Journal of Colloid and Interface Science 343 (2010) 71–78

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Journal of Colloid and Interface Science

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Hydrothermal synthesis and luminescent propertiesof YVO4:Ln3+ (Ln = Eu, Dy, and Sm) microspheres

Fei He a, Piaoping Yang a,*, Na Niu a, Wenxin Wang a, Shili Gai a, Dong Wang a, Jun Lin b,*

a College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR Chinab State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China

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

Article history:Received 26 September 2009Accepted 7 November 2009Available online 14 November 2009

Keywords:Hydrothermal synthesisDMFPVPLuminescenceYVO4

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.11.025

* Corresponding authors. Fax: +86 431 85698401.E-mail address: piaoping@ciac.jl.cn (P. Yang).

Rare-earth ions (Eu3+, Dy3+, Sm3+) doped YVO4 microspheres with uniform morphologies were success-fully prepared via a simple hydrothermal route using N,N-dimethylformamide (DMF) as the solventand polyvinylpyrrolidone (PVP) as protective agent. X-ray diffraction (XRD), scanning electron micros-copy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectra, and the kineticdecays were employed to examine the resulting phase formation, particle morphology and luminescentproperties. The XRD results reveal that all the doped samples are of high crystallization which areassigned to the pure tetragonal phase of YVO4. Additionally, the DMF/H2O volume ratio and the concen-tration of PVP both have obvious effects on the morphologies and sizes of the as-synthesized products.The sample prepared at 180 �C for 24 h with the DMF/H2O volume ratio of 3/1 and 0.4 g/L PVP concen-tration exhibits uniformly spherical shape with the diameter of 1–2 lm. Upon excitation by ultravioletradiation or low-voltage electron beams excitation, the YVO4:Ln3+ (Ln = Eu, Dy, and Sm) samples showstrong light emissions with different colors from the doped Ln3+ ions. These phosphors exhibit potentialapplications in the fields of fluorescent lamps and light emitting diodes (LEDs).

� 2009 Elsevier Inc. All rights reserved.

1. Introduction

As an important optical material, yttrium orthovanadate (YVO4)has been widely used as an excellent polarizer [1,2] and laser hostmaterial [3,4] owing to its outstanding characteristics, such asgood thermal, mechanical and optical properties. Moreover, rare-earth ions doped yttrium orthovanadate matrix has been employedto produce phosphors which can emit various colors because of thehigh luminescence quantum yields originating from the f–f transi-tions [5]. Especially, Eu3+-doped YVO4 powder, as a significantcommercial red-emitting phosphor, has been extensively utilizedin optical devices, such as color television, field emission displays(FEDs), cathode ray tubes (CRTs) and plasma display panels (PDPs)[6–8]. Besides europium ion, Dy3+ and Sm3+ ions have also beenshown to be good activators [9,10].

In recent years, much effort has been devoted to the synthesis ofinorganic phosphors with fine particle size, controlled morpholo-gies and high luminescence efficiencies [11,12]. Many syntheticroutes have been developed to prepare YVO4 doped with rare-earth ions. For instance, high-temperature solid-state reaction isa conventional method, which is simple but has several disadvan-tages, such as high energy consumption and agglomeration of par-ticles [13,14]. Compared with the conventional method, wet

ll rights reserved.

