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Thin Solid Films 516 (2008) 6088 – 6094 www.elsevier.com/locate/tsf
Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates Debasis Bera a,b,c,⁎, Satyanarayana V.N.T. Kuchibhatla b,c , S. Azad c , L. Saraf c , C.M. Wang c , V. Shutthanandan c , P. Nachimuthu c , D.E. McCready c , M.H. Engelhard c , O.A. Marina d , D.R. Baer c , S. Seal b,⁎, S. Thevuthasan c,⁎ a
Department of Materials Science and Engineering, 202 Rhines Hall, P.O. 116400, University of Florida, Gainesville, FL 32611-6400, USA b Surface Engineering and Nanotechnology Facility; Advanced Materials Processing and Analysis Center; Department of Mechanical, Materials and Aerospace Engineering; University of Central Florida, 4000 Central Florida Blvd, AMPAC-381, Eng-1, Orlando, FL 32816, USA c W. R. Wiley Environmental Molecular Sciences Laboratory; Pacific Northwest National Laboratory, Box 999 MS K8-93 [US Mail]; 33335 Q Ave [Direct delivery abd Fedex] Richland, WA 99352, USA d Pacific Northwest National Laboratory, Richland, WA 99352, USA Received 13 March 2007; received in revised form 29 October 2007; accepted 2 November 2007 Available online 12 November 2007
Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and zirconia (ZrO2) (111)-buffered sapphire (Al2O3) (0001) substrates using oxygen plasma-assisted molecular beam epitaxy to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on sapphire substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3 (0001) substrate and the ceria films. In contrast, the films, grown on a thin layer of ZrO2 (111) buffered sapphire substrate, appear to be highly oriented in nature with predominant double domain (111) orientation. Oxygen ionic conductivity of these gadolinia-doped ceria films was measured as a function of gadolinium concentration and found to be efficient at relatively lower temperature operation compared to that of bulk polycrystalline, single crystalline yttria stabilized zirconia and gadolinia-doped polycrystalline ceria. Relative improvement in ionic conductivity of highly oriented gadolinia-doped ceria films (in the lower temperature regime) can be ascribed to the increased oxygen vacancies due to presence of Gd as well as high quality of the oriented thin films. © 2007 Elsevier B.V. All rights reserved. Keywords: Highly oriented gadolinia-doped ceria; Ionic conductivity; Solid oxide fuel cell; X-ray diffraction pole-figure analysis; Transmission electron microscopy
1. Introduction Electrolyte materials with exclusive oxygen ion-conductivity are being considered as potential candidates for modern solidstate devices, such as oxygen sensors, solid oxide fuel cells (SOFC), batteries and three-way catalysts of automobiles [1–6]. ⁎ Corresponding authors. Bera is to be contacted at Tel.: +1 352 846 3331; fax: +1 352 392 4911. Seal, Tel.: +1 407 882 1119; fax: +1 407 882 1462. Thevuthasan, Tel.: +1 509 376 1375; fax: +1 509 376 5106. E-mail addresses: [email protected] (D. Bera), [email protected] (S. Seal), [email protected] (S. Thevuthasan). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.11.007
Yttria-stabilized zirconia (YSZ), an almost exclusive oxygen ion-conducting electrolyte for SOFC devices, exhibits oxygenion conductivity as high as 0.1 S/cm at 1275 K [7]. However, the ionic conductivity of YSZ at lower temperatures is significantly low [8]. Therefore, the operation of YSZ-based SOFC is limited only at higher temperatures. Increased ionic transport at lower temperatures would allow the use of a wide range of low cost materials for electrochemical devices including SOFC. In order to improve the efficiency of solid-state electrochemical devices at lower temperatures, a new electrolyte material is needed with higher oxygen ionic conductivity. A significant amount of recent research has been devoted to develop doped-
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ceria based electrolyte [7,9–16]. Particularly, ceria doped with a divalent or trivalent oxide, such as gadolinia, samaria, is known to exhibit higher ionic conductivity compared to YSZ at lower temperatures [7,9–11]. Interface controlled nucleation and growth of two-and threedimensional nanostructured materials show unique physical, chemical, and electronic properties that are different from bulk materials. Atomic and ionic transport, which is crucial for many technological applications including catalysis and fuel-cell technology, is also expected to be enhanced in nanostructured materials due to effects related to size, electronic structure, space charge and multiple interfacial pathways. It is experimentally well known that atomic transport in nanocrystalline materials differs substantially from that in coarse-grained or single-crystalline materials due to high interface diffusion in nanocrystalline materials compared to volume diffusion in single-crystalline materials [17]. However, the role of featuresize on atomic and ionic transport in nanostructured materials is still elusive. Highly oriented growth of ceria on insulating substrates has been a focus of materials research in the recent years for many technologically important applications [5,6,18,19]. In these applications, involvement of highly mobile oxygen is very important. Over wide range of working temperatures, such as, room temperature to 1000 °C, ceria plays two key roles: (i) releasing and storing oxygen, and (ii) promoting noble-metal activity and dispersion [20]. Such roles are very important for the application of ceria in SOFC or related applications. Keeping such applications in view, highly oriented pure and gadolinia-doped ceria films have been grown on pure and ZrO2 (111) buffered Al2O3(0001) substrates using oxygen plasmaassisted molecular beam epitaxy (OPA–MBE). The growth was monitored in situ using a reflection high energy electron diffraction (RHEED). In addition, these films were characterized by several ex situ surface and bulk sensitive techniques, such as, X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and atomic force microscopy. The objective of the current research is to understand the growth characteristics and ionic transport processes in ceriabased oxide thin films that may facilitate the use of new types of electrolytes with enhanced ionic conductivity at lower temperatures for electrochemical device applications. In this article, we discuss the growth of several highly oriented pure and gadolinia-doped ceria thin films on pure and thin layer zirconiabuffered sapphire (0001) using OPA–MBE. As discussed above, these films were characterized using several surface and bulk sensitive techniques to understand the growth mechanisms and the influence of Gd concentration on the total conductivity of the films. 2. Experimental details 2.1. Film growth Approximately 180 nm thick ceria films with various dopant concentrations of Gd on pure and zirconia-buffered sapphire (0001) were grown using OPA–MBE. Growth of thin films and
