J Nanopart Res (2008) 10:369–375 DOI 10.1007/s11051-007-9257-z
BRIEF COMMUNICATION
Gadolinium doped Ceria nanocrystals synthesized from mesoporous silica Emma Rossinyol Æ Eva Pellicer Æ Anna Prim Æ So`nia Estrade´ Æ Jordi Arbiol Æ Francesca Peiro´ Æ Albert Cornet Æ Joan Ramon Morante
Received: 23 February 2007 / Accepted: 16 May 2007 / Published online: 20 July 2007 Ó Springer Science+Business Media B.V. 2007
Abstract Highly crystalline and thermally stable gadolinium doped ceria (GDC) particles have been synthesized by hard template route for the first time. This oxide is being recognized as an intermediate temperature (500–700 °C) electrolyte material for applications in solid-oxide fuel cells. The GDC particles show high crystallinity and nanometric size (2.83 ± 0.05 nm in diameter) and Raman analyses confirm the formation of the solid solution instead of a CeO2 and Gd2O3 mixture. EDX and EELS studies indicate a stoichiometry coherent with the Gd0.2Ce0.8O1.9 phase. The synthesized nanometric powder is expected to be used in solid oxide fuel cells as well as in the catalytic treatment of automobile exhaust fumes.
E. Rossinyol (&) Microscopy Service, Faculty of Sciences, Universitat Auto`noma de Barcelona, Cerdanyola del Valles, Barcelona 08193, Spain e-mail:
[email protected] E. Rossinyol E. Pellicer A. Prim S. Estrade´ J. Arbiol F. Peiro´ A. Cornet J. R. Morante EME/CeRMAE/IN2UB, Departament d’Electro`nica, Universitat de Barcelona, Barcelona 08028, Spain J. Arbiol TEM-MAT, Serveis Cientificote`cnics, Universitat de Barcelona, Barcelona 08028, Spain
Keywords Cerium Electron microscopy Fuel cell Gadolinium Mesoporous materials and synthesis design
Introduction Fuel cells are now attracting much attention as environmentally friendly electric power generating systems. Solid oxide fuel cells (SOFCs) have a great potential to be the cleanest, most efficient, and versatile technologies for chemical-to-electrical energy conversion. However, the cost of materials and fabrication must be dramatically reduced to be economically competitive. For this purpose, the operating temperature must be reduced, so that interconnection, heat exchanges, and structural components may be fabricated from relatively inexpensive metal components (Tok et al. 2004). With high ionic conductivity between 500 and 700 °C, doped ceria have been extensively studied as electrolyte in intermediate-temperature SOFCs (Navarro et al. 1997; Go¨dickemeier et al. 1997; Nakagawa et al. 2001; Oishi et al. 2005). The conduction of ceria-based ceramics is made possible by the existence of oxygen vacancies, whose concentration can be increased by doping the material with metal oxide having smaller valence. Cerium dioxide, CeO2, has fluorite structure (CaF2 type) in which eight oxygen atoms surround a cerium atom. Gadolinium oxide, Gd2O3, has C-type crystal structure (Mn2O3 type) in
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which six oxygen atoms surround a gadolinium atom. C-type structure is obtained by removing two oxygen atoms of eight in fluorite structure in a regular manner (Nakagawa et al. 2001). Therefore, gadolinium-doped ceria (GDC) is a solid solution formed by replacing the Ce4+ sites of the CeO2 fluorite lattice by Gd3+ cations. GDC is considered to be one of the most promising electrolytes for SOFCs to be operated below 650 °C, as it has higher ionic conductivity compared to other commonly used materials such as LSGM [La0.9Sr0.1Ga0.8Mg0.2O2.85] (Minh et al. 1993) and YSZ [(ZrO2)0.9(Y2O3)0.1], which has sufficient ionic conductivity only arround 1000 °C. Moreover, GDC has been successfully used as part of anodes for SOFCs, especially those using hydrocarbon fuels. Studies on a Ni-GDC cermet anode have demonstrated the importance of microstructure control in obtaining a good electrode yield (Schouler et al. 1987; Watanabe et al. 1997; Tsai et al. 1998; Sato et al. 2004; Wandekar et al. 2006). Several methods have been reported for the synthesis of GDC nanoparticles, such as hydrothermal treatment, precipitation (for