characterization of samarium doped ceria powders prepared by hydrothermal synthesis for use in solid state oxide fuel cells

j m a t e r r e s t e c h n o l 3;2(2):141–148 www.jmrt.com.br Original article Characterization of samarium-doped ceria powders prepared by hydrothermal synthesis for use in solid state oxide fuel cells Sea-Fue Wang ∗ , Chun-Ting Yeh, Yuh-Ruey Wang, Yu-Chuan Wu Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan a r t i c l e i n f o a b s t r a c t Article history: In this study, Ce1−x Smx O2−ı (x = 0.0, 0.1, 0.2, 0.3, 1.0) were synthesized for use in solid oxide Received 20 November 2012 fuel cells (SOFCs) using an environment-friendly method of coprecipitation followed by Accepted 31 January 2013 hydrothermal treatment XRD and TGA results revealed that the gels after coprecipitation Available online 15 June 2013 appeared to comprise a cubic CeO2 phase with some water or hydroxyl groups attached and a Sm(OH)3 precipitate After subsequent hydrothermal treatment, the samples with CeO2 Keywords: and Sm(OH)3 precipitates were observed to be converted into a single-phase fluorite struc- Solid oxide fuel cell tured Ce1−x Smx O2−ı , as confirmed by Raman spectra, whereas the sample with pure Sm(OH)3 Hydrothermal method precipitates remained unchanged after treatment FESEM and HRTEM images showed that the synthesized Ce1−x Smx O2−ı nanopowders appeared to be spherical-like particles with Ceria a single-crystal structure and a uniform particle size of 10–30 nm The samarium dopant, when increased to 30 mol% in the Ce1−x Smx O2−ı , seemed to trigger the formation of a few nanowires with a length of ≈400 nm The sintered Ce0.8 Sm0.2 O2−ı ceramics registered an electrical conductivity of 0.048 S/cm at 700 ◦ C and an activation energy of 0.73 eV, similar or superior to those reported in the literature The feasibility of using the Ce1−x Smx O2−ı nanopowders prepared by coprecipitation-hydrothermal method in SOFCs was confirmed © 2013 Brazilian Metallurgical, Materials and Mining Association Published by Elsevier Editora Ltda All rights reserved Introduction Fluorite structured CeO2 is an important and promising rareearth oxide and has attracted increasing attention due to its wide spectrum of uses in catalysts, ultraviolet absorbers, pollutant removers, polishing materials, oxygen sensors, optical devices, hydrogen storage materials, and solid oxide fuel cells (SOFCs) [1] Doped CeO2 materials in particular serve as a vital electrolyte for use in intermediate temperature ∗ SOFCs (IT-SOFCs) operating at temperatures ranging from 500 ◦ C to 700 ◦ C, since they possess high ionic conductivity [2,3] Considerable efforts have been invested on the development of methods for preparing CeO2 in various particle sizes and morphologies, such as solid state reaction, wet chemical method [4], molecular precursor route, hydroxide coprecipitation, flame spray pyrolysis [5], sol–gel, micro-emulsion, spray drying, and hydrothermal process [6,7] Though most of these preparation techniques are capable of yielding a high degree of chemical purity, only a few of them offer the ability to Corresponding author E-mail addresses: sfwang@ntut.edu.tw, seafuewang@yahoo.com (S.