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in situ characterizations were carried out in a dual chamber ultrahigh vacuum (UHV) system described else where [21]. The molecular beam epitaxy (MBE) chamber consists of three metal evaporation sources and a UHV compatible electron cyclotron resonance (ECR) oxygen plasma source, as well as RHEED for real-time characterization of film growth. High purity cerium (Ce) and zirconium (Zr)-rods were used as the source materials in separate electron beam evaporators and gadolinium (Gd) was evaporated from an effusion cell. Growth rate of these films was monitored using quartz crystal oscillators. 10 mm × 10 mm × 1 mm sapphire substrates were cleaned ultrasonically in isopropanol and acetone for 5 min each. The substrates were introduced into the MBE chamber and cleaned at a temperature of 875 K for 10 min by exposing to activated oxygen plasma from ECR plasma source at an oxygen partial pressure of ∼ 2 × 10− 3 Pa. In the case of gadolinia-doped ceria growth without the zirconia buffer layer on sapphire (0001), Gd and Ce metal beams were co-evaporated in the presence of low pressure oxygen plasma. In the case of zirconia buffer layer, Zr metal was first evaporated in the presence of low pressure oxygen plasma followed by the gadolinia doped ceria growth. During growth, temperature and oxygen partial pressure were optimized to 925 K and 2.67 × 10− 3 Pa, respectively, and the growth was monitored by RHEED. Gd concentration in these films was varied between 6–24%. 2.2. Characterization techniques In addition to the in situ RHEED, the samples were characterized using various ex situ methods. RBS data was collected for random and channeling directions at room temperature using 2.0 MeV He+ ions to determine specimen thickness and crystal quality. A silicon surface barrier detector at 150° scattering angle was used to collect the backscattered ions. Experimental RBS spectra were simulated using a SIMNRA software in order to quantify the concentrations of gadolinium dopant in various samples. X-ray photoelectron spectroscopy (XPS) depth profiling measurements were made on a Physical Electronics Quantum 2000 Scanning ESCA Microprobe, which consists of a focused Al Kα X-ray source (1486.7 eV) and a high energy resolution hemispherical analyzer. Sample sputtering was performed using 2 kVAr+ ions rastered over a 2 mm × 2 mm area of the specimen. The sputter rate for these ion gun conditions was calibrated at a rate of 4.4 nm/min for a known SiO2 reference material. The sample was rotated at 2 RPM during sputtering. Photoemission spectra were collected using a pass energy of 46.95 eV. The XPS spectra were referenced to an energy scale with binding energies for Cu 2p3/2 at 932.67 ± 0.05 eVand Au 4f at 84.0 ± 0.05 eV. Low energy electrons (∼ 1 eV) and argon ions (∼5 eV) were used for specimen neutralization. XRD measurements were performed using Philips X'pert θ–2θ MRD diffractometer operating at 45 kV and 40 mA with a fixed Cu anode. The analyses of diffraction data were carried out using JADE 6.0 from Materials Data Inc. Transmission electron microscopy (TEM) sample preparation was carried out using a standard tripod wedge polishing method
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followed by Ar-ion beam thinning to electron transparency. High-resolution transmission electron microscopic analysis (HRTEM) was carried out using a JEOL 2010 TEM with a point to point resolution of 0.194 nm. The images are recorded with a CCD camera (1024 X 1024 pixels) followed by processing with Digital Micrograph® software. Total conductivity in the films was measured using four probe impedance spectroscopy. This capability was customized to measure the total conductivity in oxide films with thickness in the range of nanometers. A Solartron LCR meter and impedance analyzer controlled by computer is used to control the input parameters such as current and voltage in a frequency range of 10 Hz–1 GHz. This typically gives an output with real and imaginary parts of impedance. 3. Results and discussion The RHEED patterns from the sapphire (0001) substrate and the zirconia buffer layer are presented along with the pattern from gadolinia doped ceria film in Fig. 1. The RHEED pattern from the blank sapphire substrate after plasma cleaning is shown in Fig. 1(a). As zirconia thin film deposited on the sapphire substrate, the RHEED streaks changed into the features associated with zirconia (111) and remain the same until the end of the zirconia growth. The streak RHEED pattern after the deposition of a 4 nm zirconia thin film is shown in Fig. 1(b) while the pattern after deposition of the ceria thin film doped with gadolinia is shown in Fig. 1(c). As it is seen in Fig. 1(c), spotty features started to show up in the RHEED pattern from gadolinia doped ceria films, in addition to the streaks. These results indicate that the film surface appears to be rough due to the fact of possible development of a small amount of different phase during deposition along with the preferred orientation. We will discuss this observation in the later part of the discussion section. XPS was carried out to monitor the chemical composition and elemental distribution of as-deposited gadolinia-doped ceria films. Fig. 2(a) shows a representative XPS survey scan of a gadoliniadoped ceria film on zirconia buffered sapphire substrate. Absence of aluminum (Al 2p: 74.4 eV) and zirconium (Zr 3d5/2:180.0 eV) in the spectrum uniform coverage of the films and indicates that there was no major diffusion during film growth. A high resolution XPS scan of Gd 4d shown in the inset of Fig. 2(a) confirms the doping of Gd in the film. A XPS depth profile of undoped ceria thin film on zirconia buffered-sapphire is shown in Fig. 2(b). The depth profile results clearly demonstrate that there is no Zr and Al diffusion into the bulk of the film. The atomic coverage of Ce and oxygen (O) in depth profile of the film appears to be uniform throughout the film-thickness. Thickness of the film was found to be about 150 nm, consistent with RBS results. XRD spectra from the gadolinia-doped ceria films grown on pure and zirconia-buffered sapphire substrates are shown in Fig. 3(a) and (b) respectively. The film grown without the buffer layer shows polycrystalline features possibly due to structural deformations resulting from the large lattice mismatch between the Al2O3 (0001) substrate and the ceria film. In addition to the (111) peaks of ceria, the peaks associated with (200), (220), (311) and (331) are visible in this spectrum.
Fig. 1. The RHEED patterns of sapphire (0001) substrate (a) after plasma cleaning and before zirconia deposition, (b) after zirconia thin film deposition and (c) after zirconia buffered gadolinium doped ceria thin film deposition.
