Figure 1 shows the XRD patterns of the SDC ceramic, as-deposited and wet-annealed SDC thin films. The SDC ceramic target was polycrystalline, and the thin film was preferentially grown along the [111] direction. For this study, we prepared a nanocrystalline SDC ceramic which, while exhibiting admittedly poor crystallinity, did exhibit sufficient proton conductivity to allow us to discuss the differences between the SDC ceramic and thin film. The positions of the 111 peak of the SDC ceramic and as-deposited thin film are at ~ 29.02° and ~ 28.31°, and the calculated spacing of the (111) plane (d 111 ) is 3.070 and 3.151 Å, respectively. The d 111 of the thin film was expanded by 2.6% from that of the ceramic target, so as to compensate for the lattice mismatch between SDC and Al 2 O 3 . In addition, at 3.091 Å, the d 111 of the wet-annealed thin film was 1.9% less than that of the as-deposited thin film. This shrinkage of d 111 is due to the chemical absorption of water molecules by oxygen vacancies through wet annealing, as in the following reaction [32]:

$$ {\mathrm{H}}_2\mathrm{O}+{\mathrm{V}}_{\mathrm{O}}^{\bullet \bullet }+\frac{1}{2}{\mathrm{O}}_2\to 2{\left(\mathrm{OH}\right)}^{\bullet } $$ (1)

Fig. 1 XRD patterns of the as-deposited, wet-annealed SDC thin films and SDC ceramic. The two solid vertical lines are the CeO 2 (111) and (200) planes Full size image

A weak wet annealing peak, at ~ 38.0°, is assigned to the 111 peak of the Ag electrode used for the conductivity measurement.

Figure 2a shows the Ce 3d 5/2 XAS spectrum of the dry SDC thin film. The Ce 3d 5/2 spectrum corresponds to the transition from the Ce 3d 5/2 core level to the unoccupied Ce 4f states. The overall shape and peak position of the thin film were in good agreement with those of the CeO 2 thin film [3, 4, 33]. Using Gaussian fitting, the peak positions of on-1 and on-2 indicated in the spectrum were estimated to be Ce3+ and the peak positions of on-3 was estimated to be Ce4+. This result indicates that the SDC thin film has mixed valence states of Ce4+ and Ce3+. There was no significant difference in the spectrum shapes between the dry- and wet-annealed thin films, indicating that the resolution of the XAS method is not sufficient to detect the effect of proton insertion on the electronic structure. Therefore, as shown in the next section, we applied the resonant PES method to the SDC thin films, which method has significantly better resolution.

Fig. 2 a Ce 3d XAS spectrum of the as-deposited SDC thin film. The labels on-1, on-2, and on-3 indicate the excitation energies for the resonant PES measurements. b Resonant PES spectra of the as-deposited and wet-annealed SDC thin films measured at on-1, on-2, and on-3 in a. The green and blue curves are the Ce3+ and Ce4+ states, respectively, obtained from Gaussian fitting Full size image

Figure 2b shows the resonant PES spectra of the as-deposited and wet-annealed SDC thin films, measured at photon energies indicated by on-1, on-2, and on-3 in Fig. 2a. The PES spectra examined in this study reflect the surface electronic structure, since the mean free path of a photoelectron is less than 2 nm [34]. The intensities of these spectra were normalized by the acquisition times and beam current. The spectral intensities are resonantly enhanced at on-1, on-2, and on-3. The PES spectra are explained as follows: (i) the resonant PES spectra measured at on-1 and on-2 have peaks at a binding energy of ~ 2.0 eV, which corresponds to the Ce3+ state (3d94f1 L ) hybridized with the O 2p state. Here, L is ligand hole in the O 2p state; (ii) the spectra measured at on-3 has a peak at a binding energy of ~ 4.3 eV, which corresponds to the Ce4+ state (3d94f0) hybridized with the O 2p state. In the as-deposited thin film, the abundance ratio of Ce4+ at ~ 4.3 eV and Ce3+ at ~ 2.0 eV is estimated to be 88:12. This result provides additional evidence for the mixed-valence states of Ce4+ and Ce3+, as shown in Fig. 2a. The peak intensity of Ce3+ at ~ 2.0 eV is lower in the wet-annealed thin film, indicating that the oxygen vacancies are occupied by oxygen ions in a wet atmosphere.

Figure 3 shows the Arrhenius plots of the electrical conductivities of the SDC thin films and bulk ceramics measured in dry and wet atmospheres. In the dry atmosphere, the SDC thin film and bulk ceramic exhibit Arrhenius-type thermal activation behaviors over the whole temperature range. The activation energies (E A ) of the thin film and bulk ceramic are 0.70 and 1.1 eV, respectively. The conductivity of the polycrystalline SDC ceramic was two orders of magnitude lower than that of the SDC thin film, due to the influence of grain boundaries. The same activation energy and similar conductivity have been reported for Gd-doped CeO 2 polycrystals and thin films [4, 18].