oxalate, carbonate, peroxide, and hydroxide) and sol-gel (Yamashita et al. 1995; Trovarelli et al. 1996; Huang et al. 1998; Higashi et al. 1999). But these synthesis pathways yield a high average crystalline size, generally about several tens of nanometer. Nanostructured mesoporous materials have been widely studied in the development of catalytic systems (Arbiol et al. 2002, 2004; Rossinyol et al. 2005, 2007) due to their large, controllable pore size and high surface area. In this context, novel methodologies have been developed by using mesoporous materials as host to accommodate different oxides (Ryoo et al. 1996; Zhao et al. 1998; Han et al. 2000; Fukuoka et al. 2001; Zhang et al. 2002; Zhu et al. 2003; Yang et al. 2003; Tian et al. 2004). According to this synthesis pathway, the voids of a preformed mesoporous solid are impregnated with desired precursors and removal of the former solid templates leads to mesostructures with other compositions and crystalline frameworks. The pore structure, such as pore size and channel conductivity can be tunned depending on the practical application (Yu et al. 2003). In this context, we report on the synthesis and structural characterization of nanostructured GDC in
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view of its use in fuel cell devices. The control of the active surface area is one of the most important issues for tailoring the properties of materials used not only as electrolyte but also as anode. Thus, in order to improve the surface to bulk ratio, mesostructured GDC have been synthesized. Our effort relies on obtaining an active material with an improved porosity degree defined by the selected hard nanotemplate.
Results and discussion SiO2 SBA-15 was synthesized under acidic conditions by using Pluronic P123 triblock copolymer as a template and TEOS as a silicon source (Zhao et al. 1998). GDC mesoporous structure was synthesized by impregnating the silica template with appropriate precursors. Silica/GDC mixtures were obtained after calcination. Silica template was later removed with NaOH (Rossinyol et al. 2005, 2007) in order to release the GDC nanoparticles (see the experimental section for details). A representative XRD pattern of the product obtained is shown in Fig. 1. Peaks of synthesized powders were rather broad reflecting the nanocrystalline nature of the product. The pattern agrees with the formation of a single cerium oxide phase with a cubic fluorite structure. The indexing of the diffraction peaks would also be consistent with the lattice planes of the fluorite structure Gd0.2Ce0.8O1.9 (S.G.: ˚ (JCPDS database 75-0162). Fm3m) with a = 5.423 A However, the XRD pattern could be also compatible with a CeO2 in a cubic fluorite structure or with a Ctype Gd2O3 structure, since it is difficult to clearly distinguish between those structures and the GDC solid solution. In the fluorite unit cell structure, the Ce4+ cations are placed in the FCC lattice sites, while the anions (O2 ) are located at the eight tetrahedral sites. Four remaining octahedral sites in the FCC lattice remain vacant. Therefore, fissions products can be accommodated in the vacant position of this large number of unoccupied octahedral interstitial sites, which allows this compound to be used as a nuclear fuel (Tok et al. 2004). A high-magnification TEM image of GDC nanoparticles has been obtained using a JEOL 2011 microscope. Figure 2a shows spherical shaped nanocrystals with rather uniform size. The mean diameter
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Fig. 1 XRD spectrum of GDC indexed from JCPDS database (75-0162). Fluorite structure is shown as an inset
Fig. 2 (a) Representative HRTEM image of GDC nanoparticles and (b) SAED patterns indexed from JCPDS database (75-0162)
