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

b rOriginal 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 Wan

w w w j m r t c o m b r

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

Article history:

Received 20 November 2012

Accepted 31 January 2013

Available online 15 June 2013

Keywords:

Solid oxide fuel cell

Hydrothermal method

Ceria

a b s t r a c t

In this study, Ce1−xSmxO2−ı(x = 0.0, 0.1, 0.2, 0.3, 1.0) were synthesized for use in solid oxide

fuel cells (SOFCs) using an environment-friendly method of coprecipitation followed by hydrothermal treatment XRD and TGA results revealed that the gels after coprecipitation appeared to comprise a cubic CeO2phase with some water or hydroxyl groups attached and

a Sm(OH)3precipitate After subsequent hydrothermal treatment, the samples with CeO2

and Sm(OH)3precipitates were observed to be converted into a single-phase fluorite struc-tured Ce1−xSmxO2−ı, as confirmed by Raman spectra, whereas the sample with pure Sm(OH)3

precipitates remained unchanged after treatment FESEM and HRTEM images showed that the synthesized Ce1−xSmxO2−ınanopowders appeared to be spherical-like particles with

a single-crystal structure and a uniform particle size of 10–30 nm The samarium dopant, when increased to 30 mol% in the Ce1−xSmxO2−ı, seemed to trigger the formation of a few nanowires with a length of≈400 nm The sintered Ce0.8Sm0.2O2−ıceramics 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−xSmxO2−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

Fluorite structured CeO2is an important and promising

rare-earth oxide and has attracted increasing attention due to

its wide spectrum of uses in catalysts, ultraviolet absorbers,

pollutant removers, polishing materials, oxygen sensors,

opti-cal 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

∗ Corresponding author.

E-mail addresses:sfwang@ntut.edu.tw,seafuewang@yahoo.com(S.-F Wang)

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 develop-ment of methods for preparing CeO2in various particle sizes and morphologies, such as solid state reaction, wet chemical method[4], molecular precursor route, hydroxide coprecipita-tion, 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

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

control the morphology, particle size and degree of

agglomera-tion Drawbacks in the fabrication of reliable CeO2powders 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

syn-thesis 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 CeO2powders[8–11]

As reported in the literature, CeO2and doped Ce1−xMxO2−ı

(M: Ce3+, Gd3+, Sm3+, Bi3+, La3+, and Ca2+) nanomaterials have

been successfully prepared using hydrothermal synthesis

through modification of starting material, pH value,

miner-alizer, 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

prod-ucts when processed at low temperature pose additional

problems Removing the attached species usually requires

subsequent heat treatment that in turn often leads to

seri-ous 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

syn-thesis In this study, CeO2 powders with various amounts

of samarium-ion were synthesized through

coprecipitation-hydrothermal 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−xSmxO2 −ı

powders were investigated and discussed

Reagent grade (Aldrich, USA) cerium nitrate

hexahy-drate [Ce(NO3)3·6H2O] and samarium nitrate hexahydrate

[Sm(NO3)3·6H2O] in proportions according to the formula of

Ce1−xSmxO2 −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−xSmxO2−ıceramics

The resulting solutions with a concentration of 0.3 M were

then titrated to a pH value of 9.8 with ammonia (28% NH3·H2O)

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−xSmxO2−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

soak-ing temperature of 200◦C at a heating rate of 3◦C/min The

hydrothermal reactions were performed under an equi-librium vapor pressure of aqueous solution at various set-temperatures (120–132 psi) While the Ce1−xSmxO2−ı pow-ders were synthesized for reaction times ranging from 1 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 fil-tered, 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 inTable 1 In order to elucidate the effect of hydrothermal treatment, the precursors including SC0, SC20, and SC100 were further char-acterized for comparison, together with the hydrothermally synthesized Ce1−xSmxO2 −ıpowders.

