-
Processing and Application of Ceramics 13 [3] (2019) 310–321
https://doi.org/10.2298/PAC1903310M
The influence of short thermal treatment on structure,
morphologyand optical properties of Er and Pr doped ceria
pigments:Comparative study
Dragana Mićović1, Maja C. Pagnacco2, Predrag Banković2,
Jelena Maletaškić3,Branko Matović3, Veljko R. Djokić4, Marija
Stojmenović3,∗
1Faculty of Physical Chemistry, University of Belgrade,
Studentski trg 12, 11000 Belgrade, Serbia2Institute of Chemistry,
Technology and Metallurgy, University of Belgrade, Department of
Catalysis and
Chemical Engineering, Njegoševa 12, 11000 Belgrade,
Serbia3Vinča Institute of Nuclear Sciences, University of
Belgrade, P. O. Box 522, Belgrade, Serbia4Faculty of Technology and
Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade,
Serbia
Received 28 March 2019; Received in revised form 7 August 2019;
Accepted 26 August 2019
Abstract
Potential non-toxic pink and red ceramic pigments based on CeO2
were successfully synthesized by self-propagating room temperature
method and thermally treated at 600, 900 and 1200 °C for 15 min.
The structure,morphology and optical properties, as well as thermal
stability of Ce1-xErxO2-δ and Ce1-xPrxO2-δ (x = 0.05)were examined.
Single-phase composition of all obtained CeO2 pigments was
confirmed using XRPD methodand Raman spectroscopy and it was not
dependent on temperature. The mechanism of structural behaviourwas
thoroughly examined using Raman and FTIR spectroscopy. Nanometric
dimensions of the crystallites ofall pigments were confirmed using
XRPD, TEM and FE-SEM analysis. Colour properties were dependent
onthe temperature treatment, and their position in the chromaticity
diagram was studied using UV/VIS spec-trophotometry. Colour
efficiency measurements were supplemented by colorimetric analysis.
It is proved thatall samples are thermally stable in the
investigated temperature range (up to 1200 °C), and their
potentialapplication as environmentally friendly pigments of
desired colour is confirmed.
Keywords: CeO2, Er and Pr doping, structure, optical properties,
colour, pigments
I. Introduction
Recently, almost all fields of science have been ded-icated to
solving different problems in order to protectthe environment. One
of the fields of science that dealswith this problem is the field
of development of newnon-toxic environmentally pure inorganic
pigments. To-day, inorganic pigments are frequently used for
thecolouration of many materials in the industry includ-ing the
production of plastics, ceramics, inks, rubbers,paints and glasses
[1,2]. Traditional pigments, such asmatrix of lead oxide (Pb3O4)
and tin oxide (SnO2), areused in large extents [3]. Iron oxide
(Fe2O3) encapsu-lated in zircon (ZrSiO4) matrix, which gives pale
red orpink colours [3], and Cd(SxS1-x)-ZrSiO4 as red-orange
∗Corresponding author: tel: +381 11 340 8860,e-mail:
[email protected]
pigment [4], are also widely used. However, the appli-cation of
the Cd(SxS1-x)-ZrSiO4 system at temperaturesabove 900 °C is very
toxic and unstable [4]. Althoughsome inorganic pigments containing
elements such asPb, Sn, Fe, Cr, Se, Cd, Hg, Sb have shown
excellentproperties, their application is strictly controlled
andregulated by legislation and regulations adopted by gov-ernments
in many countries due to their high toxicity(not only for the
environment but also for human health)[5–7].
Lately, many attempts have been made in order toeliminate the
application of above mentioned toxic andunstable pigments [8–11].
With a few exceptions, ma-jority of ceramic pigments is based on
inorganic ox-ides. Researches have been increasingly focused onthe
development of new nanosized ceramic pigments,in particular based
on cerium(IV)-oxide (CeO2), with
310
https://doi.org/10.2298/PAC1903310M
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
the goal of spreading the range of applications of ce-ramic
pigments [12–15]. Thanks to their high thermalresistance, chemical
stability (low corrosion) and verygood optical and colouristic
properties, ceramic pig-ments based on CeO2 are increasingly used
nowadays[6,7,9,10,12,13,16]. They attract particular attention
be-cause of ability to filter ultraviolet (UV) radiation
andprotection capabilities from the effects of solar light(UV, VIS
and IR light) when applied in the form ofcoloured coating
[13,14].
In order to enable development of environmentallynon-harmful
ceramic pigments based on CeO2, manyproblems have to be solved
starting with those relatedto powder synthesis. The syntheses of
ceramic pigmentsare each time more focused on obtaining CeO2
pigmentsin the form of fine nanosized particles with high
surfacearea, because these features influence colour
intensities[12,17,18]. Namely, the properties of the obtained
pig-ments can be controlled by the incorporation of anotherelement
into the CeO2 lattice using different processesof synthesis and
modification [12,19–23]. The colour-ing mechanism is based on the
charge transfer transitionfrom O2p to Ce4f within the CeO2 band
structure, whichcan be modified by the introduction of an
additionalelectronic level between the anionic O2p valence bandand
the cationic Ce4f conduction band [7]. The pro-cess of ceria (CeO2)
doping with different ions (Ru
3+/4+,Yb3+, Er3+, Y3+, Gd3+, Sm3+, Dy3+, Nd3+, Pr3+/4+) byusing
the self-propagating reaction at room temperature(SPRT method) has
proven to be a very easy and uniqueway to obtain pigments with
stable structure, morphol-ogy and optical properties [12,23–27].
The SPRT pro-cedure, which is one of the most promising because
ofseveral advantages over conventional methods [12,23–27], is based
on the self-propagating room temperaturereaction between metal
nitrates and sodium hydroxide,whereby the required equipment is
extremely simpleand inexpensive. The reaction is spontaneous,
whereasthe stoichiometry of the final product precisely matchesthe
tailored composition.
Because of the above mentioned advantages,CeO2 doped with Er
3+ and Pr3+ (Ce1-xErxO2-δ andCe1-xPrxO2-δ; x = 0.05), as new
potential environ-mentally friendly non-toxic ceramic pink and
redpigments, were synthesized using the SPRT method[12,18,24,26].
The parameter δ denotes oxygen defi-ciency, i.e. departure from
stoichiometry, both becauseof the introduction of dopant cations
(x), and becauseof intrinsic non-stoichiometry. In our previous
research[12], various shades of pink colour were obtained forEr3+
doped CeO2 (Ce1-xErxO2-δ; x = 0.05–0.20) at roomtemperature (25 °C)
and after thermal treatment at 600,900 and 1200 °C for 4 h in an
air.
