Enhancement ofopticalpropertiesanddependenceofthecrystalstructure, morphological propertiesofPrPO4 by microwave-assisted-hydrothermal synthesis

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Ceramics International 4 www.elsevier.com/locate/ceramint
Enhancement of optical properties and dependence of the crystal structure,morphological properties of PrPO4 by microwave-assisted-hydrothermal

synthesis

D. Palma-Ramíreza, M.A. Domínguez-Crespoa,n, A.M. Torres-Huertaa, E. Ramírez-Menesesb,E. Rodrígueza, H. Dorantes-Rosalesc, N. Cayetano-Castrod

aCICATA-Unidad Altamira, Instituto Politécnico Nacional, IPN, Km 14.5, Carretera Tampico-Puerto Industrial Altamira, C. P. 89600 Altamira, Tamps., MéxicobUniversidad Iberoamericana, Departamento de Ingeniería y Ciencias Químicas, Prolongación Paseo de la Reforma 880, Lomas de Santa Fe C. P. 01219,

México D.F, MexicocInstituto Politécnico Nacional, ESIQIE, Departamento de Metalurgia, C. P. 07300 Mexico, D.F, México

dInstituto Politécnico Nacional, Centro de Nanociencias Micro y Nanotecnologías, Departamento de DRX, C. P. 07300 Mexico, D.F, México

Received 24 June 2015; received in revised form 31 August 2015; accepted 1 September 2015Available online 8 September 2015

Abstract

In this paper, PrPO4 nanostructures with hexagonal (rhabdophane phase) and monoclinic (monazite phase) structures were obtained by amicrowave-assisted-hydrothermal method and characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), FourierTransform Infrared Spectroscopy (FT-IR), Dynamic Light Scattering (DLS) and Reflectance Diffuse Spectroscopy (DRS) and High ResolutionTransmission Electron Microscopy (HRTEM). A first study of the experiment was done in order to evaluate the reaction time (130 and 180 1C),synthesis time (15 and 30 min) and sintering temperature (400 and 600 1C). Subsequently, the medium pH was adjusted to 1, 3, 5, 9 and 11 undertwo previously selected conditions. The results highlight the significant effects exerted by the synthesis parameters on the structure, crystal andparticle sizes, morphology type, reaction mechanism as well as on the PrPO4 absorption/transmission region.& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Praseodymium phosphate; Microwave-assisted-hydrothermal synthesis; Nanorod-like morphologies; Optical properties

1. Introduction

Rare-earth phosphates have ideal characteristics to be usedin fluorescent lighting, fluorescent hosts, scintillators and laserdevices as single crystals, powder and glass [1,2]. Phosphorsconsist of a host lattice with a luminescent ion which has theability to convert short wavelength radiation into mostly lowerenergy radiation in the visible (VIS) range [3]. The synthesis toobtain activated phosphate particles and further embedded indifferent polymer matrix has been used to obtain structures aswaveguides, photonic crystals, coatings, biomedical diagnos-tic, energy conversion, telecommunications and bulk glasses

/10.1016/j.ceramint.2015.09.00215 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author.ss: [email protected] (M.A. Domínguez-Crespo).

including spheres, rings and other geometries exploited inoptical resonators fabrication [3–6]. In particular, nanoparticlesof praseodymium phosphate have been hardly studied and theiroptical properties can be of interest, e. g., as UV absorbers inplastics, either to conserve the structural properties fromdegradation by UV light, or in the form of composites whichprotect UV sensitive materials [7]. It has been stated that inthese industrial applications, the synthesis method of PrPO4

plays an important role in the production of nanostructureswith a specific morphology [8]. A lot of effort has gone intosynthesizing monazite (monoclinic structure) and rhabdophane(hexagonal phase) rare-earth phosphates under different para-meters and methods (e. g, hydrothermal, sol–gel, wet-chemicaland high-temperature-solid–solid reaction methods) [3,9,10];however, these methods have noticeable disadvantages. Firstly,

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in order to increase and improve the synthesis rate, theirchemical homogeneity and morphology, the initial solutionsare heated up under elevated pressure and temperatures(hydrothermal method), which results in segregation due tothe solubility differences in the individual components [8]. Italso requires long reaction times of about 4 [2] to 12 h [11],even 40 days when the compounds are obtained at roomtemperature [10] as well as a large quantity of reactants arerequired for the synthesis [3,8]. More recently, the ongoingmicrowave assisted hydrothermal method was proved to be agreen approach for synthesizing nanomaterials which havesome advantages over the conventional hydrothermal methodsuch as the requiring of lower synthesis temperatures to reachthe complete reaction and shorter reaction times [12,13].However, to the best of our knowledge, the study on tailoredPrPO4 nanostructures by microwave-assisted-hydrothermalsynthesis has yet to be investigated. Herein, we propose a fastsynthesis to obtain PrPO4 nanostructures with varied morphol-ogies and structures through a microwave-assisted-hydrother-mal approach. The evolution from nanorod-like to spherical-like morphologies was modulated by simply changing the pHin the aqueous solution. Additionally, the dependence of theoptical and morphological properties of the as-prepared PrPO4

materials on the solution pH is also discussed. The process wasfast and reproducible in comparison with other conventionaltechniques like sol–gel.

