Accepted Manuscript

Thermal stability of Ce1-xBixO2-δ (x = 0.1 – 0.5) solid solution

Marija Prekajski, Viktor Fruth, Cristian Andronescu, Lidija V. Trandafilović,

Jelena Pantić, Aleksandar Kremenović, Branko Matović

PII: S0925-8388(13)01166-3

DOI: http://dx.doi.org/10.1016/j.jallcom.2013.05.006

Reference: JALCOM 28510

To appear in:

Received Date: 26 December 2012

Revised Date: 19 April 2013

Accepted Date: 2 May 2013

Please cite this article as: M. Prekajski, V. Fruth, C. Andronescu, L.V. Trandafilović, J. Pantić, A. Kremenović, B.

Matović, Thermal stability of Ce1-xBixO2-δ (x = 0.1 – 0.5) solid solution, (2013), doi: http://dx.doi.org/10.1016/

j.jallcom.2013.05.006

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1

THERMAL STABILITY OF Ce1-xBixO2-δ (x = 0.1 – 0.5) SOLID SOLUTION

Marija Prekajski1, Viktor Fruth2, Cristian Andronescu2, Lidija V. Trandafilović1,

Jelena Pantić1, Aleksandar Kremenović3, Branko Matović1

1 Vinča Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11001 Belgrade,

Serbia

2 Institute of Physical Chemistry, Romanian Academy, 202 Splaiul Independentei Street, Bucharest

060021, Romania

3 Faculty of Mining and Geology, University of Belgrade, Đušina 7, 11000 Belgrade, Serbia

Abstract: Single phase solid solution Ce1-xBixO2-δ with the composition of x = 0.1

– 0.5 were successfully synthesized at room temperature using simple and fast Self

Propagating Room Temperature procedure (SPRT). Thermal stability of these solid

solutions with different concentration of Bi cation was investigated at various

temperatures up to 1400 °C by applying thermogravimetric analysis (DTA/DTG/TG).

Powders were characterized by X-ray powder diffraction (XRD), scanning electron

microscopy (SEM) and Infra Red spectroscopy (IR). It was revealed that all samples with

concentration of bismuth higher than 10 at. % are unstable during thermal treatment,

resulting in Bi leaving the structure of ceria and formation of β-Bi2O3 as second phase.

Moreover, at a certain temperatures bismuth begins to evaporate.

Key words: Ce1-xBixO2-δ; nanostructured materials; thermal stability; X-ray

diffraction; thermal analysis

2

* Corresponding author. Tel./fax:+381 11 3408224 E-mail address: prekajski@vinca.rs (M. Prekajski)

1. Introduction

Ceria-base nanocrystalline powders have been considered as an interesting

material for application in catalysis [1], fuel cells [2], ultraviolet absorbers [3], hydrogen

storage materials [4], oxygen sensors [5], optical devices [6] polishing materials [7] and

others. In CeO2, the facile Ce4+/Ce3+ redox cycle often leads to a higher oxygen-storage

capacity with reversible addition and removal of oxygen in the fluorite structure of ceria.

The better redox properties than those of CeO2 alone were obtained by the incorporation

of metal ions into the CeO2 lattice forming Ce1−xMxOy solid solutions, such as Al, Zr, Hf,

Si, and La [8–14]. Recently, much effort has been focused on the doping with cations that

have oxidation states lower than 4+. The purpose is to modify the chemical and physical

properties by creating oxygen vacancies inside the parent oxide [15–17]. Surface oxygen

and oxygen vacancies are involved in catalytic activity, and enhanced oxygen mobility

will enable the occurrence of redox processes at lower temperatures.

Solid solutions (CeO2)1−x(BiO1.5)x might be of a high interest for catalytic

applications [18] and integration in gas sensors [19]. Moreover, system is expected to be

a novel electrolyte exhibiting new electrochemical transport properties because ceria-

based solid solutions doped with lower valence ions usually possess oxide ion

3

conductivity higher than yttrium-stabilized zirconia (YSZ) and because the oxide ionic

conductivity of δ-Bi2O3 is the highest to date [20].