In this work, structure, morphology and optical prop-erties of
Ce1-xErxO2-δ and Ce1-xPrxO2-δ (x = 0.05) atroom temperature (25
°C), and after thermal treatment at600, 900 and 1200 °C for 15 min
in air, were compared.Thermal treatment lasted only for 15 min
because the
ceramic pigment (Ce1-xPrxO2-δ; x = 0.05) very quicklyreached
different hues of red colour at 600, 900 and1200 °C. In addition,
praseodymium (Pr3+) was selectedas a very suitable candidate for
obtaining ceramic pig-ments with a range of shades of red colour
[18], be-cause of its lower valence state, comparing with ceria,and
good incorporation in its crystal lattice. Further-more, Pr3+ doped
CeO2 has still been unused up to nowas a ceramic pigment for
colouring of different oxidesand glass for the application in
industrial production[28,29]. Here presented research was focused
on the de-velopment of a novel class of potential
environmentallyfriendly non-toxic pigments based on CeO2 with
differ-ent shades of pink and red colour, aiming to expand therange
of ceramic pigments application in the industry.
In this study, optical properties of solid solutionsbased on
CeO2, after very short thermal treatment in air,were investigated
in order to replace toxic red pigmentsin a potential application in
industrial production.
II. Experimental section
2.1. Materials and methods
Starting reactants used in the experiments werecerium nitrate
hexahydrate (Ce(NO3)3 · 6 H2O; Aldrich,USA), erbium nitrate
pentahydrate (Er(NO3)3 · 5 H2O;Aldrich, USA), praseodymium nitrate
hexahydrate(Pr(NO3)3 · 6 H2O; John Mathey) and sodium hydrox-ide
(NaOH, Vetprom-Chemicals). The desired composi-tions of the solid
solutions were calculated from the ion-packing model equation [30].
The concentrations of thestarting reactants were calculated in
accordance with thechemical formula of the final products
(Ce0.95Er0.05O2-δand Ce0.95Pr0.05O2-δ) [24,26], which were derived
fromthe reactions (1) and (2):
(1−x)Ce(NO3)3 · 6 H2O + xEr(NO3)3 · 5 H2O + 3 NaOH +
(1/2−δ)O2 −−−→ Ce1−xErxO2−δ + 3 NaNO3 + yH2O (1)
2 (1−x)Ce(NO3)3 · 6 H2O + xPr(NO3)3 · 6 H2O + 6 NaOH +
(1/2−δ)O2 −−−→ 2 Ce1−xPrxO2−δ + 6 NaNO3 + 15 H2O (2)
The starting chemicals were hand mixed [18,26] inalumina mortar
for 15 min and then exposed to air for3 h, which provided required
energy for total termina-tion of reaction according to the equation
(1). For theremoval of NaNO3 the entire mixture was dispersed
inwater, and rinsing was performed in a Centurion 102 Dcentrifuge
at 3000 rpm for 10 min. The procedure wasperformed three times with
distilled water and twicewith ethanol. The absence of NaNO3 was
confirmed bythe titration analysis of the powder on Na using EDTAas
the titrant. After drying at 100 °C for 24 h, the pow-ders were
thermally treated at 600, 900 and 1200 °C for15 min in ambient
atmosphere.
2.2. Characterization
All powders were characterized at room temperatureby X-ray
powder diffraction (XRPD) using an Ultima
311
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
IV Rigaku diffractometer, equipped with Cu Kα1,2 radi-ation,
using a generator voltage of 40.0 kV and a gen-erator current of
40.0 mA. The 2θ range of 20–80° wasused for all powders in a
continuous scan mode with thescanning step size of 0.02° and the
scan rate of 2 °/min.Phase analysis was performed using the PDXL2
soft-ware (version 2.0.3.0), with reference to the patterns ofthe
International Centre for Diffraction Data database(ICDD), version
2012. XRPD method was also usedto evaluate the crystallite size
(DXRPD) and lattice pa-rameter (aXRPD) as a function of
temperature. Assum-ing that line broadening (β = β′ + β′′) is the
sumof the contributions attributed to the crystalline size(DXRPD)
and microstrain (eXRPD), which can be writ-ten as β′ = 1/(DXRPD ·
cos θ) and β′′ = 4eXRPD · tan θ,the crystallite size and
microstrain can be determinedfrom the linear relationship between β
cos θ and 4 sin θ,where θ is the Bragg diffraction angle, and
microstrainis eXRPD = ∆d/d (∆d is the displacement of the
lattice)[31]. Before measurement, the angular correction wasdone
using high quality Si standard. Lattice parameterswere refined from
the data using the least square proce-dure. Standard deviation was
about 1%.
Raman spectra were collected in the spectral rangefrom 300–700
cm-1 using a DXR Raman microscope(Thermo Scientific, USA) equipped
with a diodepumped solid state high-brightness laser (532 nm),
anOlympus optical microscope and a CCD detector. Laserpower was
kept constant at 1 mW. The analysis of thescattered light was
carried out using a spectrograph withgrating of 900 lines/mm.
Transmission electron microscopy (TEM) analysis ofinvestigated
samples was carried by using JEOL JEM-2100 at 200 kV.
Microstructure and morphology of the investigatedsamples were
observed using the field emission-scanning electron microscopy
(FE-SEM) analysis(TESCAN Vega TS5130MM). The samples were
pre-coated with a several-nanometre thick layer of gold be-fore
observation. A Fine Coat JFC-1100 ION SPUT-TER Company JEOL device
was used for the coatingprocedure.
Particle size distribution was determined from the ob-tained TEM
and FE-SEM micrographs, immediately af-ter they had been taken,
using the Digital Micrographsoftware. Approximately 40 particles
from each micro-graph were chosen for the measurements. The
diameterof the particles was measured manually, on screen andthe
result was recalculated into particle sizes using theDigital
micrograph software. The mean value was takenas the relevant
particle size of the powder.
The EDS analysis was carried out at the invasive elec-tron
energy of 30 keV by means of a QX 2000S deviceby Oxford
Microanalysis Group. The maximum resolu-tion was 0.4 nm.
Fourier transform infrared (FTIR) spectra of the sam-ples were
collected before and after the heat treatmentat different
temperatures by using a Perkin Elmer Spec-trum Two FTIR
spectrometer in the transmission mode.Pressed KBr pellets
(technique 1 : 100) were recordedin the range from 450 to 4000 cm-1
with the resolutionof 4 cm-1.
Optical properties were analysed by using a dif-fuse reflectance
(DR) Thermo Electron Nicolet Evo-lution 500 spectrophotometer.
Labsphere USRS-99-010 was used as the reflectance standard. The
spec-tra were registered in the wavelength range from 350to 800 nm
and corresponding diffuse reflectance curveswere recorded. Scan
speed, step recording and band-width were 240 nm/min, 1 nm and 4
nm, respectively.
The analysis of colour characteristics of all sampleswas
performed according to the CIE L*a*b* (1976)standard, using
illuminant C spectral energy distribu-tion. In this system, L* is
the colour lightness (L* =0 for black and L* = 100 for white), a*
is the green(−)/red (+) axis, and b* is the blue (−)/yellow (+)
axis,and their values were calculated according to the CIEL*a*b*
standard.
III. Results and discussion
The X-ray diffraction patterns of all obtained dopedceria
powders (Ce1-xErxO2-δ and Ce1-xPrxO2-δ; x = 0.05)are shown in Fig.