-F Wang) 2238-7854/$ – see front matter © 2013 Brazilian Metallurgical, Materials and Mining Association Published by Elsevier Editora Ltda All rights reserved http://dx.doi.org/10.1016/j.jmrt.2013.01.004 142 j m a t e r r e s t e c h n o l 3;2(2):141–148 control the morphology, particle size and degree of agglomeration Drawbacks in the fabrication of reliable CeO2 powders by conventional solid state reaction route at high temperatures include lower chemical activity, higher impurity content, and large particle sizes; CeO2 powders prepared by this method are thus unsuitable for device applications Hydrothermal synthesis as a low temperature method to synthesize nano-sized ceramic powders in aqueous solutions offers the potential to control the morphology and the degree of agglomeration of the prepared CeO2 powders [8–11] As reported in the literature, CeO2 and doped Ce1−x Mx O2−ı (M: Ce3+ , Gd3+ , Sm3+ , Bi3+ , La3+ , and Ca2+ ) nanomaterials have been successfully prepared using hydrothermal synthesis through modification of starting material, pH value, mineralizer, solvent, and surfactant [6,7,9,12,13] Though powders with nanosize and controlled morphology can be achieved, the high soaking temperature (>250 ◦ C) required for their preparation remains a major setback [14–19] Moreover, the water, hydroxyl groups, and hydrocarbon that continues to attach to the surface of the powders in the final products when processed at low temperature pose additional problems Removing the attached species usually requires subsequent heat treatment that in turn often leads to serious agglomeration or aggregation of particles [7,20–22] These drawbacks highlight the need to better study and understand the formation of the nanoparticles through hydrothermal synthesis In this study, CeO2 powders with various amounts of samarium-ion were synthesized through coprecipitationhydrothermal synthesis method, without the use of organic solvent, organometallic precursors, and metal surfactant complexes The effects of samarium-ion contents on the structure and physical characteristics of the Ce1−x Smx O2−ı powders were investigated and discussed Experimental procedure Reagent grade (Aldrich, USA) cerium nitrate hexahydrate [Ce(NO3 )3 ·6H2 O] and samarium nitrate hexahydrate [Sm(NO3 )3 ·6H2 O] in proportions according to the formula of Ce1−x Smx O2−ı (x = 0, 0.1, 0.2, 0.3, 1.0) were mixed in de-ionized water, resulting in a clear aqueous solution that was used as starting materials to synthesize the Ce1−x Smx O2−ı ceramics The resulting solutions with a concentration of 0.3 M were then titrated to a pH value of 9.8 with ammonia (28% NH3 ·H2 O) to co-precipitate the cationic species The aqueous solutions after titration appeared to contain translucent colloidal precipitates, which were used as precursors for subsequent hydrothermal treatment or were filtered, washed using hot water, dried at 75 ◦ C in oven for 24 h, and then subjected to further characterizations After being ground and screened, the dried powders were designated as SC0, SC20, SC100, etc., in which the Arabic number represents the percent molar fraction of samarium ions, i.e 100% times x value in Ce1−x Smx O2−ı The hydrothermal experiments were conducted in an autoclave using Teflon-lined steel vessels The solutions with various colloidal precipitates were then sealed into the steel vessel, placed in an oven, and heated to the soaking temperature of 200 ◦ C at a heating rate of ◦ C/min The hydrothermal reactions were performed under an equilibrium vapor pressure of aqueous solution at various set-temperatures (120–132 psi) While the Ce1−x Smx O2−ı powders were synthesized for reaction times ranging from to 24 h, most of the characterization studies were performed on the samples prepared for a 10-h reaction duration After hydrothermal treatment, the solid reaction products were filtered, washed three times in distilled water, and dried in an oven at 75 ◦ C for 24 h The dried powders were then designated, respectively, as SDC0, SDC10, SDC20, SDC30, and SDC100 according to their nominal composition shown in Table In order to elucidate the effect of hydrothermal treatment, the