Differential thermal analysis (DTA/TGA) was performed

in a Pt crucible at a heating rate of 5◦C/min under flow-ing air usflow-ing 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 con-centrations of the Ce4+ and Sm3+ ions in the Ce1−xSmxO2 −ı

powders as well as the supernatant after both the coprecipi-tation 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␣ radi-ation at a scan speed of 2◦/min in the 2 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◦ 2 val-ues Available Sm0.2Ce0.8O2−ı powder (Fuel Cell Materials, USA; BET surface area = 6.2 m2g−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 syn-thesized particles were semi-quantitatively determined using

a field emission electron microscope (Hitachi S-4100, Japan)

In addition, a JEOL-2010 high-resolution transmission elec-tron microscope (HRTEM) operating at 200 kV was used to examine the crystallography and microstructure of the as synthesized Ce1−xSmxO2−ı powders The room temperature Raman spectra of Ce1−xSmxO2−ıpowders 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 tempera-tures ranging from 25◦C to 800◦C in air using Keithley 2400

3 Results and discussion

Fig 1shows the XRD patterns of the dried and washed col-loidal 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 resul-tant SC20 precipitates appeared to be comprising a cubic CeO2

phase and a small amount of Sm(OH)3 It was found in a previ-ous study that hydrolysis of the Ce4+and Sm3+ions took place and resulted in Ce(OH)4and Sm(OH)3initially, and the original precipitates were not CeO or CeO2−ı[7] Our results revealed

Table 1 – Designations and their synthesis steps for the powders prepared in this study.

Ce(NO3)3·6H2O (mol%) Sm(NO3)3·6H2O (mol%) Titration Hydrothermal treatment Filtering and drying

that dehydration of Ce(OH)4proceeded 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,

respec-tively, 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 Sm2O3emerge 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−xSmxO2 −ı

powders synthesized at 200◦C for 10 h are shown in Fig 3

The results revealed the presence of a single-phase fluorite

structured Ce1−xSmxO2−ı for the SDC0, SDC10, SDC20, and

(a) SC0

(b) SC 20

CeO2

CeO2

Sm0.2Ce0.8O1.9(SDC20)

Sm(OH)3

JCPDS: 43-1002 JCPDS: 75-0158 JCPDS: 83-2036

2θ (degree)

Sm(OH)2

(c) SC 100

Fig 1 – XRD patterns of SC0, SC20 and SC100 washed and

dried colloidal precipitates, used as precursors for

hydrothermal synthesis.

SDC30 samples, whereas pure Sm(OH)3remained unchanged for SDC100 after the same hydrothermal treatment The pre-ferred orientations of the powders synthesized are found to be (1 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 CeO2particles and the dehydra-tion of the Sm(OH)3precipitates during hydrothermal process

Fig 4presents the slow-scanned XRD patterns from 40◦

to 51◦ 2 values for the Ce1−xSmxO2−ıpowders prepared by hydrothermal synthesis, as compared to those of the com-mercial Ce0.8Sm0.2O2 −ıpowders The results indicate that the

(2 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 sub-stituted into the CeO2lattices after hydrothermal treatment The completion of the samarium-ion substitution in CeO2 lat-tices was also verified by the absence of (2 1 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 par-ticles retained the form of Sm(OH)3phase after hydrothermal treatment, similar to that observed in the SDC100 sample

102

SDC 20

SC 20

SDC 30

SDC 10 CeO2

Sm(OH)3

101 100 99 98 97 96 95 94 93 92 91 90 89 88 87 86

50 100 150 200 250 300 350 400 450 550 650

Temperture (ºC)

Fig 2 – TGA curves for Ce0.8Sm0.2O2−ı powders with and without hydrothermal treatment.

Table 2 – Chemical compositions, lattice parameters, and crystalline sizes of Ce 0.8 Sm 0.2 O 2 −ı powders prepared in this

study.