Reports about CeO2 – Bi2O3 compounds showed that the possibility of obtaining a

pure single-phase solid solution was depending on the synthesis route. In our previous

work, nanosized solid solution was successfully synthesized at room temperature by

using Self Propagating Room Temperature procedure (SPRT) [21].

Nonequilibrium conditions are responsible for high solubility of Bi3+ in CeO2

lattice and it can be said that solubility of Bi3+ is highly dependent on synthesis condition,

primarily temperature condition. Authors who had used higher temperatures for synthesis

(260 – 600 °C) [19-20, 22] as a result had a solubility only up to 10 or 15%. On the other

hand, we have used room temperature synthesis procedure and found that solubility of Bi

in CeO2 is about 50%. Similar results were obtained also by Gangsehe and coauthors [23]

that had used low hydrothermal synthesis route operating at 240 °C.

If a compound has to be used as electrolyte material in SOFC’s than it has to be

stable in certain temperature range. However, very few data are available about the

thermal stability of these solid solutions at higher temperatures. That is why we wanted to

investigate thermal stability of these solid solutions synthesized by SPRT method, with

different concentration of Bi, at various temperatures.

2. Experimental

Nanocrystalline CeO2 – Bi2O3 was synthesized by a SPRT method using cerium

nitrate hexahydrate (Riedel-de Haën, 99% purity), bismuth nitrate pentahydrate (Riedel-

de Haën, 99% purity) and sodium hydroxide (Lach-Ner, 99% purity) as starting

4

materials. Amounts of reactants for synthesis of Ce1-xBixO2-δ nanopowders was calculated

according to equation:

(1-x) Ce(NO3)3.6H2O + xBi(NO3)3

.5H2O + 3NaOH + (½-δ) O2 → Ce1-xBixO2-δ + 3NaNO3

+ yH2O

(1)

Synthesis of nanopowders was carried out in an alumina mortar mixing reactants

for 5-7 minutes, allowing rapid progress of the reaction at room temperature in the air.

After being exposed to air for 3 h, the entire volume of the powder was dissolved in water

and subjected to centrifugation at Centurion 1020D centrifuge at 3000 rpm, for 10

minutes. Rinsing procedure was repeated four times with distilled water and twice with

ethanol, order to eliminate NaNO3 from the synthesized powder mixture. At the end,

material was dried out at 60 °C in ambient atmosphere.

Synthesized powders were characterized and details can be found in our previous

work [21]. X-ray powder diffraction analysis of as synthesized samples showed that the

ceria powders with up to 50% of Bi are solid solutions with fluorite type of structure [21].

All synthesized powders have particle size in nanometric range (less than 4 nm) [21].

Raman spectral studies confirmed that all synthesized powders are single-phase solid

solutions [21].

Samples of Ce1-xBixO2-δ solid solutions with composition x = 0.1 – 0.5 which

were previously obtained at room temperature, were calcinated for 1 h at different

5

temperatures from 600 º up to 1200 ºC and then quenched to room temperature. Heating

rate was 10 ºC/min.

The X-ray powder analysis was used to identify the crystalline phases as well as

to calculate lattice parameters of solid solutions of obtained powders. Powder XRPD

patterns of all samples heat treated at different temperatures were recorded on Siemens

D-500 X-ray Powder Diffractometer with Ni filtered Cu Kα1,2 radiation. The

measurements were performed in the 2θ range from 20º to 80º with the step of 0.02º and

scanning time of 12 s per step. Calculation of the average crystallite size (D) was

performed on the basis of the full width at half maximum intensity (FWHM) of the 111,

200, 220 and 311 reflections of CeO2 by using Scherrer’s formula:

(1)

where λ is the wavelengths of the X-rays, θ is diffraction angle, β is corrected half-width

for instrumental broadening β = (βm – βs ), βm observed half-width and βs is half-width of

the standard CeO2 sample.

