microwave-assisted hydrothermal synthesis of cepo4 nanostructures: correlation between the...

Journal of Alloys and Compounds xxx (2015) xxx–xxx

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Journal of Alloys and Compounds

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Microwave-assisted hydrothermal synthesis of CePO4 nanostructures:Correlation between the structural and optical properties

http://dx.doi.org/10.1016/j.jallcom.2014.12.0530925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (M.A. Domínguez-Crespo).

Please cite this article in press as: D. Palma-Ramírez et al., J. Alloys Comp. (2015), http://dx.doi.org/10.1016/j.jallcom.2014.12.053

D. Palma-Ramírez a, M.A. Domínguez-Crespo a,⇑, A.M. Torres-Huerta a, H. Dorantes-Rosales b,E. Ramírez-Meneses c, E. Rodríguez a

a Instituto Politécnico Nacional, CICATA-Unidad Altamira, Km 14.5, Carretera Tampico-Puerto Industrial Altamira, C.P. 89600 Altamira, Tamps, Mexicob Instituto Politécnico Nacional, ESIQIE, Departamento de Metalurgia, C.P. 07300 México D.F., Mexicoc Universidad Iberoamericana, Departamento de Ingeniería y Ciencias Químicas, Prolongación Paseo de la Reforma 880, Lomas de Santa Fe, C.P. 01219 México D.F., Mexico

a r t i c l e i n f o a b s t r a c t

Article history:Available online xxxx

Keywords:CePO4

NanostructuresMicrowave-assisted hydrothermal method

In this work, the microwave-assisted hydrothermal method is proposed as an alternative to the synthesisof cerium phosphate (CePO4) nanostructures to evaluate the influence of different synthesis parameterson both the structural and optical properties. In order to reach this goal, two different sets of experimentswere designed, varying the reaction temperature (130 and 180 �C), synthesis time (15 and 30 min) andsintering temperature (400 and 600 �C), maintaining a constant pH = 3. Thereafter, two experimentalconditions were selected to assess changes in the properties of CePO4 nanopowders with pH (1, 5, 9and 11). The crystal structure and morphology of the nanostructures were characterized by X-ray diffrac-tion (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respec-tively. Diffuse reflectance properties of CePO4 with different microstructures were studied. The resultsdemonstrated that by using the microwave-assisted hydrothermal method, the shape, size and structuralphase of CePO4 can be modulated by using relatively low synthesis temperatures and short reactiontimes, and depending on pH, a sintering process is not needed to obtain either a desired phase or size.Under the selected experimental conditions, the materials underwent an evolution from nanorods tosemispherical nanoparticles, accompanied by a phase transition from hexagonal to monoclinic.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, CePO4 nanostructures have become increasinglyimportant in a variety of applications such as fluorescence, ionexchange, catalytic materials and ceramic composite materialswith high mechanical properties [1] as a result of their low dimen-sionality and the quantum confinement effect. The technologicalapplication of these nanostructured materials is strongly depen-dent on their morphology, crystalline phase and particle size.CePO4 presents two phases: monoclinic and hexagonal [2,3]. Fewstudies on the production of CePO4 nanostructured materials havebeen found in the literature. The hexagonal phase can be easilyobtained at low temperatures, while the monoclinic phase couldbe prepared via the solid state reaction and hydrothermal methodat high temperatures [2]. Among these methods, the hydrothermalsynthesis seems to be potentially useful to obtain CePO4 nano-structures; the preparation of inorganic nano or micromaterials

by this technique features advantages such as the use of simpleequipment, low cost, high-uniform area production, low processtemperatures, catalyst-free growth, environmental friendlinessand very-easy-to-control particle sizes; however, it usuallyrequires long reaction times (about 24 h) to reach the nanoscale[3,4]. As an enhancement of the typical hydrothermal method,the microwave-assisted route has the advantage of producingnanostructures with shorter synthesis times as a consequence ofthe efficient and fast heating during the reaction process [5]. Par-ticularly for CePO4, Ekthammathat et al. [6] have reported thatmonoclinic nanorods can be synthesized from Ce(NO3)3�6H2Oand Na3PO4�12H2O at pH 1 by a simple microwave radiation for60 min, while Patra and coworkers [7] obtained rhabdophane-typehexagonal nanorods by the microwave-assisted solvothermalsynthesis, using a domestic microwave oven.

To the best of our knowledge, there have been few publishedstudies related to the synthesis of CePO4 nanostructures usingmicrowave energy. The aim of this work is to determine the effectsof the synthesis parameters on the structural, morphologicaland optical properties of CePO4 nanostructures produced by the

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Table 1Experimental design used to obtain CePO4 nanoparticles using a pH �3.

Experimentat pH �3

Reactiontemperature(Tr, �C)

Synthesistime(t, min)

Sinteringtemperature(Ts, �C)

1 130 15 4002 180 15 4003 130 30 4004 180 30 4005 130 15 6006 180 15 6007 130 30 6008 180 30 600

2 D. Palma-Ramírez et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx

microwave-assisted hydrothermal method. For this purpose, vari-ables such as reaction time, synthesis and sintering temperatureas well as pH of the media have been evaluated.

2. Experimental details

2.1. Preparation of CePO4 nanostructures

Tripolyphosphoric acid solution (0.035 mol�1, H5P3O10) was obtained by using acation exchange resin (Dowex 50W X4 100-200 mesh) for conversion of sodium tri-polyphosphate, Na5P3O10�5H2O (purum p.a., P98.0% (T), Sigma–Aldrich).