2. Experimental procedure

The synthesis experiment for PrPO4 powders was done bymodifying an earlier hydrothermal method reported by Julianaet al. for the synthesis of CePO4 [14]. Ce3þ has similar ionicradius and 4f shell configuration as Pr3þ , so the same behaviorwould be expected in the synthesis of PrPO4. The hydrothermal

Table 1Set of experiments for the synthesis of PrPO4 displaying the phase composition an

Experiment pH Reaction temperature (Tr, 1C) Synthesis time (t, m

1 2 130 152 180 153 130 304 180 305 130 156 180 157 130 308 180 304 1 180 304 34 54 94 118 18 38 58 98 11

synthesis was assisted by microwave energy using a microwaveoven manufactured by CEM Corporation. The experiment wascarried out as follows: 50 mL of Pr(NO3)3 � 6H2O (0.10 mol)(Sigma-Aldrich, 99% purity) were added dropwise to 25 mL of atripolyphosphoric acid solution (0.035 mol�1, H5P3O10) pre-viously prepared from the ion exchange of sodium tripolypho-sphate Na5P3O10 � 5H2O (pure p.a., Z98.0% (T), Sigma-Aldrich)using a cation exchange resin (Dowex 50 W X4 100–200 mesh),then deionized water was added to adjust a final volume of100 mL. The pH of the final solution was �2.2. The solutionswere transferred into a container (HP-500 vessel from CEMCorporation). The autoclave was sealed and introduced into theoven (Frequency 2.45 GHz, power of �200 W) which has anattached pressure sensor and inserted fiber optic temperaturesensor.An initial set of experiments (see Table 1) was prepared to

assess the optimal conditions by varying the reaction tempera-ture, synthesis time and sintering temperature (Tr, t, and Ts,respectively). Once the initial set of experiments was com-pleted, the conditions labeled as 4 and 8 were selected toestablish the pH at which the monoclinic phase could befavored at lower sintering temperatures (400 and 600 1C) incomparison with the traditional process where a temperatureabove 800 1C is required [9]. The pH values were adjustedusing NH4OH (30%) and HNO3 (37%) to reach pH values of1, 3, 5, 9 and 11 for each condition.

2.1. Characterization of nanopowdersPrPO4 powders were evaluated by X-ray powder diffraction

(XRD) using a Bruker D8 Advance diffractometer equippedwith Lynxeye detector and Cu Kα radiation (λ¼1.5405 Å) at35 kV and 25 mA. The data were recorded at room temperaturein the 2θ range of 15–701, step size of 0.0161 and step timeof 0.5 s.

d synthesis parameters.

in) Sintering temperature (Ts, 1C) Phase composition (wt%)

Monazite Rhabdobphane

400 19.95 80.05400 58.74 41.26400 47.69 52.31400 69.64 30.36600 100 0.0600 100 0.0600 92.92 7.08600 100 0.0400 94.68 5.32

60.26 39.7473.8 26.282.75 17.2586.45 13.55

600 100 0.098.15 1.85100 0.099.75 0.25100 0.0

20 30 40 50 60 70

Monoclinic

Hexagonal

Inte

nsity

(a. u

.)

T r130°C t15min

Tr180°C t15min

Tr130°C t30min

(241

)

(003

)

(103

)(3

01)

(300

)(1

03)(2

11)

(112

)(102

)(2

00)

(020

)

2θ (degrees)

Tr180°C t30min

(101

)

(140

)(3

02)

(203

)

(202

)

20 30 40 50 60 70

Tr180°C t30minTs600°C



(241

)(0

14)

(114

)(1

40)

(132

)(3

22)

(023

)(1

03)

(231

)(2

12)

(311

)(0

31)




Tr180°C t15minTs400°CHexagonal

MonoclinicIn

tens

ity (a

. u.)

(112

)(2

02)(012

)(1

20)

(200

)(1

11)

(111

)(1

10)

(101

)

(302

)(2

03)

(003

)

2θ (Degrees)


(112

)

(101

)

(102

)(2

00)

(301

)

(003

)(2

11)

Fig. 1. X-ray diffraction patterns of: (a) non-sintered PrPO4 powders obtainedunder the conditions mentioned in Table 1, (b) PrPO4 powders sintered at twodifferent temperatures (400 and 600 1C).

D. Palma-Ramírez et al. / Ceramics International 42 (2016) 774–788776

Morphology and texture of nanopowders were examined byhigh resolution scanning electron microscopy (HRSEM/EDS)using a JEOL JSM-6701F equipment. Transmission electronmicroscopy (TEM) micrographs were obtained with a JEM-ARM200CF, JEOL, (Lattice resolution 78 picometers, accel-eration voltage 200 kV).

FT-IR (Fourier Transform Infrared Spectroscopy) spectrawere recorded on a spectrum one Perkin Elmer spectrometer(4 cm�1 of resolution setting) in order to evaluate thevibrational properties of the PrPO4 bands. Samples werescanned at least 40 times in the range of 1500–450 cm�1 byusing KBr pellets.

