Effects of Dopant Ionic Radius on Cerium Reduction in Epitaxial Cerium Oxide Thin Films

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1 Effects of Dopant Ionic Radius on Cerium Reduction in Epitaxial Cerium Oxide Thin Films Nan Yang, a,* Pasquale Orgiani, c,d Elisabetta Di Bartolomeo, e Vittorio Foglietti, b Piero Torelli, d Anton V. Ievlev, f Giorgio Rossi, g Silvia Licoccia, e Giuseppe Balestrino, b Sergei V. Kalinin f and Carmela Aruta b* a. School of Physical Science and Technology, ShanghaiTech University, Shanghai, China b. National Research Council CNR-SPIN and Department DICII, University of Roma Tor Vergata Rome, Italy c. National Research Council CNR-SPIN, University of Salerno, Fisciano, I-84084, Salerno, Italy d. National Research Council CNR-IOM, TASC National Laboratory, I Trieste, Italy e. Dept. of Chemical Science and Technologies, University of Rome Tor Vergata, Via della Ricerca Scientifica, Rome, Italy f. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States g. Deparment of Physics, University of Milano, I Milano, Italy *Carmela Aruta, carmela.aruta@spin.cnr.it *Nan Yang, yangnan@shanghaitech.edu.cn S1. X-Ray diffraction Stuctural properties of RE 0.1 Ce 0.9 O 2-δ (RECO) films with rare-earth RE ions having different ionic radius, i.e. La, Sm, Gd and Yb (LCO, SCO, GCO and YCO) were investigated by X-Ray diffraction (XRD). Figure S1 shows the /2 x-ray diffraction measurements of all the targets used to grow the films. XRD investigation of pure ceria CeO 2 target only shows diffraction S1

2 peaks belonging to the desired phase with no trace of any impurities. Similarly, doped ceria targets does not show any extra peaks related to some spurious phase; interestingly, XRD analysis at high diffraction angles (see Figure S1 right) does show both diffraction peaks correlated to rare-earth substituted compounds as well as a fraction of unreacted pure CeO 2 (highlighted by the yellow box). Consistently with the predictions, diffraction peaks related to doped ceria are measured at lower and lower angles, therefore indicating an increase of the average lattice parameters. Figure S1. /2 scans of all the targets used to grow the RECO and CeO 2 films in the 2 range degrees (left panel). The region of the (224) Bragg reflection is highlighted by the yellow box and reported in the right panel. /2 measurements (where is the angle between the incident scattering vector and the sample surface, while the 2 angle is formed between the incident and the outgoing scattering vectors) were performed on all the RECO films both at low resolution with Co K radiation, reported in Figure S2(a) and high resolution with Cu K radiation, reported in Figure S2(b). At high resolution we also show the measurement of a bare CeO 2 film grown in the same deposition condition, as a comparison. The low resolution measurements on a wider angular scan demonstrate the single c-axis orientation of the film and the absence of spurious phases. The c-axis value was obtained from the high resolution measurements of Figure S2(b). S2

3 Figure S2. (a) /2 scans at low resolution with Co K radiation in the 2 range degrees. (b) /2 scans at high resolution with Cu K radiation of the RECO films, together with the bare CeO 2 film grown in the same deposition condition, in the 2 range degrees. Figure S3 shows the rocking curves of the (002) diffraction peaks for the LCO, SCO and YCO. The low crystal structure mosaicity is confirmed by the small FWHM value of the (002) reflection. The rocking curves are fitted by Lorentz curves and values are 0.22 for LCO, 0.31 for SCO and 0.33 for YCO. Figure S3. Rocking curve scans for LCO,SCO and YCO. Reciprocal space maps (RSM) in symmetrical and asymmetrical configuration, as shown in Figure S4, allow to obtain deeper information on out-of-plane and in-plane structural properties, respectively. RSM in symmetrical configuration (top panels) were performed around the (330) reflection of NGO substrate and (004) of the RECO S3

4 films indicating that films are monodomain and perfectly aligned along the out-ofplane crystal direction. RSM in asymmetrical configuration (bottom panels) were performed around the (33-4) and (-2-24) reflections of NGO and RECO respectively, indicating that both in plane lattices have a square symmetry. The in-plane matching between film and substrate is demonstrated by the occurrence of the substrate and film peaks at the same Q x value within a resolution of 0.02Å. The smooth increasing of the Q y value moving from the LCO (highest ion radius size) to the YCO (lowest ion radius size) indicates the decreasing c-axis parameter, while keeping fixed the inplane lattice parameter. Our detailed analysis demonstrates that RECO films grown on NGO substrate are highly epitaxial. Figure S4. The symmetrical (upper panels) and asymmetrical (down panels) reciprocal space maps (RSM) of RECO films grown on NGO substrates. Red lines are a guide for the eyes. In Figure S5 we show the plot of c/c vs. a/a, where c/c is the relative variation of the out-of-plane lattice parameter of RECO film with respect to the bulk, and a/a is the relative variation of the lattice parameter of bulk RECO with respect to the NGO substrate (mismatch). The Poisson coefficient can be calculated with the formula S4