CePO4 nanostructures were synthesized by mixing Ce(NO3)3�6H2O (0.10 mol)(Sigma–Aldrich, 99% trace metals basis) and H5P3O10 (0.10 mol). The microwavereaction was performed in a microwave oven (CEM-MARS, frequency 2.45 GHz,power of 200 W). The synthesis process was as follows: 50 ml of Ce(NO3)3�6H2O

20 30 40 50 60 70

(a)

Tr130°Ct15min

Tr130°Ct30min

Tr180°Ct15min

Tr180°Ct30min

2θ (degrees)

Rhabdophane (hexagonal)

(321

)

(303

)

(104

)

(310

)

(212

)(3

01)

(112

)

(110

)

(101

)

(200

)(1

02)

(211

)(0

03)

(302

)(2

20)

Tr130t30Ts600pH11

Tr130 t30Ts0pH11

Tr130t30Ts0pH9

Tr130t30Ts600pH5

Tr130t30Ts0pH5

Tr130t30Ts600pH1(120

)(0

12)

Monoclinic

(142

)(132

)

(031

)

(402

)

(033

)

(121

)

(103

)(3

21)

(231

)

(311

)

Tr130t30Ts600pH9

(112

)

Tr130t30Ts0pH1

(111

)

(212

)(2

02)

(022

)(2

10)

(212

)

(c)

(022

)(2

10)

(114

)

(310

)(3

02)

(003

)(211

)

(200

)(2

00)

(110

)

(101

)

Inte

nsity

(a.u

.)

(112

)(012

)

(301

)

(113

)(2

20)

(222

)(1

04)

(311

)(2

13)

(041

)(3

12)

(321

)(4

02)

Hexagonal

20 30 40 50 60 702θ (degrees)

Inte

nsity

(a.u

.)

Fig. 1. X-ray diffraction patterns of (a) non-sintered CePO4 powders obtained under the(400 and 600 �C), (c) samples synthesized under experiment 7 conditions at different sdifferent solution pH values.

Please cite this article in press as: D. Palma-Ramírez et al., J. Alloys Comp. (20

were added dropwise to 25 ml of H5P3O10 under stirring; then, deionized waterwas added to adjust a final volume of 100 ml at pH �3. Thereafter, solutions weretransferred into a Teflon container (autoclave). The autoclave was sealed and heatedfor each experiment by varying both the synthesis temperature (130 and 180 �C)and time (15 and 30 min). Subsequently, the precipitate was separated by filtrationand dried for 24 h at 90 �C and sintered at two different temperatures (see Table 1).

From these initial experiments, the synthesis conditions of the samples labeledas 7 and 8 were chosen to evaluate changes in the morphology and particle size ofCePO4 nanopowders at different pH values. These effects were analyzed at pH = 1, 5,9, and 11. HNO3 (37%) and NH4OH (30%) were used to adjust the pH values either inalkaline or acid medium.

2.2. Characterization of nanopowders

The structure of the CePO4 nanoparticles was determined by X-ray powder dif-fraction (XRD) with a Bruker D8 Advance diffractometer equipped with a Lynxeyedetector and Cu Ka radiation (k = 1.5405 Å) at 35 kV and 25 mA. Data were collectedat room temperature in the 2h range of 15–70�, step size of 0.016 and step time of0.5 s. To evaluate the effects of the reaction parameters and sintering temperatureon the structural and vibrational properties of CePO4, Fourier transform infraredspectroscopy (FTIR) was carried out. The spectra were recorded on a Perkin Elmerspectrometer using KBr pellets, 4 cm�1 of resolution setting and range of 1300–450 cm�1. The samples were scanned 40 times. Scanning electron microscopywas used to evaluate morphological changes in the CePO4 nanopowders using aJEOL JSM-6300 apparatus (20 kV), whereas transmission electron microscopy wasused to corroborate the structure and phase composition of the samples by usinga JEOL-2000 FX-II working at an accelerating voltage of 100 kV. The analysis ofthe average particle size and size distribution were determined by dynamic lightscattering, using deionized water as dispersant medium in a Malver Zetasizer NanoZSP, model ZEN5600. The particle size of the dispersions was described by thecumulants mean diameter, and the size distribution was described by the polydis-persity index and the size distribution plot. The optical properties of the sampleswere followed by ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DR)using a 110-mm-diameter-integrating-sphere accessory mounted on a Cary 5000

(b) Hexagonal

Tr130t15Ts600

Tr180t15Ts600

Tr130t15Ts400

Tr180t15Ts400

Tr130t30Ts400

Tr180t30Ts400

Tr130t30Ts600

(211

)

(012

)

(200

)(0

20)

(111

)(0

11)

(103

)

(311

)(0

31)

(212

)(2

02)

(212

)

(241

)(120

)

(004

)

(140

)(1

32)

(023

)

(142

)

Tr180t30Ts600Monoclinic

(321

)

(104

)(3

10)

(302

)(2

00)

(212

)(1

03)

(112

)

(101

)

Inte

nsity

(a.u

.)

HexagonalMonoclinic

Tr180t30Ts600pH5

Tr180t30Ts600pH11

(200

)

Tr180t30Ts600pH9

Tr180t30Ts0pH11

(012

)

(402

)

(311

)

(121

)

Tr180t30Ts0pH9

(214

)

(033

)

Tr180t30Ts0pH1

Tr180t30Ts600pH1

(041

) (233

)

(040

)(0

23)(320

)(2

12)

(221

)(0

31)

(212

)(2

02)

Tr180t30Ts0pH5

(101

)

(101

)

(d)

(011

)

(211

)(1

20)

(020

)

Inte

nsity

(a.u

.)