1. Regardless of the dopant type or
Figure 1. XRPD patterns of: a) Ce0.95Er0.05O2-δ and b)
Ce0.95Pr0.05O2-δ powders, thermally treated in air for 15 min at
differenttemperatures
312
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
Table 1. Lattice parameters obtained by ion-packing model
(aipm), lattice parameter (aXRPD), crystallite size (DXRPD)
andmicrostrain (eXRPD) obtained by XRPD analysis, and particle size
obtained by TEM and FE-SEM of investigated samples
at different temperatures
Sampleaipm,Er3+ aipm,Pr3+ aipm,Pr4+ aXRPD DXRPD eXRPD Particle
size
[Å] [Å] [Å] [Å] [nm] [%] [nm]25 °C
Ce0.95Er0.05O2-δ [24] 5.4091 - - 5.4023 3.72 0.87
3.5*Ce0.95Pr0.05O2-δ 5.4231 5.4040 5.4410 2.39 1.07 2.3*
600 °CCe0.95Er0.05O2-δ 5.4091 - - 5.3996 12.68 0.35
12.2*Ce0.95Pr0.05O2-δ - 5.4231 5.4040 5.4181 10.66 0.87 10.1*
900 °CCe0.95Er0.05O2-δ 5.4091 - - 5.3895 32.23 0.11
32.0**Ce0.95Pr0.05O2-δ 5.4231 5.4040 5.4120 29.63 0.12 27.9**
1200 °CCe0.95Er0.05O2-δ 5.4091 5.3854 36.81 0.08
37.9**Ce0.95Pr0.05O2-δ 5.4231 5.4040 5.4103 35.06 0.07 34.5**
* TEM, ** FE-SEM
heat treatment temperature, all obtained solid
solutionsexhibited the single-phase fluorite crystal structure
withFm3m space group [18]. High solubility of the dopantsin the
obtained solid solutions and retaining of the fluo-rite crystalline
structure can be attributed to their nano-metric dimensions. The
structure information was takenfrom the American Mineralogist
Crystal Data StructureBase (AMCDSB). The reflections for the
as-preparedpowders, that had not been thermally treated, were
sig-nificantly broadened (Fig. 1), indicating small crystal-lite
size (DXRPD) and/or microstrain (eXRPD) (Table 1).Because of very
low crystallinity, the XRPD patternsexhibited very diffuse
diffraction lines, in such a waythat some XRD peaks are not visible
(i.e. 222, 400, 331,420). However, after the thermal treatment, the
powderswere depicted by XRPD diagrams with sharper diffrac-tion
lines, which is in line with the increase of crystallitesizes DXRPD
(Table 1).
The results of lattice parameters (aXRPD), crys-tallite size
(DXRPD) and microstrain (eXRPD) of theCe0.95Pr0.05O2-δ and
Ce0.95Er0.05O2-δ powders are shownin Table 1. It was found that the
crystallite size ofthe doped ceria powders increases with the
increaseof temperature from 25–1200 °C. This is in a goodagreement
with the lattice parameter values, which de-creased with increasing
thermal treatment temperaturedecreased, and approached the value of
the standardpure ceria crystal lattice [23], as well as the
theoreticalvalues obtained by ion-packing model (aipm) (Table
1).Namely, according to Shannon’s compilation [32], theionic radii
of Ce4+, Ce3+, Er3+, Pr3+ and Pr4+ are 0.970,1.143, 1.004, 1.126
and 0.960 Å, respectively, for eight-fold coordination.
Additionally, it is known that CeO2crystal lattice contains Ce4+
and Ce3+ ions per core-shellmodel [33, 34]. It was found that most
of Ce3+ is locatedat the surface [33,34]. All above mentioned
indicatesthat doping process can lead to the substitution of
Ce4+
and Ce3+ ions with Er3+ and Pr3+ ions (confirmed by Ra-man
spectroscopy results presented further in the text).
Besides, it is also known that praseodymium ions caneasily
change the oxidation state (Pr3+↔Pr4+) [35,36].Since the ionic
radius of Er3+ is smaller than the ionicradius of Ce3+ and larger
than the ionic radius of Ce4+,the doping process can lead to the
dilation of the latticeand the reduction of the lattice parameter
values in com-parison with the pure CeO2 [23]. With increasing
tem-perature, the lattice parameter value of the CeO2 dopedwith
Er3+ ions further decreased. Such behaviour can beexplained by the
loss of oxygen vacancies because of theinfluence of high
temperature. Namely, theoretical cal-culations predict that oxygen
vacancies tend to migratefrom the nanoparticle’s interior to the
surface [24,37].According to our results it seems that the
migrationof oxygen vacancies was intensified when the sampleswere
exposed to higher temperatures. In the case of Er3+
doped ceria subjected to the high temperature heat treat-ment
(900 and 1200 °C), oxygen vacancies probably mi-grated from CeO2
nanoparticles’ interior to the surface,and furthermore, left the
sample surface thus leading tolattice stabilization. This
assumption can be supportedby the obtained Raman spectra (Fig. 2a),
where the Ra-man intensity of “vacancies mode” decreased as
theconsequence of the heat treatment. On the other hand,the lattice
of the ceria doped with praseodymium ionsexhibited expansion, which
can be explained in termsof higher oxygen vacancy concentration
because of pos-sible presence of Ce3+, Pr3+ and Pr4+ ions.
Besides,the lattice parameter of the CeO2 doped with Pr
3+/4+
ions gradually decreased with increasing temperature,although
Raman spectroscopy confirmed increased Ra-man intensity of
“vacancies mode”. The reason for thisbehaviour would have to be the
change of the oxidationstate of Pr3+ in Pr4+ (0.960 Å), with
similar ionic radiusas Ce4+ ion (0.970 Å), which explains the
decrease inlattice parameter with the formation of a new
oxygenvacancy (confirmed by Raman spectroscopy; Fig.
2b).Additionally, all treated samples exhibited strong influ-ence
of temperature on the microstrain, where with in-
313
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
Figure 2. De-convoluted Raman spectra of: a) Ce0.95Er0.05O2-δ
and b) Ce0.95Pr0.05O2-δ at different temperatures
creased temperature the values of the crystallite sizes
in-creases together with lowering the microstrain (Table
1).According to our findings, the decrease of microstraincan lead
to lattice stabilization, which was achieved bytwo completely
different processes. Thus, in the caseof the samples containing
Er3+ that lacks in the ox-idized form of erbium (Er4+) lattice
stabilization (re-flected in decreasing microstrain and lattice
parameter)was achieved by oxygen vacancies leaving the
samplesurface. On the other hand, with the samples
containingpraseodymium ions, the change in the oxidation state(from
Pr3+ to Pr4+) that has similar ionic radius and thesame oxidation
state as Ce4+ ions, leads to the decreasedmicrostrain and lattice
parameter values and general lat-tice stabilization.
