Materials Science and Engineering B 156 (2009) 42–47

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Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

Na2EDTA-assisted hydrothermal synthesis and luminescent properties ofYVO4:Eu3+ with different morphologies in a wide pH range

Juan Wanga,b, Yunhua Xua,∗, Mirabbos Hojamberdieva, Jianhong Penga, Gangqiang Zhuc

a Shaanxi Key Laboratory of Nano-materials and Technology, Xi’an University of Architecture and Technology, Xi’an 710055, PR Chinab School of Science, Xi’an University of Architecture and Technology, Xi’an 710055, PR Chinac School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, PR China

a r t i c l e i n f o

Article history:Received 3 August 2008Received in revised form28 September 2008Accepted 4 November 2008

Keywords:

a b s t r a c t

Nano- and micro-scaled Eu-doped yttrium orthovanadate (YVO4:Eu3+) powders had been fabricatedvia disodium ethylenediamine tetraacetate (Na2EDTA)-assisted morphology controllable hydrothermalmethod in a wide pH range at 180 ◦C for 24 h. The as-synthesized samples were characterized by X-raypowder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM),and photoluminescence spectroscopy (PL). The results showed that the pH value of the synthesis solutionplayed a key role in the formation of the final products with different morphologies, including ball-likemicro-spheres, micro-spheres composed of submicron cubes and flower-like structures containing nano-

Hydrothermal synthesisMorphologyP

plates. The photoluminescence measurement revealed that the luminescent properties of the sampleswere changed by varying their morphologies. The significant ball-like micro-spheres of YVO :Eu3+ parti-

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hotoluminescencecles had been synthesized

. Introduction

Inorganic nano-crystals have exhibited many fascinatingovel size- and shape-dependent properties [1–3]. In particular,orphology-controlled synthesis of nano-structured optical mate-

ials has great significance in systematic fundamental studies ofrystal growth and exploring new applications. Yttrium orthovana-ate (YVO4), an important optical material, has many profitableharacteristics, that is, good thermal, mechanical and optical prop-rties, which make it play an important role in many devicenvolving the artificial production of light and display fields, suchs excellent polarizer [4,5] and laser host material [6,7]. Dopinghe YVO4 powder with Eu3+ has been used as a red phosphor inolor television and cathode ray tubes (CRTs), owing to its high-uminescence efficiency upon electron-beam excitation [8,9].

It is well-known that preparation methods have high effects onaterial micro-structure and physical properties. YVO4:Eu3+ can be

repared either by solid-state reaction at high temperatures [10] ory solution chemistry method including hydrothermal synthesis11–14], hydrolyzed colloidal reaction technique [15], and induced

recipitation [16]. However, the hydrothermal synthetic route hasdvantages to obtain high-crystallized powders with narrow parti-le size distribution and high purity without further heat treatmentt high temperatures. Morphology and size of the particles can be

∗ Corresponding author. Tel.: +86 29 82202531; fax: +86 29 82202886.E-mail address: yunhuaxu@yahoo.com.cn (Y. Xu).

921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2008.11.008

4

the luminescence intensity of them is the strongest one among all products.© 2008 Elsevier B.V. All rights reserved.

controlled via the hydrothermal process as well by adjusting thesource species, reaction temperature and time, etc. Recently, sev-eral researches have been done by hydrothermal synthesis to obtaindifferent size and shape of YVO4:Eu3+ nano-materials. Wu et al.[11] have synthesized YVO4 nano-rods/micro-tubes by a hydrother-mal reaction of (NH4)0.5V2O5 nano-wires templates with Y3+ atpH 5–6. Wang et al. [17] prepared single phase of Eu3+-dopedYVO4 nano-phosphors by a mild hydrothermal method at differ-ent pH values (pH 7–11). Wu et al. [18] once tailored the sizeand shape of YVO4 crystallites by changing the reaction condi-tion in both strongly acidic and basic media. The morphologicalcontrol of YVO4:Eu3+ nano-cystallites has been performed by vari-ation of the pH value (8–14) in the basic solution and pointed outthat luminescence intensity increased from bundle-like, rice-liketo rhombus-like nano-crystals [19]. Wu et al. [20] have fabricatedthe rod-like, olive-like, and pineapple-like nano-crystals of theYVO4:Eu3+ under hydrothermal conditions by using porous sili-con substrates, V2O5 nano-wires and CTAB additives at pH 6–7.The results of those experiments are interesting, but the pureYVO4:Eu3+ had been synthesized by variation of the narrow pHvalue or the complex routes, and the morphology of the YVO4:Eu3+