Values of lattice parameters with standard deviation [24] of all powders were calculated

based on collected XRPD data according to the formula:

θβλ

cosKDhkl =

6

2

222

21

2

2sin4

a

lkh

hkld

++==λ

θ

(2)

Thermogravimetric and thermodifferential analysis (DTA/TGA) were performed

using a Mettler Toledo TGA/SDTA851e equipment at a heating rate of 10 °C/min-1, in air,

with typically 20 mg sample.

FTIR spectroscopic analyses were carried out at room temperature using a Nicolet

380 spectrophotometer in the spectral range from 400 to 4000 cm−1, with a resolution of

4 cm−1 . The datasets were averaged over 200 scans.

For microstructural studies it was used JOEL JSM-6610LV scanning electron

microscope (SEM). Semi-quantative analysis was determinated by making a use of

energy dispersive X-ray analysis (EDX) on X-Max Large Area SDD EDX spectrometer.

3. Results and discussion

The XRPD patterns of annealed CeO2 samples with concentrations of Bi3+ from

10 to 50% are represented at Figures 1 (a-c). Several general observations can be made

from Figure 1:

- in all samples CeO2 with fluorite type of structure is principle phase,

- it is notable that with increasing temperature peaks become more narrow

and more intense due to crystal structure arrangement and crystal growth,

7

- in samples with higher concentration of Bi3+ there is segregation of

additional two phases with increasing of temperature,

- further increasing of temperature leads to the disappearance of one

additional phase.

Observing the diagrams of Ce0.90Bi0.10O2-δ sample (Figure 1) it seems that the

powder with 10% of Bi is stable and always monophased in investigated temperature

range.

However, in sample with 20% of Bi at temperatures of 900 and 1000 ºC a Bi–rich

secondary phase was formed (Figure 2). That phase has been identified as tetragonal β'

phase of Bi2O3 [19] and it is marked with an asterisk in Figure 1. It is interesting that at

temperatures above one thousand degrees there are no additional peaks that correspond to

β' phase of Bi2O3. This could be explained by evaporation of small amount of Bi. It is

known that Bi has a high vapor pressure at relatively low temperatures, therefore melts,

and easily evaporates at experimental condition [25].

In samples with 30% and 40% of Bi3+ onset of the second phase in the form of β'–

Bi2O3 begins around 800 ºC. However, at 1000 ºC the phase evolution becomes more

complex. Namely, tetragonal β–Bi2O3 appears as a third phase (filled circles in Figure

1). Concentration of this Bi phase reaches its maximum at 1100 ºC, and begins to

decrease at 1200 ºC due to evaporation.

Finally at very high concentrations of Bi of even 50% – segregation of secondary

β' phase occurs already at 700 ºC. Further increase in temperature leads to the formation

of a third phase in the form of β–Bi2O3 at a temperature of 900 °C. Since all analyzed

samples in these work were recorded on the same device and the same conditions was

8

used during recording, X-ray powder patterns can be compared with each other. In this

respect, peak intensities show the concentration of the given phase in the sample. By

observing the diagram (Figure 1. c) it can be noted that with increasing temperature there

is a decrease in the intensity of the β'–Bi2O3 peak phase until their complete

disappearance (asterisk in Figure 1). Simultaneously, peak intensities of the β–Bi2O3

increase with temperature up to 1200 ºC when it starts to decline (filled circles in Figure

1). It is notable that increasing concentration of β–Bi2O3 phase, amount of β’–Bi2O3

decrease, which suggest that on higher temperatures Bi has a greater tendency to form β–

Bi2O3.

It is known that CeO2 can stabilize β–Bi2O3 phase. Namely, ceria may act as an

extrinsic nucleus for the formation of β–Bi2O3 because CeO2 lowers its free energy of

formation compared to that of α–Bi2O3 built on lattice vectors (2a√2, 2a√2, a) [23, 26].

