Correlation between surface potentials and surface oxygen exchange coefficents of CeO2
Synthesis and Properties of Functional Materials
Final Report Abstract
The Fermi energy at CeO2 surfaces measured by photoemission can vary by up to 2 eV depending on doping and processing conditions. The measured values largely follow bulk defect chemistry calculations, indicating that surface space charge regions cannot extend more than 1-2 nm. For not too strongly oxidising or reducing conditions, the surfaces exhibit a similar content of Ce3+ . Nominally undoped and donor-doped CeO2 exhibit similar ionisation potentials, irrespective of surface orientation. Surface faceting and the chemistry of Ce with its different oxidation states is believed to be responsible for this behaviour. An important consequence of this is that it is not possible to vary the ionisation potential and thereby the work function within the accessible rather narrow range of oxygen partial pressure either by changing the oxygen partial pressure or the surface orientation. One major experimental approach of this project to verify the hypothesis of the relation between work function and oxygen surface exchange coecients could therefore not be applied. The ionisation potential of acceptor-doped CeO2 is lower than that of undoped and donordoped material. The potential enhancement of oxygen exchange rate by the higher Fermi energy in donor-doped compared to acceptor-doped CeO2 is fully compensated by the higher ionisation potential. Finally, the work functions of donor- and acceptor CeO2 are comparable. The lower ionisation potential of Gd-doped CeO2 might have two different explanations: i) the segregation of acceptor dopants and the associated higher concentration of oxygen vacancies; ii) the presence of a very narrow surface space region. Within the project it was not possible to identify, which of the two contributions is more important. Electrical conductivities of Gd-doped thin films are dominated by oxygen ion conductivity and do not depend on oxygen partial pressure in the accessible range in agreement with bulk defect chemistry. The electrical conductivity of nominally undoped and Nb-doped CeO2 is dominated by electron transport. While the magnitude of conductivities and activation energies of ceramics can be well described using defect chemistry, those of thin films can only be explained with the contribution of grain boundaries. Electron accumulation/depletion in the space charge layers has to be assumed for nominally undoped/Nb-doped films, respectively, to explain the behaviour. This corresponds to segregation of oxygen vacancies/interstitials to the grain boundary core. Consistent values for the diffusivity and migration enthalpy of oxygen interstitial ions in donordoped ceria bulk ceramics were obtained by two different experiments: conductivity relaxation and oxygen tracer exchange. Defect formation enthalpies and entropies for oxygen interstitials and polarons in donor-doped ceria were derived. Molecular dynamic simulations yielded diffusivities for oxygen vacancies and interstitials in bulk ceria, which were comparable to experimental results. Defect interactions for both defect species were studied, with defect interactions between vacancies result in a decrease of the diffusivity and an increase of the migration enthalpy; for interstitials the opposite effect is observed, diffusivity goes up, while migration enthalpy goes down. Significant surface contamination with (Ca,Al,Si)Ox impurity phase(s) prevented the determination of surface exchange coecients for Nb-doped bulk ceramics. Different oxygen surface exchange coecients were determined using SIMS from two differently oriented epitaxially grown thin films, indicating that surface orientation indeed may play a role, regardless of the similar surface potentials. Regardless of these diculties, the oxygen surface exchange coecients of all studied samples and those obtained from literature exhibit a rather general behaviour without pronounced differences for acceptor-, donor- and nominally undoped samples, which agrees well with the comparable work functions of the differently doped films.
Publications
- Surface Potentials of (111), (110) and (100) oriented CeO2−x thin films, Appl. Surf. Sci. 377 (2016), 1
H. Wardenga, and A. Klein
(See online at https://doi.org/10.1016/j.apsusc.2016.03.091) - A space-charge treatment of the increased concentration of reactive Ce3+ at the surface of a ceria solid solution, Angew. Chem. Int. Ed. 56 (2017), 14516
A.F. Zurhelle, X. Tong, A. Klein, D.S. Mebane, and R.A. De Souza
(See online at https://doi.org/10.1002/anie.201708118) - The Concentration and Diffusivity of Oxygen Interstitials in Niobia-Doped Ceria, J. Phys. Chem. C
S. P. Waldow, H. Wardenga, S. Beschnitt, A. Klein, and R. A. De Souza
(See online at https://doi.org/10.1021/acs.jpcc.8b10613)