chemistry methods have been proved a more powerful techniquefor preparing single-phased and well-crystalline YVO4, such assol–gel process [8,15], solution combustion synthesis [16], co-pre-cipitation reaction [17], and low temperature hydrothermal meth-ods [9,18,19]. Among all the methods for the synthesis of YVO4:Lnphosphors, hydrothermal method is one of the most importanttechniques, because it can synthesize particles with high-crystal-linity, narrow size distribution and high purity at a mild reactioncondition without milling or further calcination treatment [20].In a hydrothermal process, chemical reactions are carried out inan autoclave in the presence of an aqueous solvent above roomtemperature and at elevated pressure. Furthermore, hydrothermalmethod can easily control the size and morphology of crystalsthrough regulation of the starting materials quantum, reactiontemperature and time. Wu et al. [21] reported the synthesis ofwell-defined YVO4 microcrystal with clear facets in strongly acidicmedia and nanoflakes with dimensions of 5–50 nm in basic media.Wang et al. [22] prepared single phase of Eu3+-doped YVO4 nano-phosphors at different pH values (pH = 7–11) by a mild hydrother-mal method. Wu et al. [23] synthesized YVO4 nanorods/microtubesby a hydrothermal reaction of (NH4)0.5V2O5 nanowires templateswith Y3+. This group has also synthesized rod-, olive-, and pineap-ple-shaped nanocrystals of YVO4:Eu3+ using porous silicon sub-strates, V2O5 nanowires, and CTAB additives [24]. Wang et al.[25] prepared nano- and micro-scaled Eu-doped yttrium orthovan-adate powders via Na2EDTA-assisted morphology controllable

72 F. He et al. / Journal of Colloid and Interface Science 343 (2010) 71–78

hydrothermal method in a wide pH range. However, modern FEDsrequire the phosphors possess uniformly spherical morphologyand suitable particle size range (1–3 lm), which can offer the pos-sibility of brighter cathodoluminescent performance, high defini-tion, and much improved screen packing due to their highpacking density and reduction of light scattering [26–29]. Thus,developing a facile approach for the synthesis of YVO4:Ln3+ micro-spheres with narrow size distribution should be highly potential.

Herein, a simple and novel hydrothermal process was proposedfor the synthesis of lanthanons doped YVO4 particles with high-crystallinity and uniform morphology, in which N,N-dimethyl-formamide (DMF) was firstly used as hydrothermal solvent andpolyvinylpyrrolidone (PVP) as protective agent. So far, there wereonly limited studies discussing the influences of hyrothermalparameters on the properties of rare earth doped crystallineYVO4 phosphors, we altered the synthesis conditions includingDMF/water volume ratio, PVP concentration, doped rare-earth ionsand calcination temperature to modify the particle shape andluminescent properties. The obtained samples were well charac-terized by means of XRD, FESEM, TEM, optical spectra as well asthe kinetic decay times.

nsity

(a. u

.)

(a)

(b)

2. Experimental section

2.1. Synthesis

All the reagents including N,N-dimethylformamide (DMF) (A. R.,Beijing Beihua Chemical Co., Ltd.), polyvinylpyrrolidone (PVP) (A.R., Beijing Beihua Chemical Co., Ltd.), NH4VO3 (A. R., Tianjin DamaoChemical Co., Ltd.), Y2O3, Eu2O3, Dy2O3, Sm2O3 (99.99%, Science andTechnology Parent Company of Changchun Institute of AppliedChemistry), HNO3 (A. R., Beijing Beihua Chemical Co., Ltd.) wereanalytic grade and used without further purification.

In a typical process, 0.117 g of NH4VO3 was dissolved in 1 mL ofHNO3. Then the H2O and DMF mixed solvent with a specific DMF/H2O volume ratio of 1, 2 or 3 were added. Different amounts of PVP(0.4 g, 0.8 g or 1.2 g) and 1 mL of 0.05 mol L�1 Eu(NO3)3 were addedin the solution. The doping concentration of Eu3+ was 5 mol% to Y3+

in YVO4:Eu3+. After that, 0.107 g of Y2O3 was dissolved in 3 mL ofdilute HNO3 with stirring and heated to drive off the superfluousHNO3 until the Y(NO3)3 powders were obtained. Then the previoussolution was added immediately with stirring. After stirred inten-sely for 30 min, the resulting solution was then transferred into a50 mL sealed Teflon autoclave and heated at 180 �C for 24 h. Afterthe autoclave was naturally cooled to the room temperature, thefinal products were separated by centrifugation, and washed sev-eral times with the ethanol and distilled water. Finally, the ob-tained samples were dried at 80 �C for 12 h. The annealingprocess was performed at 600, 700, 800, 900 �C in air for 3 h at aheating rate of 1 �C min�1, respectively. The synthetic process de-scribed above has been used to prepare YVO4 crystals doped withDy3+ and Sm3+, respectively. The doping concentration of Dy3+ andSm3+ is 2 mol% that of Y3+ in YVO4 host, which has been optimizedin our previous report [10].