20 30 40 50 60 702θ (degrees)

20 30 40 50 60 702θ (degrees)

stated conditions, (b) CePO4 nanoparticles sintered at two different temperaturesolution pH values and (d) samples synthesized under experiment 8 conditions at

15), http://dx.doi.org/10.1016/j.jallcom.2014.12.053


Table 2Crystal size of CePO4 nanostructures estimated from Scherrer equation.

Experiment Crystal size of powdersobtained at �pH 3

Crystal size of powdersobtained at pH 1, 5, 9 and 11

Ts0 (nm) Ts400 or 600�C (nm) Ts0 (nm) Ts400 or 600�C (nm)

1 16 15 – –2 14 15 – –3 17 15 – –4 16 17 – –5 16 19 – –6 14 21 – –7 17 20 52 (pH 1) 15 (pH 1)

11 (pH 5) 11 (pH 5)15 (pH 9) 14 (pH 9)16 (pH 11) 17 (pH 11)

8 16 20 29 (pH 1) 18 (pH 1)8 (pH 5) 12 (pH 5)13 (pH 9) 14 (pH 9)10 (pH 11) 10 (pH 11)

Table 4Particle z-average diameter (Dz), polydispersity index (PDI), the highest mean number(%) and their sizes of CePO4 nanopowders.

Condition Dz (nm) PDI The highestmean number (%)

Size (nm)

1 529.2 1.16 26 3962 541.3 1.1 31 4593 445.3 1.12 28 3424 533.4 1.13 24 4595 315 1.1 31 2556 396 1.11 27 3427 pH 1 (335.4) pH 1 (1.82) 14 (pH 1) 122 (pH 1)

pH 3 (497.7) pH 3 (1.16) 24 (pH 3) 396 (pH 3)pH 5 (270.8) pH 5 (1.39) 18 (pH 5) 141 (pH 5)pH 9 (228.1) pH 9 (1.31) 21 (pH 9) 122 (pH 9)pH 11 (214.4) pH 11 (1.48) 21 (pH 11) 91 (pH 11)

8 pH 1 (370.7) pH 1 (1.35) 19 (pH 1) 190 (pH 1)pH 3 (513.9) pH 3 (1.14) 24 (pH 3) 255 (pH 3)pH 5 (295.7) pH 5 (1.19) 27 (pH 5) 190 (pH 5)pH 9 (266) pH 9 (1.06) 56 (pH 9) 14 (pH 9)pH 11 (201.3) pH 11 (1.27) 22 (pH 11) 122 (pH 11)

D. Palma-Ramírez et al. / Journal of Alloys and Compounds xxx (2015) xxx–xxx 3

spectrophotometer. Samples were scanned with an average time of 0.1 s, scan rateof 600 nm/min and data intervals of 1 nm within the 700–200 nm range. The back-ground reflectance of polytetrafluoroethylene (PTFE) was measured before. TheKubelka Munk function F(R1) was applied to convert the diffuse reflectance spectrainto the equivalent absorption spectra and determine the optical band gap from theslope of the linear part in the (F(R1)hv)2 vs. hv plot (photon energy).

3. Results and discussion

3.1. Structure and morphology of the samples

Fig. 1a and b shows the results obtained from the X-ray diffrac-tion analysis of CePO4 nanoparticles before and after being sub-jected to sintering in the first experiment series. As it can beseen in Fig. 1a, the peaks of the CePO4 powders match with thehexagonal phase, known as rhabdophane (PDF card # 04-012-5051). The main differences between the synthesis temperaturesin the microwave process are observed in the (101) reflection,which disappears at 180 �C. A strong relationship between thestructure type and temperature has been reported in the literaturefor CePO4 materials [4,8,9], where the rhabdophane structurebegins to be transformed into the monoclinic phase by heatingthe nanopowders at temperatures above 400 �C. Regularly, ananhydrous form of the hexagonal phase is obtained between 100and 400 �C; however, depending on the cation used during the syn-thesis, a transition of monoclinic monazite can be completelyreached at 650 �C [10]. In our case, the hexagonal phase tends to

Table 3Crystal size of CePO4 and treatment of linear plots to obtain the size o

Condition As-prepared elnKk=L ¼ 0:94x0:154051L

L

1 at �pH 3 e�4:4404 ¼ 0.0117 12 at �pH 3 e�4:5628 ¼ 0.0104 13 at �pH 3 e�4:6342 ¼ 0.0097 14 at �pH 3 e�4:58 ¼ 0.0102 15 at �pH 3 e�4:4404 ¼ 0.0117 16 at �pH 3 e�4:5628 ¼ 0.0104 17 at �pH 3 e�4:6342 ¼ 0.0097 18 at �pH 3 e�4:58 ¼ 0.0102 17 (pH 1) e�5:955 ¼ 0.0025 57 (pH 5) e�4:5992 ¼ 0.0010 17 (pH 9) e�4:8082 ¼ 0.0081 17 (pH 11) e�4:8469 ¼ 0.0078 18 (pH 1) e�5:70206 ¼ 0.0033 48 (pH 5) e�4:5758 ¼ 0.0102 18 (pH 9) e�4:70319 ¼ 0.0090 18 (pH 11) e�4:51318 ¼ 0.0109 1


grow with preferential direction at 180 �C without the monaziteformation.