In contrast, the ceria films, grown on a 3 nm ZrO2 (111) buffer layer on top of Al2O3 (0001), appear to be highly oriented in (111) direction as shown in Fig. 3(b). Most of the polycrystalline features are not visible in this spectrum except less intense (200) in comparison to Fig. 3(a). This is likely due to the much smaller lattice mismatch between cubic zirconia and ceria compared to the substantial differences between the lattice parameters of Al2O3(0001) and ceria. The lattice parameters mismatch between ceria and sapphire is high because the lattice parameter of ceria in a fluorite structure is 0.541 nm, whereas, lattice parameters of sapphire in a corundum structure are a: 0.475 nm and c: 1.3 nm. Therefore, a buffer layer was chosen with an intermediate lattice parameter of
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the spectrum also matched with the reference data from cubic fluorite ceria and zirconia structures. Average crystallite sizes of doped and undoped ceria samples were also evaluated from XRD patterns using Scherrer equation assuming that the peak broadening was only due to small crystallite size. Contribution of non-uniform strain to the peakbroadening is expected to be same for both doped and undoped samples as the full-width-at-half-maximum (FWHM) is same for all samples. According to the Scherrer equation, the crystallite size was approximated to be 8 nm extracted from (222) peak for both doped and undoped samples (Fig. 3(a) and (b)). It was also found that samples with different amount of Gd showed the same crystallite size as FWHM of (222) XRD peak is same for all doped samples. This is due to fact that ionic radii of Ce ions (0.092 nm for Ce4+ and 0.103 nm for Ce3+) are comparable or higher than that of Gd ion (0.094 nm for Gd3+) [22]. Note that the peak positions (two-theta values) are unchanged for undoped polycrystalline and doped highly crystalline ceria. Such a trend implies that there was no uniform strain present in the doped samples. A bright-field HRTEM micrograph of the cross-section of zirconia buffered gadolinia-doped ceria is presented in Fig. 4(a). Energy dispersive X-ray spectroscopic analyses (EDS) were carried out at the interfaces of sapphire, zirconia and ceria films, and as well as along the thickness of the films. The results reveal Fig. 2. (a) XPS survey spectrum of gadolinia-doped ceria thin film on zirconia buffered sapphire substrate; Inset shows a high resolution Gd 4d envelop. (b) XPS depth profile from a zirconia buffered pure ceria thin film on sapphire substrate.
sapphire and ceria. The lattice parameter of zirconia is 0.511 nm and it is stabilized as cubic structure in thin film. Note that the XRD pattern shows two high intense peaks of (111) planes and two low-intense peaks of (100) planes of ceria, indicating preferential growth of (111) plane of ceria. The peak positions corresponding to the gadolinia-doped ceria and pure zirconia in
Fig. 3. XRD plots for (a) ceria thin film on sapphire (0001) substrate (b) heterolayers of gadolinia-doped ceria and zirconia thin film on sapphire (0001).
Fig. 4. (a) HRTEM micrograph of the cross-section of a ceria thin film grown on zirconia buffered sapphire (0001) substrate. EDS analyses shows almost uniform distribution of Gd along the cross section of the thin film.
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that there is no diffusion of gadolinium into the sapphire or zirconia during the deposition of films. A series of EDS spectra was acquired along the thickness of the gadolinia-doped ceria film for quantitative analysis. Quantitative analysis of these spectra showed that the amount of Gd was uniformlydistributed throughout the ceria thin film (see Fig. 4(b)). RBS was carried out on the samples to quantify the thickness of the film and Gd present in the samples. The qualitative and semi-quantitative information on concentration of Gd obtained from quartz crystal oscillators and EDS, respectively, could be substantiated by RBS measurements. A spectrum collected from RBS along with simulated results using SIMNRA software is shown in Fig. 5. The concentration of Gd from simulated spectra is in well-agreement with concentration data from EDS. The simulated results of ceria samples doped with different percentage of Gd show that the thickness of ceria and zirconia are to be approximately 150 nm and 3 nm, respectively. The percentage Gd was found to be 5.3%, 8.4%, and 19.2% in three samples. 4. Orientation of OPA–MBE grown thin films As mentioned earlier the growth of multilayer thin films was monitored by the in situ RHEED. The RHEED streaks, presented in Fig. 1(c), are showing some distorted features. It may be indicative of the presence of little amount of a differently oriented ceria in the highly oriented ceria thin film. This result is consistent with XRD results presented in Fig. 3(b). To assess the preferred orientation of the thin film relative to the orientation (0001) of sapphire substrate, pole figures of the ceria (111), ceria (200) and alumina (202) were collected, as shown in Fig. 6. Theoretically, ceria (111) film with a single inplane domain shows three (111) poles at 70° tilt in addition to the out-of-plane (111) pole at 0° tilt. In the present study, however, six poles at 70° tilt relative to the central pole are observed (see Fig. 6(a)). It suggests that the ceria (111) film has two in-plane domains. Similarly, Fig. 6(b) represents ceria (200) pole-figure showing six (200) poles at 54.74° tilt which is the inter-planner angle between (111) and (200) for face-centered cubic lattice. The pole at 0° corresponds to the out-of-plane (200) orientation and
Fig. 6. Pole figures with respect to (a) ceria (111), (b) ceria (200) and (c) sapphire (202) planes.
Fig. 5. RBS spectrum for highly oriented zirconia-buffered gadolinium doped ceria thin film on sapphire (0001) with superimposed simulated pattern from SIMNRA.