The chemical compositions from ICP analysis, and the

lat-tice parameters and crystalline sizes calculated from the XRD

results of the hydrothermally synthesized Ce1−xSmxO2−ı

pow-ders are listed in Table 2 The as-synthesis Ce1−xSmxO2 −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

liter-ature[10,23,9] The ratios of cerium ion and samarium ion in

the Ce1−xSmxO2−ıpowders were in accord with the nominal

compositions; this was expected since the ICP analysis

con-firmed that the residual samarium and cerium contents left

SDC20 SDC30

2θ (degree)

SDC10

SDC0

SDC100 CeO2

Sm(OH)3

Sm0.2Ce0.8O1.9(SDC20)

CeO2

Sm(OH)3

JCPDS: 75-0158

JCPDS: 43-1002

(222) (400) (331) (420)

JCPDS: 83-2036

80 70

60 50

40 30

20

Fig 3 – XRD patterns of Ce1−x SmxO2−ı powders prepared by

hydrothermal synthesis.

in supernatant after precipitation and hydrothermal synthe-sis were trivial Rise in the samarium content was observed

to trigger a gradual increase in the lattice constant ‘a’ from

5.4020 ˚A (SDC0) to 5.4390 ˚A (SDC30) The particle sizes of the hydrothermally synthesized Ce1−xSmxO2 −ıpowders appeared

to be very similar, ranging from 8 nm (SDC0) to 15 nm (SDC20)

Fig 5displays the Raman spectra of the hydrothermally synthesized Ce1−xSmxO2−ıpowders, which confirm the for-mation of the cubic fluorite phase shown in Fig 3 The intensive band at 460–470 cm−1corresponding to the allowed Raman mode (F2g) of fluorite metal dioxides belonged to the O5

h

(Fm3m) space group[24,25], referred to as cubic Ce1−xSmxO2−ı

in the present case For SDS0 (pure CeO2) powders, the Raman spectrum was symmetric around 465 cm−1and the F2gmode corresponded to the symmetric vibration of oxygen ions

Lab SDC30

0.36º

0.25º

0.29º

0.14º

Lab SDC20

Lab SDC10

Lab SDC100 Lab SDC 0

2θ (degree)

Standard SDC20

Sm0.2Ce0.8O1.9(SDC20)

CeO

2

Sm(OH)3 CeO2

Sm(OH)

3

JCPDS: 75-0158 JCPDS: 43-1002 JCPDS: 83-2036

Fig 4 – Slow-scanned XRD patterns of Ce 1−x SmxO 2−ı powders prepared by hydrothermal synthesis, as compared

to that of a commercial Ce0.8Sm0.2O2−ı (SDC20) powder.

SDC20

SDC10

Wavenumber(cm–1)

CeO2

Fig 5 – Raman spectra of the hydrothermally synthesized

Ce1−x SmxO2−ı powders.

around Ce4+ions in the CeO6octahedra[26] In the samarium

doped CeO2, the F2gband became asymmetrical and slightly

shifted to low frequencies, due to the cell expansions

result-ing 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

broad band in the range of 530–620 cm−1was observed in the samples with samarium doped CeO2and 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 CeO2lattices according to the defect equation as follows:

Sm2O3CeO−→ 2Sm2 Ce+ V··

o Fig 6 illustrates the FESEM micrographs of the hydrother-mally synthesized Ce1−xSmxO2−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−xSmxO2−ıpowders revealed larger grain sizes than the crystalline sizes listed in Table 2 Spherical-like shaped particles with a uniform particle size of 10–30 nm were found for the Ce1−xSmxO2 −ıpowders As the samarium

content increased in the Ce1−xSmxO2 −ı powders, the

parti-cles 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 CeO2nanowires was due to the effect of the surface’s absorbing Cl−ions during hydrothermal treatment while only nanoparticles were formed in the solution with

NH4NO3 present[9] In the present study, the Ce1−xSmxO2−ı powders were prepared free of Cl− and with only NH4NO3

present in the solution The appearance of nanowires in SDC30

Fig 6 – SEM micrographs of the hydrothermally synthesized (a) SDC0, (b) SDC10, (c) SDC20, and (d) SDC30 powders.