Particle sizes were measured in a Malver Zetasizer NanoZSP, model ZEN5600 equipped with an Argon laser (λ¼633nm) at room temperature. Previously, PrPO4 powders werediluted with water in a 1/100 ratio. Data were calculated fromthe particle diameter moments (number-average diameter Dn;weight-average diameter Dw and z-average diameter Dz), usingEqs. (1)–(4) and the polydispersity index (PDI) was alsodetermined as follows:

niDi

niDn

1= Σ

Σ ( )

niDi

niDiDw

2

4

3= Σ

Σ ( )

niDi

niDiDz

3

6

5= Σ

Σ ( )

D

DPDI

4w

n

=( )

where ni is the number of CePO4 nanoparticles withdiameter Di.

Diffuse reflectance of the samples was evaluated by Ultra-violet–visible diffuse reflectance spectroscopy (UV–vis DR)using a 110-mm-diameter-integrating sphere accessorymounted on a Cary 5000 Spectrophotometer.

The fluorescence intensity measurements were performedusing a Carl ZEISS Microscope, LSM 700 confocal microscopeand the built software ZEN of the LSM 710. The emissionspectra were acquired at 405 nm and room temperature.

3. Results and discussions

The crystallization process of PrPO4 powders was investi-gated by the X-ray diffraction technique. The XRD patterns ofPrPO4 samples before and after being thermally treated at 400and 600 1C (pH¼2.2) are shown in Fig. 1a and b. Theobtained patterns can be indexed to the hexagonal andmonoclinic phases of the praseodymium phosphate hydrateknown as rhabdophane (PrPO4 �H2O, a¼b¼7.0 Å, c¼6.43Å, ICDD 00-020-0966) and praseodymium phosphate knownas monazite (PrPO4, a¼6.434 Å, b¼6.760 Å, c¼6.982 Å,ICDD 01-083-0653). In general, non-significant changes in the

structural phases were observed by varying either the reactiontemperature (from 130 to 180 1C) or reaction time (15 and30 min), Fig. 1a. The resulting patterns reveal that a mixture ofphases (mainly rhabdophane) is obtained with the proposedsynthesis approach. It has been stated that the crystal structurebelonging to the hexagonal system (named rhabdophane in thecase of the light lanthanide orthophosphates, Ce3þ , Ln3þ andPr3þ ) requires calcination temperatures above 800 1C totransform it into monazite [9]. However, we found in thiswork that such phase transformation can take place at lowertemperatures (130 and 180 1C) when the rare earth phosphate(PrPO4) is synthesized by the microwave-assisted-hydrother-mal method.Commonly, rhabdophane is a very stable structure and of

particular interest in tribological applications [15], whereas themonazite phase has been reported to be used in tribologicalapplications too, it can display a high optical emissivityaccompanied by a high fusion temperature [16] and recently,this structure has shown interesting properties as UV absorberin plastics.

Table 2Crystal size of PrPO4 and treatment of linear plots to obtain the crystallite sizefrom Scherrer modified equation.

Experiment pH As-prepared

eln x

L

0.94 0.154051K L/=

λL (nm) Sintered

eln x

L

0.94 0.154051K L/=

λL (nm)

1 2 0.01372 11 0.0148 102 0.01235 12 0.0116 123 0.0116 12 0.0110 134 0.0122 12 0.0123 125 0.01372 11 0.0003 116 0.01235 12 0.0012 127 0.0116 12 0.0007 128 0.0122 12 0.0004 124 1 0.0074 18 0.0218 74 3 0.0118 12 0.0119 114 5 0.0103 13 0.0115 124 9 0.0028 48 0.0628 34 11 0.0035 39 0.0410 38 1 0.0074 18 0.0511 38 3 0.0118 12 0.0096 128 5 0.0103 13 0.0187 128 9 0.0028 48 0.0108 138 11 0.0035 39 0.0060 23


Thus, to obtain a high percentage of the monoclinicstructure, the samples were thermally treated from 400 to800 1C. During the XRD measurements, however, non-impor-tant changes were observed with the sintering temperatures upto 600 1C. The as-treated samples at 400 and 600 1C and theXRD patterns are shown in Fig. 1b. The XRD patterns of thesamples sintered at 400 1C are mainly composed of themonoclinic phase (58.74 and 69.64 wt%) when a reactiontemperature of 180 1C is applied during the synthesis. As thesintering temperature is increased from 400 to 6001 C, severalpeaks of the hexagonal phase tend to shift to lower 2θ angles,indicating the transformation to the monoclinic phase (ICDD01-083-0653), Table 1.

A quantitative estimation of the domain size for PrPO4

nanopowders was evaluated from the three most intense peaksusing the Scherrer modified equation developed by A. Monshiet al. [17]. The method assumes that there are N peaks ofspecific nanocrystals which must present identical L values inthe range of 0–1801 (2θ) or 0–901 (θ), assumption that is notnecessarily true and consequently, the mathematically errors inthe calculation must be decreased by using the least squaresmethod to obtain the following equations:

K

L cos

K

L cos.

15

β λθ

λθ

= = ( )

K

L cos

K

L cosIn In

.In In

16

β λθ

λθ

= = + ( )

After applying the least squares method, the slope calculatedfrom the Inβ vs cosIn 1/ θ( ) plot and a K LIn /λ( ) intercept canbe obtained using the following expression [17].