additional six (200) poles are the in-plane ceria (111) film. It further confirms the presence of two in-plane domains in the ceria (111) film. Sapphire (202) pole figure presented in Fig. 6(c) was used for comparison. According to these pole-figures and 2θ scans, ceria (111) certainly has very high intensity compared to
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ceria (200). Based on the intensities it is quite evident that amount of ceria with (200) orientation present in sample is very minor. Therefore, it is reasonable to say that the ceria film on a zirconia buffered sapphire substrate was highly (111) oriented with two inplane domains. 5. Evaluation of oxygen ionic conductivity The electrical conductivity of godolinia-doped ceria on zirconia buffered sapphire (0001) films was measured using a four-probe van der Pauw technique [23]. Sapphire was used as substrate to minimize the interference of electrical conductivity from the substrate to the measured conductivity. Oxygen ionic conduction is approximately zero in the sapphire. Note that the electronic conductivity in ceria or zirconia based oxides is significantly less compared to ionic conductivity, especially, at low temperatures [9]. Therefore the electronic conductivity of these oxides can be neglected and assumed that the majority of the total conductivity is dominated by the oxygen ionic conductivity. Hence the total conductivity is interchangeably dealt as oxygen ionic conductivity in doped ceria and zirconia systems. Oxygen ionic conductivity results from samples with different concentration of Gd have been presented in Fig. 7. For comparison, the oxygen ionic conductivity data from polycrystalline [1], single crystalline YSZ [24] and gadolinia-doped polycrystalline ceria [9] are also shown in the figure. In general, these highly oriented films showed enhanced conductivity compared to the bulk polycrystalline YSZ and single crystalline YSZ. In comparison to the polycrystalline gadolinia doped ceria, these highly oriented films showed better ionic conductivity values at lower temperatures. In addition, there is a significant amount of increase in conductivity with increase in concentration of Gd (∼5–19%) in the ceria films. In order to understand the relationship between the concentration of dopant in the samples and ionic conductivity, values of oxygen ionic conductivity from different gadolinia-doped ceria samples at a temperature 770 °C are plotted against experimentally found Gd concentration, as shown in Fig. 7(b). Conductivity values of single crystalline YSZ [24] and gadolinia-doped polycrystalline ceria [9] are also shown for comparison. Fig. 7(b) shows a linear relationship (linear regression coefficient (R2) 98% and standard deviation 0.29) between ionic conductivity and amount of dopant. Recall that the crystallite size was unchanged for all the samples regardless the concentration of Gd in the films. In other words, the effect of crystallite size on ionic conductivity data is either negligible or same for all doped samples. Therefore, increase in oxygen ionic conductivity values with increasing amount of Gd from 5 to 19% in ceria films at the 770 °C is mainly due to generation of higher amount of oxygen vacancies in the samples. Generally, trivalent elements, such as Gd3+, create oxygen vacancies in ceria films which consequently increase the ionic conductivity of the films [7,9–11]. Texture of the films also affects the ionic conductivities of the film. In order to compare the effect of the orientation on oxygen ionic conductivity, conductivity data of gadolinia-doped polycrystalline ceria at 770 °C is introduced in the Fig. 7(b). It shows a significantly lower ionic conductivity value than corresponding that of gadolinia doped highly crystalline ceria
Fig. 7. Arrhenius plot for of oxygen ionic conductivity of zirconia-buffered gadolinia-doped ceria thin film on sapphire. The data from polycrystalline YSZ [1], single crystalline YSZ thin films [24] poly crystalline gadolinia-doped ceria [9] are also shown. (b) Conductivity values of single crystalline YSZ [24], gadolinia doped polycrystalline ceria [9] and gadolinia-doped highly crystalline ceria on zirconia-buffered sapphire (0001) substrate at 770 °C.
film. Therefore, texture of the sample plays an important role on overall ionic conductivity. In conclusion, these highly oriented gadolinia doped ceria thin films on zirconia-buffered sapphire substrate can be potential candidates for the use as electrolytes in solid oxide fuel cell devices at lower temperatures due to comparatively higher oxygen ionic conductivity values. Oxygen ionic conductivity depends on concentration of dopants in the samples as well as on the texture of the samples. Although current work mainly focuses on the growth of the highly oriented gadolinia-doped ceria film along with some information on texture and ionic conductivity, our future study on these film is expected to reveal more on the relationship among texture, doping concentration, grain size and ionic conductivity. 6. Summary Growth of highly oriented zirconia-buffered ceria thin film on sapphire substrate is demonstrated. Thin films with and without a
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buffer-layer were characterized by several surface and bulk characterization techniques. The orientation of films was monitored and characterized by in situ RHEED and ex situ XRD analyses. Dopant concentration, film thickness evaluated using different techniques were found to be identical. Despite the small amount of ceria (200) orientation, the films grown with zirconia buffer layer are highly oriented in nature with predominant double domain (111) orientation compared to the films without the buffer layer. Oxygen ionic conductivity of these films was measured and found to be efficient at relatively lower temperature operation compared to that of bulk polycrystalline, single crystalline YSZ and gadolinia-doped polycrystalline ceria. Among 5.3, 8.4 and 19.2% gadolinia doped ceria samples, higher extent of gadolinia-doped sample shows better efficiencies at lower temperature. Relative improvement in ionic conductivity of highly oriented gadolinia-doped ceria films (in the lower temperature regime) can be ascribed to the increased oxygen vacancies due to presence of Gd as well as high quality of the oriented thin films. Acknowledgement This research work was supported in part by the Office of Fellowship Program of Pacific Northwest National laboratory (PNNL) and NSF-REU program at University of Central Florida. The experiments were performed in the laboratory of Environmental Molecular Sciences Laboratory and supported by the US Department of Energy's Office of Biological and Environmental Research. Authors would like to thank Y. Du and M. Mckinley for their assistance during the course of the experiments and data analysis. Bera would like to thank PNNL for Summer Research Fellowship, and Materials Research Society (MRS) for Symposium Student Assistantship to present this work at MRS 2004 Fall Meeting at Boston. References [1] N.Q. Minh, T. Takahashi, Science and Technology of Ceramic Fuel Cells, Elsevier, Amsterdam, 1995. [2] N. Izu, W. Shin, N. Murayama, Sens. Actuators B 93 (2003) 449.