precursors including SC0, SC20, and SC100 were further characterized for comparison, together with the hydrothermally synthesized Ce1−x Smx O2−ı powders Differential thermal analysis (DTA/TGA) was performed in a Pt crucible at a heating rate of ◦ C/min under flowing air using a Netzsch Calorimeter, STA 409PC, on powders to evaluate the possible reactions during heating ICP-AES (Kontron S-35) analysis was conducted to analyze the concentrations of the Ce4+ and Sm3+ ions in the Ce1−x Smx O2−ı powders as well as the supernatant after both the coprecipitation and the hydrothermal treatment The crystal structure and phase composition of the powders were determined using an X-ray diffractometer (Rigaku D/max, Japan) with Cu K␣ radiation at a scan speed of 2◦ /min in the 2Â range between 20◦ and 80◦ A slow-scanned measurement at a rate of 0.2◦ /min was also performed on the powders from 40◦ to 80◦ 2Â values Available Sm0.2 Ce0.8 O2−ı powder (Fuel Cell Materials, USA; BET surface area = 6.2 m2 g−1 ) was further measured and taken as a standard for comparison The phase compositions of the powders were studied using the lattice parameters calculated from XRD The morphology and size of the synthesized particles were semi-quantitatively determined using a field emission electron microscope (Hitachi S-4100, Japan) In addition, a JEOL-2010 high-resolution transmission electron microscope (HRTEM) operating at 200 kV was used to examine the crystallography and microstructure of the as synthesized Ce1−x Smx O2−ı powders The room temperature Raman spectra of Ce1−x Smx O2−ı powders were recorded on a Raman microspectrometer (Jobin Yvon) with He–Cd laser at 325 nm A standard four-probe method was used to measure the electrical conductivity of the specimens in the temperatures ranging from 25 ◦ C to 800 ◦ C in air using Keithley 2400 Results and discussion Fig shows the XRD patterns of the dried and washed colloidal precipitates, including SC0, SC20, SC100, after titration by ammonia For SC0 precipitates, the diffraction peaks of the XRD pattern are indexed to a cubic fluorite structure of CeO2 (JCPDS Card No 43-1002), while the peaks referred to as pure Sm(OH)3 (JCPDS Card No 83-2036) were found for SC100 precipitates With 20 mol% samarium-ion additions, the resultant SC20 precipitates appeared to be comprising a cubic CeO2 phase and a small amount of Sm(OH)3 It was found in a previous study that hydrolysis of the Ce4+ and Sm3+ ions took place and resulted in Ce(OH)4 and Sm(OH)3 initially, and the original precipitates were not CeO2 or CeO2−ı [7] Our results revealed 143 j m a t e r r e s t e c h n o l 3;2(2):141–148 Table – Designations and their synthesis steps for the powders prepared in this study Designation Concentration in solution Ce(NO3 )3 ·6H2 O (mol%) SC0 SC20 SC100 SDC0 SDC10 SDC20 SDC30 SDC100 Processing steps Sm(NO3 )3 ·6H2 O (mol%) Titration Filtering and drying 100 80 0 20 100 Yes No Yes 90 80 70 0 10 20 30 100 Yes Yes Yes that dehydration of Ce(OH)4 proceeded rapidly and form CeO2 based precipitates which were likely to contain some water or hydroxyl groups; Sm(OH)3 , however, remained hydrolyzed The findings were confirmed by the TGA results presented in Fig 2, using the precipitates of SC100 and SC20 dried at 120 ◦ C to remove the water absorption before measurement Weight losses of 13.2% and 10.7% up to 700 ◦ C were observed, respectively, for SC100 and SC20 precipitates On the other hand, by theoretical calculation, weight losses for SC100 and SC20 precipitates associated with the reaction converting Sm(OH)3 into Sm2 O3 emerge to be 3.4% and 3.1% The measured (13.2%) and the calculated values (13.4%) of the SC100 sample are close enough to