of the individual nanoparticle is measured to be 2.83 ± 0.05 nm (based on 100 particles from the set of TEM images acquired). The lattice fringes visible in the HRTEM image displayed in Fig. 2a are indicative of the high crystallinity of these particles. The indexing of the lattice parameters of the selected area electron diffraction (SAED) shown in Fig. 2b would agree with the structure proposed from the XRD results (fluorite structure Gd0.2Ce0.8O1.9, S.G.: ˚ ). As stated before, C-type Fm3m with a = 5.423 A Gd2O3, CeO2 in fluorite structure and GDC solid solution give quite similar diffraction patterns, which makes peaks assignation difficult. The interplanar ˚ could be indexed to distance corresponding to 3.13 A plan (111). TEM studies have also been focused on the structural characterization of this material (Fig. 3). Both High Angle Anular Dark Field (HAADF) and Bright Field (BF) images clearly show the
mesostructre organization of the sample. Rings shown in the SAED image displayed as an inset in Fig. 3d reveal that despite the clear orientation of the silica channels, the nanoparticles arrays are randomly oriented along the mesostructured framework. The presence of gadolinium has been confirmed by energy dispersive X-ray analysis (EDX) and is depicted in Fig. 4. The spectrum exhibits signals corresponding to Ce and Gd, which could be compatible with a stoichiometry coherent with the Gd0.2Ce0.8O1.9 phase (copper presence is due to the TEM grid). Electron Energy Loss Spectroscopy (EELS) analysis has also been performed in order to verify the homogeneity of the sample and the average Gd/Ce proportion. Figure 5 shows the results of the EELS quantification. Notice that the fine structure of the O K peak remains the same along the considered region, and that the variation in the Gd/Ce proportion is below the experimental
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Fig. 3 TEM micrograph of the mesostructured framework of SBA-15 GDC replica (a) HAADF (b) and (c) BF STEM. (d) Detailed image of the nanoparticle array
Fig. 4 EDX microanalysis of GDC sample. Copper presence is due to the TEM grid
precision. The atomic ratio between Gd/Ce seems to be slightly below the stoichiometry proposed. Since the quantification of an EELS spectrum requires a
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more complex mathematical treatment, we can not determine the exact stoichiometry but we can assure the homogeneity of the sample. Furthermore, the
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Fig. 5 (a) HAADF STEM image of the sample, (b) O K peak variation along the highlighted line in (a), (c) Gd M5,4 and Ce M5,4 peak variation along the highlighted line in (a), and (d) Gd/Ce quantification along the highlighted line in (a)
difference between the observed average Gd/Ce proportion and the nominal Gd/Ce proportion is also below the experimental precision. However, it is not possible to confirm the formation of gadolinium-doped ceria solid solution instead of a mixture of CeO2 and Gd2O3 by means of EDX analysis. As for EELS analysis, it shows that the expected stoichiometry is found in a sub-nanometric volume (spot size being about 0.2 nm), which seems to suggest that only one oxide is found instead of a mixture of two. Therefore, Raman spectra have been obtained in order to corroborate the formation of Gd0.2Ce0.8O1.9. The magnified region of the 200– 550 cm 1 wavelength (obtained using a Jobin Yvon T64000 spectrograph equipped the Ar+ coherent INNOVA 300 laser as a excitation source) is shown in Fig. 6. If a mixture of CeO2 and Gd2O3 is formed, the Raman spectra will show a characteristic vibrational mode at 360 cm 1, attributed to the main
strongest band of the Gd2O3 cubic phase (GarciaMurillo et al. 2002; Godinho et al. 2007). This vibrational mode is not present in our sample. By contrast, a band at 465 cm 1 corresponding to the F2g symmetry of the CeO2 cubic phase (Matta et al. 2002) can be found. Thereby, we can assume the existence of a single cubic phase. On indexing the XRD patterns it was observed that GDC retained the cubic symme˚ ) is close try of CeO2 as the ionic radii of Gd (0.938 A ˚ ). Thus, 20% of gadolinium oxide to that of Ce (0.87 A is completely dissolved into the ceria fluorite structure, which means that the Ce4+ ions of the lattice are partially replaced by the Gd3+ ions.