K

Le

7KL

In λ= ( )λ

For the calculations, a K value of 0.94 was considered sincethe shape morphology of the obtained PrPO4 has been reportedas rods [18] and the corresponding data are shown in Table 2.The crystallite size seems to be very stable in the 1st set ofexperiments even before the sintering process (10–13 nm).Under these conditions, the sintering temperature has not greatinfluence on the crystallite size.

The molecular vibration of the as-prepared PrPO4 nano-powders was investigated using Fourier Transform Infrared(FT-IR) Spectroscopy, Fig. 2a and b. The spectra displayed thecharacteristic bands c.a. 956 cm�1 and 618 cm�1, 567 cm�1

and 541 cm�1 which correspond to the bending ( v4) andstretching ( v3) vibrations of PO4

3−, respectively [19–21],however, the intensity of the bands and additional bandsemerging in the analysis highlight that the synthesis timeexerts a great influence on the formation of bonds. Forexample, the sample synthesized at 180 1C and 15 min issimilar to those obtained at 130 1C (with 15 or 30 min),whereas if the time is increased to 30 min at the same synthesistemperature, a new vibrational band appears at 578 cm�1. Thisband is probably related to the structural re-arrangement ofPrPO4 since it is found in all the samples sintered at 600 1C

(Fig. 2b). Another relevant characteristic observed in this workis that by increasing the sintering temperature from 400 to600 1C, the signal at 956 cm�1 becomes sharper as thetemperature is increased and it is related to the vibration ofthe P–O bond in the monoclinic structure [22].The particle size distribution graphs, the average particle

size (Dz), and polydispersity index (PDI) of the PrPO4 powdersare shown in Fig. 3 and Table 3. The results in Table 3 showthat the mean average sizes (Dz) from DLS measurements forthe as-obtained PrPO4 nanostructures range from 324 to867 nm. However, the expected PDI was found to be above1.5, suggesting that the PrPO4 nanoparticles are mostlycomposed of multiple particle populations, i.e., poly-dispersedsystems of particles [23].The morphology of the as-prepared samples is shown in

Fig. 4. It is noticeable that agglomerates of rod-like and semi-spherical particles were formed independently of the synthesisconditions. The nanorod morphology has been reported to beobtained during the synthesis of PrPO4 [11]. These nanorodstend to stack and agglomerate to each other forming largerparticles. It was not possible to measure with accuracy thelength and diameter of the rods; however, a preliminaryestimation of the length and diameter is about �200 nm and�25 nm, respectively.Results of different syntheses have shown that pH is a

significant parameter in altering morphology [24]. During thefirst set of experiments, the synthesis pH was maintainedconstant to evaluate morphology changes. For this reason andin order to obtain more information on the pH effect on themorphology and properties, two conditions were selected toevaluate the dependency on the acid or alkaline initial solutionas well as the sintering temperature. Previously, it has beenfound for CePO4 that by pH modulating, it is possible to obtaina monoclinic structure at low sintering temperatures. Thus, the

1400 1200 1000 800 6000

20

40

60

80

100

1400 1200 1000 800 6000

20

40

60

80

100

1400 1200 1000 800 6000

20

40

60

80

100

1400 1200 1000 800 6000

20

40

60

80

100

Tran

smitt

ance

(%)

Tran

smitt

ance

(%)

Tran

smitt

ance

(%)

1400 1200 1000 800 6000

20

40

60

80

100

1400 1200 1000 800 6000

20

40

60

80

100

1400 1200 1000 800 6000

20

40

60

80

100

1400 1200 1000 800 6000

20

40

60

80

100

Tran

smitt

ance

(%)

Tran

smitt

ance

(%)

Tran

smitt

ance

(%)

Tran

smitt

ance

(%)

Fig. 2. FT-IR spectra of the PrPO4 nanopowders sintered at: (a) 400 1C, and (b) 600 1C under the conditions of the first set of experiments.


conditions of experiments 4 and 8, constant reaction tempera-ture (Tr¼180 1C) and time (30 min) were selected. Duringthese experiments, pH was adjusted at 1, 3, 5, 9 and 11, usingtwo sintering temperatures (400 and 600 1C); the experimentalmatrix is also presented in Table 1.

Structural dependence of the solution pH was investigatedfor these experiments. The pH effect on the structural vibra-tions was characterized for these experiments and the resultsare shown in Fig. 5a and b. The spectra display the

characteristic bending (at 957 cm�1) and stretching (at 615,580, 566 and 542 cm�1) vibrations of PO4

3−, which are morepronounced as pH goes from alkaline to acid medium. Themost notable difference by adjusting the pH is that the band at615 cm�1 is slightly shifted to lower wavenumbers and theregion between 1200 and 1000 cm�1 is well defined as thesintering temperature is increased to 600 1C (Fig. 5a and b).The XRD patterns of the samples obtained at different pH

values are presented in Fig. 6a–c. The analyzes of the XRD

0 200 400 600 800 10000

5

10

15

20

25Tr180t15Ts400

Dz = 648PDI = 1.6

Freq

uenc

y

Diameter (nm)0 200 400 600 800 1000

0

5

10

15

20

25 Tr180t15Ts400

Dz = 867PDI = 4.2

Freq

uenc

y

Diameter (nm)