To evaluate possible effects of the synthesis temperature of theCePO4 nanostructures on the hexagonal–monoclinic phase trans-formation, the samples were sintered at two different tempera-tures (400 and 600 �C) and the results are presented in Fig. 1b.The XRD patterns indicate that at 400 �C, both structural phases(hexagonal and monoclinic) coexist, while at 600 �C, the peaksagree with the monoclinic structure of the CePO4 powders (PDFcard # 04-007-2786). The (211) and (�311) planes, correspondingto the hexagonal and monoclinic structures, respectively, over-lapped each other at 400 �C, but a combination of the synthesisand sintering temperatures split the peak ca. 42�, forming a new(031) plane. Due to the fact that at 400 �C only the hexagonalphase is reported, the existence of both phases can be stronglyinfluenced by the microwave supplied energy. Furthermore, thephase transformation between the rhabdophane-type structure(hexagonal system) and the monazite-type structure (monoclinicsystem) has been observed from about 650 �C [11], but in this case,the monoclinic structure can be completely achieved at low sinter-ing temperatures, highlighting the effect of the microwave energy.

The interesting point here is that by using microwave energy,there is a great influence on the microstructure and as a conse-quence, nanostructures with specific properties can be evaluated.To evaluate the dependence of the nanopowder structure on pH,a second set of experiments was designed by adjusting the pH

f crystallites by Scherrer modified formula.

(nm) Sintered elnKk=L ¼ 0:94x0:154051L

L (nm)

2 e�4:581 ¼ 0.0102 144 e�4:5222 ¼ 0.0108 135 e�4:63076 ¼ 0.0097 154 e�4:6 ¼ 0.0100 142 e�4:825 ¼ 0.0080 184 e�4:938 ¼ 0.0071 205 e�4:851 ¼ 0.0078 194 e�4:824 ¼ 0.0080 186 e�4:22391 ¼ 0.00146 104 e�2:9174 ¼ 0.0540 38 e�4:7569 ¼ 0.0085 178 e�4:9299 ¼ 0.0072 203 e�4:6425 ¼ 0.0096 154 e�2:9742 ¼ 0.0510 36 e�4:9476 ¼ 0.0071 203 e�4:5768 ¼ 0.0102 14



20 nm

50 nm

20 nm

100nm

20 nm

100nm

50 nm

100nm

(a)

(b)

(c)

(d)

(1) m (011)

(2) m (020)

(3) m ( )

(4) m ( )

(5) m (013)

(6) m ( )

(7) m ( )

(1) m (201)

(2) m (002)

(3) m ( )

(4) m (131)

(5) m (301)

(6) m (310)

(7) m ( )

(1) h (100)

(2) m (101)

(3) m (020)

(4) m (002)

(5) h (012)

(6) m ( )

(1) h (100)

(2) h (101)

(3) m ( )

(4) m ( )

(5) h (112)

(6) h ( )

(7) m (310)

Fig. 2. Selected TEM micrographs and SAED patterns of CePO4 nanopowders synthesized under different experimental conditions: (a) Tr130t15Ts400, (b) Tr130t30Ts400, (c)Tr180t15Ts600 and (d) Tr180t30Ts600. In the figure, m and h represent the monoclinic and hexagonal phases, respectively.


solution at 1, 5, 9, and 11, using the synthesis conditions of samples7 and 8. Additionally, to highlight the structural modification withthe sintering temperature, XRD patterns of these samples werescanned at room temperature and were labeled as Ts0.

Fig. 1c and d shows the XRD patterns of CePO4 synthesized atsolution pH values of 1, 5, 9 and 11 (Tr = 130 �C). The XRD resultsshow some structural changes with pH even before starting thesintering process. For example, for the samples synthesized underthe conditions of experiment seven (Fig. 1c), it is seen that by usinga solution with pH = 1, the hexagonal phase is favored, whereas atransformation of this phase into the monoclinic one took placeat pH = 5, where both structural phases (hexagonal and mono-clinic) coexist; such transformation seems to be reached in alkalinemedium (pH = 9 and 11). It is important to mention that there is adiscrepancy with a previous work using similar experimental con-ditions, where the monoclinic structure was observed at pH 1,


whereas the hexagonal phase coexists by adjusting the solutionin the pH range of 2–5 [6]. The mismatch in the results is mainlycorrelated with the type of precursor used in the synthesis(Na3PO4�12H2O or H5P3O10) and its interaction with Ce(NO3)3

during the synthesis. Even, the authors obtained similar resultswith other rare earth elements to obtain LaPO4 nanorods [12]. Thisobservation pointed out the influence of the acid or alkalinecharacter of the raw materials to obtain a desired structure. Asexpected, after the thermal treatment at 600 �C, the samplescrystallized into the monazite form.

On the other hand, by increasing the synthesis temperature upto Tr = 180 �C for 30 min, the monoclinic structure is favored, evenafter the treatment at 600 �C (Fig. 1d). In this case, the XRD pat-terns evaluated before the sintering process displayed less reflec-tions of the hexagonal structure at pH = 1, indicating that at thisreaction temperature (180 �C), the transformation from hexagonal



Experiment 7 (Tr130,t30,Ts600)

Experiment 8 (Tr180,t30,Ts600)

100 nm

(a)

pH= 1 100 nm

(b)

pH= 5100 nm

(c)

pH= 9100 nm

(d)

pH= 11

100 nm

(e)

pH= 1100 nm

(f)

pH= 5100 nm

(g)

pH= 9

(h)

pH= 11

Fig. 3. SEM micrographs of CePO4 nanopowders obtained under the conditions of experiments: (a–d) 7 and (e–h) 8.

Table 5Band gap of CePO4 nanopowders calculated from the Kubelka–Munk modifiedspectra.

Experiment (�pH 3) Band gap (eV)

1 3.0 –2 3.07 –3 3.0 –4 3.07 –5 3.06 –6 3.1 –7 3.02 pH 1 3.22

pH 5 3.04pH 9 3.25pH 11 3.25

8 3.07 pH 1 3.15pH 5 3.25pH 9 3.08pH 11 3.23


to monoclinic has begun. Evidently, a pure monazite structurewithout impurities of the rhabdophane phase was obtained aftersintering the samples at 600 �C. The monazite structure is clearlyobtained at pH values of 5, 9 or 11 without the annealing processand the polycrystallinity of the phase is enhanced at 600 �C.