Conclusions In summary, we have reported for the first time the synthesis of gadolinium-doped-ceria by hard template
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Fig. 6 Raman spectra of gadolinium-doped ceria
route. The GDC particles show high crystallinity and nanometric size (around 2.8 nm in diameter). The average particle diameter is lower than those reported for GDC particles synthesized by other methods. EDX spectra show gadolinium peaks and a Gd/Ce ratio close to 0.25 has been estimated from EELS analyses. The TEM images reveal that single Gd0.2Ce0.8O1.9 nanocrystallites are randomly oriented in the mesostructured framework. The formation of the solid solution instead of a CeO2 and Gd2O3 mixture has been unambiguously confirmed by Raman studies. The synthesized nanometric powder is expected to be used in solid oxide fuel cells as well as in the catalytic treatment of automobile exhaust fumes.
respectively). After 30 min stirring, the dispersion was dried at room temperature in air atmosphere and calcined at 350 °C for 4 h. The silica/GDC mixture was dispersed again in ethanol with 0.2 g of precursors, dried at room temperature in air atmosphere and calcined at 750 °C. Finally the silica template was removed with NaOH (Rossinyol et al. 2005). In all cases chemicals were of analytical grade and water used had been distilled twice and deionised with a Millipore Milli-Q system.
Experimental section
References
A solution with Pluronic P123 (6 g, purchased from BASF Corporation) was dissolved in distilled H2O (195 g) and concentrated HCl (30 g, 35%) was added and stirred for 6 h at 35 °C. TEOS (12.49 g) was added at the mixture and stirred for 24 h at 35 °C, and then heated at 100 °C for another 24 h as a hydrothermal treatment. The solid product was filtered, washed, dried at room temperature in air atmosphere and calcined at 550 °C for 4 h. For the synthesis of GDC mesoporous oxide, SBA-15 silica (0.15 g) were dispersed in ethanol and contacted with 0.4 g nitrate precursors (in an atomic proportion of 20% Gd(NO3)36H2O and 80% Ce(NO3)36H2O
Arbiol J, Cabot A, Morante JR, Chen F, Liu M (2002) Distributions of noble metal Pd and Pt in mesoporous silica. Appl Phys Lett 81:3449 Arbiol J, Rossinyol E, Cabot A, Peiro´ F, Cornet A, Morante JR, Chen F, Liu M (2004) Noble metal nanostructures synthesized inside mesoporous nanotemplate pores. Electrochem Solid State Lett 7:J17 Fukuoka A, Sakamoto Y, Guan S, Inagaki S, Sugimoto N, Fukushima Y, Hirahara K, Iijima S, Ichikawa M (2001) Novel templating synthesis of necklace-shaped mono- and bimetallic nanowires in hybrid organic-inorganic mesoporous material. J Am Chem Soc 123:3373 Garcia-Murillo A, Le Luyer C, Garapon C, Dujardin C, Bernstein E, Pedrini C, Mugnier J (2002) Optical properties of europium-doped Gd2O3 waveguiding thin films prepared by the sol-gel method. Opt Mater 19:161
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Acknowledgements This work was partially supported by E.U. Nanos4 project. EME is with CeRMAE, center on Advanced Materials for Energy of the Generalitat de Catalunya. We acknowledge the technical support from the Microscopy Service of the Universitat Autonoma de Barcelona.
J Nanopart Res (2008) 10:369–375 Go¨dickemeier M, Sasaki K, Gauckler LJ, Riess I (1997) Electrochemical characteristics of cathodes in solid oxide fuel cells based on ceria electrolytes. J Electrochem Soc 144:1635 Godinho MJ, Gonc¸alves RF, Santos LPS, Varela JA, Longo E, Leite ER (2007) Room temperature co-precipitation of nanocrystalline CeO2 and Ce0.8Gd0.2O1.9-d powder. Mater Lett 61:1904 Han YJ, Kim JM, Stucky GD (2000) Preparation of noble metal nanowires using hexagonal mesoporous silica SBA15. Chem Mater 12:2068 Higashi K, Sonoda K, Ono H, Sameshima S, Hirata Y (1999) Synthesis and sintering of rare-earth-doped ceria powder by the oxalate coprecipitation method. J Mater Res 14(3):957 Huang K, Feng M, Goodenough JB (1998) Synthesis and electrical properties of dense Ce0.9Gd0.1O1.95 ceramics. J Am Ceram Soc 81(2):357 Matta J, Courcot K, Abi-Aad E, Boukays A (2002) Identification of vanadium oxide species and trapped single electrons in interaction with the CeVO4 phase in vanadium-cerium oxide systems. 