The identification of the Raman spectrum of thematerials based
on CeO2 allowed the informationabout their potential behaviour and
applications. Es-pecially, based on Raman signature for oxygen
vacan-cies in the above mentioned materials, the mechanismof their
behaviour under the influence of temperaturecan be defined. The
Raman spectra of the as-preparedCe0.95Er0.05O2-δ and
Ce0.95Pr0.05O2-δ powders (25 °C)and those thermally treated in air
for 15 min at differ-ent temperatures (600, 900 and 1200 °C) are
presentedin Fig. 2. It is well documented that the main featurein
the Raman spectrum of the pure and stoichiometricCeO2 with fluorite
structure is a single allowed Ramanmode of the first order (F2g
symmetry), positioned atthe wavenumber of 465 cm-1 [23]. This mode
occurs asa result of the symmetric breathing mode of O atoms
around each cation (CeO8, the lattice vibration). In thepresent
work, the obtained nanopowders at room tem-perature (25 °C; Fig. 2)
exhibited shifting of this modeto lower energies (457 and 450 cm-1)
with increasedline width and expressed asymmetry at the lower
en-ergy side. These changes in Raman peak profile [18,23]may be due
to the nanosized effects such as phonon con-finement, inhomogeneous
strain and non-stoichiometry.After thermal treatment in air during
15 min at dif-ferent temperatures (600, 900 and 1200 °C), the
Ra-man mode was shifted to higher frequencies both
forCe0.95Er0.05O2-δ (460, 462 and 463 cm
-1, respectively),and for the Ce0.95Pr0.05O2-δ (456, 459 and 463
cm
-1, re-spectively), with the reduction of line width (Table
2)and the appearance of symmetry. All these character-istics of the
Raman peak profile showed that thermaltreatment led to grain growth
and the formation of bet-ter ordered structures, which was
confirmed by XRPD(Fig. 1; Table 1), TEM (Figs. 3a and 3b) and
FE-SEManalyses (Figs. 3c and 3d).
In addition to the mentioned Raman mode of the firstorder, the
Raman spectra of the powders based on CeO2showed the presence of
additional second order modespositioned at about 550 and 600 cm-1
[12,23], and de-fined as Raman signature for oxygen vacancies.
Themodes positioned at about 550 and 600 cm-1 are as-signed to
intrinsic and extrinsic oxygen vacancies, gen-erated in CeO2
lattice with reduction in particle size anddoping process,
respectively [12,23]. Namely, with par-ticle size decreasing the
overall free surface of the pow-der increases, enabling easier
release of oxygen from
314
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
the lattice, thus leaving a vacancy and two electrons lo-calized
on a cerium atom. This process causes the for-mation of Ce3+ ions
(lowering of Ce4+ valence, whichis due to electroneutrality
demands) and the emergenceof the Raman mode at around 600 cm-1
(Fig. 2) [23].On the other hand, for the doped nanopowders
addi-tional Raman mode at around 550 cm-1 was attributedto oxygen
vacancies introduced into the ceria latticewhen Ce4+ ions were
replaced with cations of lowervalence state [12,23] (in this case
oxidation numberswere 3+). At 25 °C the occurrence of the mode
orig-inating from the oxygen vacancies can be explainedby the
substitution of two Ce4+ ions with two dopantions (Er3+ or Pr3+),
when one oxygen vacancy is in-troduced into the ceria lattice in
order to maintain theelectrical neutrality [24,26]. However, with
increasingheat treatment temperature, the mentioned mode forthe
Ce0.95Er0.05O2-δ was slowly disappearing [12], whilefor the
Ce0.95Pr0.05O2-δ these modes merged into one atabout 570 cm-1,
whose intensity increased with increas-ing temperature [18,24].
This behaviour of the modeoriginating from the oxygen vacancies
introduced intothe ceria lattice can be explained by two different
mech-anisms for the two dopants:1. With increasing temperature, the
Raman intensity of“vacancies mode” related to the Er3+ doped CeO2
de-creases (Fig. 2a), which can be due to the intensifiedmigration
of oxygen vacancies towards the surface ofthe sample and their loss
under the influence of tem-perature. The loss of oxygen vacancies
due to the influ-ence of temperature was in agreement with
theoreticalcalculations which also predict their migration from
thenanoparticles’ interior to the surface [24,37]. Thus, un-der the
influence of increased temperature, the migra-tion of oxygen
vacancies was intensified, followed bytheir leaving of the sample
surface at higher tempera-tures. The higher temperature, the more
eased leavingof sample surface was. This mechanism was supportedby
the recorded Raman spectra (Figs. 2a and 3), whereit is clear that
Raman intensity of “vacancies mode” de-
creases with increasing temperature of the heat treat-ment.2.
With increasing temperature, the Raman “vacanciesmodes” related to
the praseodymium doped CeO2 ap-peared originating from the presence
of Ce3+ ions. Theyunderwent merging in one mode at about 570
cm-1,whose intensity increased with increasing temperature(Fig.
2b). This can be due to the presence of Ce3+ andPr3+, but also Pr4+
ions (0.960 Å) in CeO2 structure,whose ionic radii are similar to
that of Ce4+. Namely,the praseodymium ions can easily change the
oxidationstate (Pr3+↔Pr4+) [24,36], which allows the stabiliza-tion
of the crystal lattice, formation of a new oxygenvacancy and
increase of the Raman intensity of “vacan-cies mode” (Figs. 2b and
3).
Thus, according to the literature [18,23,24] and herepresented
results (Figs. 2 and 3), the intensity of peaks at550 and 600 cm-1
is dependent on the number of vacan-cies. Their absence or presence
may indicate the mech-anisms of vacancy behaviour with increasing
tempera-
Figure 3. Vacancies modes area as a result of Raman peak’sarea
sum at 550 and 600 cm-1 illustrating two different
mechanisms of vacancy behaviour with increasingtemperatures
Table 2. Results of the de-convolution of Raman spectra of the
Ce0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δ powders, at roomtemperature
(25 °C) and those thermally treated at different temperatures (600,
900 and 1200 °C), in air for 15 min
SampleArea [%]
Peak 1 FWHM Peak 2 Peak 3 Peak 4(465 cm-1) (465 cm-1) (550 cm-1)
(600 cm-1) (480 cm-1)
25 °CCe0.95Er0.05O2-δ 86.95 60.05 7.47 5.57 -Ce0.95Pr0.05O2-δ
85.03 42.14 5.87 9.10 -
600 °CCe0.95Er0.05O2-δ 88.74 53.21 6.59 4.67 -Ce0.95Pr0.05O2-δ
77.10 15.77 22.90 -
900 °CCe0.95Er0.05O2-δ 90.82 31.98 - 3.87 5.31Ce0.95Pr0.05O2-δ
51.96 11.87 48.04 -
1200 °CCe0.95Er0.05O2-δ 95.36 12.88 - - 4.64Ce0.95Pr0.05O2-δ
45.58 11.85 54.42 -
315
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
tures, and improvement of properties for potential appli-cations
[12,25–27]. It is evident from Fig. 3 that the totalvacancies
content for the Er +3 doped CeO2 decreased,while it increased in
the case of Pr3+/4+ doped CeO2.
Additional mode positioned at about 480 cm-1, whichoccurs in the
case of the Ce0.95Er0.05O2-δ thermallytreated at the temperature at
900 °C and above, can bea result of the surface mode appearing in
the case ofvery small crystallites like CeO2-x [37]. The
appearanceof this infrared active mode in the Raman spectra canbe
explained by the lack of long-range order in smallcrystallites and
by the relaxation of the selection rulesas a consequence of the
defective structure. With fur-ther increase of temperature, the
intensity of the men-tioned mode decreases (Fig. 2a), which
confirmed theformation of ordered CeO2 structures. The occurrenceof
this mode is in correlation with the FTIR results, andaccording to
our knowledge, its presence in both Ramanand FTIR spectra of the
doped CeO2 nanocrystals is firstreported here (discussed
below).
