Interfacial properties of ceria films on yttria‐stabilized zirconia

Abstract

Ceria (CeO 2 ) and yttria‐stabilized zirconia (YSZ) are two promissing electrolyte materials used in solid oxide fuel cells due to their high ionic conductivities. Theory predicts that the formation energy of oxygen vacancies is decreased at free surfaces and internal interfaces. Oxygen vacancies are expected to segregate at interfaces and thus might provide an easy path for rapid ion conduction. 1 Aims of the present work are: (i) to obtain insights into the structure and chemistry of interfaces between ceria films and YSZ substrates, and (ii) quantitative assessments of ceria/zirconia intermixing and oxidation state of Ce at interfaces. Analytical scanning transmission electron microscopy (STEM) is the method of choice for a comprehensive materials characterization in terms of structure and chemistry down to atomic levels. 2 In the present study high‐resolution electron spectroscopic imaging was performed in an advanced TEM/STEM system (JEOL JEM‐ARM200CF) equipped with a cold field‐emission gun, a probe C s ‐corrector (DCOR, CEOS/Heidelberg), and X‐ray and electron spectrometer attachments (JEOL Centurio SDD; GATAN GIF Quantum ERS). Electron energy‐loss spectroscopy (EELS) in STEM enables to probe the local oxidation state of Ce ions in interface regions by utilizing valence sensitive features in the energy‐loss near‐edge fine structures of Ce‐M 4,5 edges. Continuous epitaxial films of pure and 10 mol% Gd doped ceria were grown by pulsed laser deposition on (111) YSZ substrates (Fig. 1a). The film‐substrate lattice mismatch is accommodated by misfit dislocations (extra atomic planes in the YSZ substrate). 3 Atomic column‐resolved EEL spectroscopic images (ESI) were acquired in selected regions of interest (ROI) across ceria‐YSZ interfaces. ESI enables the visualization of Ce 4+ àCe 3+ reduction in narrow interface regions using multiple linear least‐square (MLLS) fitting methods (Fig. 1c). ESI setup parameters: ROI 134x39 pixels, pixel size 0.051nm, dwell time 0.02 s/pixel, probe current 140 pA, camera length 15 mm, collection angle 110 mrad, energy resolution 0.5 eV, total acquisition time 108 s. Chemical and valence changes across interfaces were quantitatively assessed by EELS line scans (Figs. 2,3) with acquisition parameters: probe size 0.1 nm, probe current 140 pA, convergence angle 28 mrad, collection angle 110 mrad, number of measured points 160, step width 45 pm, range 7 nm, dwell 1 s/pixel, acquisition time 163 s. Parallel recording of EELS and HAADF signals in STEM‐EELS enables a precise correlation of EELS spectra and structural features at atomic levels regardless of eventual sample drift. Thus, atomically resolved EEL spectra were extracted column‐by‐column from line scans (integration windows 0.25 nm) and quantitatively evaluated for each individual (111) atom plane crossed by the line scan. The Ce‐M 5 /M 4 intensity ratios were measured by the second derivative method 3 (Fig. 3a), from which the fraction of Ce 3+ can be deduced (ratios of 0.95 and 1.26 correspond to Ce 4+ and Ce 3+ in CeO 2 and CePO 4 reference materials, respectively) 3 . Chemical profiles indicate a Ce/Zr intermixing zone extending over seven (111) lattice planes (Fig.3b). No noticeable differences were observed between pure and Gd doped ceria films. In both doped and non‐doped ceria films, Ce 4+ is gradually reduced in a region of 7 to 9 (111) lattice planes wide with a maximum fraction of 0.9 to 1.0 for Ce 3+ in a single atomic layer at the interface (Fig. 3a). Assuming charge balance, the presence of Ce 3+ ions is seen as evidence of oxygen vacancy formation in narrow interface regions. In summary, it is concluded that advanced analytical TEM/STEM methods enable the elucidation of local non‐stoichiometry, which is crucial not only for understanding charge transport mechanisms in these hetero‐structured materials, but also for understanding the catalytic properties of ceria. 3,4