changed unremarkable. It is well-known that tuning the pH valueof the growth solution is a crucial step for the control of the mor-

phology transformation. A number of studies demonstrated thatthe morphology of the product has a strong dependence on thekind of surfactant. The surfactant-assisted hydrothermal synthe-sis had been successfully used to control the morphology andsize of particles. Recently, ethylenediaminetetraacetic (EDTA) has

and Engineering B 156 (2009) 42–47 43

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een effectively employed in the hydrothermal process as a chelat-ng ligand and structure-directing agent to produce nano-crystals

ith different morphologies [21,22]. However, the reports on theydrothermal synthesis of YVO4:Eu3+ using disodium ethylenedi-mine tetraacetate (Na2EDTA) as the surfactant are few.

Therefore, in this paper, we report a simple Na2EDTA-ssisted hydrothermal process to prepare nano- and micro-scaledVO4:Eu3+ powders with various morphologies by the variationf pH value (in the range of 1–14). The phase structures, mor-hologies and optical properties of the as-synthesized samples areharacterized by X-ray powder diffraction (XRD), scanning elec-ron microscopy (SEM), transmission electron microscopy (TEM)nd photoluminescence spectroscopy (PL), respectively.

. Experimental

.1. Preparation of Y0.95Eu0.05VO4 powders

Y0.95Eu0.05VO4 powders were prepared via surfactant-assistedydrothermal synthetic route as follows. The stoichiometricmounts of Y2O3 (99.99%), Eu2O3 (99.99%) and NH4VO3 (99%) wereissolved in 25 ml HNO3 (1 mol/L). The amount of Na2EDTA playsn important role for the final morphology. The shape of YVO4 wille changed if the amount of Na2EDTA is more or less. In this experi-ent, the same molar amount of Na2EDTA with the amount of YVO4as added into the mixed solution as a complex agent. The solutionas stirred for 10 min to form a yellow-colored mixture. Subse-

uently, sodium hydroxide solution was added drop-wise underigorous stirring until the pH value of the mixture was adjusted to, 4, 7 and 14. After that, the well-stirred solution was poured into a0-mL Teflon-lined stainless steel autoclave with a filling capacityf 75% and heated at 180 ◦C for 24 h. As the autoclave was cooledown to room temperature, the as-synthesized precipitate was col-

ected, washed with distilled water and absolute ethanol severalimes, and dried in vacuum at 80 ◦C for 10 h.

.2. Characterization

The phase composition of as-prepared powders were charac-erized by an X-ray powder diffraction using a CuK� radiationt � = 1.5406 Å at 40 kV and 50 mA (Model D/MAX2550V, Rigakuo., Tokyo, Japan). The morphology of as-synthesized powdersas observed using a scanning electron microscopy (FEI Quanta

00 ESEM, The Netherlands). The images of transmission elec-ron microscopy, high-resolution transmission electron microscopyHRTEM) and selective area electron diffraction (SAED) werebtained on a JEM-2100F electron microscope (JEOL, Tokyo, Japan)ith an accelerating voltage of 200 kV. The excitation and emission

Fig. 2. SEM images of the YVO4:Eu3+ powders synthesized at 180 ◦C for

Fig. 1. XRD patterns of YVO4:Eu3+ crystals synthesized at 180 ◦C for 24 h with dif-ferent pH values: (a) 1; (b) 4; (c) 7; (d) 14.

photoluminescence spectra were measured using a PerkinElmerLS55 fluorescence spectrometer (PerkinElmer, Shelton, USA).

