atomic Layer Deposition of Nanometer-Sized CeO₂ Layers in Ordered Mesoporous ZrO₂ Films and Their Impact on the Ionic/Electronic Conductivity

The physicochemical properties of thin metal oxide layers strongly depend on the layer thickness and thus differ significantly from their bulk counterpart. In this work, we present the growth of defined thin layers of CeO2 within mesostructured ZrO2 thin films using atomic layer deposition (ALD). The prepared films consist of a cubic ordered arrangement of 15 nm spherical mesopores induced by the used diblock copolymer poly(isobutylene)-block-poly(ethylene oxide) (PIB50-b-PEO45), which allows studying the growth process and the successful coating of the interior pore surfaces via the combination of scanning electron microscopy (SEM), time-of-flight mass spectrometry (ToF-SIMS), and laser ellipsometry. These methods prove the CeO2 layer growth and impregnation of the pores up to 100 ALD cycles, at which the interconnecting channels between the mesopore layers are filled completely impeding further transport of the gaseous CeO2 precursors. X-ray photoelectron spectroscopy (XPS) and diffractometry (XRD) measurements point out the increased amount of Ce3+ after a low number of ALD cycles and show the presence of cubic CeO2 with increasing amount of ALD cycles, respectively. Impedance spectroscopic investigation further proves the formation of a continuous CeO2 path through the entire porous network of the insulating ZrO2 film and shows a strong influence of the layer thickness on the conductivity. All in all, our work presents the preparation of novel hybrid CeO2/ZrO2 model systems, which enable us to tailor their physicochemical properties by changing the thickness of the active oxide layer, and promises improvements for their use as catalysts in oxidation reactions such as the HCl oxidation reaction or as a three-way catalytic converter in automotives.