Fig. 3 Arrhenius plots of the electrical conductivities in the in-plane of the SDC thin films and bulk ceramics, measured in dry and wet atmospheres Full size image

In contrast, due to the proton migration, the conductivities of the thin film and the bulk ceramic measured in a wet atmosphere gradually increase as the temperature decreases to below 100 and 250 °C, respectively. In particular, the increase in the conductivity ratio was more marked in the thin film. Single crystals and micropolycrystalline CeO 2 do not exhibit proton conductivity, but since such proton conduction is caused by absorbed protons at the surface, nanopolycrystals and porous CeO 2 do exhibit proton conductivity [19, 20].

In general, the room temperature surface proton conduction of fluorine-type oxides such as CeO 2 or YSZ is explained by the Grotthuss mechanism [14,15,16,17,18]. According to this mechanism, physisorbed H 2 O forms OH− and H 3 O+ ions on the surface at room temperature and the H 3 O+ proton transfers from one H 2 O molecule to a neighboring H 2 O molecule, as in the following reaction:

$$ {\mathrm{H}}_2{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2{\mathrm{O}}^{+} $$ (2)

Similar behavior was observed in the CeO 2 and YSZ thin films and bulk ceramic [14,15,16,17,18,19,20,21,22,23,24].

The dependence of relative humidity on the resistivity of the wet-annealed SDC thin film, at room temperature, is shown in Fig. 4a. The resistivity decreased greatly as the relative humidity increased and decreased by three orders of magnitude when the humidity was increased from 50RH% to 100RH%. The dramatic increase in the conductivity of the SDC thin film at room temperature, as shown in Fig. 3, is due to the increase in physisorbed water on the SDC surface as the relative humidity increases. The red plot shows the resistivity of dry-annealed SDC thin film measured in a 100RH% wet atmosphere at 22 °C, which resistivity was two orders of magnitude higher than that of the wet-annealed SDC thin film. This indicates that the proton absorption at the SDC surface, by wet-annealing, increases the surface proton conductivity. Figure 4(b) show the Cole-Cole plot of the wet-annealed thin film measured at 22°C. The spectrum is shown in order to distinguish the bulk resistance and electrode interface resistance at the low temperature region shown in Fig. 3. The wet-annealed thin film exhibits one semicircle and the tail of a second semicircle, indicating that the conducting carrier is surface conducting protons. Figure 5 shows the O 1s PES spectra of the dry- and the wet-annealed thin films. Both exhibited a double-peak structure and a sharp peak at ~529.5 eV, which corresponds to O2- in oxygen sites. On the other hand, the positions of the weaker peaks are different, and can be interpreted as follows: (i) the broad peak at ~532 eV in the as-deposited thin film corresponds to the OH- absorbed at the surface created by chemisorbed water.; and (ii) the peak at 533 eV in the wet-annealed thin film corresponds to H 2 O molecules physisorbed at the surface [35]. The same peak structures have been reported in YSZ thin film with surface proton conduction at room temperature [15, 36]. The peak ratio of physisorbed H 2 O was increased from 7.8% to 24% by wet-annealing. Thus, the increase in conductivity by wet-annealing, shown in Fig. 4, reflects an increase in the physically adsorbed water molecules at the SDC surface. A proton conductivity of 5.98×10-5 S⁄cm was achieved at room temperature in the preferentially oriented thin film, which is two orders of magnitude higher than that of polycrystalline ceramics. Such conductivity is applicable to practical devices [26,27,28,29,30]. Most notable was the ~10-2 S/cm proton conductivity exhibited in a high humidity atmosphere, as shown in Fig. 4(a), which is considerably higher than the highest proton conductivities reported so far; approximately ~10-4 S/cm for Gd-doped CeO 2 thin films [19] and ~10-6 S/cm for Gd-doped CeO 2 polycrystals [18]. Such high proton conductivity is considered to derive from two features of the preferentially oriented SDC thin film with oxygen vacancy. The first feature is high water adsorbability on the SDC (111) surface. In the O1s PES spectrum, 16.9% of the detected oxygen was attributed to chemically adsorbed water and 24% was attributed to physically adsorbed water. This means that there are layers of physisorbed water on the SDC surface that can acts as proton conducting paths. The second feature is the dissociation of adsorbed water at the SDC (111) surface. The reduced CeO 2-δ (111) surface promotes the dissociation of water molecules and the formation OH- and H+, which contribute to proton conduction [37, 38]. Dissociated protons can migrate through a physically adsorbed water layer by the Grotthus-mechanism. Therefore, the preferentially oriented SDC thin film contributed to such high proton conduction.

Fig. 4 a The relative humidity dependence of the wet-annealed SDC thin film and b Cole-Cole plots of the wet-annealed SDC thin film, measured in 100 RH% wet air at 22 °C Full size image