0 100 200 300 400 500 6000

5

10

15

20

25

30

35Tr130t30Ts400

Dz = 324PDI = 3.8

Freq

uenc

y

Diameter (nm)0 200 400 600 800 1000

0

5

10

15

20

25

30

35 Tr180t30Ts400

Dz = 851PDI = 2.5

Freq

uenc

yDiameter (nm)

0 100 200 300 400 500 6000

10

20

30

40 Tr130t15Ts600

Dz = 377PDI = 4.4

Freq

uenc

y

Diameter (nm)0 100 200 300 400 500 600 700 800

0

5

10

15

20

25

30

35Tr180t15Ts600

Dz = 462PDI = 3.5

Freq

uenc

y

Diameter (nm)

0 200 400 600 800 10000

5

10

15

20

25

30Tr130t30Ts600

Dz = 617PDI = 3.7

Freq

uenc

y

Diameter (nm)0 100 200 300 400 500 600 700

0

5

10

15

20

25

30

Freq

uenc

y

Diameter (nm)

Tr180t30Ts600

Dz = 433PDI = 4.0

Fig. 3. Particle size distribution of the 1st experiment particles sintered at 400 and 600 1C.


patterns of the as-prepared PrPO4 (Fig. 6a) show a clear pHdependence of the PrPO4 final structure. It is easy to perceivethat by using pH¼1, a percent of the monoclinic phase can bereached without requiring sintering temperatures, where otherpH conditions favored a phase mixture of the monoclinic/hexagonal (pH¼3–5) and monoclinic/hexagonal structureswith small reflections emerging from the tetragonal phase atpH¼9–11 (PDF #00-033-1077).

In the case of the samples synthesized under experiment4 conditions and sintered at 400 1C (Fig. 6b), the XRD patternsshowed that the characteristic reflections of both phases stillpersist within the evaluated pH interval. However, at low pH, a

high percentage of the monazite structure (94.68 wt%) isfavored; whereas these concentrations decrease and slightlyincrease as pH goes from 3 to 5 (see Table 1). It was observedthat a high percentage can be reached by increasing thesolution pH. On the other hand, samples thermally treated at600 1C displayed XRD peaks that matched predominantly wellwith the monazite structure, Table 2. Using this sinteringtemperature, the monazite structure is reached independentlyof the solution pH (Fig. 6c).The pH influence on the PrPO4 crystal size was also

calculated through the Scherrer modified equation. Duringthe second stage of experiments (4 and 8), the crystallite size


tended to increase with pH from 12 to 48 nm, but it gotreduced in the 3–23 nm interval with the sintering process(Table 2). The pH influence can be divided into three aspects:(i) the crystal size in acid medium (pH¼1) displays a decreaseafter 400 1C, (ii) at intermediate acid character (pH¼3–5) thesize is almost stable and it is similar to one obtained in the firstset of experiments at pH¼2 and (iii) at higher pH values (9and 11), the crystal size tends to decrease with the sinteringtemperature. A similar trend to reduce the crystal size with pH

Table 3Z-average particle diameter (Dz) and polydispersity index (PDI) of PrPO4

nanopowders.

Condition no. Dz(nm) PDI

1 648 1.62 867 4.23 324 3.84 pH¼1 (885) pH¼1 (1.0)

pH¼2 (851) pH¼2 (2.5)pH¼3 (724) pH¼3 (1.3)pH¼5 (417) pH¼5 (2.4)pH¼9 (756) pH¼9 (4.4)pH¼11 (528) pH¼11 (2.0)

5 377 4.46 462 3.57 617 3.78 pH¼1 (338) pH¼1 (1.8)

pH¼2 (433) pH¼2 (4.0)pH¼3 (273) pH¼3 (1.5)pH¼5 (287) pH¼5 (1.3)pH¼9 (615) pH¼9 (1.2)pH¼11 (799) pH¼11 (1.1)

Tr130,t15,Ts400 Tr180,t15,Ts400

Tr130,t15,Ts600 Tr180,t15,Ts600

100 nm 100 nm

100 nm 100 nm

Fig. 4. SEM micrographs of the 1st set of experiments: (a) Tr130t15Ts400, (b) Tr18(g) Tr130t30Ts600 (g), and (h) Tr180t30Ts600.

has been reported during the synthesis of LaPO4 nanopowders[25]. It is well known that the microwave-assisted-hydrother-mal method reduces the activation energy of the materials, thuslowering the synthesis temperatures of the materials, ascompared with the conventional ceramic method [26]. There-fore, the microwave-assisted-hydrothermal method along withan adequate choice of the synthesis parameters can help tomodulate the structural phases during the synthesis of PrPO4

nanoparticles.To corroborate our assumption regarding the dependence of

the structure, phase composition and particle sizes on pH,HRTEM measurements were carried out. HRTEM and FourierTransform-Filtered Images (inset) of samples prepared underthe conditions of experiments 4 (Tr180t30Ts400) and 8(Tr180t30Ts600) at different pH values are presented inFigs. 7 and 8, respectively.The HRTEM images proved that the nanoparticles are

composed of two types of morphologies: rods and sphericalnanoparticles, which are quite dependent of the pH in thereaction medium. As expected, the final products synthesizedat pH 1 consist mainly of long rods whose length is affected bythe sintering temperature; powders thermally treated at 400 1Cshowed diameters (D) between 12 and 30 nm with lengths (L)ranging from 100 to 350 nm, whereas at 600 1C, shorternanorods were obtained (D¼10–26 nm, L¼40–200 nm). Itis also clearly seen that by increasing the pH at 3, the nanorodsstart evolving to shorter and narrower sizes with lengths c.a.20–173 nm (sintered at 400 1C) and up to 15–47 nm (sinteredat 600 1C). Upon rising to pH 5, the length appearance is moreevident since the analysis of the micrographs showed particlesizes between 7 and 50 nm, independently of the sintering