The TEM images of the as-prepared Ce0.95Er0.05O2-δand
Ce0.95Pr0.05O2-δ nanopowders at 25 °C and thosethermally treated at
600 °C for 15 min in air, are pre-sented in Figs. 4a and 4b. In all
the images the pres-ence of the agglomerates and formed aggregates
can beseen, which results from the natural tendency of
crys-tallites to agglomerate [12,23,26]. The first reason forthis
is energetically more stable configuration of the ag-glomerated
form, while the second reason is the pos-sibility of the
crystallite growth. The additional infor-mation on the morphology
and microstructure of theCe0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δ
nanopowders ther-mally treated at 900 and 1200 °C for 15 min in air
were
obtained by using FE-SEM technique (Figs. 5a and 5b).These two
powders show homogeneous structure andgrain growth upon thermal
treatment can be traced, indi-cating good thermodynamical
stability. It is noteworthythat the mean particle size calculated
from TEM (Fig.4) and FE-SEM images (Fig. 5) shows increase
withincreasing heat treatment temperature for both dopedCeO2
powders (Table 1). The results obtained from theTEM images indicate
that the mean particle size wasbelow 4 nm and that it differed at
most by 1 nm fromthose obtained by XRPD. Besides, the results
indicatethat the average grain size measured from the FE-SEMimages
was smaller than 40 nm and it differed at mostby 2 nm from those
obtained by XRPD. This confirmsthe consistence of the TEM and
FE-SEM results withthe results obtained by XRPD.
Corresponding EDS spectra of the Ce0.95Er0.05O2-δand
Ce0.95Pr0.05O2-δ nanopowders thermally treated at1200 °C in air for
15 min (Fig. 6), and mean valuesof the Ce3+/4+/Er3+ and
Ce3+/4+/Pr3+/4+ chemical ratiosconfirmed that Er3+ and Pr3+/4+ ions
in the concentra-tions of 5% (Ce3+/4+/Er3+ = 95.23/4.77,
Ce3+/4+/Pr3+/4+
= 95.11/4.89) are successfully incorporated into the
hostmatrix.
In Fig. 7, FTIR spectra of the as-preparedCe0.95Er0.05O2-δ and
Ce0.95Pr0.05O2-δ powders (25 °C)and those thermally treated at
different temperatures(600, 900 and 1200 °C) in air for 15 min are
pre-sented. Both spectra show a large absorption band lo-cated at
around 480 cm-1. This band can be attributedto the Ce–O stretching
vibration [12,38,39] and corre-sponds to the F1u IR active mode of
the CeO2 fluoritestructure. Additionally, the mode positioned at
about
Figure 4. TEM images of investigated samples: a) as-prepared and
b) thermally treated at 600 °C
Figure 5. FE-SEM images of investigated samples thermally
treated at: a) 900 and b) 1200 °C
316
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
Figure 6. EDS spectra of: a) Ce0.95Er0.05O2-δ and b)
Ce0.95Pr0.05O2-δ samples thermally treated at 1200 °C
480 cm-1 was observed in Raman spectrum of the sam-ple
Ce0.95Er0.05O2-δ and can be ascribed to a surfacemode appearing in
very small crystallites such as CeO2-xwhose energy lies between the
infrared active TO andLO phonons of CeO2 [37]. As already
mentioned, theappearance of this IR active mode in the Raman
spectracan be explained by the lack of long-range order in
smallcrystallites and by the relaxation of the selection rulesas a
consequence of defective structure. Furthermore,the bands located
at around 720, 840, and 1060 cm-1
can be attributed to the CO2 asymmetric stretching vi-bration,
CO 2 –3 bending vibration, and C–O stretch-ing vibration,
respectively [40]. These bands arise asthe consequence of the
reaction of atmospheric CO2with water and sodium hydroxide during
the synthesisprocess, which leads to the adsorption of
atmosphericCO2 on the cations [21] and formation of
“carbonate-like” species on the particle surfaces [38]. The bands
ataround 1360 and 1520 cm-1 can be attributed to the vi-brations of
carbonate species [38,40]. The attenuationof the bands after heat
treatment indicates that the car-bonate species were thermally
decomposed. The bandlocated at around 1640 cm-1 is attributed to
the H–O–Hbending vibration [21] and indicates the presence of
wa-ter. Both groups’ spectra contain a large band with themaximum
located at around 3400 cm-1, which can beattributed to the O–H
stretching vibration [12]. It con-firms the presence of moisture
and structural water inthe sample before its exposure to heat
treatment. Sincethe band is attenuated in the spectrum collected
afterheat treatment at 1200 °C during 15 min in air, it can
beconcluded that some moisture was absorbed after calci-nation.
The temperature influence on the optical properties ofthe
Ce0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δ, particularly thereflectance
and band gap energy, was examined usingDR UV-VIS spectrometry. Both
samples have showedabsorption maximum (complementary reflectance
min-imum) at around 380 nm, which is attributed to thecharge
transfer of O2p→Ce4f [41]. The reflectancecurves for
Ce0.95Pr0.05O2-δ samples showed slow growthwith increasing
wavelength, reaching reflectance maxi-mum at around 650 nm (red
region). On the other hand,the reflectance curves for the
Ce0.95Er0.05O2-δ samplesshowed a sudden increase in reflectance
with increasingwavelength and plateau was reached at around 450
nm(blue region). In addition, the diffuse reflectance spectraof the
Ce0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δ showed theabsorption
difference between the as-prepared (25 °C)and thermally treated
(600, 900 and 1200 °C) sam-ples (Fig. 8), respectively. For the
Ce0.95Er0.05O2-δ andCe0.95Pr0.05O2-δ samples that were not
thermally treated,intensive reflectance occurred along the full
range ofelectromagnetic spectrum. It is evident from Fig. 8
thatincreasing heat treatment temperature resulted in in-creasing
absorption (decreasing of reflectance plateau),especially for the
samples treated at 1200 °C. The inten-sive absorption peaks at 490,
520, 546, 652 and 677 nmwere found on Ce0.95Er0.05O2-δ reflectance
plateau. Dueto the increasing absorption at these wavelengths
withincreasing Er3+ content in ceria lattice, these peaks
areassigned to erbium ion transitions [12]. Since dopantEr3+ ions
replace Ce4+ ions, they appear as new lightabsorbing species. The
corresponding absorption bandsand transition energies are presented
in Fig. 9. Ac-cording to our findings, the absorption peaks at
490,
Figure 7. FTIR spectra of: a) Ce0.95Er0.05O2-δ and b)
Ce0.95Pr0.05O2-δ powders, thermally treated at different
temperatures
317
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
Figure 8. The DR UV-VIS spectra (a and c) and chromatic diagram
(b and d) of the Ce0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δpowders,
thermally treated at different temperatures in air for 15 min
Figure 9. Scheme of the energy levels and energy transitionsof
Ce0.95Er0.05O2-δ
520, 550 and 660 nm are assigned to following energytransitions
4I15/2→
4F7/2,4I15/2→
2H11/2,4I15/2→
4S3/2 and4I15/2→
4F9/2 [15,41].Beside differences in the reflectance plateau R
[%],
the temperature influence on the visible light absorptionis
evidenced as the shift of the reflectance curves to-
wards larger wavelengths and it had much greater im-pact on the
Ce0.95Pr0.05O2-δ samples. The low energyshift signifies partial
reduction of cerium to its triva-lent state [42,43]. The reduction
process (Ce4+→Ce3+)for the applied experimental conditions occurred
in thepraseodymium doped CeO2 samples rather than with er-bium
ions. This finding indicates that the Ce4+→Ce3+
reduction process has important role in the lattice
sta-bilization. Additionally, the band gap energies (Eg) ofthe
Ce0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δ samples treatedat different
temperatures 25, 600, 900 and 1200 °C werefound (Table 3.). The
band gap energy calculations wereperformed on the basis of the Tauc
plot [44], and theKubelka-Munk function [45]. The Eg values were
de-termined by the extrapolation of the linear part of the[F(R) ×
E]2 versus energy E curves, where F(R) isKubelka-Munk function F(R)
= (1 − R)2/(2R), and Ris reflectance.