10 20 30 40 50 60 70 80

Inte

2θ (degree)

101 11

222

0

301 31

2

420

332

224

512

200

(d)

(c)

Fig. 1. XRD patterns of YVO4:Eu3+ (a), YVO4:Dy3+ (b), YVO4:Sm3+ (c), obtained at180 �C for 24 h with DMF/H2O volume ratio of 3/1 and 0.4 g/L PVP concentration,and the standard data for tetragonal YVO4 (JCPDS No. 17-0341) (d).

2.2. Characterization

Powder X-ray (XRD) patterns were obtained on a Rigaku TR IIIdiffractometer with Cu Ka radiation (k = 0.15405 nm). The acceler-ating voltage and emission current are 40 kV and 200 mA. Fieldemission scanning electron microscope (FESEM) images were in-spected on a XL30 microscope (Philips) equipped with an energy-dispersive X-ray spectrum (EDS, JEOL JXA-840). Transmission elec-tron microscope (TEM) and high-resolution transmission electronmicroscope (HRTEM) were performed on a FEI Tecnai G2 S-Twin

transmission electron microscope with a field emission gun oper-ating at 200 kV. The PL excitation and emission spectra were re-corded on a Hitachi F-4500 spectrofluorimeter equipped with a150 W xenon lamp as the excitation source. The PL lifetimes ofthe samples were obtained from a Lecroy Wave Runner 6100 Dig-ital Oscilloscope (1 GHz) using a 250 nm laser (pulse width = 4 ns,gate = 50 ns) as the excitation source (Continuum Sunlite OPO).The powders of the samples used for lifetime measurement wereheld tightly and smoothly in a special groove for PL measurement.The cathodoluminescent (CL) measurements were carried out in anultrahigh-vacuum chamber (<10�8 Torr), where the samples wereexcited by an electron beam at a voltage range of 1.5–3.5 kV withdifferent filament currents, and the spectra were recorded on an F-4500 spectrophotometer. All the measurements were performed atroom temperature.

3. Results and discussion

3.1. Phase, structure and morphology

The composition and phase of the products were first examinedby XRD. Fig. 1 shows the XRD patterns of the as-synthesizedYVO4:Ln3+ (Ln = Dy, Sm, and Eu) products and the standard datafor tetragonal YVO4 peak position (JCPDS No. 17-0341). It can beseen from Fig. 1 that all the samples exhibit the characteristicpeaks of pure crystalline tetragonal YVO4 with a space group ofI41/amd, which are in good agreement with the values of the stan-dard card (JCPDS No. 17-0341). The sharpening of the diffractionpeaks indicates that the products are well-crystalline. No otherphase related with doped components can be detected in theXRD patterns, suggesting that rare-earth ions have been homoge-nously incorporated into the host lattice. The calculated unit-cellparameters of the samples are summarized in Table 1.

As shown, the calculated cell parameters of a = b = 7.0924 Å,c = 6.2978 Å for YVO4:Eu3+, a = b = 7.0828 Å, c = 6.2925 Å forYVO4:Dy3+, and a = b = 7.0919 Å, c = 6.2879 Å for YVO4:Sm3+ arewell consistent with the standard data of a = b = 7.1192 Å,c = 6.2898 Å for pure tetragonal YVO4 (JCPDS No. 17-0341).