A quantitative estimation of the domain size for CePO4 nano-powders was evaluated and shown in Table 2 from Scherer equa-tion, using the most intense peak for each XRD pattern:

L ¼ Kkb cos h

ð1Þ

where L = nanocrystallite size, k = wavelength of X-ray radiation (CuKa, k = 0.15405 nm), b = FWHM, full width at half maximum ofpeaks (radians), h = Bragg angle, K = is the shape factor, which canvary from 0.62 to 2.08, where 0.89 is usually taken for sphericalcrystals [13]. Thus, K depends on at least three things: ‘breadth’ def-inition, crystallite shape and crystallite-size distribution [14]. In theanalyses, the factor value (K) was considered for a spherical shape(0.89). Additionally, Scherrer results were compared with theresults obtained by a Scherrer modified method developed by Mon-shi et al. [15]. The modification used in this work correct the sys-tematic error stemming from the assumption that there are Ndifferent peaks of specific nanocrystals at either the 0–180� (2h)or 0–90� (h) range; then, all these N peaks must present identicalL values for the crystal size, which is not necessary true. Thus, to


obtain the average L value through all the peaks (or some selectedpeaks), the mathematical errors in the calculation must be reducedby using the least square method to obtain the following equations:

b ¼ KkL � cos h

¼ KkL

1cos h

ð2Þ

ln b ¼ lnKk

L � cos h¼ ln

KkLþ ln

1cos h

ð3Þ

After applying the least square method, the slope ln KkL can be

obtained and the plot intercept can be calculated using the follow-ing expression [15].

elnKkL ¼ Kk

Lð4Þ

For the calculations, the K value was considered to vary from 0.90,for semispherical nanoparticles, to 0.94 for nanorod [16] structureand the results from both set of experiments have been summa-rized in Tables 2 and 3.

The first set of experiments displays crystal sizes around 14–21 nm, which indicates that the sintering temperature exerts nogreat influence on the crystallite size, while a similar trend wasobtained during the second stage. Under the conditions of experi-ments 7 or 8 at pH values of 5, 9 and 11, the CePO4 nanopowderspresent values in the 8–16 nm and 10–17 nm ranges at room tem-perature and 600 �C, respectively. On the other hand, the CePO4

nanopowders obtained at pH = 1 in both experiments displayedthe largest crystallite sizes (52 and 29 nm) with an importantreduction after the sintering process (29 and 18 nm). In the analy-ses, it was assumed that the particle shapes were spherical, how-ever, it has been reported that CePO4 can have either a rod orspherical morphology [8]. Thus, the crystal size change after thesintering process could be due to the transformation from rods intospherical particles, which could be related to the smaller amount ofrods present in the sample, and therefore, the broadening of thediffraction peaks due to the formation of smaller clusters isobserved [17].

As it can be seen in Table 3, strong associations were foundbetween the crystal size obtained by Scherrer and Scherrer modi-fied equations. The first and second sets of experiments revealcrystal sizes between 12–20 nm and 13–18 nm, respectively. Bycomparing the results obtained from Scherrer equation, it is con-firmed that the behavior of the crystal size shows no big differ-ences among the reaction temperature, sintering temperature



0

10

20

30

Tr130t30Ts600Tr180t15Ts600

Tr130t15Ts600Tr180t30Ts400

Tr130t30Ts400

Tr180t15Ts400

Tr130t15Ts400

Num

ber (

%)

Particle size (nm)

(a)

0

5

10

15

20

25

Num

ber (

%)

Particle size (nm)

(b)pH 11

pH 9

pH 5

pH 1

Tr130t30Ts600

0 100 200 300 400 500 600 700 800 900 0 100 200 300 400 500 600

0 100 200 300 400 500 600 700 8000

10

20

30

40(c)

Num

ber (

%)

Particle size (nm)

pH 11

pH 9

pH 5

pH 1

Tr180t30Ts600

Fig. 4. Particle size distributions of: (a) 1st set of experiments, (b) Tr130t30Ts600 at different pH values and (c) Tr180t30Ts600 at different pH values.


and synthesis time for the case of samples obtained below pH = 3.As expected, the crystal sizes in experiments 7 (56 nm) and 8(43 nm) were decreased after the sintering process when the initialsolution has the lowest pH value (1). From these comparisons, thismethod led us to a more accurate value of L from all differentselected peaks (three most intense).

Therefore, it can be inferred that by adjusting the pH from 5 to9, monoclinic type CePO4 nanostructures began to be formed,whereas at pH 11, the hexagonal phase was transformed totallyto monazite in the synthesis independently of the reaction temper-ature (130 and 180 �C) without needing further sinteringtreatment.

The structure and morphology of selected CePO4 nanostruc-tures, using both sets of experiments, were evaluated by the TEMand SEM techniques as well as by selected area electron diffractionpatterns (SAED). In the first set of experiments, by varying the reac-tion (Fig. 2a and b), the SAEDs corroborated the presence of twostructural phases at 400 �C of sintering temperature, which grewpredominantly in nanorod shape and some particle-like agglomer-ate morphologies with a wide variety of sizes; rod nanostructuresconsist of diameters from 5 to 12 nm and lengths up to 221 nm. Byincreasing the sintering temperature to 600 �C, the nanorods disap-peared to form semi-spherical agglomerates with diameters below20 nm (Fig. 2c and d). As expected, at 600 �C, a strong relationshipbetween the sintering temperature and microstructure wasobtained.