3. Results and discussion

The XRD patterns of the as-synthesized powders with differentpH (1, 4, 7, and 14) at 180 ◦C for 24 h are shown in Fig. 1. Despite thebroadening caused by the small size effect of the particles, all thepeaks can be indexed as a tetragonal phase, approaching the stan-dard values for the bulk YVO4, which is in good agreement withliterature data (JCPDS Card No. 17-0341). The sharpening of thediffraction peaks indicates that the products are well-crystallized.No characteristic peaks are observed for other impurities, whichmean a pure YVO4:Eu3+ can be obtained under the current condi-tions.

The pH value of the reaction solution plays an important rolein controlling the morphologies of the resulting products. The SEMand TEM images of the samples obtained at pH 1, 4, 7 and 14 areillustrated in Figs. 2, 3, 4 and 5, respectively. It is well known thatspherical morphology of YVO4:Eu3+ is necessary for obtaining highbrightness and high-resolution due to the high packing densitiesand low scattering of light [23]. However, the reports on uniformball-like micro-spheres of YVO4:Eu3+ are scarce. Fig. 2 indicates the

as-synthesized YVO4:Eu3+ powders prepared at 180 ◦C for 24 h withpH 1. It is clearly shown that the large-scale uniform ball-like micro-spheres product has regular morphology and well-crystalline statewith the diameter in the range of 2–3 �m. A fewer twinning sphereswith a clear interface are included as shown in Fig. 2b (arrows).

24 h with pH 1: low-magnification (a) and high-magnification (b).

44 J. Wang et al. / Materials Science and Engineering B 156 (2009) 42–47

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ig. 3. SEM and TEM images of the YVO4:Eu3+ powders synthesized at 180 ◦C for 24EM image of several dispersed submicron cubes; (e) TEM image; (f) HRTEM image

n contrast, the ball-like micro-spheres of YVO4:Eu3+ are differentrom the nano-belts synthesized without the addition of Na2EDTA15]. The environment of the Na2EDTA solution has influenced theormation of the shape of YVO4:Eu3+ crystals.

When the pH value of the solution increased to 4, the morphol-gy of the YVO4:Eu3+ powders (Fig. 3) was quite different fromhe sample synthesized at pH 1. Fig. 3a shows a low-magnificationEM image of the as-synthesized sample that micro-spheres withifferent size distribution were formed. Fig. 3b and c shows high-agnification SEM images that the surfaces of these twinningicro-spheres are composed of uniform submicron cubes. To

urther investigate crystal status and morphology of YVO4:Eu3+

owders, the HRTEM, SAED, and corresponding TEM images ofhe final products are represented in Fig. 3d–g, respectively.ig. 3d shows several dispersed and well-defined submicron cubesith uniform edge length in the range of 400–600 nm. Low-agnification TEM image in Fig. 3e reveals the micro-structure is

h pH 4: (a) low-magnification SEM image; (b, c) high-magnification SEM image; (d)e edge of the submicron cube; (g) SAED pattern.

composed of submicron cubes with a hollow characteristic. Furtherinsight into the cube nano-crystal was gained by HRTEM observa-tion on the edge of the cube as shown in Fig. 3e. The lattice fringe of0.36 and 0.47 nm in the observed submicron-crystallites agree wellwith the (2 0 0) and (1 0 1) lattice planes as shown in Fig. 3f. TheSAED pattern (Fig. 3g) taken on the cube submicron-crystal showsa single nature with well-ordered and linearly arranged spots, sim-ilar to single-crystal-like diffraction spots, which indicates that thecubes formed are single crystallines with a preferred growth orien-tation. The corresponding SAED pattern is well-consistent with theresult of HRTEM.

Fig. 4 shows the SEM and TEM images of the YVO4:Eu3+ pow-

ders synthesized with the pH value of 7. A SEM image of theas-synthesized powders with flower-like structures composed ofnano-plates with a uniform diameter about 30 nm is illustrated inFig. 4a. The TEM, SAED, and HRTEM images are shown in Fig. 4b–e,respectively. TEM image in Fig. 4b reveals a flower-like micro-

J. Wang et al. / Materials Science and Engineering B 156 (2009) 42–47 45

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ig. 4. SEM and TEM images of the YVO4:Eu3+ powders synthesized at 180 ◦C for 2igh-magnification TEM image; (d) HRTEM image on the edge of the nano-plate; (e

tructure composed of many nano-plates. A high-magnificationEM image shows the nano-plates with the diameter in the rangef 10–50 nm (Fig. 4c). Fig. 4d indicates the HRTEM image that theattice fringe of 0.36 and 0.47 nm in the observed nano-crystallitesgree well with the (2 0 0) and (1 0 1) lattice planes. The SAED pat-ern (Fig. 4e) taken on a nano-plate shows a single nature withinearly arranged spots and well agreed with the result of HRTEM.