The morphology, size and microstructure details were investi-gated by SEM and TEM techniques. Fig. 2 shows the SEM imagesof YVO4:Eu3+, YVO4:Dy3+, YVO4:Sm3+, TEM and HRTEM images ofthe as-prepared YVO4:Eu3+, respectively. The SEM images (Fig. 2aand b) reveal that all the YVO4:Ln3+ samples are composed of rel-

Fig. 2. SEM images of YVO4:Eu3+ (a), YVO4:Dy3+ (b), YVO4:Sm3+ (c), TEM (d), and HRTEM (e) images of the as-prepared YVO4:Eu3+. All the YVO4:Ln3+ phosphors are obtained at180 �C for 24 h with DMF/H2O volume ratio of 3/1 and 0.4 g/L PVP concentration.

Table 1Unit-cell parameters and the deviations for the as-prepared YVO4:Eu3+, YVO4:Dy3+, and YVO4:Sm3+ microspheres prepared at 180 �C for 24 h with DMF/H2O volume ratio of 3/1and 0.4 g/L PVP concentration.

Samples a, b (Å)/deviations c (Å)/deviations c/a Vol (Å3)/deviations

JCPDS 17-0341 7.1192 6.2898 0.8835 318.8YVO4:Eu3+ 7.0924/0.0268 6.2978/0.0080 0.8879/0.0044 316.8/2.0YVO4:Dy3+ 7.0828/0.0374 6.2925/0.0027 0.8884/0.0049 315.67/3.13YVO4:Sm3+ 7.0919/0.0273 6.2879/0.0009 0.8866/0.0031 316.25/3.55

F. He et al. / Journal of Colloid and Interface Science 343 (2010) 71–78 73

atively uniform spherical particles with the particle diameter of1–2 lm. The TEM image (Fig. 2c) of the as-prepared YVO4:Eu3+

sample clearly indicates well-dispersed and uniform sphericalmicrospheres with narrow size distribution are obtained, whichare much similar to the SEM images. Moreover, the lattice fringesin the HRTEM image (Fig. 2d) are apparent, suggesting the high-crystallinity of the sample. The lattice fringes of (200) planeswith an interplanar distance of 0.35 nm are marked by the arrows.The calculated interplanar distance between the adjacent latticefringes are in good agreement with the d200 spacing of the standardvalue (0.352 nm) (JCPDS No. 17-0341), which further confirms thatthe high-crystallinity and single crystal feature of as-preparedsample.

3.2. Factors influencing the formation of the products and theformation process

Fig. 3 shows the SEM images of YVO4:Eu3+ samples preparedwith different PVP concentrations. It can be seen that both the par-ticle size and morphology change with the increase of the PVP con-centrations. In the absence of PVP (Fig. 3a), the sample depicts anon-uniform size and inhomogeneous shape. For sample with thePVP concentration of 0.4 g/L (Fig. 3b), well-dispersed microsphereswith the particle diameter of about 1–2 lm are observed. However,when the PVP concentration was increased to 0.8 g/L, someagglomerates appear, as shown in Fig. 3c. And the agglomeratesare more obvious when 1.2 g/L PVP was added (Fig. 3d). The resultssuggest that PVP concentration plays a critical role to the size and

morphology of the as-prepared YVO4:Eu3+ particles and the sampleprepared with the PVP concentration of 0.4 g/L has the optimumsize and morphology.

The SEM images of YVO4:Eu3+ samples synthesized with differ-ent DMF/H2O volume ratios are exhibited in Fig. 4. Clearly, thereare some differences in the morphology and size of the YVO4:Eu3+

particles. As shown in Fig. 4a, some block shaped particles can beobtained when pure water is used as the solvent, and no self-assembled microspheres can be found. For the sample synthesizedwith the DMF/H2O volume ratio of 1/1 (Fig. 4b), some sphericalparticles begin to appear, but they are irregular because a lot ofsmall pieces can’t be fully assemble. When the DMF/H2O volumeratio is increased to 3/1, regular microspheres with the particlediameter of 1–2 lm are obtained, as shown in Fig. 4c. Whereas,there is an obvious agglomeration to form large particles whenuse pure DMF as solvent. On basis of above analysis, we can deducethat proper ratio of DMF in the mixed solvent is vital to the forma-tion of final spherical YVO4:Eu3+ product and the favorable volumeratio of DMF/H2O is 3/1.