Fig 7 – (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

impor-tant role in triggering the formation of the Ce1−xSmxO2−ı

nanowires.Fig 7shows 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

inFig 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

visu-ally free of impurities, as observed from the images ofFig 7(c)

and (d)

The Ce1−xSmxO2−ıpowders, including SDC10, SDC20, and

SDC30 were pelletized and sintered at 1400◦C for 2 h The

sintered samples appeared to be dense and their electrical

conductivities were subsequently measured in air from room temperature to 800◦C.Fig 8shows the Arrhenius plot of the electrical conductivity of the sintered Ce1−xSmxO2−ı ceram-ics 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 conductiv-ity of SDC20 sample prepared in the present study, however, was larger than that of commercial Ce0.8Sm0.2O2−ı sample (0.031 S/cm) and similar or superior to the results reported in the literature[2,3] Therefore, Ce1−xSm O2−ınanopowders for

–2.5

–3.5

–4.5

–3.0

–4.0

–5.0

–6.0

–7.0

–8.0

0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35

1000/T (K–1)

T(ºC)

–1)

–5.5

–6.5

–7.5

SDC10 SDC20 SDC30

Ea ~ 0.73 eV

Ea ~ 0.78 eV

Ea ~ 0.96 eV

Linear fit

Fig 8 – Arrhenius plot of electrical conductivity of

Ce1−xSmxO2 −ı ceramics prepared from hydrothermally

synthesized powders.

use in SOFCs can be prepared using an environment-friendly

method of coprecipitation followed by hydrothermal

treat-ment

Ce1−xSmxO2−ı nanopowders were synthesized using

coprecipitation-hydrothermal method After

coprecipita-tion, the gels consisted of a cubic CeO2 phase with some

water or hydroxyl groups attached, and a Sm(OH)3

pre-cipitate Disappearance of the Sm(OH)3 precipitates was

concurrent with the substitution of Sm3+ into the CeO2

lat-tices during hydrothermal synthesis, leading to the formation

of a single fluorite phase of Ce1−xSmxO2 −ı The synthesized

Ce1−xSmxO2−ınanopowders appeared to be spherical

parti-cles 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 Ce0.7Sm0.3O2 −ınanopowders The

electrical properties of the sintered Ce0.8Sm0.2O2 −ıceramics

confirmed their feasibility for use in SOFCs

Conflicts of interest

The authors declare no conflicts of interest

r e f e r e n c e s

[1] Dos Santos ML, Lima RC, Riccardi CS, Tranquilin CS, Bueno

PR, Varela JA, et al Preparation and characterization of ceria

nanospheres by microwave-hydrothermal method Mater

Lett 2008;62:4509–11

[2] Fergus JW Electrolytes for solid oxide fuel cells J Power

Sources 2006;162:30–40

[3] Ivers-Tiffée E, Weber A, Herbstritt D Materials and

technologies for SOFC components J Eur Ceram Soc

2001;21:1805–11

[4] Rambabu B, Ghosh S, Jena H Novel wet-chemical synthesis and characterization of nanocrystalline CeO2and

Ce0.8Gd0.2O1.9as solid electrolyte for intermediate temperature solid oxide fuel cell (IT-SOFC) applications J Mater Sci 2006;41:7530–6

[5] Seo DJ, Ryu KO, Park SB, Kim KY, Song RH Synthesis and properties of Ce1−xGdxO2−x/2solid solution prepared by flame spray pyrolysis Mater Res Bull 2006;41:359–66

[6] Hungría AB, Martínez-Arias A, Fernández-García M, Iglesias-Juez A, Guerrero-Ruiz A, Calvino JJ, et al Structural Morphological, and Oxygen Handling Properties of Nanosized Cerium-Terbium Mixed Oxides Prepared by Microemulsion Chem Mater 2003;15:4309–16

[7] Cheng MY, Hwang DH, Sheu HS, Hwang BJ Formation of

Ce0.8Sm0.2O1.9nanoparticles by urea-based low-temperature hydrothermal process J Power Sources 2008;175:

