Oxygen Surface Exchange and Tracer Diffusion in Differently Oriented Thin Films of Gd-Doped CeO2

Isotope Exchange Raman Spectroscopy (IERS): A Novel Technique to Probe Physicochemical Processes In Situ

A novel in situ methodology for the direct study of mass-transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges, is developed. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, which is not accessible by conventional methods, enabling complementary insights for the study of ion-transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) is demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary-ion mass spectrometry (ToF-SIMS) characterization and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness, and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate the understanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research, and beyond.

Surface potentials of acceptor- and donor-doped CeO2 thin films and their relation to oxygen surface exchange

Surface Fermi level positions, ionisation potentials, and work functions of acceptor-, donor-, and nominally undoped CeO₂ have been determined by means of in situ photoelectron spectroscopy on films grown with different surface orientation and preparation conditions. The Fermi energy varies in accordance with the doping and oxygen activity. The ionisation potentials are largely unaffected by the preparation conditions and surface orientation. They are comparable for nominally undoped, 1% donor-doped, and 1% acceptor-doped films. The majority of the 10% Gd-doped films exhibit a 0.5 eV lower ionisation potential, which might be related to the presence of a surface space-charge region. The lower ionisation potential of the 10% Gd-doped films compensates for their lower Fermi energies and eventually results in work functions being largely independent on doping. Oxygen surface exchange coefficients determined using secondary ion mass spectrometry and conductivity relaxation experiments reveal similar magnitudes and are not strongly affected by doping type, concentration, and surface orientation. The results indicate that surface space-charge regions are crucial for oxygen surface exchange but do not allow to finally identify the rate determining step for oxygen incorporation into CeO₂-based materials.

Kinetics of competing exchange of oxygen and water at the surface of functional oxides

Kinetics of concomitant exchange reactions of oxygen from molecular oxygen and from water are determined from IEDP SIMS experiments.