Ce0.8Sm0.2O2 laminated film

Solid State Ionics 230 (2013) 16–20

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Electronic and oxide ion conductivity in Pr2Ni0.71Cu0.24Ga0.05O4/Ce0.8Sm0.2O2 laminated film Junji Hyodo a, Shintaro Ida a, b, John A. Kilner b, c, Tatsumi Ishihara a, b,⁎ a b c

Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka, Japan Department of Materials, Imperial College London, London SW7 2AZ, UK

a r t i c l e

i n f o

Article history: Received 1 May 2012 Received in revised form 6 October 2012 Accepted 18 October 2012 Available online 14 November 2012 Keywords: Nanoionics Pr2NiO4 CeO2 Layer-by-layer structure

a b s t r a c t Pr2Ni0.71Cu0.24Ga0.05O4/Sm0.2Ce0.8O2 (PNCG/SDC) layer-by-layer thin film was prepared by pulsed laser deposition (PLD) method in this study. Dense and uniform PNCG/SDC film was successfully deposited on MgO substrate. Structure of the film was analyzed with secondary ion mass spectroscopy (SIMS). Although small diffusion of cation into MgO substrate was observed, the obtained film consists of PNCG and SDC nano-sized film laminated. The electrical conductivity measurements were performed as a function of layer thickness of PNCG or SDC. The conductivity decreased with decreasing PNCG layer thickness. In contrast, the conductivity increased with decreasing SDC layer thickness. Change in electrical conductivity seems to be related with the change in lattice constant of PNCG and SDC. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Recently, nano-size effects on ionic conductivity are attracting much attention from improving the ionic conductivity because the large change in electrochemical properties is induced by nano-size effects. There are several approaches for nano-size effects on ionic conducting materials, i.e., decreasing the grain size [1–3] and producing thin films [4–6] or laminated structures [7,8]. Because of negative charge at grain boundary, it is reported that nano size grain in oxide ion conductor generally decreases electrical conductivity [9]. In contrast, lamination of oxide ion conductor shows positive effects on oxide ion conductivity. Therefore, laminated film structure with nanometer thickness was focused in this study. Among various ionic conductors, mixed ionic and electronic conductors (MIECs) show the ionic and electronic conduction simultaneously. Thus, MIECs are expected as a promising cathode material for SOFCs or an oxygen permeation membrane. Some MIECs show even higher oxide ionic conductivity than conventional pure ionic conductors although the electronic conduction is still high [10]. In our previous study, it was found that Pr2NiO4 with K2NiO4 structure shows high hole as well as oxide ion conductivity. In addition, from calculation of the oxygen permeation rate using PNCG, doping Cu and Ga for the Ni-site (denoted as PNCG) further improved oxide ionic conductivity, which is higher than that of Sr- and Mg-doped LaGaO3 (LSGM) [11,12]. Interstitial oxygen in

⁎ Corresponding author at: Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Fax: +81 92 802 2871. E-mail address: [email protected] (T. Ishihara). 0167-2738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2012.10.018

rock salt layers was assigned to the dominant ionic carrier in PNCG from neutron diffraction analysis [13]. Considering nano-size effects, it was confirmed that electrochemical properties of PNCG were changed by decreasing the film thickness, which is similar to the case of pure ionic conductors. In fact, improvement of electrical conductivity in PNCG is observed at nano size film and in particular, PNCG film with 320 nm shows much higher electrical conductivity because of improved mobility of electronic hole [14]. Jongsik et al. reported a nano-size effect on the electrical conductivity in the vertically aligned film of MIECs and pure oxide ionic conductors (Sr-doped LaCoO3 and Gd-doped CeO2) with epitaxial columnar growth [15] and it was used for cathode of solid oxide fuel cells (SOFCs). The observed maximum power density with columnar structure was three times larger than that without columnar one. However, details of nano-size effects on oxide ionic conductivity in laminated film of MIECs and ionic conductors were still not clear although improvement in electrical conductivity was suggested. Therefore, the change in electrical conductivity of PNCG film laminated with samarium-doped ceria (SDC), which was prepared by pulsed laser deposition (PLD) method, was studied systematically in this study. 2. Experimental PLD method was used for preparing the laminated films. All samples in this study were four alternating layers of which top layer film was always SDC one. The laminated film was deposited on the MgO polycrystalline substrate at 1073 K, PO2 = 0.1 Pa. The laser power was set at 180 mJ/pulse, and the used frequency for laser irradiation was 10 Hz.

J. Hyodo et al. / Solid State Ionics 230 (2013) 16–20

3. Results and discussion Fig. 2 (a) shows the XRD pattern of PNCG/SDC laminated film after annealing in air. Although the intensity of peaks assigned to Pr2NiO4 was weak, all peaks could be assigned to Pr2NiO4 and ceria except those for the MgO substrate. The reason for the weak intensity of X-ray diffraction from PNCG could be the thin thickness of PNCG film and the second layer in laminated film. On the other hand,

(b)

(a) V SDC

I,V PMCG

Au electrode

MgO

I Fig. 1. Electrode configuration for the conductivity measurement (a) Van der Pauw method with dc measurement and (b) parallel electrode with ac measurement.

