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Acceptor-doped ceria ( (Ce_1-x Me__x^+IIIO_2-x/2) ) is a versatile material which exhibits a high oxygen ion conductivity at temperatures higher than 500 °C and is used e.g. in oxygen sensors or as component in solid oxide fuel cells. At the same time, ceria also shows a high catalytic activity, even at room temperature. For temperatures between 600-1200 °C the defect chemistry and charge transport mechanisms have been investigated intensively over the past 50 years, but experimental data for conductivity at room temperature are still limited. Also, the theory of the charge transport at temperatures below 400 °C has not been fully understood, yet.

Kelvin Probe Force Microscopy (KPFM) is an AFM-based measurement method which can detect the local surface potential (or from the physics view: the Volta potential) of a sample surface. The surface potential is a measure for local changes of the defect chemistry, as it is directly related to the Fermi niveau [1]. Hence, in a combined KPFM-polarization setup, KPFM mapping can be used to measure the surface potential difference directly at the polarized contact area and can also determine the time-dependent relaxation process of the defect concentration gradient introduced during polarization. The relaxation data can then be used to calculate chemical diffusion coefficients for the material.

The advantage of this technique is that in addition to the sole relaxation data it also gives information about the local surface potential distribution, lateral extent of the gradient, variations of the shape of the gradient, interaction at grain boundaries etc. [2, 3].

Starting from single crystal ceria materials and also including nanocrystalline and epitaxial thin film as well as bulk ceramic ceria samples, we were able to verify that the relaxation process at room temperature is dependent on the ratio of grains/grain boundaries and on the dopant concentration, as was to expect from high temperature behavior [2, 4].

Most recently, the technique was deployed to measure diffusion coefficients of single ceria grains in a dual-phase oxygen permeation membrane made of a spinel (FeCo₂O₄) as electron conductor and acceptor-doped ceria as an oxygen ion conductor [5].

Acknowledgements: This work has been funded by the Deutsche Forschungsgemeinschaft (project# 387282673). 

Literature:

1.           Zhang, Z.; Yates, J. T. Chem Rev 2012, 112 (10), 5520-5551.

2.           Neuhaus, K.; Schulze-Küppers, F.; Baumann, S.; Ulbrich, G.; Lerch, M.; Wiemhöfer, H.-D. Solid State Ionics 2016, 288, 325-330.

3.           Lee, W.; Lee, M.; Kim, Y.-B.; Prinz, F. B. Nanotech 2009, 20 (44), 445706.

4.           Neuhaus, K.; Gregori, G.; Maier, J. ECS J Solid State Sci Tech 2018, 7 (8), P362.

5.           Neuhaus, K.; Schmidt, C.; Fischer, L.; Meulenberg, W. A.; Ran, K.; Mayer, J.; Baumann, S. Beilstein J Nanotech 2021, 12 (1), 1380-1391.