After adjusting pH to 14, the shapes of the micro-structures wereransferred into nano-plates (Fig. 5a) with a width of about 200 nmnd lengths of around 1.0 �m. Fig. 5b shows a high-magnificationEM image that the nano-plate has smooth surface and the aspectatio is about 5. Fig. 5c shows the HRTEM image that the latticeringe of 0.36 and 0.475 nm in the observed nano-crystallites agreeell with the (2 0 0) and (1 0 1) lattice planes. The SAED pattern

Fig. 5d) taken on the nano-plate shows a single nature with linearlyrranged spots and also well-agreed with the result of HRTEM.

Consequently, the morphology of YVO4:Eu3+ could be manip-lated by adjusting the pH value of hydrothermal solution. Aseported by Ropp and Carroll [24], the form of vanadium ions werextremely sensitive to the pH of the solution, the vanadium ionsxisted as VO2

+ when the solution was in strong acidity. When theH rose to 2, vanadium ions were in the form of V10O28

6− prin-ipally; while the pH further increased to 5, vanadium ions wereiable to the form of V3O9

3−. VO43− existed in the solution when

he pH values were above 7. In our case, since the pH of the solutionanges from 1 to 14, the following reactions might occur:

2O3 + Eu2O3 + HNO3 → Y(NO3)3 + Eu(NO3)3 + H2O

VO2+ + Eu3+ + Y3+

hydrothermal process−→ YVO4 : Eu3+(spherical structures) (pH 1)

ith pH 7: (a) low-magnification SEM image; (b) low-magnification TEM image; (c)pattern of the nano-plate.

V10O286− + Eu3+ + Y3+

hydrothermal process−→ YVO4 : Eu3+(spherical structures) (pH 4)

VO43− + Eu3+ + Y3+ + EDTA2−

hydrothermal process−→ YVO4 : Eu3+(plate-like structures) (pH 7 or 14)

It is well-known that the addition of additive agents can affectthe nucleation and growth of particles, which consequently canmodify particle morphology and size. Because different pH valuecan induce different modality of Na2EDTA, when the pH value ofsynthesis solution ranged from 1 to 4, the EDTA2− ions are few,so the process of formation YVO4:Eu3+ structures are mainly thehomogeneous precipitation process. The homogeneous nucleationand growth in aging process is essential for the preparation ofspherical phosphors. When the pH value increased to 7 and 14,the Na2EDTA leads to the anisotropic growth of nano-particles.Because it is clear that a strong ligand (Na2EDTA) is not only neededto form a stable complex with Y3+, but also the ligand binds tothe surface of the crystal, which directly affects the growth direc-tion and crystal structure of the nano-crystals. Moreover, as tothe tetragonal YVO4:Eu3+, the atom density on the (2 0 0) crys-tallographic plane is larger than those on other planes, whichmight provide more absorbed opportunity of the carboxylic rad-icals on the (2 0 0) plane. This preferred adsorption on the (2 0 0)plane further affected the aggregation of the tetragonal YVO4:Eu3+

nano-particles to ultimately results in the formation of plate-like structures. The great change of morphology of YVO4:Eu3+

powders had been fabricated by this way is different from theother surfactant-assisted hydrothermal methods in a pH range[13,19,20].