Band gap width decreased with increasing heattreatment
temperatures for both Ce0.95Er0.05O2-δ andCe0.95Pr0.05O2-δ samples
(Table 3). The greatest bandgap values are determined for the
Ce0.95Er0.05O2-δ(3.25 eV) and Ce0.95Pr0.05O2-δ (3.05 eV) samples
thatwere not thermally treated. Because of slightly smallerband gap
energy value for the praseodymium than forthe as-prepared erbium
doped sample (25 °C), it can beconcluded that Pr3+/4+ ion
incorporation in CeO2 en-hanced lattice stabilization to a greater
extent, compar-ing to Er3+ ion. The samples treated at 600 °C
exhibited
318
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
Table 3. Calculated band gap energy, Eg [eV], and colour
coordinates (L*a*b*) for the Ce0.95Er0.05O2-δ and
Ce0.95Pr0.05O2-δpowders, thermally treated at different
temperatures, in air for 15 min
SampleBand gap Colour measurementEg [eV] L* a* b*
25 °CCe0.95Er0.05O2-δ [15] 3.25 78.880 -1.560 10.683
Ce0.95Pr0.05O2-δ 3.05 70.351 1.548 22.616600 °C
Ce0.95Er0.05O2-δ 3.22 78.144 -1.268 9.103Ce0.95Pr0.05O2-δ 2.30
45.031 27.288 35.832
900 °CCe0.95Er0.05O2-δ 3.20 75.127 1.572 2.319Ce0.95Pr0.05O2-δ
2.27 43.491 30.892 44.19
1200 °CCe0.95Er0.05O2-δ 3.17 62.663 1.281 2.944Ce0.95Pr0.05O2-δ
2.13 33.633 34.299 49.291
greater band gap reduction (the greatest band gap en-ergy drop)
in comparison with the as-prepared samples.Low energy shift of the
Er3+ doped samples insignif-icantly varies with the increase of
heat treatment tem-peratures. The observed energy drop was
significant forthe Ce0.95Pr0.05O2-δ samples. It is related to
oxygen va-cancies (detected with Raman spectroscopy, Table 2),
aswell as the Ce4+→Ce3+ reduction process. The latter re-sults in
the increase of Ce3+ concentration that can causeenergy shift to a
lower level [43].
Regarding the colour of samples, it is well-knownthat the colour
of absorbed light includes the band gapenergy, but also all colours
of higher energy (shorterwavelength), because electrons can be
excited from thevalence band to a range of energies in the
conductionband. Thus, we can easily calculate absorbed light (λ
<1240.8/Eg) using Eg = hc/λ, where h is Planck’s con-stant and c
is the speed of light in vacuum. Therefore,wide band gaps obtained
for Ce0.95Er0.05O2-δ at differenttemperatures (from 25 to 1200 °C)
correspond to 382,385, 388 and 391 nm, respectively and thus shades
fromyellow toward light pink appeared. Otherwise, bandgaps obtained
for Ce0.95Pr0.05O2-δ at different temper-atures (from 25 to 1200
°C) correspond to 407, 539.5,547 and 583 nm, respectively. The
Ce0.95Pr0.05O2-δ at25 °C has a band gap of 3.05 eV and thus
absorbslight with λ < 407 nm and appears yellow,
whileCe0.95Pr0.05O2-δ at 600 and 900 °C with λ < 539.5 nm,λ <
547 nm appear reddish-orange (the colours of lightreflected)
because it absorbs green, blue and violet light.The
Ce0.95Pr0.05O2-δ at 1200 °C absorbs light with λ <583 nm and
appears a darker shade of reddish-orangebecause it besides green,
blue and violet light absorbsand yellow light. Additionally, it can
be concluded thatabundance of defects related to oxygen vacancies
aswell as the Ce4+→Ce3+ reduction process, resulted inthe narrowed
band gap and enhanced optical absorptionin Ce0.95Pr0.05O2-δ at
elevated temperatures.
Based on certain changes of band gap widthand colour
characteristics of the Ce0.95Er0.05O2-δ andCe0.95Pr0.05O2-δ samples
(Figs. 8a and 8c), the values of
chromatic coordinates for both groups of compounds atroom
temperature (25 °C), and heat treated at 600, 900and 1200 °C for 15
min in air are illustrated in chromaticdiagrams (Figs. 8b and 8d).
The calculated values of theband gap for both Ce0.95Er0.05O2-δ and
Ce0.95Pr0.05O2-δsamples are presented in Table 3. From the
correspond-ing L*a*b* coordinates (Table 3), it is obvious
thatcolour changes are very intensive for the heat
treatedCe0.95Pr0.05O2-δ sample. On the other hand, changesof the
colour characteristics of the Ce0.95Er0.05O2-δ aresomewhat lower in
intensity (Table 3). At the same time,increasing contribution of
the coordinates a* and b* wasin compliance with the progressive
decrease of lumi-nosity (L*), especially for the Ce0.95Pr0.05O2-δ
sample(from 70.351 to 33.633). Therefore, the colour shiftedwith
increasing temperature from yellow toward lightpink hue (increasing
a*) and became more saturated(decreasing L*) for the
Ce0.95Er0.05O2-δ sample. For theCe0.95Pr0.05O2-δ sample the colour
shifted from yellowto dark-red hue. Successful incorporation of
Er3+ as wellas Pr3+/4+ ions in the lattice of CeO2 is thus
confirmed.The most pink and red hues (the highest values of
co-ordinates a*) are achieved for the pigments thermallytreated at
the highest temperature (1200 °C). Thus, ac-cording to Figs. 8b and
8d, and their corresponding in-sets (visual appearance), the colour
and intensity of thepigments was dependent on the composition
(dopant iontype) and heat treatment temperature, and the colour
ofthe pigments could vary from white-pink to dark-redhue.