Calculated lattice parameters for all samples are shown in Table 1. In the whole

temperature range the lattice parameters increase with increasing of Bi3+ concentration

according to the Vegard’s rule. According to Shannon’s compilation the ionic radii of

Ce4+ and Bi3+ for CN 8, are 0.97 and 1.17 Å, respectively [27]. Thus, increasing Bi3+

concentration will keep on enlarging cell lattice. On the other hand, increasing of

temperature leads to surprising reduction in the unit cell size that can be explained by at

least by two effects. First, increasing of temperature leads to better crystallization of

ceria with fluorite type of structure. As crystal becomes larger, cell parameters decrease

[28]. Second reason lies in fact that at certain temperature Bi ion leaves the fluorite

structure of Ce1-xBixO2-δ and appears in the form of β'–Bi2O3 and β–Bi2O3. However, unit

9

cell parameter of compounds with different concentration of Bi at each studied

temperature increase with increasing of Bi content, which indicates that in the crystal

lattice of CeO2 there is still a certain amount of Bi. Crystallite size values (Table 2)

decreases with increasing of Bi concentration that is common phenomena in doped CeO2

systems [29]. Nevertheless, crystallite size increases with increasing calcinations

temperature owing to crystallization. This can be seen at the diagrams (Figures 1) as

sharpening effect of reflections.

TG-DTA curves of the ceria powders with concentration of Bi from 10 to 40 mol

% are shown in Fig. 2. Complete thermal process can be divided into four stages. At the

first one, DTA curves show an endothermic peak at ~80 °C, followed by weight loss of

~8 %. This weight loss probably corresponds to the evaporation of water absorbed by

crystalline ceria. In the second stage, exothermal peak with weight loss of ~5% exists at

about 230 to 260 °C. It was assumed that it comes from the decomposition of residual

nitrates groups.

Total weight loss for these two stages is ~13 % (Table 3). Significant weight lost

indicates that as prepared samples were also slightly hydrated. These mass loss

corresponds to the decomposition of Ce(OH)3 or Ce(OH)4/CeO2·2H2O, indicating either a

partially hydrated (CeO2·H2O) or a mixture (CeO2+CeO2·H2O) of ceria phases [30, 31].

In addition, an exothermic peak around 270–350 °C, correlated to a weight loss of ~5%,

must be considered as combination of decomposition of hydrated CeO2, the

crystallization of the residual amorphous phase and decomposition of the nitrates group.

According to literature, Ce4+ ions could be hydrolyzed and could form complexes

with water molecules or OH− to give [Ce(OH)x(H2O)y](4 − x)+, where x+y represent the

10

coordination number of Ce4+. In an aqueous solution, H2O, being a polar molecule, tends

to take protons away from coordinated hydroxide, leading to the formation of

CeO2·nH2O. This process can be described by the following equations [32]:

(3)

(4)

On the other hand, presence of residual nitrates is not surprising, forasmuch

synthesis procedure in which metal nitrates were used as starting materials. In these

stages, major weight loss has occurred up to ~200 °C, but much higher temperature of

~500 °C seems necessary for a complete dehydration. During heating of hydrated

samples, it comes to the decomposition of adsorbed water molecules and residual nitrates.

Resulting gases are released by evaporation. After the temperature reaches 500 °C, the

mass of the sample keeps invariableness up to 1200 °C.

The third stage shows small exothermic reactions in the temperature region from

200 to 700 °C with maximum at ~550 °C. This reaction is not accompanied by any

changes in the mass. Based on XRPD diagrams it can be attributed to the crystallization

reaction of Bi reach secondary phases.

The last phase in termogravimetric evolution of examined samples is major

endothermic reaction at about 1400 °C, which represents melting and evaporating process

of Bi compounds. For every analyzed sample with different concentration of Bi these

reaction starts at 1200 °C but the end of reaction depends on the concentration of Bi.

[ ] 1)4(

224 O)(HCe(OH)OHOHCe

+−+ ⎯→⎯++x

yxyx

[ ] +−++⋅⎯→⎯+ OHOHOHCeOOHO)(HCe(OH) 32222

)4(

2 nnx

yx

11

Namely, as the amount of Bi increase, end of the reaction is shifted towards higher

temperature. The amount of evaporated bismuth is higher as its initial concentration in

the sample was greater.

