Room Temperature Polarization Phenomena in Nanocrystalline and Epitaxial Thin Films of Gd-Doped Ceria Studied by Kelvin Probe Force Microscopy

Abstract

Thin films of the composition Ce 0.9 Gd 0.1 O 2-δ and with a thickness of 200 nm were prepared on an Al 2 O 3 (0001) substrate at 720 • C or room temperature, to yield epitaxial or nanocrystalline materials. The effect of the microstructure on room temperature polarization were assessed by a combination of polarization with up to ±5 V using an AFM tip and a large silver back contact followed by surface potential mapping with Kelvin probe force microscopy. The nanocrystalline sample showed a low degree of polarization, which was reversible, and a fast relaxation process. The epitaxial thin film, in contrast, was already polarized at smaller biases and showed irreversible changes of the surface potential after polarization with +5 V. For lower biases, two distinct relaxation reactions were identified: a fast redistribution of charge over a large area in the first seconds after end of polarization and a slower process in the vicinity of the polarization contact, which took several minutes to relax. The slower polarization behavior is due to a bulk chemical diffusion process with a coefficient D δ on the order of 10 −17 m 2 • s −1 , whereas the faster polarization can be assigned to a charge carrier redistribution within a space charge layer.

Measurement of polarization effects in dual-phase ceria-based oxygen permeation membranes using Kelvin probe force microscopy

“…In contrast to positive polarization, polarizing the sample with a negative bias referring to the AFM tip led to a lowered surface potential in the direct vicinity of the contact, which is in good accordance with the results previously published for singlephase ceria materials [21][22][23][24][25]. Similar to the effect observed with +1 V polarization, polarization with a tip bias of −1 V led in some cases to a parallel oxidation effect of an area adjacent to the contact area (Figure 5).…”

“…Similar to KPFM-based polarization-relaxation analyses performed for single-phase ceria materials [21][22][23][24][25], positive polarization of CSO grains in the CSO-FC2O composite with a tip bias of +3 V led to a locally increased surface potential reaction, which decreased over time.…”

“…This is in good accordance with the conductivity measurements, where the conductivity was slightly lower and the activation energy slightly higher for experiments with negative polarization. The Table 1: Calculated time constants and diffusion coefficients at room temperature in comparison to literature values for single-phase Ce 0.9 Gd 0.1 O 1.95 thin films from [24] obtained with the same method. The diffusion length L was determined from the average radius of the area affected by polarization, which could be measured in the KPFM map.…”

“…The surface potential gradient was also shown to be reversible over time as long as the applied voltage was kept in a certain range. The time dependence was shown to vary with dopant concentration (e.g., abundance of oxygen vacancies in ceria) and also depends on the ratio of grain boundary/grain bulk [22][23][24][25]. Single ceria grains in a mixed ion/electronconductive composite have so far not been addressed by AFMbased electrochemical studies.…”

“…In this way, polarization-induced, reversible surface potential changes have already been monitored for a variety of single-phase acceptordoped ceria materials [21][22][23]. Time-resolved relaxation curves calculated from the KPFM images can be used to determine room temperature diffusion coefficients [24]. These data are not easily accessible by oxygen permeation experiments or state-of-the-art electrochemical experiments with macroscopic electrodes, because the electrical conductivity of ceria-based materials is very low below 400 °C.…”