A possible self-assemble formation process of the sphericalstructure particles is presented in Scheme 1. Typically, the amor-phous YVO4 nuclei are covered with PVP, which form the primaryparticles of precursor. Then the crystal nucleus began to grow onan isotropic growth through a random aggregation to form spher-ical aggregates. The spherical crystals become bigger and biggerwith the continuous growth of precursor under the hydrothermaltreatment. Finally, the microspherical particles formed after fur-ther growth and recrystallization. In this process, PVP act as an

Fig. 4. SEM images of YVO4:Eu3+ samples synthesized with DMF/H2O volume ratio of: 0 (a), 1/1 (b), 3/1 (c), and pure DMF solvent (d).

Fig. 3. SEM images of YVO4:Eu3+ samples prepared with different PVP concentrations: (a) 0, (b) 0.4 g/L, (c) 0.8 g/L, and (d) 1.2 g/L.

74 F. He et al. / Journal of Colloid and Interface Science 343 (2010) 71–78

protective and capping agent to direct the structure and size of thefinal particles. The YVO4:Ln3+ microspheress may probably beformed through a mechanism which is similar to that for the prep-aration of La(OH)3, CdSe, and Y2O3:Eu [30–32].

3.3. Photoluminescence (PL) and kinetic properties

The PL properties of the as-prepared YVO4:Eu3+ phosphors werecharacterized by the PL excitation and emission spectra, as shownin Fig. 5a. The excitation spectrum (Fig. 5a, left) consists of a broadabsorption band with a maximum at 278 nm, which can be

assigned to the charge transfer from the oxygen ligands to the cen-tral vanadium atom inside the VO3�

4 groups [33]. From the view-point of molecular orbital theory, it corresponds to transitionsfrom the 1A2 (1T1) ground state to 1A1 (1E) and 1E (1T2) excitedstates of the VO3�

4 ion [34]. The excitation lines of Eu3+ ion canhardly be detected because the absorption intensity of the generalf–f transitions of the Eu3+ ions in the longer wavelength region isvery weak with respect to that of the VO3�

4 groups, suggesting thatthe excitation of the Eu3+ ions is mainly through the energy trans-fer from the VO�4 groups to Eu3+ ions [8,35]. Upon excitation intothe vanadate group at 278 nm, the emission spectrum (Fig. 5a,

Scheme 1. Schematic diagram showing experimental process for the synthesis ofYVO4:Ln3+ (Ln = Eu, Sm, Dy).

1 For interpretation of color in Fig. 5, the reader is referred to the web version ofthis article.

F. He et al. / Journal of Colloid and Interface Science 343 (2010) 71–78 75

right) exclusively contains the strong transition lines of 5D0 ?7F1

(593 nm), 5D0 ?7F2 (612 nm) and the very weak lines of 5D2 ? 7F0

(476 nm), 5D1 ? 7F1 (538 nm), 5D0 ?7F3 (649 nm), and 5D0 ?

7F4

(698 nm). No emission from the VO3�4 groups are observed, indi-

cating the efficient energy transfer from the VO3�4 group to Eu3+

ions. The 5D0 ?7F2 emission of Eu3+ ions belongs to hypersensitive

transitions with DJ = 2, which is strongly influenced by the outsidesurroundings. This emission transitions is dominant in the emis-sion spectra when the Eu3+ ion is located at a low-symmetry localsite (without an inversion center). It is well known that YVO4 pos-sesses a tetragonal structure with a space group of I1/amd. Thepoint symmetry of Y atom in YO8 dodecahedra is D2d, without aninversion center, while V atom is in the center of VO4 tetrahedra(Td) [34]. When the Eu3+ ions occupy the sites of Y3+ ions in