5 reported in the plot. According to the elasticity theory, is expected to be in the range The region compatible with the strain effect is highlighted in yellow. As it can be inferred from the plot, the lattice parameters variation of CeO 2 and YCO only can be discussed in terms of elasticity theory (i.e. strain). Figure S5. Plot of c/c vs. a/a. The yellow region indicates where the Poisson coefficient in less than 0.5. S2. X-ray Photoemission Spectroscopy Core level spectra were in situ measured at APE beamline with a photon excitation energy of 1254 ev. Fitting Ce 3d core level we estimated the Ce 3+ concentration. For the fitting procedure, we subtracted a Shirley background from the spectra and used a mixed Gaussian and Lorentzian line shape for each component. The full width at half maximum (FWHM) was fixed at the same value for the 3d 5/2 and 3d 3/2 corresponding couple peaks for each sample, as well as the orbital splitting distance between Ce 3d 3/2 and Ce 3d 5/2 so that the constituent peaks are at the same binding energy for all the samples. For the spin-orbit intensity ratio of Ce 3d we used the degeneracy ratio 2:3. In particular, Ce 3d spectra is fitted using a total of 10 components caused by the spin-orbit splitting of the Ce 3d 5/2 and Ce 3d 3/2 and by a redistribution of the binding energy after a core hole is created. There are four peaks (v 0, v, u 0, u ) that are derived S5

6 from Ce 3+, and the other six are from the Ce 4+ valence state. 1,2 Therefore, the concentration of Ce 3+ can be estimated by the following equations: Ce 3+ = v 0 + v +u 0 +u Ce 4+ = v +v + v + u +u + u [Ce 3+ ]= Ce 3+ /(Ce 3+ + Ce 4+ ) Figure S6. Ce 3d XPS core level spectrum of CeO 2 thin film grown on NGO substrate. As an example, we report in Figure S6 the Ce 3d core level spectrum together with the fitting result of CeO 2 thin film grown on NGO substrate. We further calculate the oxygen vacancy concentration by using the following chemical composition (Re 3+ ) 0.1 (Ce 3+/4+ ) 0.9 O 2-δ. Assuming that the oxygen deficiency corresponds to the oxygen vacancy concentration [.. ] induced by cation substitution, giving rise to [Ce 3+ ] to preserve the charge neutrality, we can simply write: [.. ] =. Where we assume the nominal composition for the Re cations since the complex 3d and 4d XPS spectra prevents the careful determination of the concentration. The obtained [Ce 3+ ] and [.. ] values are reported in table S1. S6

7 Sample CeO 2- RE 0.1 Ce 0.9 O 2- RE= Yb Gd Sm La [Ce 3+ ] mol% [.. ] mol% Table S1. [Ce 3+ ] obtained from the Ce 3d XPS analysis and calculated [.. ] values for the different RE cation substitutions are reported in mol% units compared to the CeO 2 thin film grown in the same conditions. Figure S7. c/c vs. Ce 3+ concentration. In figure S7 we report the behavior of the relative variation of the out-of-plane lattice parameter with respect to the bulk as a function of the Ce 3+ concentration. No clear correlation is observed, thus demonstrating that the Ce 3+ formation cannot be related to the elastic strain. S3. Detailed rare earth elements spectra analysis In the case of La 3d and Sm 3d core level spectra reported in Figure 3 (a) and (b) of the main text, the spin-orbit splitting gives rise to the two couples of peaks attributed to the 3d 5/2 and 3d 3/2. For the La 3d the main peaks are at ev and ev, while the satellites are at ev and ev. The interpretation of the satellites is still under debate, but most of the studies explain them in terms of charge-transfer S7

8 effects. 1, 3 For the Sm 3d the major peaks at binding energy at ev and ev as can be seen in the Figure 3 (b) of the main text are attributed to the ionization of Sm 3+, while the additional peaks at ev and ev may correspond to metallic Sm segregation still in the trivalent state. 4 However, we can rule out the presence of divalent Sm ions, because the corresponding ionic (metallic) peaks should be present at about 9 ev (8 ev) below the ionic (metallic) peaks of trivalent Sm ions. The interpretation of the Gd 4d and Yb 4d core level spectra, reported in Figure 3 (c) and (d) of the main text, is even more controversial and a clear distinction between the 4d 3/2 and 4d 5/2 spin-orbit components is not feasible. However, such core levels are particularly useful because they are sensitive to the presence of different valence states. 5-7 For the Gd 4d it has been reported that the two peak components, indicated as A and B used in Figure 3 (c) of the main text include the multiple structures due to the Gd 4d-4f interaction. 8 However, the life-time broadening was also invoked to explain the Gd 4d spectrum considered mainly as split into two peaks by the spin-orbit interaction. 9 But further experimental and theoretical works confirmed that the 4d 4f interaction was as strong as the 4d spin-orbit coupling. 10 The exact interpretation of the spectral components is out of the scope of the present study. We mainly note here that the spectra of Figure 3 (c) of the main text has the typical shape profile of Gd 2 O 3. 5 Also for the Yb 4d the lifetime broadening and the 4d-4f Coulomb-exchange interactions contribute to the spectrum. 9 In addition, the close proximity with the Ce 4p core level signal 1 makes the fitting procedure quite complex. To visualize the main spectral contributions, we fit the convoluted structure with four components for Yb 4d (A, A, B, B ) and the additional four components for the Ce 4p (a, a, b, b in figure S8). 1 The absence of a lower binding energy feature about 4 ev below the main Yb 3+ 4d component indicates no detectable presence of Yb 2+ content. 6 S8