The origin of mass loss that occurs during thermal treatment up to 500 °C was

tested using FTIR method. Figure 3. shows FTIR spectra of as prepared Ce0,7Bi0,3O2-δ

sample, and the same sample thermally treated at different temperatures at which the

DTA analysis reported peaks. FTIR spectrum of the as-prepared CeO2 sample, Fig. 6,

shows a group of strong intense bands at 3237, 1461, 1306 cm-1 and below 500 cm-1. The

spectrum also shows peaks of lower intensity at 1636, 1035 and 836 cm-1.

In lights of the proposed assignments of such peaks in other similar systems, [33-

35] the observed bands can be assigned as follows: wide band at about 3237 cm−1 is

typical for O–H [30], which corresponds to the physically adsorbed water on the sample.

Band at 1636 cm−1 corresponds to the δ(OH) mode [33]. The existence of residual or

adsorbed water molecules and hydroxide groups in the ceria-based samples is very

common [36-39]. The wide band at 1306 cm−1 consists of the symmetrical stretching

mode of N=O and the inside bending mode of N–H [36]. The outside bending mode of

N–H is at 836 cm−1. Minor mode which occurs at 1461 cm-1 corresponds to vibrations of

CH3 molecules. The existence of these organic molecules is the result of processing

where the last step is rinsing of the samples with alcohol. Intense absorption band that

appears at wavelengths below 500 cm-1 represents the stretching vibration of molecules in

the structure of cerium oxide, i.e. δ (Ce-O-Bi) mode [36, 40].

FTIR spectra of thermally treated samples show a gradual disappearance of the

absorption bands, which corresponds to vibrations of adsorbed water molecules and

12

residual nitrates (Figure 3.). First, there is a decrease and then loss of bands at the 3237

and 1636 cm-1 which corresponds to adsorbed water. These bands almost completely

disappear at 230 °C (Figure 3.). Temperatures above 500 °C lead to the decomposition of

nitrates groups, and thus the disappearance of the vibration bands that belong to them. In

the end, there remains only the peak corresponding to δ (Ce-Bi-O) mode (Figure 3). The

results of the FTIR analysis confirmed the assumption that the mass loss that occurs

during the heat treatment up to 500 °C of the Ce1-xBixO2-δ samples originates from

adsorbed water (i.e. moisture) and residual nitrate.

SEM images of samples Ce0,90Bi0,10O2-δ and Ce0,50Bi0,50O2-δ calcinated at 700 and

1200 °C are represented at Figure 4 (a-d) respectively. It reveals that morphologies of

powders annealed at 700 oC is cemented in a form of agglomerated lumps. Further

increase in annealing temperature changes the morphology of the powder. Fig. 4b and 4d

show that agglomerates in sample annealed at 1200 ºC are joined into bigger body but

still without clear crystal forms. The change in morphology can be ascribed to high

annealing temperature that provides fast diffusion necessary for crystallite growth. It can

be roughly estimated that the particle size of sample annealed at 1200 oC lies in the range

of 100 to 200 nm.

Semi-quantitative chemical analysis was done on samples doped with 50% Bi,

and the results are shown in Table 4. It revealed that as prepared sample and sample

thermally treated at 700 °C have chemical composition very similar with desired one,

which is not the case with samples calcinated at higher temperatures that are subject to

significant loss of Bi. These results confirm the assumption that at high temperature

(above 1000 °C) comes to a loss of Bi from the samples, by the process of evaporation.