Tr130,t30,Ts400 Tr180,t30,Ts400

Tr130,t30,Ts600 Tr180,t30,Ts600

100 nm 100 nm

100 nm 100 nm

0t15Ts400, (c) Tr130t30Ts400, (d) Tr180t30Ts400, (e) Tr130t15Ts600, (f) Tr180t15Ts600,


conditions. By further increasing the pH up to alkalinemedium, a further decrease in morphology from nanorods tospherical particles was provoked.

The interplanar spacing of the PrPO4 crystals was deter-mined from the inverse Fourier Transform of the HRTEMmicrographs. It is clearly seen that at pHs 1 and 3, the samplesynthesized under no. 4 conditions (sintered at 400 1C) showsa fringe spacing value between 6.2 and 6.8 Å, which matchesto the theoretical value of (100) of the hexagonal PrPO4,

indicating that the growth direction is parallel to it (Fig. 7).This fringe value persists in the samples that were synthesized

1400 1200 1000 800 600

957996

580 566pH 1

542

615

pH 3

Tr180

t30

Ts400

pH 5

pH 9

Tran

smitt

ance

(%)

Wavelength (cm-1)

pH 11

1400 1200 1000 800 600

614

580 566pH 1

539

957pH 3

Tr180

t30

Ts600

pH 5

pH 9

Tran

smitt

ance

(%)

Wavelength (cm-1)

pH 11

Fig. 5. FT-IR spectra of the PrPO4 nanopowders: (a) samples synthesizedunder experiment 4 conditions at different pH values and (b) samplessynthesized under experiment 8 conditions at different pH values.

at pH 5. Different crystals belonging to the monoclinic systemat (100) (d¼3.9 Å) plane are also seen. Planes (111¯ ), (002) and(020) are evident in the spherical particles synthesized inalkaline medium.On the other hand, when samples are sintered at 600 1C

(condition no. 8), the monoclinic fringe space is more evidentindependently of the morphology type: nanorods or sphericalparticles (Fig. 8).The monodispersity analysis of the PrPO4 synthesized under

these experimental conditions showed a strong dependence ofthe particle size and shape on the pH and sintering tempera-ture. The PDI values for the Tr180t30Ts400 samples showmonodisperse particles in acid medium (pH¼1–3), however,the sintering temperature modifies the particle size distributionas it is shown by the samples prepared at Tr180t30Ts600. In thelast case, only at pH between 5 and 11 monodisperse systemscan be obtained. The particle size estimated by DLS wascompared with TEM images, which are inset in the particlesize distribution graphs in Figs. 9 and 10. In agreement withthe TEM micrographs, the highest particle size, estimated byDLS, is caused by nanopowder agglomeration. In fact, withexception of the samples obtained under the Tr180t30Ts400(pH¼1) conditions, where nanorods are seen (23 nm dia.and 94 nm length on average), all the samples tend to formagglomerates.The synthesis of PrPO4 by the hydrothermal method is

associated with the interaction of the dipole molecules withhigh frequency electromagnetic radiation. However, when it isassisted by the microwave method, a microwave diffusesvolumetrically through the material being heated; in suchway, the polar molecules (such as H2O) absorb microwaveenergy and provoke energetic rotational movements as aconsequence of the polarization of ions [13], providing aplatform to reduce the synthesis time and control the nuclea-tion and growth of PrPO4 nanostructures. It has been statedthat some rare earth ions like La3þ and PO4

3− are stackedalternately along the [001] direction in the hexagonal crystalstructure of LaPO4; thus, it is reasonable to expect thepraseodymium ion to be of similar ionic radii and the chargemust display a comparable behavior at low pH. It is believedthat the highest Hþ ion concentration produces the adhesion ofthese ions on the crystal facets, producing an increase in theelectrostatic potential and allowing the union of the sphericalparticles to form the nanorod-type morphology as it is shownin Fig. 11. In these systems, the largest particles grew at thecost of the small ones, due to the solubility energy differencebetween the largest particles and the smallest ones, accordingto the well-known Thomson–Gibb law [27]. Thus, the stackingleads to an amount of net charges and the strongest polarity for(100) facets in comparison with other lattice planes. In suchcase, the activation energy for crystal growth along the c axisof the hexagonal phase is lower than that perpendicular to the caxis from a thermodynamic perspective, which implies that thegrowth rate along the c axis is higher than that perpendicular tothe c axis. All these affecting factors drive 1D nanorods togrow preferentially along the [100] direction. Consequently,the promotion of the anisotropic growth of the as-prepared

20 30 40 50 60 70

Monoclinic

Hexagonal

*

*

Tr180°Ct30min pH11*

*Tetragonal

(024

)(3

11)

(311

)

(112

)

(200

)

*

(320

)(1

03)

(120

) Tr180°Ct30min pH9

Inte

nsity

(a.

u.)