signify existence of nearly pure Sm(OH)3 in SC100 For SC20, the obvious distance between the measured value (10.7%) and the calculated value (3.1%) indicates the presence of water or hydroxyl group attached to the CeO2 product The X-ray diffraction patterns of the Ce1−x Smx O2−ı powders synthesized at 200 ◦ C for 10 h are shown in Fig The results revealed the presence of a single-phase fluorite structured Ce1−x Smx O2−ı for the SDC0, SDC10, SDC20, and (a) SC0 CeO2 Sm(OH)2 SDC30 samples, whereas pure Sm(OH)3 remained unchanged for SDC100 after the same hydrothermal treatment The preferred orientations of the powders synthesized are found to be (1 1) The TGA results of the SDC0, SDC10, and SDC30 samples registered very low weight losses of 1.48%, 0.65%, 0.25%, and 0.57%, respectively, confirming both the detachment of water or hydroxyl groups from the CeO2 particles and the dehydration of the Sm(OH)3 precipitates during hydrothermal process Fig presents the slow-scanned XRD patterns from 40◦ to 51◦ 2Â values for the Ce1−x Smx O2−ı powders prepared by hydrothermal synthesis, as compared to those of the commercial Ce0.8 Sm0.2 O2−ı powders The results indicate that the (2 0) peak of the hydrothermally synthesized SDC10, SDC20 and SDC30 powders shifted to a lower angle at a value of 0.14◦ , 0.25◦ , 0.36◦ as compared to that of the SDC0 (pure CeO2 ) sample at 47.62◦ , which is caused by the larger ionic radius of samarium ion (0.1079 nm) compared with that of cerium ion (0.097 nm) It is apparent that samarium ions were substituted into the CeO2 lattices after hydrothermal treatment The completion of the samarium-ion substitution in CeO2 lattices was also verified by the absence of (2 1) peak of Sm(OH)3 phase at 40.85◦ of the XRD patterns of SDC10, SDC20, and SDC30 powders Any samarium ion left outside the CeO2 particles retained the form of Sm(OH)3 phase after hydrothermal treatment, similar to that observed in the SDC100 sample (b) SC 20 Relative intensity Hydrothermal treatment 102 101 SDC 20 SDC 30 100 99 98 (c) SC 100 SDC 10 CeO2 97 CeO2 JCPDS: 43-1002 Sm0.2Ce0.8O1.9(SDC20) JCPDS: 75-0158 Weight% 96 95 94 93 SC 20 92 91 90 Sm(OH)3 JCPDS: 83-2036 89 Sm(OH)3 88 20 30 40 50 60 70 80 2θ (degree) Fig – XRD patterns of SC0, SC20 and SC100 washed and dried colloidal precipitates, used as precursors for hydrothermal synthesis 87 86 50 100 150 200 250 300 350 400 450 500 550 600 650 700 Temperture (ºC) Fig – TGA curves for Ce0.8 Sm0.2 O2−ı powders with and without hydrothermal treatment 144 j m a t e r r e s t e c h n o l 3;2(2):141–148 Table – Chemical compositions, lattice parameters, and crystalline sizes of Ce0.8 Sm0.2 O2−ı powders prepared in this study Powder Composition from ICP Ce (mol%) SDC0 SDC10 SDC20 SDC30 Commercial SDC Lattice parameter, a (Å) Crystalline size, D (nm) Sm (mol%) 100 90.0 80.4 70.6 80.4 10.9 20.2 30.3 19.5 5.4020 5.4273 5.4305 5.4390 – (111) The chemical compositions from ICP analysis, and the lattice parameters and crystalline sizes calculated from the XRD results of the hydrothermally synthesized Ce1−x Smx O2−ı powders are listed in Table The as-synthesis Ce1−x Smx O2−ı powders certainly are of very high purity, since only inorganic cerium and samarium salts and ammonia were used in this synthesis; impurities caused by the other anionic and organic species were thus eliminated The absence of anionic and organic residues from the precursors disengaged their effects on the particle size and morphology as reported in the literature [10,23,9] The ratios of cerium ion and samarium ion in the Ce1−x Smx O2−ı powders were in accord with the nominal compositions; this was expected since the ICP analysis confirmed that the residual