[3] D.L. Maricle, T.E. Swarr, S. Karavolis, A.C. Khandkar, W. Weppner, Solid State Ion. 52 (1992) 173. [4] G.E. Meng, Q.X. Fu, S.W. Zha, C.R. Xia, X.Q. Liu, D.K. Peng, Solid State Ion. 148 (2002) 533. [5] G.A. Deluga, J.R. Salge, L.D. Schimdt, X.E. Verykios, Science 303 (2004) 993. [6] A. Trovarelli, Catalysis by ceria and related Materials, Imperial College Press, London, 2002. [7] S. Thevuthasan, S. Azad, O.A. Marina, V. Shutthanandan, D.E. McCready, L. Saraf, C.M. Wang, L. Lyubinetsky, C.H.F. Peden, V. Petrovsky, Proceeding of IEEE-NANO 2003, vol. 2, 2003, p. 550. [8] O. Yamamoto, Electrochim. Acta 45 (2000) 2423. [9] S. Azad, O.A. Marina, C.M. Wang, L. Saraf, V. Sutthanandan, D.E. McCready, A. El-Azab, J.E. Jaffe, M.H. Engelhard, C.H.F. Peden, S. Thevuthasan, Appl. Phys. Letts. 86 (2005) 131906. [10] H. Yahiro, Y. Eguchi, K. Eguchi, H. Arai, J. Appl. Electrochem. 18 (1988) 527. [11] B.C.H. Steele, Solid State Ion. 129 (2000) 95. [12] V.V. Kharton, F.M. Figueiredo, L. Navarro, E.N. Naumovich, A.V. Kovalevsky, A.A. Yaremchenko, A.P. Viskup, A. Carneiro, F.M.B. Marques, J.R. Frade, J. Mater. Sci. 36 (2004) 1105. [13] A. Tsoga, A. Gupta, A. Naoumidis, P. Nikolopoulos, Acta Mater. 48 (2000) 4709. [14] R. Gerhardt, A.S. Nowick, J. Am. Cer. Soc. 69 (1986) 641. [15] Z. Tianshu, P. Hing, H. Huang, J. Kilner, Solid State Ion. 148 (2002) 567. [16] T. Ohashi, S. Yamazaki, T. Tokunaga, Y. Arita, T. Matsui, T. Harami, K. Kobayashi, Solid State Ion. 113-115 (1998) 559. [17] N. Sata, K. Eberman, K. Eberl, J. Maier, Nature 408 (2000) 946. [18] K. Otsuka, T. Ushiama, I. Yamanaka, Chem. Lett. 9 (1993) 1517. [19] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 365. [20] F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornaciero, G. Comelli, R. Rosei, Science 309 (2005) 752. [21] S.A. Chambers, T.T. Irans, T.A. Hileman, J. Mater. Res. 9 (1994) 2944. [22] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, Edition 55, CRC Press, Cleveland, Ohio, 1974. [23] L.J. van der Pauw, Philips Res. Rep. 13 (1958) 1. [24] S. Ikeda, O. Sakurai, K. Uematsu, N. Mizutani, M. Kato, J. Mater. Sci. 20 (1985) 4593.
Thin Solid Films 516 (2008) 6088 – 6094 www.elsevier.com/locate/tsf
Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates Debasis Bera a,b,c,⁎, Satyanarayana V.N.T. Kuchibhatla b,c , S. Azad c , L. Saraf c , C.M. Wang c , V. Shutthanandan c , P. Nachimuthu c , D.E. McCready c , M.H. Engelhard c , O.A. Marina d , D.R. Baer c , S. Seal b,⁎, S. Thevuthasan c,⁎ a
Department of Materials Science and Engineering, 202 Rhines Hall, P.O. 116400, University of Florida, Gainesville, FL 32611-6400, USA b Surface Engineering and Nanotechnology Facility; Advanced Materials Processing and Analysis Center; Department of Mechanical, Materials and Aerospace Engineering; University of Central Florida, 4000 Central Florida Blvd, AMPAC-381, Eng-1, Orlando, FL 32816, USA c W. R. Wiley Environmental Molecular Sciences Laboratory; Pacific Northwest National Laboratory, Box 999 MS K8-93 [US Mail]; 33335 Q Ave [Direct delivery abd Fedex] Richland, WA 99352, USA d Pacific Northwest National Laboratory, Richland, WA 99352, USA Received 13 March 2007; received in revised form 29 October 2007; accepted 2 November 2007 Available online 12 November 2007
Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and zirconia (ZrO2) (111)-buffered sapphire (Al2O3) (0001) substrates using oxygen plasma-assisted molecular beam epitaxy to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on sapphire substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3 (0001) substrate and the ceria films. In contrast, the films, grown on a thin layer of ZrO2 (111) buffered sapphire substrate, appear to be highly oriented in nature with predominant double domain (111) orientation. Oxygen ionic conductivity of these gadolinia-doped ceria films was measured as a function of gadolinium concentration and found to be efficient at relatively lower temperature operation compared to that of bulk polycrystalline, single crystalline yttria stabilized zirconia and gadolinia-doped polycrystalline ceria. Relative improvement in ionic conductivity of highly oriented gadolinia-doped ceria films (in the lower temperature regime) can be ascribed to the increased oxygen vacancies due to presence of Gd as well as high quality of the oriented thin films. © 2007 Elsevier B.V. All rights reserved. Keywords: Highly oriented gadolinia-doped ceria; Ionic conductivity; Solid oxide fuel cell; X-ray diffraction pole-figure analysis; Transmission electron microscopy
1. Introduction Electrolyte materials with exclusive oxygen ion-conductivity are being considered as potential candidates for modern solidstate devices, such as oxygen sensors, solid oxide fuel cells (SOFC), batteries and three-way catalysts of automobiles [1–6]. ⁎ Corresponding authors. Bera is to be contacted at Tel.: +1 352 846 3331; fax: +1 352 392 4911. Seal, Tel.: +1 407 882 1119; fax: +1 407 882 1462. Thevuthasan, Tel.: +1 509 376 1375; fax: +1 509 376 5106. E-mail addresses: [email protected] (D. Bera), [email protected] (S. Seal), [email protected] (S. Thevuthasan). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.11.007
Yttria-stabilized zirconia (YSZ), an almost exclusive oxygen ion-conducting electrolyte for SOFC devices, exhibits oxygenion conductivity as high as 0.1 S/cm at 1275 K [7]. However, the ionic conductivity of YSZ at lower temperatures is significantly low [8]. Therefore, the operation of YSZ-based SOFC is limited only at higher temperatures. Increased ionic transport at lower temperatures would allow the use of a wide range of low cost materials for electrochemical devices including SOFC. In order to improve the efficiency of solid-state electrochemical devices at lower temperatures, a new electrolyte material is needed with higher oxygen ionic conductivity. A significant amount of recent research has been devoted to develop doped-
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ceria based electrolyte [7,9–16]. Particularly, ceria doped with a divalent or trivalent oxide, such as gadolinia, samaria, is known to exhibit higher ionic conductivity compared to YSZ at lower temperatures [7,9–11]. Interface controlled nucleation and growth of two-and threedimensional nanostructured materials show unique physical, chemical, and electronic properties that are different from bulk materials. Atomic and ionic transport, which is crucial for many technological applications including catalysis and fuel-cell technology, is also expected to be enhanced in nanostructured materials due to effects related to size, electronic structure, space charge and multiple interfacial pathways. It is experimentally well known that atomic transport in nanocrystalline materials differs substantially from that in coarse-grained or single-crystalline materials due to high interface diffusion in nanocrystalline materials compared to volume diffusion in single-crystalline materials [17]. However, the role of featuresize on atomic and ionic transport in nanostructured materials is still elusive. Highly oriented growth of ceria on insulating substrates has been a focus of materials research in the recent years for many technologically important applications [5,6,18,19]. In these applications, involvement of highly mobile oxygen is very important. Over wide range of working temperatures, such as, room temperature to 1000 °C, ceria plays two key roles: (i) releasing and storing oxygen, and (ii) promoting noble-metal activity and dispersion [20]. Such roles are very important for the application of ceria in SOFC or related applications. Keeping such applications in view, highly oriented pure and gadolinia-doped ceria films have been grown on pure and ZrO2 (111) buffered Al2O3(0001) substrates using oxygen plasmaassisted molecular beam epitaxy (OPA–MBE). The growth was monitored in situ using a reflection high energy electron diffraction (RHEED). In addition, these films were characterized by several ex situ surface and bulk sensitive techniques, such as, X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and atomic force microscopy. The objective of the current research is to understand the growth characteristics and ionic transport processes in ceriabased oxide thin films that may facilitate the use of new types of electrolytes with enhanced ionic conductivity at lower temperatures for electrochemical device applications. In this article, we discuss the growth of several highly oriented pure and gadolinia-doped ceria thin films on pure and thin layer zirconiabuffered sapphire (0001) using OPA–MBE. As discussed above, these films were characterized using several surface and bulk sensitive techniques to understand the growth mechanisms and the influence of Gd concentration on the total conductivity of the films. 