Scanning electron microscope images of the progress of CePO4

nanostructures with the pH values (1, 5, 9 and 11) for the condi-tions used with experiments 7 and 8 are presented in Fig. 3a–h,respectively. Hexagonal nanorods of approximately 100 nm–1 lmin length and 10–100 nm in diameter were obtained at pH 1(Fig. 3a and e). At intermediate pH = 5, the morphology begins tochange and transforms into agglomerates of semi-spherical nano-particles (Fig. 3b and f). Under these conditions, both morphologies


coexist: nanorods and agglomerated particles. By increasing the pHup to 9, the predominant morphologies are spherical particles, butthin and fine nanorods are also detected in some areas (Fig. 3c andg). Interestingly, the nanorods become more evident by increasingthe reaction temperature (130 �C). Finally, at pH = 11, the CePO4

nanopowders only consist of semi-spherical particles.From these observations, it is clear that the morphological fea-

tures of the CePO4 nanostructures can be controlled by the pH ofthe reaction medium.

The significance of a morphology-controlled synthesis and thespecial properties of CePO4 nanopowders have been well acknowl-edged [18] due to the fact that different shapes often display differ-ent surface structures and always expose different active planes.Their sizes, surface structures, and interparticle interactions candevelop some unique properties and improved performances forfuture applications. Therefore, it is evident that the materials willhave a different performance as a function of the reaction medium.

Particle diameter moments (number-average diameter Dn;weight-average diameter Dw and z-average diameter) were calcu-lated using Eqs. (5)–(7), and the polydispersity index (PDI) wasdetermined using Eq. (8):

Dn ¼RniDi

Rnið5Þ

Dw ¼RniD

4i

RniD3i

ð6Þ

Dz ¼RniD

6i

RniD5i

ð7Þ

PDI ¼ Dw

Dnð8Þ

where ni is the number of CePO4 nanoparticles with diameter Di.The particle size distribution (PSD) graphs as well as average



578 564

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smitt

ance

(%)

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smitt

ance

(%)

Tr180

t30

Ts400

Tr180

t15

Ts400

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Ts400

Tr130

t15

Ts400

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618

955

955

1140-983

1140-983

1140-983

955

541

1140-983

955 618

578 564

538

(a)

1400 1200 1000 800 600 1400 1200 1000 800 600

1400 1200 1000 800 600 1400 1200 1000 800 600

T r180 t15 T s600

955

537

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smitt

ance

(%)

Tran

smitt

ance

(%)

578564

616

578564

Tr180

t30

Ts600

Tr130

t30

Ts600

10911069 1016

995

Tr130

t15

Ts600

9551091

1069 1016995

537616

(b)

Wavenumber (cm-1)

Wavenumber (cm-1)Wavenumber (cm-1)

Fig. 5. FT-IR spectra of CePO4 nanopowders sintered at: (a) 400 �C and (b) 600 �C, under the conditions of the first set of experiments.


particle size (Dz), polydispersity index (PDI), the highest meannumber (%) and sizes are shown in Table 4 and Fig. 4.

The results of the size distribution analyses show that themajority of the samples feature a broad distribution. For example,the average particle size of the first set of experiments (at pH = 3) isfound between 541–315 nm with PDI values above 1, indicatingthe high polydispersity in the system. At pH = 1, the distributionis slightly displaced to lower sizes; sample 7 showed two groupsat about 42 and 122 nm, confirming the mixed morphology seenby TEM and SEM micrographs, while the nanopowders at pH = 5,9 and 11 displayed the highest % of nanoparticles at 140–190 nm, 121–531 nm, and 90–125 nm, respectively. From theseobservations and the associations with TEM and MEB, we can con-clude that the powders consist of polydispersed agglomerateswhich need to be separated into individual particles.

The effect of the microwave-assisted hydrothermal reactionparameters on the CePO4 nanoparticles were also analyzed by


means of FT-IR, and the results of nanostructures treated at 400and 600 �C for the first set of experiments in the 1500–450 cm�1

region are shown in Fig. 5a and b.The IR spectra of the 1–4 and 5–8 materials synthesized under

the specified experimental conditions and sintered at 400 �C arepresented in Fig. 5a, respectively. The spectra of the 1–4 conditionsdisplay peaks at about 1140–983 cm�1 (antisymmetric stretchingvibration of the P–O bond (m3)), 618, 578, 564, and 539 cm�1

(asymmetric bending modes of the PO43� group (m4)) [19].

The FTIR spectra for samples 5–8 (Fig. 5b) show a similar ten-dency in the 950–450 cm�1 range. However, there are some differ-ences between the analyzed sintering temperatures. One of themost remarkable characteristics of the samples corresponding tothe 1–4 condition is observed at the 955 cm�1 signal (symmetricstretching vibrations). It is observed that the intensity of the peakis more evident as the sintering temperature is increased from400 to 600 �C; this is due to the vibration of the P–O bond in the



CePO4 complex

CePO4 spherical particles

CePO4 rods like-particles

H+

OH-

Nucleation

Nucleation

Faster ionic motion

Lower ionic motion CePO4

CePO4H+

OH-

Fig. 6. Schematic representation of the formation of nanospherical particles andnanorods.