YVO4, the hypersensitive transition of Eu3+ 5D0 ?7F2 domain the

emission spectra.The PL spectra of YVO4:Dy3+ and YVO4:Sm3+ crystals are shown

in Fig. 5b and Fig. 5c, respectively. As for YVO4:Dy3+, the excita-tion spectrum (Fig. 5b, left) consists of an intense band with amaximum at 280 nm assigned to VO3�

4 absorption. Two character-istic peaks from 4F9/2 ? 6H15/2 (484 nm, blue) and 4F9/2 ? 6H13/2

(573 nm, yellow1) are predominant bands in the emission spec-trum of YVO4:Dy3+ upon 280 nm excitation (Fig. 5b, right). Theintensity of its yellow emission is stronger than that of the blueemission because the point symmetry of Dy3+ ions is D2d withoutan inverse center in the host of YVO4. Obviously, the strong emis-sion of Dy3+ is also due to an efficient energy transfer from theVO3�

4 group to Dy3+ in YVO4:Dy3+. Similarly, a strong band with amaximum at 307 nm associated with the absorption of the VO3�

4

group has been detected in the excitation spectrum of YVO4:Sm3+

(Fig. 5c, left). Excitation into the vanadate group at 307 nm yieldsthe characteristic orange emission of Sm3+ at 562 nm (4G5/2 ?6H5/2), 602 nm (4G5/2 ? 6H7/2), and 645 nm (4G5/2 ? 6H9/2), asshown in Fig. 5c. No emission from the VO3�

4 groups is observed,revealing that, similarly to the situations for Eu3+ and Dy3+ ions,an efficient energy transfer also occurs from VO3�

4 to Sm3+. The4G5/2 ? 6H5/2 (602 nm) transition is the dominate emission peakfor YVO4:Sm3+ sample.

Fig. 6 presents the corresponding CIE (Commission Intematio-nale de I’Eclairage 1931 chromaticity) coordinates positions,which show the different emission colors for the as-preparedYVO4:Eu3+, YVO4:Dy3+, and YVO4:Sm3+ phosphors, respectively.The YVO4:Eu3+ sample can emit bright red light and its chroma-ticity coordinates are x = 0.6074, and y = 0.3267 (point a inFig. 6). The CIE coordinates for the emission spectrum ofYVO4:Dy3+ are determined as x = 0.3892, and y = 0.3859, locatedin the yellow region (point b in Fig. 6). The YVO4:Sm3+

sample shows the luminescence color of orange, and the chro-maticity coordinates are x = 0.5601, and y = 0.4144 (point c inFig. 6).

To study the decay behaviors of rare-earth ions in more de-tails in the YVO4:Ln3+ phosphors, the typical decay curves forthe luminescence of Eu3+ in YVO4:Eu3+, Dy3+ in YVO4:Dy3+, andSm3+ in YVO4:Sm3+ are measured, as shown in Fig. 7. It can beseen that all the curves can be well-fitted into a double-exponen-tial function as I = A1exp (�t/s1) + A2exp (�t/s2), in which s1 ands2 are the fast and slow components of the luminescence life-times, A1 and A2 are the fitting parameters, and the fitting resultsare shown in Fig. 7. This indicates that the coordination environ-ment of the rare-earth ions is homogeneous in YVO4. Accordingto the formula s ¼ ðA1s2

1 þ A2s22Þ/(A1s1 + A2s2) [9,34], the average

lifetime for 5D0 ?7F2 (612 nm) of Eu3+, 4F9/2 ? 6H13/2 (573 nm)

of Dy3+, and 4G5/2 ? 6H5/2 (602 nm) of Sm3+ in the YVO4 host lat-tices are determined to be 0.79, 0.19, and 0.52 ms, respectively. Itis well accepted that the decay curve depends on the number ofluminescent centers, defects, and impurities in host. Differentsites in host lattices may generate several luminescent centerswhich lead to a multi-exponential behavior. In the YVO4:Ln3+

crystals, Ln3+ occupy the sites of the Y3+ ions with the D2d sym-metry, indicating only one luminescent center exist. The multi-exponential behavior may be due to the adsorbed impuritieswhich lead to the defects and the quenching centers. The dou-ble-exponential decay behavior of the activator is often observedwhen the excitation energy is transferred from the donor toacceptor.