2. Experimental details 2.1. Film growth Approximately 180 nm thick ceria films with various dopant concentrations of Gd on pure and zirconia-buffered sapphire (0001) were grown using OPA–MBE. Growth of thin films and
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in situ characterizations were carried out in a dual chamber ultrahigh vacuum (UHV) system described else where [21]. The molecular beam epitaxy (MBE) chamber consists of three metal evaporation sources and a UHV compatible electron cyclotron resonance (ECR) oxygen plasma source, as well as RHEED for real-time characterization of film growth. High purity cerium (Ce) and zirconium (Zr)-rods were used as the source materials in separate electron beam evaporators and gadolinium (Gd) was evaporated from an effusion cell. Growth rate of these films was monitored using quartz crystal oscillators. 10 mm × 10 mm × 1 mm sapphire substrates were cleaned ultrasonically in isopropanol and acetone for 5 min each. The substrates were introduced into the MBE chamber and cleaned at a temperature of 875 K for 10 min by exposing to activated oxygen plasma from ECR plasma source at an oxygen partial pressure of ∼ 2 × 10− 3 Pa. In the case of gadolinia-doped ceria growth without the zirconia buffer layer on sapphire (0001), Gd and Ce metal beams were co-evaporated in the presence of low pressure oxygen plasma. In the case of zirconia buffer layer, Zr metal was first evaporated in the presence of low pressure oxygen plasma followed by the gadolinia doped ceria growth. During growth, temperature and oxygen partial pressure were optimized to 925 K and 2.67 × 10− 3 Pa, respectively, and the growth was monitored by RHEED. Gd concentration in these films was varied between 6–24%. 2.2. Characterization techniques In addition to the in situ RHEED, the samples were characterized using various ex situ methods. RBS data was collected for random and channeling directions at room temperature using 2.0 MeV He+ ions to determine specimen thickness and crystal quality. A silicon surface barrier detector at 150° scattering angle was used to collect the backscattered ions. Experimental RBS spectra were simulated using a SIMNRA software in order to quantify the concentrations of gadolinium dopant in various samples. X-ray photoelectron spectroscopy (XPS) depth profiling measurements were made on a Physical Electronics Quantum 2000 Scanning ESCA Microprobe, which consists of a focused Al Kα X-ray source (1486.7 eV) and a high energy resolution hemispherical analyzer. Sample sputtering was performed using 2 kVAr+ ions rastered over a 2 mm × 2 mm area of the specimen. The sputter rate for these ion gun conditions was calibrated at a rate of 4.4 nm/min for a known SiO2 reference material. The sample was rotated at 2 RPM during sputtering. Photoemission spectra were collected using a pass energy of 46.95 eV. The XPS spectra were referenced to an energy scale with binding energies for Cu 2p3/2 at 932.67 ± 0.05 eVand Au 4f at 84.0 ± 0.05 eV. Low energy electrons (∼ 1 eV) and argon ions (∼5 eV) were used for specimen neutralization. XRD measurements were performed using Philips X'pert θ–2θ MRD diffractometer operating at 45 kV and 40 mA with a fixed Cu anode. The analyses of diffraction data were carried out using JADE 6.0 from Materials Data Inc. Transmission electron microscopy (TEM) sample preparation was carried out using a standard tripod wedge polishing method
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followed by Ar-ion beam thinning to electron transparency. High-resolution transmission electron microscopic analysis (HRTEM) was carried out using a JEOL 2010 TEM with a point to point resolution of 0.194 nm. The images are recorded with a CCD camera (1024 X 1024 pixels) followed by processing with Digital Micrograph® software. Total conductivity in the films was measured using four probe impedance spectroscopy. This capability was customized to measure the total conductivity in oxide films with thickness in the range of nanometers. A Solartron LCR meter and impedance analyzer controlled by computer is used to control the input parameters such as current and voltage in a frequency range of 10 Hz–1 GHz. This typically gives an output with real and imaginary parts of impedance. 3. Results and discussion The RHEED patterns from the sapphire (0001) substrate and the zirconia buffer layer are presented along with the pattern from gadolinia doped ceria film in Fig. 1. The RHEED pattern from the blank sapphire substrate after plasma cleaning is shown in Fig. 1(a). As zirconia thin film deposited on the sapphire substrate, the RHEED streaks changed into the features associated with zirconia (111) and remain the same until the end of the zirconia growth. The streak RHEED pattern after the deposition of a 4 nm zirconia thin film is shown in Fig. 1(b) while the pattern after deposition of the ceria thin film doped with gadolinia is shown in Fig. 1(c). As it is seen in Fig. 1(c), spotty features started to show up in the RHEED pattern from gadolinia doped ceria films, in addition to the streaks. These results indicate that the film surface appears to be rough due to the fact of possible development of a small amount of different phase during deposition along with the preferred orientation. We will discuss this observation in the later part of the discussion section. XPS was carried out to monitor the chemical composition and elemental distribution of as-deposited gadolinia-doped ceria films. Fig. 2(a) shows a representative XPS survey scan of a gadoliniadoped ceria film on zirconia buffered sapphire substrate. Absence of aluminum (Al 2p: 74.4 eV) and zirconium (Zr 3d5/2:180.0 eV) in the spectrum uniform coverage of the films and indicates that there was no major diffusion during film growth. A high resolution XPS scan of Gd 4d shown in the inset of Fig. 2(a) confirms the doping of Gd in the film. A XPS depth profile of undoped ceria thin film on zirconia buffered-sapphire is shown in Fig. 2(b). The depth profile results clearly demonstrate that there is no Zr and Al diffusion into the bulk of the film. The atomic coverage of Ce and oxygen (O) in depth profile of the film appears to be uniform throughout the film-thickness. Thickness of the film was found to be about 150 nm, consistent with RBS results. XRD spectra from the gadolinia-doped ceria films grown on pure and zirconia-buffered sapphire substrates are shown in Fig. 3(a) and (b) respectively. The film grown without the buffer layer shows polycrystalline features possibly due to structural deformations resulting from the large lattice mismatch between the Al2O3 (0001) substrate and the ceria film. In addition to the (111) peaks of ceria, the peaks associated with (200), (220), (311) and (331) are visible in this spectrum.