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ctan

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)

1,32

56

7,4

8

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(a)

λ (nm)

130°C, 15 min

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0

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500λ (nm)

8(pH5)

7(pH5)

8(pH1)

7(pH11)

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use

refle

ctan

ce (%

)

7(pH1)

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0

5

10

15

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25

20 0 2 2 0 2 40 2 6 0 28 0 300

7 (p H 9)

λ (n m )

8 (pH 5 )

7 (p H 11 )

8 (pH 1 1)

7 (p H 9 )

Diff

use

refle

ctan

ce (%

)

7 (pH 1 )

8 (pH 9)8 (p H 1 )

(b)

Fig. 7. Diffuse reflectance spectra obtained for all the as-prepared powders, beforeand after the sintering process.


monoclinic structure (m1), indicating the rearrangement of thehexagonal structure to form the monoclinic one. These factscorroborate and confirm the observations made in the XRD pat-terns for the samples sintered at 400 �C, where the transformationof rhabdophane was observed.

It is worth mentioning that in addition to this change, there is aremarkable difference in the broad band region between 1140 and983 cm�1, where it is observed how the peaks start to be moredefined and evident at about 1091, 1069, 1016 and 995 cm�1

(asymmetric stretching of P–O bonds). This behavior indicates acomplete rearrangement and it has been observed by calcinationstudies at different temperatures by Pusztai et al. [20].


The formation of nanomaterials, nucleation rates and growthprocess are influenced by many reaction conditions such as pH,pressure, temperature, precursor concentration, solvent [21]. Insuch a case, the controlled addition of precursors or surfactantsor templates is often used as a feasible means to control the nucle-ation rates and growth process, thus reaching the aim of control-ling the crystal size. However, the synthesis method isparticularly important. Ions such as NO�3 ;PO3�

4 come mainly fromthe cerium salt precursor and the precipitant [18]. These anionshave a selective interaction with specific facets [22], which theanions present during the synthesis of CePO4 nanomaterials viathe hydrothermal/solvothermal method, which leads to the growthof nanocrystals with varying morphologies. In such a case, the for-mation mechanism of nanorods or spherical particles depends onthe acid or alkaline medium as follows:

In the case of acid medium, the hydrothermal reaction systemconsists of Ce(NO)3�H2O, H2O, HNO3 and H3PO4 coming from thedissolution of H5P3O4. The mechanism most probably involvesthe release of PO3�

4 ions present in the H3PO4 solution, which arecombined with Ce3+ ions to form an amorphous precipitate in anaqueous solution. During the experimental work, it was observedthat as the pH value was decreased, the Ce3+PO4

3� complex wasmore dissolved. According to Y. Zhang, this dissolution behaviorresults in a faster ionic motion in the solution, leading to a higherconcentration of Ce3+ and HnPO3�

4 , where the chemical potential isincreased [23] and promotes the preferential adsorption of H+ ontothe crystal facets, which seems to raise the electrostatic potentialon the crystal surfaces [6]. Thus, the preferential adsorption of H+

leads to the hexagonal nanorod morphology as shown in Fig. 6.This process can be described with Eqs. (9)–(11).

H5P3O10 aqð Þ þH2O aqð Þ ! H4P2O7 aqð Þ þH3PO4 aqð Þ ð9ÞH4P2O7 þH2O! 2H3PO4 ð10ÞCeðNO3Þ3 aqð Þ � 6H2OþHNO3 aqð Þ þH3PO4 aqð Þ ! CePO4 sð Þ #þ 4HNO3 aqð Þ þ nH2OðaqÞ ð11Þ

On the other hand, the main reaction might take place according toEqs. (12)–(14). The precipitates of the Ce3+PO4

3� complex were moreevident when the pH of the initial solution was adjusted in alkalinemedium. This precipitate leads to a lower concentration of Ce3+ andHnPO3�

4 , decreasing the chemical potential, and the OH� ions fromNH4OH are absorbed on the crystal face, altering the growth ofthe nanorods to form spherical particles belonging to the mono-clinic system as it is also shown in Fig. 6.

H5P3O10 aqð Þ þH2O aqð Þ ! H4P2O7 aqð Þ þH3PO4 aqð Þ ð12ÞH4P2O7 þH2O! 2H3PO4 ð13ÞCeðNO3Þ3 aqð Þ � 6H2OðaqÞ þ NH4OH aqð Þ þH3PO4ðaqÞ! CePO4ðsÞ # þ2NH4NO3ðaqÞ þ nH2 ð14Þ

3.2. Optical properties of CePO4 nanostructures

The optical absorption properties of the CePO4 materials werecharacterized by diffuse reflectance (DR) measurements. Fig. 7aand b shows the diffuse reflectance spectra of CePO4 nanoparticlesobtained under the conditions of the aforementioned first and sec-ond sets of experiments. The measurements were carried outbefore and after the sintering process in the ultraviolet light range,which can be divided into three sub-intervals: the UV-C (100–280 nm), UV-B (280–315 nm), UV-A (315–400 nm) and visiblelight (400–700 nm).

The frequent use of diffuse reflectance spectroscopy for thedetermination of the electronic transitions in solid materials ishighly justified due to the optical excitation of the electrons fromthe valence band to the conduction band, which is evidenced by



Tr130t30Ts600

pH 1

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225253

300

(F(R

)hv)

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3.22 eV

hv (eV)

(F(R

)hv)

2

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215

257

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3.0 eV

Tr130

t15

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pH 5

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3.23 eV

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Tr130t30Ts600

pH 11

3.25 eV200 220 240 260 280 300 200 220 240 260 280 300

hv (eV)1.0 1.5 2.0 2.5 3.0 3.5 4.0

hv (eV)1.0 1.5 2.0 2.5 3.0 3.5 4.0

hv (eV)1.0 1.5 2.0 2.5 3.0 3.5 4.0

hv (eV)1.0 1.5 2.0 2.5 3.0 3.5 4.0

hv (eV)1.0 1.5 2.0 2.5 3.0 3.5 4.0

(F(R

)hv)

2

λ (nm)

(F(R

)hv)

2

λ (nm)

(F(R

)hv)

2

λ (nm)

(F(R

)hv)

2

λ (nm)

(F(R

)hv)

2

λ (nm)

(F(R

)hv)

2(F

(R)h

v)2

(F(R

)hv)

2(F

(R)h

v)2

(F(R

)hv)

2

Fig. 8. Kubelka–Munk modified spectra and its equivalence in hm vs k (inset) of selected CePO4 samples.


an increase in the absorbance at a given wavelength (band gapenergy) [24].