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Thickness of each layer in the laminated films was controlled by deposition time. After depositing each layer, samples were post annealed in air at 1073 K for 1 h in order to crystallize completely. The preparation methods of substrates and targets are as follows. For the substrate, polycrystalline MgO was used because of compatible thermal expansion coefficient between the substrate and laminated film. Commercial MgO powder (Tateho Chemical Ind., Co., Ltd.) was uniaxially pressed into a disc shape at 5 MPa followed by a cold isostatic press (CIP) at 300 MPa for 0.5 h. Prepared discs were sintered at 1973 K for 10 h and polished using diamond pastes with the grain size ~ 0.25 μm to obtain smooth surface. In order to prepare a target disc of PNCG for PLD, Pr(NO3)3·6H2O (Mitsuwa Chemicals Co., Ltd.), Ni(CH3COO)2·4H2O (Wako Pure Chemical Industries, Ltd.), Cu(NO3)2·3H2O (Wako Pure Chemical Industries, Ltd.) and Ga(NO3)3·8H2O (Soekawa Chemical, Co., Ltd.) were used as starting material. Proper amount of powders were dissolved into de-ionized water and mixed homogeneously with heating. After evaporating the water, the obtained powder was calcined at 673 K for 2 h to remove NOx originated from starting materials, and then pre-sintered at 1073 K for 6 h. The target discs prepared by using CIP at 300 MPa were sintered at 1523 K for 6 h. The commercial SDC powder (Daiichi Kigenso Kagaku Kogyo Co., Ltd.) was also used for the target after sintering at 1773 K for 6 h. The crystal structure of deposited film was analyzed by X-ray diffraction (XRD, RINT2500, RIGAKU) with CuKα radiation. The composition of deposited film was measured using X-ray fluorescence (XRF, XRF-1800, Shimadzu) as well as secondary ion mass spectroscopy (SIMS, Atomica 4100) with Cs primary ion source, and it was confirmed that the observed compositions of SDC and PNCG were similar with those of bulk samples by XRF analysis. The electric conductivity measurement was carried out by Van der Pauw method using Ag electrodes in the temperature range of 673–1073 K in air. Electrode configuration used for Van der Pauw method is shown in Fig. 1 (a). By using different current and potential probes, we can estimate the bulk conductivity of the film. Conductivity of the sample was also measured with AC two probe method with parallel electrode configuration as shown in Fig. 1 (b). Temperature of the sample was monitored by using K-type thermocouple located close to the samples. For oxygen partial pressure dependence of the conductivity of the laminated films, conductivity measurements were performed from pure oxygen to nitrogen at 673 K. The oxygen partial pressure was monitored with an oxygen sensor using CaO stabilized zirconia electrolyte which was set close to the sample.

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2 /degree Fig. 2. XRD pattern of (a) PNCG/SDC laminated film and (b) PNCG film on MgO substrate after annealing at 1073 K for 2 h.

evidently, strong diffraction peaks were observed for SDC. Since no diffraction peaks from secondary phase were observed, laminated film consisting of PNCG and SDC layers may be successfully deposited on MgO substrate. On the other hand, Fig. 2 (b) shows XRD peaks of PNCG single film deposited on MgO substrate under the same deposition condition with 287 nm. Strong diffraction peaks were observed and evidently, PNCG film was successfully deposited by PLD. Since diffraction peaks from (004) plane were strong, PNCG might be oriented to c-axis and so epitaxial like film deposition seems to be observed for PNCG film. Although layer-by-layer structure cannot be observed directly by SEM, it is clearly observed that dense and uniform film was successfully deposited on polycrystalline MgO substrate, and it was found that layers were tightly connected to each other and no interface between PNCG/SDC film was observed by SEM. Fig. 3 shows SIMS depth analysis of the PNCG/SDC laminated film. Although some diffusion of Pr and Ce into MgO substrate was observed and the composition of the film at the most bottom PNCG layer deviated from the objective one, almost no counter diffusion of elements was observed between PNCG/SDC at the top three layers. Therefore, PNCG/SDC laminated film with layer-by-layer structure is obtained apart from the bottom PNCG layer. SEM image of the obtained 100 nm SDC/100 nm PNCG laminated film was also shown in Fig. 4. It is obvious that the uniform and dense film was obtained on MgO substrate and so contact between SDC and PNCG layers is highly tight and it seems that contact resistance is not large. The estimated thickness of the total film is ca. 440 nm which is in good

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SDC

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CeOSmOPrONiOCuOGaO-

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Thickness/nm Fig. 3. SIMS depth analysis of the PNCG/SDC laminated film.