(120

)

Tr180°Ct30min pH5

2 θ (degrees)

(211

)

(033

)

(323

)

(302

)(2

03)

(301

)

(112

)

(102

)(2

00)

Tr180°Ct30min pH3

(011

)

(340

)(0

14)

(004

)

(200

)

(023

)(3

22)

(132

)(1

40)

(014

)

(103

)(2

31)

(212

)(1

03)

(031

)(1

12)(012

)

(020

)

Tr180°C

t30min pH1

(101

)

(120

)

(202

)

(111

)

20 30 40 50 60 70

Hexagonal

Monoclinic

Tr180°Ct30minTs400° pH1





(32

2)

(112

)

(103

)

(132

)

(020

)

Inte

nsi

ty (

a.u

.)

(101

)

(400

)

(103

)(3

01)

(311

)

(012

)(1

02)

(002

)

2θ (Degrees)

20 30 40 50 60 70

Monoclinic


(431

)

(120

)

(002

)

(002

) Tr180°Ct30minTs600° pH9

Inte

nsity

(a.

u.) Tr180°Ct30minTs600° pH5

2 θ (degrees)


(122

)

(111

)

(110

)

(124

)(0

14)

(004

)(1

40)

(132

)(3

22)

(023

)(1

03)

(231

)(2

12)

(103

)(0

31)

(112

)(2

02)

(012

)(0

02)

(200

)(0

20)

(111

)


Fig. 6. X-ray diffraction patterns of PrPO4 samples: (a) as-prepared samples adjusted at different pH values, (b) samples synthesized under experiment 4 conditionsat different pH values and (c) samples synthesized under experiment 8 conditions at different pH values.


nanorods (100) is mainly governed by a general cooperativeeffect including the intrinsic structural features of specificfaces, local solution details, foreign energy activation andautogenous pressure [28].

On the other hand, when increasing the pH value, a semisphe-rical-type morphology is favored. According to previous worksrelated to rare earth phosphates, colloidal precipitates have beenobserved as the pH value of the solution was decreased [29].However, in this work, precipitates were more evident in alkalinethan in acid medium. The precipitates were associated with the

presence of a small amount of free PO43− ions, where the motion

speed reached a maximum value [29].During the synthesis of PrPO4, nucleuses of tiny crystals are

formed and subsequently the crystal grows generating thesemispherical particles. There are higher OH� ion concentra-tions which are absorbed onto the crystal facets, producing adouble layer composed of cations (Hþ and NH4

þ ) and anions(NO3

- and OH�) as shown in Fig. 11.By taking into account these facts and considering those

already established in the literature [15,29], the most probable


reactions of the PrPO4 formation, either in acid or alkalinesolutions, can be proposed as follows:

Reaction in acid medium:

aq aq aq aqH P O H O H P O H PO 85 3 10 2 4 2 7 3 4( ) + ( ) → ( ) + ( ) ( )

H P O H O 2H PO 94 2 7 2 3 4+ → ( )

Fig. 7. Selected HRTEM micrographs of

aq aq aq

s aq n aq

Pr NO 6H O HNO H PO

PrPO 4HNO H O 10

3 3 2 3 3 4

4 3 2

( ) ( )⋅ + ( ) + ( )

→ ( ) ↓ + ( ) + ( ) ( )

Reaction in alkaline medium:

aq aq aq aqH P O H O H P O H PO 115 3 10 2 4 2 7 3 4( ) + ( ) → ( ) + ( ) ( )

experiment 4 at different pH values.

Fig. 8. Selected HRTEM micrographs of experiment 8 at different pH values.


H P O H O 2H PO 124 2 7 2 3 4+ → ( )

aq aq aq

s aq n aq

Pr NO 6H O aq NH OH H PO

PrPO 2NH NO H O 13

3 3 2 4 3 4

4 4 3 2

( ) ( )⋅ ( ) + ( ) + ( )

→ ( ) ↓ + ( ) + ( ) ( )

The effect of the reaction temperature, synthesis time andsintering temperature used for obtaining PrPO4 on the opticalproperties was evaluated from the diffuse reflectance spectra

and the results are presented in Fig. 12. It can be clearlyobserved that the spectra consist mainly of four strong bands at445, 472, 484, 595 and 601 nm assigned to 4f–4d transitionsof the following energy levels: 3H4-

3P2,3H4-

3P1,3H4-

3P0,3H4-

3D2 (upper) and 3H4-3D2 (lower), respec-

tively [30]. All the spectra display absorption in the UV range,indicating that they are good candidates as UV absorbers to beused in polymer applications, where degradation is needed tobe prevented. The only remarkable difference is found for the

Fig. 9. Particle size distribution of samples synthesized under experiment 4 conditions at different pH values.


powders synthesized at pH 9 and 11, which display absorptionfrom the UV region to the visible range; however, thesenanopowders can also be used for polymer applications sincethe absorption in the visible range does not affect themacromolecular backbone. From the absorption spectrumand Kubelka–Munk function, it was found that the band gapvaries from 5.1 to 5.4 eV with small variations depending onthe solution pH.