Fig. 1. The RHEED patterns of sapphire (0001) substrate (a) after plasma cleaning and before zirconia deposition, (b) after zirconia thin film deposition and (c) after zirconia buffered gadolinium doped ceria thin film deposition.
In contrast, the ceria films, grown on a 3 nm ZrO2 (111) buffer layer on top of Al2O3 (0001), appear to be highly oriented in (111) direction as shown in Fig. 3(b). Most of the polycrystalline features are not visible in this spectrum except less intense (200) in comparison to Fig. 3(a). This is likely due to the much smaller lattice mismatch between cubic zirconia and ceria compared to the substantial differences between the lattice parameters of Al2O3(0001) and ceria. The lattice parameters mismatch between ceria and sapphire is high because the lattice parameter of ceria in a fluorite structure is 0.541 nm, whereas, lattice parameters of sapphire in a corundum structure are a: 0.475 nm and c: 1.3 nm. Therefore, a buffer layer was chosen with an intermediate lattice parameter of
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the spectrum also matched with the reference data from cubic fluorite ceria and zirconia structures. Average crystallite sizes of doped and undoped ceria samples were also evaluated from XRD patterns using Scherrer equation assuming that the peak broadening was only due to small crystallite size. Contribution of non-uniform strain to the peakbroadening is expected to be same for both doped and undoped samples as the full-width-at-half-maximum (FWHM) is same for all samples. According to the Scherrer equation, the crystallite size was approximated to be 8 nm extracted from (222) peak for both doped and undoped samples (Fig. 3(a) and (b)). It was also found that samples with different amount of Gd showed the same crystallite size as FWHM of (222) XRD peak is same for all doped samples. This is due to fact that ionic radii of Ce ions (0.092 nm for Ce4+ and 0.103 nm for Ce3+) are comparable or higher than that of Gd ion (0.094 nm for Gd3+) [22]. Note that the peak positions (two-theta values) are unchanged for undoped polycrystalline and doped highly crystalline ceria. Such a trend implies that there was no uniform strain present in the doped samples. A bright-field HRTEM micrograph of the cross-section of zirconia buffered gadolinia-doped ceria is presented in Fig. 4(a). Energy dispersive X-ray spectroscopic analyses (EDS) were carried out at the interfaces of sapphire, zirconia and ceria films, and as well as along the thickness of the films. The results reveal Fig. 2. (a) XPS survey spectrum of gadolinia-doped ceria thin film on zirconia buffered sapphire substrate; Inset shows a high resolution Gd 4d envelop. (b) XPS depth profile from a zirconia buffered pure ceria thin film on sapphire substrate.
sapphire and ceria. The lattice parameter of zirconia is 0.511 nm and it is stabilized as cubic structure in thin film. Note that the XRD pattern shows two high intense peaks of (111) planes and two low-intense peaks of (100) planes of ceria, indicating preferential growth of (111) plane of ceria. The peak positions corresponding to the gadolinia-doped ceria and pure zirconia in
Fig. 3. XRD plots for (a) ceria thin film on sapphire (0001) substrate (b) heterolayers of gadolinia-doped ceria and zirconia thin film on sapphire (0001).
Fig. 4. (a) HRTEM micrograph of the cross-section of a ceria thin film grown on zirconia buffered sapphire (0001) substrate. EDS analyses shows almost uniform distribution of Gd along the cross section of the thin film.
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that there is no diffusion of gadolinium into the sapphire or zirconia during the deposition of films. A series of EDS spectra was acquired along the thickness of the gadolinia-doped ceria film for quantitative analysis. Quantitative analysis of these spectra showed that the amount of Gd was uniformlydistributed throughout the ceria thin film (see Fig. 4(b)). RBS was carried out on the samples to quantify the thickness of the film and Gd present in the samples. The qualitative and semi-quantitative information on concentration of Gd obtained from quartz crystal oscillators and EDS, respectively, could be substantiated by RBS measurements. A spectrum collected from RBS along with simulated results using SIMNRA software is shown in Fig. 5. The concentration of Gd from simulated spectra is in well-agreement with concentration data from EDS. The simulated results of ceria samples doped with different percentage of Gd show that the thickness of ceria and zirconia are to be approximately 150 nm and 3 nm, respectively. The percentage Gd was found to be 5.3%, 8.4%, and 19.2% in three samples. 4. Orientation of OPA–MBE grown thin films As mentioned earlier the growth of multilayer thin films was monitored by the in situ RHEED. The RHEED streaks, presented in Fig. 1(c), are showing some distorted features. It may be indicative of the presence of little amount of a differently oriented ceria in the highly oriented ceria thin film. This result is consistent with XRD results presented in Fig. 3(b). To assess the preferred orientation of the thin film relative to the orientation (0001) of sapphire substrate, pole figures of the ceria (111), ceria (200) and alumina (202) were collected, as shown in Fig. 6. Theoretically, ceria (111) film with a single inplane domain shows three (111) poles at 70° tilt in addition to the out-of-plane (111) pole at 0° tilt. In the present study, however, six poles at 70° tilt relative to the central pole are observed (see Fig. 6(a)). It suggests that the ceria (111) film has two in-plane domains. Similarly, Fig. 6(b) represents ceria (200) pole-figure showing six (200) poles at 54.74° tilt which is the inter-planner angle between (111) and (200) for face-centered cubic lattice. The pole at 0° corresponds to the out-of-plane (200) orientation and
Fig. 6. Pole figures with respect to (a) ceria (111), (b) ceria (200) and (c) sapphire (202) planes.