9 Figure S8. Yb 4d and Ce 4p core level spectra. S4. The EIS measurements in dry air and dry H 2 Because the authors are aware that the substrate can contribute to the total conductivity measured by impedance spectroscopy, the comparison of the film and NGO substrate resistances is reported in figure S9. It shows that the film s resistance (YCO is the most resistive film of the series) is always at least one-order of magnitude smaller than that of bare substrate in the whole temperature range and a larger difference is expected for other films. Specifically, at 600 C the substrate resistance is a factor ten larger than the film resistance, thus, the resistance of film and substrate in parallel configuration would be smaller by a factor of about 0.91 and, therefore, the conductivity would be larger by a factor of about 1.1. Such a difference results in a change of the activation energy of roughly 0.03 ev that is significant but not appreciable from the fit of the Arrhenius plots (see figure S10). Therefore, since the contribution of the substrate may affect the conductivity measurements, we focus our attention on the relative variation of the properties of the films changing the RE dopant cation. S9

10 Figure S9. Comparison between the resistance of the NGO substrate and the YCO film. Conductivity measurements were performed in the range o C both in dry air (Figure S10) and in dry H2 (Figure S11). The activation energies were estimated using the Arrhenius equation: Figure S10. Arrhenius plot of RECO films measured in dry air together with the linear fit. S10

11 We compare the RECO conductivity behavior in two different atmospheres: air and 5 vol % of H 2 in argon. In cerium oxide, reducing the oxygen partial pressure oxygen vacancies are formed due to the charge compensation effect. 11 The reduction process can be written using Kroger-Vink notation, as in the following: where is the oxygen vacancy. Therefore, especially at high temperature oxygen is removed from the lattice leaving oxygen vacancies and electrons, which may reduce the cations. Thus, from the neutrality condition n (electrons)=2[ ], electrons are dominating in the low oxygen partial pressure range and the oxygen ion conductor behaves an n-type conductor. Comparing the total conductivity measurements at two different oxygen partial pressure atmospheres, the difference should mainly depend on the n-type electronic conductivity. Hence, to evaluate the possible electronic contribution caused by the cerium reduction, we compare the temperature dependent conductivity behaviors for RECO thin films in H 2 and air. Figure S11(a) shows the conductivity plots for RECO thin films obtained by EIS measurements in the same temperature range. The total conductivities in H 2 are larger than in air. For example, the conductivity for LCO thin film is 7x10-3 S/cm in dry H 2 and 5 x10-3 S/cm in dry air. The largest values were obtained for YCO due to a much easier cerium reduction, confirmed by the highest [Ce 3+ ] observed in Ce 3d spectra. The activation energy obtained in H 2 is also larger than in air (Figure S11(b)). We plot the ratio between Ea obtained in H 2 and in air as function of the type of the dopant. A 10% increase of Ea is obtained for the case of the largest dopant LCO (0.79 in air vs in H 2 ) and almost a 40% increase for case of the smallest dopant YCO (1.16 ev in air vs. 1.6 ev in H 2 ). Such a behavior of Ea might be explained by the lattice expansion in reducing atmosphere. However, further investigation is in progress to better clarify this point. S11

12 Figure S11. (a) Arrhenius plot of RECO films involved in this study together with the linear fit obtained in H 2. (b) The histogram of the activation energy (Ea) ratio obtained in H 2 and air of RECO thin films. References 1. Burroughs, P.; Hammett, A.; Orchard, A. F.; Thornton, G. Satellite structure in the x-ray photoelectron spectra of some binary and mixed oxides of lanthanum and cerium. J. Chem. Soc. Dalton Trans. 1976, 1, Mullins, D. R.; Overbury, S. H.; Huntley, D. R. Electron spectroscopy of single crystal and polycrystalline cerium oxide surfaces. Surf. Sci. 1998, 409, Howng, W.Y.; Thorn, R. J. Investigation of the electronic structure of La 1 x (M 2+ ) x CrO 3, Cr 2 O 3 and La 2 O 3 by x-ray photoelectron spectroscopy. J. Phys. Chem. Solids, 1980, 41, Xu, Q.; Hu, S.; Cheng, D.; Feng X.; Han Y.; Zhu, J. Growth and electronic structure of Sm on thin Al 2 O 3 /Ni 3 Al (111) films. J. Chem. Phys. 2012, 136, Molle, A.; Wiemer, C.; Bhuiyan, M. D. N. K.; Tallarida, G.; Fanciulli, M. Epitaxial growth of cubic Gd 2 O 3 thin films on Ge substrates. Journal of Physics: Conference Series, 2008, 100, S12

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