51V MAS NMR, EPR, Raman, and thermal analysis studies. Chem Mater 14:4118 Minh NQ (1993) Ceramic fuel cells. J Am Ceram Soc 76(3):563 Nakagawa T, Osuki T, Yamamoto TA, Kitauji Y, Kano M, Katsura M, Emura S (2001) Study on local structure around Ce and Gd atoms in CeO2-Gd2O3 binary system. J Synchrotron Rad 8:740 Navarro L, Marques F, Frade J (1997) n-Type conductivity in gadolinia-doped ceria. J Electrochem Soc 144:267 Oishi N, Atkinson A, Brandon NP, Kilner JA, Steele BCH (2005) Fabrication of an anode-supported gadoliniumdoped ceria solid oxide fuel cell and its operation at 550 8C. J Am Ceram Soc 88:1394 Rossinyol E, Prim A, Pellicer E, Arbiol J, Herna´ndez-Ramı´rez F, Peiro´ F, Cornet A, Morante JR, Solovyov LA, Tian B, Bo T, Zhao D (2007) Synthesis and characterization of chromium-doped mesoporous tungsten oxide for gassensing applications. Accepted at Adv Func Mat doi: 10.1002/adfm.200600722 Rossinyol E, Arbiol J, Peiro´ F, Cornet A, Morante JR, Tian B, Bo T, Zhao D (2005) Nanostructured metal oxides synthesized by hard template method for gas sensing applications. Sens Act B 109:57 Ryoo R, Kim JM, Ko CH, Shin CH (1996) Disordered molecular sieve with branched mesoporous channel network. J Phys Chem 100:17718
375 Sato H, Yokoyama C (2004) Solid state fuel storage and utilization through reversible carbon deposition on an SOFC anode. Solid State Ionics 175:51 Schouler EJL, Kleitz M (1987) Electrocatalysis and inductive effects at the gas/platinum stabilized zirconia interface. J Electrochem Soc 134:1045 Tian B, Liu X, Solovyov LA, Liu Z, Yang H, Zhang Z, Xie S, Zhang F, Tu B, Yu C, Terasaki O, Zhao D (2004) Facile synthesis and characterization of novel mesoporous and mesorelief oxides with gyroidal structures. J Am Chem Soc 126:865 Tok AIY, Luo LH, Boey FYC (2004) Carbonate co-precipitation of Gd2O3-doped CeO2 solid solution nanoparticles. Mat Sci Eng A 383:229 Trovarelli A (1996) Catalytic properties of ceria and CeO2containing materials. Catal Rev Sci Eng 38(4):439 Tsai T, Barnett SA (1998) Effect of mixed-conducting interfacial layers on solid oxide fuel cell anode performance. J Electrochem Soc 145:1696 Watanabe M, Uchida H, Yoshida M (1997) Effect of ionic conductivity of zirconia electrolytes on the polarization behavior of ceria-based anodes in solid oxide fuel cells. J Electrochem Soc 144:1739 Wandekar RV, Ali (Basu) M, Wani BN, Bharadwaj SR (2006) Physicochemical studies of NiO-GDC composites. Mat Chem Phys 99:289 Yamashita K, Ramanujachary KV, Greenblatt M (1995) Hydrothermal synthesis and low temperature conduction properties of substituted ceria ceramics. Solid State Ionics 81:60 Yang H, Shi Q, Tian B, Lu Q, Gao F, Xie S, Fan J, Yu C, Tu B, Zhao D (2003) One-step nanocasting synthesis of highly ordered single crystalline indium oxide nanowire arrays from mesostructured frameworks. J Am Chem Soc 125:4724 Yu C, Tian B, Zhao D (2003) Recent advances in the synthesis of non-siliceous mesoporous materials. Curr Opin Solid State Mater Sci 7:191 Zhang Z, Dai S, Blom DA, Shen J (2002) Synthesis of ordered metallic nanowires inside ordered mesoporous materials through electroless deposition. Chem Mater 14:965 Zhao D, Feng J, Huo Q, Melosh N, Frederickson GH, Chmelka BF, Stucky GD (1998) Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 279:548 Zhu K, He H, Xie S, Zhang X, Zhou W, Jin S, Yue B (2003) Crystalline WO3 nanowires synthesized by templating method. Chem Phys Lett 377:317
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