The inﬂuence of short thermal treatment on structure ... 45 12.pdf · the titration analysis of the powder on Na using EDTA as the titrant. After drying at 100°C for 24h, the pow-ders

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


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


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


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


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


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


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


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


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


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


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

Documents

The inﬂuence of short thermal treatment on structure ... 45 12.pdf · the titration analysis of the powder on Na using EDTA as the titrant. After drying at 100°C for 24h, the pow-ders