137–44

[8] Yan L, Yu R, Chen J, Xing X Template-free hydrothermal synthesis of CeO2nano-octahedrons and nanorods:

investigation of the morphology evolution Cryst Growth Des 2008;8:1474–7

[9] Wang W, Howe JY, Li Y, Xiaofeng Q, Joy DC, Paranthaman MP,

et al A surfactant and template-free route for synthesizing ceria nanocrystals with tunable morphologies J Mater Chem 2010;20:7776–81

[10] Liu B, Li Q, Du X, Liu B, Yao M, Li Z, et al Facile hydrothermal synthesis of CeO2nanosheets with high reactive exposure surface J Alloys Compd 2011;509:6720–4

[11] Vasylkiv O, Kolodiazhnyi T, Sakka Y, Skorokhod VV

Synthesis and characterization of nanosize ceria–gadolinia powders J Ceram Soc Jpn 2005;113:101–6

[12] Ivanov VK, Kopitsa GP, Baranchikov AE, Grigor’ev SV, Runov

VV, Haramus VM Hydrothermal growth of ceria nanoparticles Russ J Inorg Chem 2009;5:1857–61

[13] Oh MH, Nho JS, Cho SB, Lee JS, Singh RK Novel method to control the size of well-crystalline ceria particles by hydrothermal method Mater Chem Phys 2010;124:

134–9

[14] Tok AIY, Boey FYC, Dong Z, Sun XL Hydrothermal synthesis

of CeO2nano-particles J Mater Process Technol 2007;190:217–22

[15] Dikmen S, Shuk P, Greenblatt M Hydrothermal synthesis and properties of Ce1−xLaxO2−ısolid solutions Solid State Ionics 1999;126:89–95

[16] Dikmen S, Shuk P, Greenblatt M, Gocmez H Hydrothermal synthesis and properties of Ce1−xGdxO2−ısolid solutions Solid State Sci 2002;4:585–90

[17] Dikmen S, Shuk P, Greenblatt M Hydrothermal synthesis and properties of Ce1−xSmxO2−x/2and Ce1−xCaxO2−xsolid solutions Chem Mater 1997;9:2240–5

[18] Yamashita K, Ramanujachary KV, Greenblatt M

Hydrothermal synthesis and low temperature conduction properties of substituted ceria ceramics Solid State Ionics 1995;81:53–60

[19] Dikmen S, Aslanbay H, Dikmen E, Sahin O Hydrothermal preparation and electrochemical properties of Gd3+and Bi3+,

Sm3+, La3+, and Nd3+codoped ceria-based electrolytes for intermediate temperature-solid oxide fuel cell J Power Sources 2010;195:2488–95

[20] Riccardi CS, Lima RC, dos Santos ML, Bueno PR, Varela JA, Longo E Preparation of CeO2by a simple

microwave-hydrothermal method Solid State Ionics 2009;180:288–91

[21] Yang Z, Yang Y, Liang H, Liu L Hydrothermal synthesis of monodisperse CeO2nanocubes Mater Lett 2009;63:1774–7 [22] Wu M, Zhang Q, Liu Y, Fang Q, Liu X Hydrothermal preparation of fractal dendrites: cerium carbonate hydroxide and cerium oxide Mater Res Bull 2009;44:1437–40

[23] Basu J, Divakar R, Winterstein JP, Carter CB Low-temperature

and ambient-pressure synthesis and shape evolution of

nanocrystalline pure, La-doped and Gd-doped CeO2 Appl

Surf Sci 2010;256:3772–7

[24] Levy C, Guizard C, Julbe A Soft-chemistry synthesis,

characterization, and stabilization of CGO/Al2O3/Pt

nanostructured composite powders J Am Ceram Soc

2007;90:942–9

[25] Mineshige A, Tajia T, Muroi Y, Kobune M, Fujii S, Nishi N,

et al Oxygen chemical potential variation in ceria-based solid oxide fuel cells determined by Raman spectroscopy Solid State Ionics 2000;135:481–5

[26] Kosacki I, Suzuki T, Anderson HU, Colomban P Raman scattering and lattice defects in nanocrystalline CeO2thin films Solid State Ionics 2002;149:99–105