14

4. Conclusions

Investigation of thermal stability of Ce1-xBixO2-δ solid solution with the

composition of x = 0.1 – 0.5 synthesized by SPRT method, was done. At room

temperature, these solid solutions are stable. Nonequilibrium conditions are responsible

for high solubility of Bi3+ in CeO2 lattice. Nevertheless, at high temperatures these solid

solutions become unstable, except the Ce0,90Bi0,10O2-δ sample which remains monophased

in investigated temperature range. Based on the X-ray powder reflection results obtained

in this work it can be said that samples with higher concentration of Bi3+ at temperatures

above 700 °C comes to segregation of secondary phases in form of β’–Bi2O3 and β–

Bi2O3. At temperatures above 1200 °C Bi tends to completely leave the mixture by

evaporating, which was confirmed by DTA/TGA analysis. According to observed results

of peak intensities analysis of powder patterns it seems that compounds with a higher

concentration of Bi are less stable, in terms of forming greater amount of undesirable

secondary phases. In addition, as DTA/TGA analysis showed that concentration of

evaporated Bi is in accordance with its initial concentration in the sample. Therefore, it

can be said that as concentration of Bi increases, stability of CeO2-Bi2O3 solid solution at

high temperatures decreases. Thermal instabilities, which were revealed in this paper,

should be kept in mind when potential application is concerned.

Acknowledgement: This project was financially supported by the Ministry of Education and Science of the Republic of Serbia (Project number: 45012).

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Highlights

Single phase solid solution Ce1-xBixO2-δ with the composition of x = 0.1 – 0.5 were

successfully synthesized at room temperature using simple and fast Self Propagating

Room Temperature procedure (SPRT).

Thermal stability of Ce1-xBixO2-δ solid solutions was investigated at various temperatures

up to 1400 °C.

It was revealed that all samples with concentration of bismuth higher than 10 at. % are

unstable during thermal treatment.

Bi leaving the structure of ceria and formation of β-Bi2O3 and β’-Bi2O3 as secondary

phases.

At temperatures above 1000 °C Bi tends to completely leave the mixture by evaporating.

Table 1. Calculated unit cell parameters of Ce1-xBixO2-δ (x = 0.1 – 0.5) samples calcinated at

different temperatures. Values are given in nanometers.

sample 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C 1200 °C

Ce0.9Bi0.1O2-δ 0.5422(3) 0.5423(3) 0.5421(2) 0.5416(1) 0.5411(6) 0.5405(8) 0.5404(8)

Ce0.8Bi0.3O2-δ 0.5439(2) 0.5433(3) 0.5428(9) 0.5429(3) 0.5417(4) 0.5405(7) 0.5404(8)

Ce0.7Bi0.3O2-δ 0.5447(4) 0.5444(4) 0.5431(7) 0.5428(5) 0.5417(6) 0.5409(5) 0.5407(5)

Ce0.6Bi0.4O2-δ 0.5462(5) 0.5457(1) 0.5449(4) 0.5432(5) 0.5425(7) 0.5417(9) 0.5414(1)

Ce0.5Bi0.5O2-δ 0.5477(7) 0.5463(6) 0.5459(6) 0.5438(2) 0.5424(9) 0.5412(3) 0.5415(1)

Table 2. Calculated crystallite size of Ce1-xBixO2-δ (x = 0.1 – 0.5) samples calcinated at different

temperatures. Values are given in nanometers.

sample 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C 1200 °C

Ce0.9Bi0.1O2-δ 10 14 27 40 66 124 158

Ce0.8Bi0.3O2-δ 8 12 19 26 44 118 153

Ce0.7Bi0.3O2-δ 9 11 16 22 37 96 154

Ce0.6Bi0.4O2-δ 10 12 19 22 36 79 133

Ce0.5Bi0.5O2-δ 9 12 16 18 54 90 142

average value 9 12 19 26 47 102 148

Table 3. Weight loss of samples Ce1-xBixO2-δ with composition x = 0.1 – 0.4 during the thermal

treatment.

Weight loss [%] Total weight

loss [%] 30 - 450 °C 1200 - 1400 °C

Ce0.9Bi0.1O2-δ 13.6 8.7 22.3

Ce0.8Bi0.3O2-δ 13.3 18.1 31.4

Ce0.7Bi0.3O2-δ 15.7 28.0 44.0

Ce0.6Bi0.4O2-δ 11.2 28.9 40.1

Table 4. EDX microanalysis of the as-prepared Ce0.5Bi0.5O2-δ sample and calcinated at different

temperatures.