46 J. Wang et al. / Materials Science and Engineering B 156 (2009) 42–47

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peak at 537 nm is related to transition from D1 to F1. Meanwhile,

ig. 5. TEM and HRTEM images of the YVO4:Eu3+ powders synthesized at 180 ◦C fof a single nano-plate; (c) HRTEM image on the edge of the nano-plate; (d) SAED pa

PL spectra of the samples were measured using a fluorescencepectrometer at room temperature. Fig. 6 shows PL excitation spec-ra of the Y0.95Eu0.05VO4 samples synthesized with different pHalues (pH 1, 4, 7, 14) under the emission of 616 nm. All the exci-ation spectra consist of abroad band ranging from 200 to 350 nm

ith a maximum peak at about 312 nm. The f–f transition within

u3+ 4f6 electron configuration can also be observed between theange of 350 and 500 nm although the intensity is weak. PL emis-ion spectra of samples synthesized with different pH values underhe excitation of 312 nm are shown in Fig. 7. The spectra composed

ig. 6. Excitation spectra of the samples with different morphologies: (a) micro-pheres; (b) micro-spheres composed of submicron cubes; (c) flower-like structuresontaining nano-plates; (d) nano-plates.

with pH 14: (a) low-magnification TEM image; (b) high-magnification TEM imageof the nano-plate.

of sharp lines ranging from 500 to 700 nm are associated with thetransitions from the excited 5D0 level to 7FJ (J = 1–4) levels of Eu3+

activators. The emission bands around 584.0 and 591.0 nm couldbe ascribed to the 5D0 → 7F1 magnetic dipole transitions; the weak

5 7

the emission peaks located at about 612 and 616 nm are mainlydue to the transitions from 5D0 to the two sublevels of 7F2 energylevel. The 5D0 → 7F2 transition is a forced electronic dipole–dipoletransition and hypersensitive to the environment. And their high

Fig. 7. Emission spectra of the samples with different morphologies: (a) micro-spheres; (b) micro-spheres composed of submicron cubes; (c) flower-like structurescontaining nano-plates; (d) nano-plates.

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ntensities are a consequence of the absence of an inversion sym-etry at the Eu3+ lattice site (D2d symmetry) [14,25]. The position of

he predominant peak of the nano-plates sample (pH 14) is changedo 612 nm. The change in the relative intensity of the two peaksan be attributed to polarization effects related to preferential ori-ntation of the nano-plates and absent in the micro-spheres, andhe preferential orientation of the nano-plates would result in theigher degree of disorder and corresponding lower symmetry ofrystal fields around Eu3+ ions. It is well-known that the 5D0 → 7F2ransition is highly sensitive to structure change and environmentffects. The difference of 5D0 → 7F2 emission peak in relative inten-ity and positions is due to the difference of the effects of the crystaleld perturbation on the individual f–f transitions. On the basis ofomprehension, the position of the predominant peak is associ-ted with the micro-structure. The luminescence intensity of theall-like micro-spheres is the strongest one among them; the nano-lates powders are stronger than the micro-spheres composed ofubmicron cubes because of different sizes and shapes result in dif-erent combinative abilities between the surface and the adsorbedpecies; so as to it produce the different quenching abilities to themission from Eu3+ ions. As the samples were prepared in water, theurface of the YVO4 may be covered with hydroxyl species, whichct as the efficient quenchers of the excited Eu3+ ions. The combi-ative abilities between quenching species and crystal surfaces isffected by the sizes and shapes of YVO4, so the differences of PLntensity of the samples may be attributed to the different quench-ng abilities of the adsorbed species on the surface to the emissionrom Eu3+ ions. Therefore, by changing the morphology of powders,uminescence properties can be effectively adjusted.

. Conclusions

Eu-doped YVO4 powders with different morphologies were syn-hesized by Na2EDTA-assisted hydrothermal process in a wide pHange at 180 ◦C for 24 h. It has been found that pH value of syn-

hesis solution plays an important role in the formation of thenal products with different morphologies. The significant ball-likeicro-spheres of YVO4:Eu3+ particles had been synthesized at pH 1,

ncreasing pH value causes the morphology transformation: micro-pheres composed of submicron cubes, and flower-like structures

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ngineering B 156 (2009) 42–47 47

containing mainly nano-plates. Photoluminescence measurementsshowed that the position of the predominant peak and relativeintensity changed with the morphology. The luminescence inten-sity of micro-spheres particles is the strongest one among allproducts.

Acknowledgement

The authors are grateful to Science Foundation of ShaanxiProvincial Education Department for the financial support (Nos.08JK346 and 08JZ38).

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