In general, from Fig. 7a–b two main features were observed inthe CePO4 powders during the diffuse reflectance study: first, allthe spectra revealed large absorption in the UV-B (280–315 nm)and UV-A (315–400 nm) regions with high transparence underthe visible light range (400–700 nm), attributable to the weakabsorption in the as-prepared nanostructures. This region can beparticularly important for practical applications in ultraviolet-light-based-white-light-emitting dioxides (UV-WLED), compo-nents for precision optical devices, windows, etc. [25]. A secondevident feature in the samples is the increase in the absorptionwith the hexagonal structure (at 400 �C), which is a desired condi-tion for polymer applications since UV-A and UV-B lights areresponsible for color loss, mechanical property alterations andstructural modifications [26].

The quantification of CePO4 nanopowders indicated that afterthe sintering process at 400 �C, the samples display higher reflec-tance percent in the 400–700 nm region. On the other hand, CePO4

nanostructures sintered at 600 �C show a tendency similar to thatof the non-sintered powders. In comparison, the pH modified sam-ples (Fig. 7b) show high absorption characteristics in the UV inter-val, while from all these samples, samples 7 and 8 reflect the best


stability in the visible region at pH = 1. Additionally, it wasobserved that CePO4 structures obtained at pH = 11 displayed thebest absorption properties. As expected, the reflectance can begreatly modified by the structural characteristics of the CePO4

nanoparticles i.e., shape, size and roughness [27].An approximation of the optical absorption coefficient was

evaluated by the Kulbeka–Munk function, using the reflectancedata to calculate the absorption coefficient (F(R1)),

F R1ð Þ ¼ ð1� R1Þ2

2R1¼ KðkÞ

sðkÞ ð15Þ

where (F(R1)) is the M–K function or re-emission function, R1 is thediffused reflectance at a given wavelength of an infinitely thicksample, K(k) is the absorption coefficient, and s(k) is the scatteringcoefficient. For direct band gap transitions, the optical bandgap value (Eg) is calculated using the relationðFðR1ÞhmÞ ¼ Aðhm� EgÞ1=2, where A is constant, hm is the photonenergy and Eg is the optical band gap. In all the cases, Eg is inferredby extrapolating linearly the long-wave-length edge of the peak inabsorbance to this zero line from the plot between ðFðR1ÞhmÞ2 and(hm) [28,29].

The band gap was calculated from the linear part of the slope.Selected results for the overall experiments are presented in




Fig. 8, whereas a summary of the entire samples is presented inTable 5. In the inset figures, the spectrum of condition 7 obtainedat pH = 1 shows remarkable similarities with respect to the absorp-tion spectra of bulk CePO4 reported by Yue-Ping Fang [30]. TheðFðR1Þhm2Þ vs. k (nm) curves can also be seen, where it was foundthat within pH 1–9 appears a region (200–300 nm) with four peaksthat corresponds to the transitions of the f–d orbitals from theground state 2F5/2(4f1) of Ce3+ to the five crystal field split levelsof the Ce3+ 2D(5d1) excited states [30]; the small displacement atpH = 5 was correlated with differences in the particle size. It isimportant to note that these transitions cannot be resolved inthe spectra when the solution pH is adjusted to 11.

It can be mentioned that the results evidence that the formationof CePO4 powders in the hexagonal or monoclinic phase are highlydependent on the combination of different factors, however, theconstant microwave energy, sintering temperature and pH seemto be the most important parameters. A structural stability of thehexagonal or monazite phases can be reached by using a micro-wave-assisted hydrothermal method at relatively low process tem-peratures by adjusting the solution pH. Finally, the studyemphasizes the clear relation among the process conditions, struc-ture and sample morphologies, which in turn reflected changes inthe optical properties.

4. Conclusions

An enhanced hydrothermal synthesis by using microwaveenergy for obtaining CePO4 nanostructures was reported. Themethod demonstrated to be a fast, low energy and capable onein the synthesis of CePO4 nanostructures, and an option to replacethe commonly used method, which needs long periods of time toreach a nanoscale. The present work also showed the strong rela-tion between pH and the final structural properties of cerium phos-phate nanoparticles. By evaluating the structural andmorphological properties, it was observed that by means of highmicrowave energy (200 W), a combination of hexagonal nanorodsand monoclinic nanoparticles can be achieved at 400 �C, whereaspure monazite was obtained by sintering the samples at 600 �C.By adjusting the reaction solution pH, nanorods are favored atpH = 1, whereas a very alkaline medium (pH = 11) caused the for-mation of CePO4 semispherical agglomerates. The results high-lighted the dependence of the structure on the pH, wheremonazite was formed under mild conditions. Finally, the low dif-fuse reflectance values in the UV region indicate that the nanopow-ders can be used as fillers to increase the UV resistance inpolymeric materials.

Acknowledgments

D. Palma-Ramírez is grateful for her postgraduate scholarshipby SIP-IPN and COFAA-IPN. The authors are also grateful for thefinancial support provided by CONACYT – México through theCB2009-132660 and CB2009-133618 projects and by IPN throughthe SIP 2014-0164 and 2014-0992 projects and SNI-CONACYT –México.

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