Nominal chemical composition EDX determinated chemical

composition

Ce0.50Bi0.50O2-δ as-prepared Ce0.48Bi0.52O1.74

Ce0.50Bi0.50O2-δ at 700 °C Ce0.50Bi0.50O1.75

Ce0.50Bi0.50O2-δ at 1000 °C Ce0.60Bi0.40O1.80

Ce0.50Bi0.50O2-δ at 1200 °C Ce0.90Bi0.10O1.95

Figure 1. XRPD patterns of Ce1-xBixO2-δ samples after heating at temperatures from 600 to 1200

°C: (a) Ce0.90Bi0.10O2-δ, (b) Ce0.80Bi0.20O2-δ, (c) Ce0.70Bi0.30O2-δ, (d) Ce0.60Bi0.40O2-δ and (e)

Ce0.50Bi0.50O2-δ. (mark “*” represents β'–Bi2O3 while and mark “●” represents β–Bi2O3).

20 30 40 50 60 70

(a)

(222)

(311)(220)

(200)

1200 °C

1100 °C

1000 °C

900 °C

800 °C

700 °C

600 °C

Rel

ativ

e In

tens

ity

2(degree)

RT

(111)

20 30 40 50 60 70

(b)

1200 C

1100 C

1000 C

900 C

800 C

700 C

Rea

ltive

Inte

nsity

2(degree)

RT

600 C

(111)

(200)

(220)(311)

(222)

20 30 40 50 60 70

(c)

1200 C

1100 C

1000 C

900 C

800 C

700 C

600 C

Rel

ativ

e In

tesi

ty

2(degree)

RT

(111)

(200)

(220)(311)

(222)

20 30 40 50 60 70

(d)

1200 C

1100 C

1000 C

900 C

800 C

700 C

600 C

Rel

ativ

e In

tens

ity

2(degree)

RT

(111)

(200)

(220)(311)

(222)

20 30 40 50 60 70

(e)

1200 C

1100 C

1000 C

900 C

800 C

700 C

600 C

2(degree)

Rel

ativ

e In

tesi

ty

RT

(111)

(200)

(220)(311)

(222)

Figure 2. DTA/DTG/TGA curves of the as-synthesized Ce1-xBixO2-δ powders with different Bi

concentration: (a) Ce0,9Bi0,1O2-δ; (b) Ce0,8Bi0,2O2-δ; (c) Ce0,7Bi0,3O2-δ and (d) Ce0,6Bi0,4O2-δ.

200 400 600 800 1000 1200 1400

65

70

75

80

85

90

95

100

TG

DTA

Wei

ght

loss

[%

]

Temperature [C]

Exo

200 400 600 800 1000 1200 1400

75

80

85

90

95

100

TG

DTA

Wei

ght

loss

[%

]

Temperature [C]

Exo

200 400 600 800 1000 1200 1400

55

60

65

70

75

80

85

90

95

100

TG

DTA

Wei

gh

t lo

ss [

%]

Temperature [C]

Exo

200 400 600 800 1000 1200 1400

55

60

65

70

75

80

85

90

95

100

TG

DTA

Wei

gh

t lo

ss [

%]

Temperature [C]

Exo

(a) (b)

(c) (d)

4000 3500 3000 2500 2000 1500 1000 500

440

1636

836

1035

1461

1306

C

230 C

85 C

Inte

sity

(a.

u.)

Wavenumbers (cm-1)

as-prepared

3237

Figure 3. FTIR patterns of Ce0,7Bi0,3O2-δ sample calcined under different temperatures,

according to the DTA/TG measurements.

Figure 4. SEM images of samples Ce0,90Bi0,10O2-δ calcinated at (a) 700 °C, (b) 1200 °C and

Ce0,50Bi0,50O2-δ calcinated at (c) 700 °C, (d) 1200 °C.

(a)

(d) (c)

(b)