IV. Conclusions
Nanostructured non-toxic Ce0.95Er0.05O2-δ andCe0.95Pr0.05O2-δ
oxides, as pink and red colour com-pounds, were successfully
synthesized by low costself-propagating room temperature method.
All ceriumoxide powders before (at 25 °C) and after heat treat-ment
(treated at 600, 900 and 1200 °C during 15 minin air) had crystal
structure of the fluorite type withFm3m space group, which
confirmed the structural
319
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
and thermal stability of the obtained solid solutions.Particle
sizes remained within the nanometric rangealong with temperature
increase from 25–1200 °Cfor all obtained pigments. The absence of
oxygenvacancies in the structure of the obtained pigments,examined
by Raman spectroscopy, may indicate theirfuture potential
applications. Furthermore, the colourchanges depended on the
composition (dopant iontype) and the heat treatment temperature.
Increasedtemperature resulted in colour variation from yellow
tolight-pink hue for Ce0.95Er0.05O2-δ, and from yellow todark-red
hue for Ce0.95Pr0.05O2-δ. Another consequenceof the increase of
heat treatment temperature was theincrease of crystallite size and
light absorption, wherethe edge of absorption of visible light
shifted towardlower energies.
The structure, morphology and optical properties ofnanometric
Ce0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δ synthe-sized in this work,
suggest that these compounds maybe interesting to be used as
pigments for a wide areaof applications such as in cosmetics,
ceramics, plastics,paints, coatings and glass enamels. Thus,
further inves-tigation related to the testing stability of this
nanomet-ric Ce0.95Er0.05O2-δ and Ce0.95Pr0.05O2-δ compounds inreal
samples, such as ceramics or glass, should be done.Anyhow, the
results obtained in this work will pave theway for the potential
alternative to the toxic pink andred pigments widely used.
Acknowledgement: This work was supported by theSerbian Ministry
of Education, Science, and Techno-logical Development through the
projects III 45012, III45001, III 45007 and 172015.
References
1. R.P. Prabhakar, M.L.P. Reddy, “Synthesis and
character-isation of (BiRE)2O3 (RE: Y, Ce) pigments”, Dyes
Pig-ments, 63 (2004) 169–174.
2. M. Jansen, H.P. Letschert, “Inorganic yellow-red
pigmentswithout toxic metals”, Nature, 404 (2000) 980–982.
3. R. Olazcuaga, G.L. Polles, A.E. Kira, G.L. Flem, P. Mae-stro,
“Optical properties of Ce1-xPrxO2 powders and theirapplications to
the coloring of ceramics”, J. Solid StateChem., 71 (1987)
570–573.
4. N. Maso, H. Beltran, R. Munoz, B. Julian, J.B.Carda, P.
Escribano, E. Cordoncillo, “Optimization ofpraseodymium-doped
cerium pigment synthesis tempera-ture”, J. Am. Ceram. Soc., 86
(2003) 425–430.
5. D.R. Swiler, Inorganic Pigments, in: Kirk-Othmer
Ency-clopedia of Chem. Technol., 5th ed., Wiley and Sons, Inc.New
York, 2006.
6. R.P. Prabhakar, M.L.P. Reddy, “(TiO2)1(CeO2)1-x(RE2O3)x –
novel environmental secure pigments”, DyesPigments, 73 (2007)
292–297.
7. S. Furukawa, T. Masui, N. Imanaka, “New environment-friendly
yellow pigments based on CeO2-ZrO2 solid solu-tions”, J. Alloys
Compd., 451 (2008) 640–643.
8. N. Khichar, S. Bishnoi, S. Chawla, “Introducing dual
ex-citation and tunable dual emission in ZnO through selec-tive
lanthanide (Er3+/Ho3+) doping”, RSC Adv., 4 (2014)
18811–18817.9. H. Li, Y. Jia, W. Sun, R. Zhao, J. Fu, J. Jiang,
S. Zhang,
R. Pang, C. Li, “Novel energy transfer mechanism insingle-phased
color-tunable Sr2CeO4:Eu
3+ phosphors forWLEDs”, Optical Mater., 36 (2014) 1883–1889.
10. E.M. Lemdek, K. Benkhouja, S. Touhtouh, K. Sbiaai,
A.Arbaoui, M. Bakasse, A. Hajjaji, Y. Boughaleb, R. Saez-Puche,
“Influence of Ca2+ doped on structural and opti-cal properties of
RPO4 (R = Ce
3+, Nd3+ and Pr3+) com-pounds”, Optical Mater., 36 (2013)
86–90.
11. G. Gheno, M. Bortoluzzi, R. Ganzerla, F. Enrichi,
“In-organic pigments doped with tris(pyrazol-1-yl)borate
lan-thanide complexes: A photoluminescence study”, J. Lumi-nesc.,
145 (2014) 963–969.
12. M. Stojmenović, M.C. Milenković, P.T. Banković, M.
Žu-nić, J.J. Gulicovski, J.R., Pantić, S.B. Bošković,
“Influ-ence of temperature and dopant concentration on struc-tural,
morphological and optical properties of nanometricCe1-xErxO2-δ (x =
0.05–0.20) as a pigment”, Dyes Pig-ments, 123 (2015) 116–124.
13. S. Tnunekawa, J.T. Wang, Y. Kawazoe, A. Kasuya,“Blueshifts
in the ultraviolet absorption spectra of ceriumoxide
nanocrystallites”, J. Appl. Phys., 94 (2003) 3654–3656.
14. S.T. Aruna, S. Ghosk, K.C. Patil, “Combustion synthesisand
properties of Ce1-xPrxO2-δ red ceramic pigments”, Int.J. Inorg.
Mater., 3 (2001) 387–392.
15. S.F. Santos, M.C. De Andrade, J.A. Sampaio, A.B. Luz,
T.Ogasawara, “Synthesis of ceria-praseodymia pigments bycitrate-gel
method for dental restorations”, Dyes Pigments,75 (2007)
574–579.
16. R.A. Eppler, “Solid state reactions in the preparation of
zir-con stains”, pp. 1021–1045 in Physics of Electronic Mate-rials,
Part B, Eds. L.L. Hench, D.B. Dove, Marcel Dekker,New York,
1972.
17. T. Masui, S. Furukawa, N. Imanaka, “Synthesis
andcharacterization of CeO2-ZrO2-Bi2O3 solid solutions
forenvironment-friendly yellow pigments”, Chem. Lett., 35(2006)
1032–1033.
18. B. Matovic, S. Boskovic, M. Logar, M. Radovic,
Z.Dohcevic-Mitrovic, Z.V. Popovic, F. Aldinger, “Synthesisand
characterization of the nanometric Pr-doped ceria”, J.Alloys
Compd., 505 (2010) 235–238.
19. A A.I.Y. Tok, F.Y.C. Boey, Z. Dong, X.L. Sun, “Hydrother-mal
synthesis of CeO2 nanoparticles”, J. Mater. Process.Technol., 190
(2007) 217–222.