Fig. 5. RBS spectrum for highly oriented zirconia-buffered gadolinium doped ceria thin film on sapphire (0001) with superimposed simulated pattern from SIMNRA.
additional six (200) poles are the in-plane ceria (111) film. It further confirms the presence of two in-plane domains in the ceria (111) film. Sapphire (202) pole figure presented in Fig. 6(c) was used for comparison. According to these pole-figures and 2θ scans, ceria (111) certainly has very high intensity compared to
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ceria (200). Based on the intensities it is quite evident that amount of ceria with (200) orientation present in sample is very minor. Therefore, it is reasonable to say that the ceria film on a zirconia buffered sapphire substrate was highly (111) oriented with two inplane domains. 5. Evaluation of oxygen ionic conductivity The electrical conductivity of godolinia-doped ceria on zirconia buffered sapphire (0001) films was measured using a four-probe van der Pauw technique [23]. Sapphire was used as substrate to minimize the interference of electrical conductivity from the substrate to the measured conductivity. Oxygen ionic conduction is approximately zero in the sapphire. Note that the electronic conductivity in ceria or zirconia based oxides is significantly less compared to ionic conductivity, especially, at low temperatures [9]. Therefore the electronic conductivity of these oxides can be neglected and assumed that the majority of the total conductivity is dominated by the oxygen ionic conductivity. Hence the total conductivity is interchangeably dealt as oxygen ionic conductivity in doped ceria and zirconia systems. Oxygen ionic conductivity results from samples with different concentration of Gd have been presented in Fig. 7. For comparison, the oxygen ionic conductivity data from polycrystalline [1], single crystalline YSZ [24] and gadolinia-doped polycrystalline ceria [9] are also shown in the figure. In general, these highly oriented films showed enhanced conductivity compared to the bulk polycrystalline YSZ and single crystalline YSZ. In comparison to the polycrystalline gadolinia doped ceria, these highly oriented films showed better ionic conductivity values at lower temperatures. In addition, there is a significant amount of increase in conductivity with increase in concentration of Gd (∼5–19%) in the ceria films. In order to understand the relationship between the concentration of dopant in the samples and ionic conductivity, values of oxygen ionic conductivity from different gadolinia-doped ceria samples at a temperature 770 °C are plotted against experimentally found Gd concentration, as shown in Fig. 7(b). Conductivity values of single crystalline YSZ [24] and gadolinia-doped polycrystalline ceria [9] are also shown for comparison. Fig. 7(b) shows a linear relationship (linear regression coefficient (R2) 98% and standard deviation 0.29) between ionic conductivity and amount of dopant. Recall that the crystallite size was unchanged for all the samples regardless the concentration of Gd in the films. In other words, the effect of crystallite size on ionic conductivity data is either negligible or same for all doped samples. Therefore, increase in oxygen ionic conductivity values with increasing amount of Gd from 5 to 19% in ceria films at the 770 °C is mainly due to generation of higher amount of oxygen vacancies in the samples. Generally, trivalent elements, such as Gd3+, create oxygen vacancies in ceria films which consequently increase the ionic conductivity of the films [7,9–11]. Texture of the films also affects the ionic conductivities of the film. In order to compare the effect of the orientation on oxygen ionic conductivity, conductivity data of gadolinia-doped polycrystalline ceria at 770 °C is introduced in the Fig. 7(b). It shows a significantly lower ionic conductivity value than corresponding that of gadolinia doped highly crystalline ceria
Fig. 7. Arrhenius plot for of oxygen ionic conductivity of zirconia-buffered gadolinia-doped ceria thin film on sapphire. The data from polycrystalline YSZ [1], single crystalline YSZ thin films [24] poly crystalline gadolinia-doped ceria [9] are also shown. (b) Conductivity values of single crystalline YSZ [24], gadolinia doped polycrystalline ceria [9] and gadolinia-doped highly crystalline ceria on zirconia-buffered sapphire (0001) substrate at 770 °C.
film. Therefore, texture of the sample plays an important role on overall ionic conductivity. In conclusion, these highly oriented gadolinia doped ceria thin films on zirconia-buffered sapphire substrate can be potential candidates for the use as electrolytes in solid oxide fuel cell devices at lower temperatures due to comparatively higher oxygen ionic conductivity values. Oxygen ionic conductivity depends on concentration of dopants in the samples as well as on the texture of the samples. Although current work mainly focuses on the growth of the highly oriented gadolinia-doped ceria film along with some information on texture and ionic conductivity, our future study on these film is expected to reveal more on the relationship among texture, doping concentration, grain size and ionic conductivity. 6. Summary Growth of highly oriented zirconia-buffered ceria thin film on sapphire substrate is demonstrated. Thin films with and without a
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buffer-layer were characterized by several surface and bulk characterization techniques. The orientation of films was monitored and characterized by in situ RHEED and ex situ XRD analyses. Dopant concentration, film thickness evaluated using different techniques were found to be identical. Despite the small amount of ceria (200) orientation, the films grown with zirconia buffer layer are highly oriented in nature with predominant double domain (111) orientation compared to the films without the buffer layer. Oxygen ionic conductivity of these films was measured and found to be efficient at relatively lower temperature operation compared to that of bulk polycrystalline, single crystalline YSZ and gadolinia-doped polycrystalline ceria. Among 5.3, 8.4 and 19.2% gadolinia doped ceria samples, higher extent of gadolinia-doped sample shows better efficiencies at lower temperature. Relative improvement in ionic conductivity of highly oriented gadolinia-doped ceria films (in the lower temperature regime) can be ascribed to the increased oxygen vacancies due to presence of Gd as well as high quality of the oriented thin films. Acknowledgement This research work was supported in part by the Office of Fellowship Program of Pacific Northwest National laboratory (PNNL) and NSF-REU program at University of Central Florida. The experiments were performed in the laboratory of Environmental Molecular Sciences Laboratory and supported by the US Department of Energy's Office of Biological and Environmental Research. Authors would like to thank Y. Du and M. Mckinley for their assistance during the course of the experiments and data analysis. Bera would like to thank PNNL for Summer Research Fellowship, and Materials Research Society (MRS) for Symposium Student Assistantship to present this work at MRS 2004 Fall Meeting at Boston. References [1] N.Q. Minh, T. Takahashi, Science and Technology of Ceramic Fuel Cells, Elsevier, Amsterdam, 1995. [2] N. Izu, W. Shin, N. Murayama, Sens. Actuators B 93 (2003) 449.
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