agreement with SIMS analysis and expected 100 nm thickness for each layer of PNCG and SDC. The electrical conductivity measurements were performed by Van der Pauw method in air. Fig. 5 shows the Arrhenius plots of the electrical conductivity of the laminated film with different thicknesses of PNCG layer. It is noted that for this experiment, SDC film thickness was kept at 100 nm. Evidently, the conductivity decreased with decreasing the thickness of PNCG, which was a similar tendency with PNCG single film on MgO substrate reported in our previous work [14]. Similar to PNCG single film, decreased hole mobility in PNCG layers may be related to the decreased electronic conductivity in laminated film [14]. In order to understand the reason for decreased electrical conductivity, change in crystal lattice parameter of PNCG was estimated from XRD measurement. Fig. 6 shows the estimated lattice parameter of 100 nm thick PNCG film as a function of temperature. Crystal structure of PNCG was tetragonal type lattice and so lattice parameter of a (= b) and c was estimated from XRD measurement. It was found that a lattice parameter in PNCG film with 100 nm was expanded; however, c lattice parameter was shrunk. This suggests that the crystal lattice was tensile in a and b planes but compressed along c lattice direction. This may also be related with decreased interstitial oxygen in rock salt in nano size thickness film. Since compressed

lattice may decrease the mobility of hole, the electrical conductivity decreased with decreasing PNCG film thickness. The conductivity of laminated film with different thicknesses of SDC layer was also measured by fixing the PNCG thickness of 100 nm as shown in Fig. 7. In contrast to PNCG film thickness, it was found that the conductivity in the laminated film increased with decreasing SDC layers. Compared with the bulk conductivity of SDC as shown in Fig. 5, observed electrical conductivity of the film increased significantly, in particular, at lower temperature. This is because activation energy for electrical conductivity decreased with decreasing SDC film thickness. Since the film thickness of PNCG was kept at 100 nm, increase in electrical conductivity could be assigned to the improved conductivity in SDC layer. Since measurement of electrical conductivity of the film was performed with Van der Pauw method which is surface 4 electrode system, conductivity of the film was also measured with AC two probe method by using parallel electrode configuration for 100 nm thickness SDC film. The obtained conductivity is also shown in Fig. 7, in which good agreement between two different methods are observed, albeit the conductivity estimated with AC two probe method was slightly higher than that of Van der Pauw method. Considering the same slope in plots, experimental errors like electrode distance and estimation of resistance from impedance

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PNCG Bulk SDC Bulk PNCG/SDC film (PNCG:96nm) PNCG/SDC film (PNCG:129nm) PNCG/SDC film (PNCG:199nm) PNCG/SDC film (PNCG:373nm) PNCG/SDC film (PNCG:535nm)

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1000/(T/K) Fig. 4. SEM image of the PNCG/SDC laminated film with 100 nm thickness for each layer.

Fig. 5. Arrhenius plots of the electrical conductivity of the laminated film with different thicknesses of PNCG layer. SDC film thickness was kept at 100 nm.

J. Hyodo et al. / Solid State Ionics 230 (2013) 16–20

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plots could be assigned to the difference in conductivity by two methods. Therefore, bulk conductivity of PCNG/SDC laminated film seems to be successfully measured in this study. It is also noted that electrical conductivity of the laminated film decreased slightly with decreasing PO2 suggesting that hole conduction seems to be dominated. However, slope of PO2 dependence of the electrical conductivity of the laminated film became smaller with decreasing SDC film thickness and at 100 nm SDC/100 nm PNCG, electrical conductivity depends on oxygen par0.018 tial pressure with PO2 , which means that conductivity was hardly dependent on PO2 from 10 − 5 to 1 atm. Change in lattice constant was also estimated with XRD measurement and it was found that the peaks from SDC shifted to a lower value with decreasing SDC film thickness suggesting that the unit lattice of SDC is expanded in the film. Fig. 8 shows the temperature dependence of lattice constant of SDC film with 100 nm thickness. Evidently, the estimated lattice constant of SDC was larger than that of bulk SDC at all temperature and difference became more significant with decreasing temperature. Therefore, SDC lattice was tensile in the film and one reason for this may be related with difference in thermal

expansion coefficient. This may also be related with the increased conductivity. It is suggested that the oxide ion conductivity is increased by applying the tensile stress, as confirmed by quantum calculation [16] and experimentally [17]. Therefore, the improved conductivity in PNCG/SDC laminated film with decreasing SDC thickness may be assigned to the improved oxide ion conductivity in SDC layer. Details of the change in oxide ion conductivity in SDC film are now under investigation and this will be reported in the future. 4. Conclusion Effects of the film thickness on the electrical conductivity in PNCG/SDC laminated film were investigated in this study. It was found that the electrical conductivity was decreased by decreasing the film thickness in case of PNCG; however, it increased in case of SDC film. The improvement in conductivity became more significant at lower temperature suggesting that activation energy for oxide ion conductivity may decrease by decreasing SDC film thickness. The estimated SDC layer conductivity in PNCG/SDC is higher than that of SDC bulk by around two orders of magnitude at 673 K. Compared with the bulk sample, there is high possibility that oxide ion conductivity in SDC nano-sized film laminated with PNCG shows a significant increase because of the expanded lattice of SDC.

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This study was financially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from Japan Science Technology. I2CNER was supported by WPI program from Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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1000/(T/K) Fig. 7. Temperature dependence of conductivity in laminated film with different thicknesses of SDC layer. PNCG film thickness was kept at 100 nm.

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