20. M. Lipińska-Chwałek, F. Schulze-Küppers, J.
Malzbender,“Mechanical properties of pure and doped cerium
oxide”,J. Eur. Ceram. Soc., 35 (2015) 1539–1547.
21. S. Phoka, P. Laokul, E. Swatsitang, V. Promarack,
“Syn-thesis, structural and optical properties of CeO2
nanoparti-cles synthesized by a simple polyvinyl pyrrolidone
(PVP)solution route”, Mater. Chem. Phys., 115 (2009) 423–428.
22. M.J. Godinho, R.F. Goncalvez, L.P.S. Santos, J.A. Varela,E.
Longo, E.R. Leite, “Room temperature co-precipitationof
nanocrystalline CeO2 and Ce0.8Gd0.2O1.9-δ powder”,Mater. Lett., 61
(2007) 1904–1907.
23. M. Stojmenović, S. Bošković, S. Zec, B. Babić, B.
Ma-tović, D. Bučevac, Z. Dohčević-Mitrović, F.
Aldinger,“Characterization of nanometric multidoped ceria
pow-ders”, J. Alloy. Compd., 507 (2010) 279–285.
24. N. Paunović, Z. Dohčević-Mitrović, R. Scurtu, S.
Škrabić,M. Prekajski, B. Matović, Z.V. Popović, “Suppression
ofinherent ferromagnetism in Pr-doped CeO2 nanocrystals”,
320
-
D. Mićović et al. / Processing and Application of Ceramics 13
[3] (2019) 310–321
Nanoscale, 4 (2012) 5469–5476.25. M. Stojmenović, M.C.
Pagnacco, V. Dodevski, J. Guli-
covski, M. Žunić, S. Bošković, “Studies on struc-tural and
morphological properties of multidoped
ceriaCe0.8Nd0.0025Sm0.0025Gd0.005Dy0.095Y0.095O2-δ (x = 0.2)
assolid solutions”, J. Spectroscopy, 2016 (2016) 5184542.
26. M. Stojmenović, S. Bošković, M. Žunić, B. Babić,B.
Matović, D. Bajuk-Bogdanović, S. Mentus, “Studieson structural,
morphological and electrical properties ofCe1-xErxO2-δ (x =
0.05–0.20) as solid electrolyte for IT-SOFC”, Mater. Chem. Phys.,
153 (2015) 422–431.
27. M. Stojmenović, M. Žunić, J. Gulicovski, V. Dodevski,M.
Prekajski, A. Radulović, S. Mentus, “Structural, mor-phological
and electrical properties of Ce1-xRuxO2-δ (x= 0.005–0.02) solid
solutions”, Ceram. Int., 42 (2016)14011–14020.
28. W. Kaewwiset, K. Thamaphat, J. Kaewkhao, P.
Limsuwan,“Er3+-doped soda-lime silicate glass: artificial pink
gem-stone”, Am. J. App. Sci., 9 (2012) 1769–1775.
29. A.R. Jha, Rare Earth Materials: Properties and
Applica-tions, CRC Press Taylor & Francis Group, 2014.
30. S.J. Hong, A.V. Virkar, “Lattice parameters and densitiesof
rare earth oxide doped ceria electrolytes”, J. Am. Ceram.Soc., 78
(1995) 433–439.
31. W. Hall, G.K. Williamson, “X-ray line broadening fromfiled
Al and W”, Acta Metal., 1 (1953) 22–31.
32. R.D. Shannon, “Revised effective ionic radii and system-atic
studies of interatomic distances in halides and chalco-genides”,
Acta Crystallogr., A32 (1976) 751–767.
33. L. Wu, H.J. Wiesmann, A.R. Moodenbaugh, R.F. Klie, Y.Zhu,
D.O. Welch, M. Suenaga, “Oxidation state and latticeexpansion of
CeO2-x nanoparticles as a function of particlesize”, Phys. Rev. B,
69 (2004) 125415–125416.
34. S. Tsunekawa, S. Ito, Y. Kawazoe, “Surface structures
ofcerium oxide nanocrystalline particles from the size de-pendence
of the lattice parameters”, Appl. Phys. Lett., 85(2004)
3845–3847.
35. S. Lütkehoff, M. Neumann, A. Ślebarski, “3d and 4d
x-ray-photoelectron spectra of Pr under gradual oxidation”,Phys.
Rev. B: Condens. Matter, 52 (1995) 13808–13811.
36. M.Y. Sinev, G.W. Graham, L.P. Haack, M. Shelef, “Kineticand
structural studies of oxygen availability of the mixedoxides
Pr1-xMxOy (M = Ce, Zr)”, J. Mater. Res., 11 (1996)1960–1971.
37. S. Aškrabić, Z. Dohčević-Mitrović, A. Kremenović,N.
Lazarević, V. Kahlenberg, Z.V. Popović, “Oxygenvacancy-induced
microstructural changes of annealedCeO2-x nanocrystals”, J. Raman
Spectrosc., 43 (2014) 76–81.
38. D. Andreescu, E. Matijevic, D.V. Goia, “Formation ofuniform
colloidal ceria in polyol”, Colloids. Surf. A, 291(2006)
93–100.
39. J. Liu, Z. Zhao, J. Wang, C. Xu, A. Duan, G. Jiang, Q.Yang,
“The highly active catalysts of nanometric CeO2-supported cobalt
oxides for soot combustion”, Appl. Catal.B, 84 (2008) 185–195.
40. S. Wang, F. Gu, C. Li, H. Cao, “Shape-controlled syn-thesis
of CeOHCO3 and CeO2 microstructures”, J. Cryst.Growth, 307 (2007)
386–394.
41. Y. Xijuan, X. Pingbo, S. Qingde, “Size-dependent
opticalproperties of nanocrystalline CeO2:Er obtained by
com-bustion synthesis”, Phys. Chem. Chem. Phys., 3
(2001)5266–5269.
42. A. Sharma, M. Varshney, J. Park, T.K. Ha, K.H. Chae,
H.J.Shin, “Bifunctional Ce1-xErxO2 (0 ≤ x ≤ 0.3) nanoparticlesfor
photoluminescence and photocatalyst applications: AnX-ray
absorption spectroscopy study”, Phys. Chem. Chem.Phys., 17 (2015)
30065–30075.
43. S.A. Ansari, M.M. Khan, M.O. Ansari, S. Kalathil, J.
Lee,M.H. Cho, “Band gap engineering of CeO2 nanostructureusing an
electrochemically active bio film for visible lightapplications”,
RSC Adv., 32 (2014) 16782–16791.
44. J. Tauc, R. Grigorovici, A. Vancu, “Optical properties
andelectronic structure of amorphous germanium”, Phys. Sta-tus
Solidi, 15 (1966) 627–637.
45. L.S. Cavalcante, V.M. Longo, J.C. Sczancoski, M.A.P.Almeida,
A.A. Batista, J.A. Varela, M.O. Orlandi, E.Longo, M.S. Li,
“Electronic structure, growth mechanismand photoluminescence of
CaWO4 crystals”, Cryst. Eng.Comm., 14 (2012) 853–868.
321
IntroductionExperimental sectionMaterials and
methodsCharacterization
Results and discussionConclusions