samarium and cerium contents left 12 15 11 – in supernatant after precipitation and hydrothermal synthesis were trivial Rise in the samarium content was observed to trigger a gradual increase in the lattice constant ‘a’ from ˚ (SDC0) to 5.4390 A ˚ (SDC30) The particle sizes of the 5.4020 A hydrothermally synthesized Ce1−x Smx O2−ı powders appeared to be very similar, ranging from nm (SDC0) to 15 nm (SDC20) Fig displays the Raman spectra of the hydrothermally synthesized Ce1−x Smx O2−ı powders, which confirm the formation of the cubic fluorite phase shown in Fig The intensive band at 460–470 cm−1 corresponding to the allowed Raman mode (F2g ) of fluorite metal dioxides belonged to the O5h (Fm3m) space group [24,25], referred to as cubic Ce1−x Smx O2−ı in the present case For SDS0 (pure CeO2 ) powders, the Raman spectrum was symmetric around 465 cm−1 and the F2g mode corresponded to the symmetric vibration of oxygen ions (220) Lab SDC30 (331) (420) (400) (311) 0.36º (222) (200) (220) SDC30 0.25º SDC20 Lab SDC20 Relative intensity Relative intensity 0.29º SDC10 SDC0 CeO2 Standard SDC20 0.14º Lab SDC10 CeO2 SDC100 Sm(OH)3 Lab SDC Sm(OH)3 Lab SDC100 Sm0.2Ce0.8O1.9(SDC20) JCPDS: 75-0158 Sm0.2Ce0.8O1.9(SDC20) JCPDS: 75-0158 CeO2 JCPDS: 43-1002 CeO2 JCPDS: 43-1002 Sm(OH)3 JCPDS: 83-2036 Sm(OH)3 JCPDS: 83-2036 20 30 40 50 60 70 80 2θ (degree) Fig – XRD patterns of Ce1−x Smx O2−ı powders prepared by hydrothermal synthesis 40 41 42 43 44 45 46 47 48 49 50 51 2θ (degree) Fig – Slow-scanned XRD patterns of Ce1−x Smx O2−ı powders prepared by hydrothermal synthesis, as compared to that of a commercial Ce0.8 Sm0.2 O2−ı (SDC20) powder j m a t e r r e s t e c h n o l 3;2(2):141–148 broad band in the range of 530–620 cm−1 was observed in the samples with samarium doped CeO2 and assigned to a band that could be attributed to the oxygen vacancies in the lattices [25] The intensity of the broad band rose with the extent of Sm3+ in the CeO2 lattices according to the defect equation as follows: Intansity (a.u.) SDC30 SDC20 CeO ·· Sm2 O3 −→2 2SmCe + VO + 3O× o SDC10 CeO2 200 300 400 500 600 145 700 Wavenumber(cm–1) Fig – Raman spectra of the hydrothermally synthesized Ce1−x Smx O2−ı powders around Ce4+ ions in the CeO6 octahedra [26] In the samarium doped CeO2 , the F2g band became asymmetrical and slightly shifted to low frequencies, due to the cell expansions resulting from the substitution of Sm3+ ions in the CeO2 lattices and the subsequent oxygen loss around cations The degree of F2g -peak shifts increased as the Sm3+ content grew, i.e the number of the oxygen vacancies increased In addition, a weak Fig illustrates the FESEM micrographs of the hydrothermally synthesized Ce1−x Smx O2−ı powders, including SDC0, SDC10, SDC20, and SDC30 The synthesized powders were prepared without hard aggregates It is noted that the SEM images of Ce1−x Smx O2−ı powders revealed larger grain sizes than the crystalline sizes listed in Table Spherical-like shaped particles with a uniform particle size of 10–30 nm were found for the Ce1−x Smx O2−ı powders As the samarium content increased in the Ce1−x Smx O2−ı powders, the particles appeared to be slightly larger and agglomeration of the particles became apparent It is interesting to note that the presence of a few nanowires with a length of ≈400 nm was also observed for SDC30 As reported in a previous study, the presence of the CeO2 nanowires was due to the effect of the surface’s absorbing Cl− ions during hydrothermal treatment while only nanoparticles were formed in the solution with NH4 NO3 present [9] In the present study, the Ce1−x Smx O2−ı powders were prepared free of Cl− and with only NH4 NO3 present in the solution