To correlate the optical properties with the final application,the fluorescence of selected samples was acquired using aZEISS LSM700 confocal microscope.Fig. 13 displays theemission spectra of selected PrPO4nanoparticles samples under405 nm of excitation at room temperature. Comparison of theemission spectrum of Tr1801Ct30minTs6001C modified in acidconditions with that obtained in alkaline media afforded thehigh association of the pH with the morphological propertiesand the final photonic properties of PrPO4nanostructures.Comparing the two emission spectra, it can be found thatPrPO4 nanorods (i.e. sample obtained at pH¼1) spectrumconsists mainly of two separated bands of fluorescence withmaxima at �510 nm and �610 nm which are due to the3P0-

3H5 and 1D2-3H4 transitions of Pr3þ in the sample

[31]. Results reveal no significant dependence of emission on

the sintering temperature. In the case of smaller particles(obtained at pH¼9), it is seen that an enhancement in OH�

ions during the powders synthesis could lead to the quenchingof the luminescence; which may explain the poor emission ofprepared samples at pH¼9.Typically semi-conductor material used as UV absorber

works by absorbing photons producing electron–hole pairswhich in turn react with oxygen, water or hydroxyls groups toform free radicals when holes and electrons join the surfacestarting the breaking bonds in the polymer [32], because thelatter will degrade more easily the backbone as a consequenceto the heat concentrated on them. Thus, the results highlightthat microwave-assisted hydrothermal method can modulatecrystallite size, structure and morphology of UV absorbersmaterials, offering the possibility to transform the high energyfrom UV photons on radiation of low energy (Visible light)instead of concentrating the energy (thermal energy). The latteris of great advantage to avoid plastic degradation.

4. Conclusions

Polycrystalline PrPO4 nanostructures were successfully pre-pared by the microwave-assisted-hydrothermal method from Pr

0

5

10

15

20

Freq

uenc

y

0 100 200 300 400 5000

5

10

15

20

25

Freq

uenc

y

Diameter (nm)

0 100 200 300 400 500 6000

5

10

15

20

25

30

Freq

uenc

y

Diameter (nm)0 200 400 600 800 1000

0

5

10

15

20

25

Freq

uenc

yDiameter (nm)

0 200 400 600 800 10000

5

10

15

20

25

Freq

uenc

y

Diameter (nm)

Fig. 10. Particle size distribution of samples synthesized under experiment 8 conditions at different pH values.

Fig. 11. Schematic representation of the formation of PrPO4 with (a) nanorod-type morphology in acid medium and (b) semispherical-type morphology in alkalinemedium at crystal level.


(NO3)3 � 6H2O and a tripolyphosphoric acid solution. Themethod was simple and easily repeated and could be devel-oped for the synthesis of different rare earth phosphate

materials. The study also demonstrates that by using thismethod and modulating the pH, the hexagonal structure istransformed into the monoclinic one. Even this transformation

200 300 400 500 600 7000

20

40

60

80

100

Tr180°C

t30min

Ts600°C

Tr130°Ct30minTs600°C

Tr180°C

t30min

Ts400°CTr180°Ct15minTs600°C

Tr180°C

t30min

Tr130°C

t15min

Ts600°C

Tr130°C

t30min

Ts400°C

Tr130°C

t30min

Tr180°Ct15min

Tr130°C

t15min


Diff

use

refle

ctan

ce (

%)

λ (nm)


200 300 400 500 600 700

0

20

40

60

80

100

8 (Tr180°C

t30min

Ts600°C

)4 (Tr

180°Ct30min

Ts400°C

)

Diff

use

refle

ctan

ce (

%)

8 (pH 11)8 (pH 9)

8 (pH 5)

8 (pH 3)

8 (pH 1)

4 (pH 11)

4 (pH 9)

4 (pH 5)

4 (pH 3)

4 (pH 1)

λ (nm)

Fig. 12. Diffuse reflectance spectra of: (a) the entire as-prepared PrPO4 powders before and after the sintering process (1st experiment) and (b) powders synthesizedunder conditions 4 and 8 and adjusted at different pH values.

450 500 550 600 6500

20

40

60

80

100

120

140

160

610 nm

Tr180°Ct30minTs400°C pH 1



Em

issi

on In

tens

ity

Emission wavelength [nm]

510 nm

Fig. 13. Fluorescence of selected PrPO4 samples under 405 nm excitation.


took place at lower temperatures in comparison with othertraditional processes such as the solid state reaction, co-precipitation, solvothermal or sol–gel techniques.

The size and shape of the as-obtained nanopowders werestrongly dependent on the pH, which can be the mostdetermining parameter during the synthesis process. Finally,the results regarding the PrPO4 optical properties reveal thehigh absorption in the UV region, indicating the possibleapplication of these materials to reinforce the structuralproperties of polymeric materials.

Acknowledgments

D. Palma-Ramírez is grateful for her postgraduate scholar-ship to CONACYT, SIP-IPN and COFAA-IPN. The authors

are also grateful for the financial support provided by CON-ACYT through the CB2009-132660 and CB2009-133618projects and to IPN through the SIP 2015-0202 and 2015-0227 projects and SNI-CONACYT.

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Enhancement ofopticalpropertiesanddependenceofthecrystalstructure, morphological propertiesofPrPO4 by microwave-assisted-hydrothermal synthesis