The appearance of nanowires in SDC30 Fig – SEM micrographs of the hydrothermally synthesized (a) SDC0, (b) SDC10, (c) SDC20, and (d) SDC30 powders 146 j m a t e r r e s t e c h n o l 3;2(2):141–148 Fig – (a) TEM images, (b) the corresponding SAED patterns, and (c and d) the high-resolution lattice fringes of the hydrothermally synthesized CeO2 powders seems to suggest that the samarium ions played an important role in triggering the formation of the Ce1−x Smx O2−ı nanowires Fig shows the TEM images, the corresponding SAED patterns, and the high-resolution lattice fringes of the hydrothermally synthesized SDC0 powders The particles are all crystalline as can be seen from the diffraction rings shown in Fig 7(b), and their sizes again fall in the range of 10–30 nm They reported a single crystalline structure based on the fact that the lattice fringes corresponding to the reflections are markedly observed The particle surfaces are clean and visually free of impurities, as observed from the images of Fig 7(c) and (d) The Ce1−x Smx O2−ı powders, including SDC10, SDC20, and SDC30 were pelletized and sintered at 1400 ◦ C for h The sintered samples appeared to be dense and their electrical conductivities were subsequently measured in air from room temperature to 800 ◦ C Fig shows the Arrhenius plot of the electrical conductivity of the sintered Ce1−x Smx O2−ı ceramics The electrical conductivities of SDC10, SDC20, and SDC30 samples at 700 ◦ C emerged to be 0.029 S/cm, 0.048 S/cm, and 0.016 S/cm, respectively The activation energies of SDC10, SDC20, and SDC30 were calculated and, respectively, read 0.79 eV, 0.73 eV, and 0.96 eV The electrical conductivity of SDC20 appeared to be larger than those of SDC10 and SDC30 samples, a finding generally in agreement with the one reported in the literature [2] The electrical conductivity of SDC20 sample prepared in the present study, however, was larger than that of commercial Ce0.8 Sm0.2 O2−ı sample (0.031 S/cm) and similar or superior to the results reported in the literature [2,3] Therefore, Ce1−x Smx O2−ı nanopowders for j m a t e r r e s t e c h n o l 3;2(2):141–148 T(ºC) 750 700 650 600 550 500 –2.5 SDC10 SDC20 SDC30 Linear fit –3.0 –3.5 log σ (S • cm–1) –4.0 Ea ~ 0.73 eV –4.5 –5.0 Ea ~ 0.78 eV –5.5 –6.0 Ea ~ 0.96 eV –6.5 –7.0 –7.5 –8.0 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1000/T (K–1) Fig – Arrhenius plot of electrical conductivity of Ce1−x Smx O2−ı ceramics prepared from hydrothermally synthesized powders use in SOFCs can be prepared using an environment-friendly method of coprecipitation followed by hydrothermal treatment Conclusions Ce1−x Smx O2−ı nanopowders were synthesized using coprecipitation-hydrothermal method After coprecipitation, the gels consisted of a cubic CeO2 phase with some water or hydroxyl groups attached, and a Sm(OH)3 precipitate Disappearance of the Sm(OH)3 precipitates was concurrent with the substitution of Sm3+ into the CeO2 lattices during hydrothermal synthesis, leading to the formation of a single fluorite phase of Ce1−x Smx O2−ı The synthesized Ce1−x Smx O2−ı nanopowders appeared to be spherical particles with a single crystalline structure and a uniform particle size of 10–30 nm Moreover, a few nanowires with a length of ≈400 nm were found in the Ce0.7 Sm0.3 O2−ı nanopowders 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41 42 43 44 45 46 47 48 49 50 51 2θ (degree) Fig – Slow-scanned XRD patterns of Ce1−x Smx O2−ı powders prepared by hydrothermal. .. Arrhenius plot of electrical conductivity of Ce1−x Smx O2−ı ceramics prepared from hydrothermally synthesized powders use in SOFCs can be prepared using an environment-friendly method of coprecipitation