200 300 400 500 600 700

Wavelength (nm)

λem

= 612 nm

PL

Inte

nsi

ty (

a. u

.)

λex

= 278 nm

Eu3+

698

5 D0-7 F

4

649

5 D0-

7 F3

593

5 D0-7 F

161

2 5 D

0-7 F2

538

5 D1-7 F

1

476

5 D2-7 F

0

278

VO4

3-

(a)

200 300 400 500 600 700

PL

inte

nsi

ty (

a.u

.)

Wavelength (nm)

VO4

3-

280

λex

= 573 nm λex

= 280 nm

Dy3+

484

4 F9/

2-6 H15

/2

477

573

4 F9/

2-6 H

13/2 (b)

200 300 400 500 600 700

PL

Inte

nsi

ty (

a. u

.)

Wavelength (nm)64

5 4 G

5/2-6 H

9/260

24 G

5/2-

6 H7/

2

562

4 G5/

2-6 H5/

2

307 λ

em = 602 nm λ

ex = 307 nm

Sm3+

VO4

3-

(c)

Fig. 5. Emission (left) and excitation spectra (right) of YVO4:Eu3+ (a), YVO4:Dy3+ (b), and YVO4:Sm3+ (c) samples prepared at 180 �C for 24 h with DMF/H2O volume ratio of 3/1and 0.4 g/L PVP concentration.

Fig. 6. CIE chromaticity diagram showing the emission colors for YVO4:Eu3+ (a),YVO4:Dy3+ (b), and YVO4:Sm3+ (c) prepared at 180 �C for 24 h with DMF/H2Ovolume ratio of 3/1 and 0.4 g/L PVP concentration.

76 F. He et al. / Journal of Colloid and Interface Science 343 (2010) 71–78

3.4. Influence of annealing on the PL properties

Influence of annealing temperatures on the PL emission intensi-ties of YVO4:Eu3+ sample is given in Fig. 8. The spectrum is domi-nated by the 5D0 – 7F2 emission of Eu3+ at 612 nm. Obviously, thePL intensity of the as-prepared sample is much weaker than that

of the samples after annealing. Additionally, it can also be seen thatthe PL intensities increase with the increase of annealing temper-ature. It is well known that the 5D0 – 7F2 transition is highly sensi-tive to the structure change and environment effects. The better ofthe crystallization and the bigger of the particles exist, the less ofthe defects appear on the surface, which can increase the lumines-cent intensity. As a result, the phenomenon can be due to the en-hanced crystallization of the YVO4:Eu3+ with the increase of theannealing temperature, which results in the increase of the lumi-nescent intensity.

3.5. Cathodoluminescence properties

The cathodoluminescence (CL) properties of the as-preparedYVO4:Eu3+ crystals has also been investigated as a function ofaccelerating voltage and filament current, as shown in Figs. 9 and10, respectively. Fig. 9 exhibits the CL spectra of YVO4:Eu3+ samplewith an increase of the accelerating voltage from 1.5 kV to 3.5 kVwhen the filament current is fixed at 100 mA. It can be seen thatthe emission intensity increases with the increasing of the acceler-ating voltage. This is mainly because the electron energy increaseand depth of the electrons penetrating into the phosphor’s bodyalso increase with the raise of the accelerating voltage, which in-duce more luminescence centers be excited. Similarly, the CLintensity increases with the filament current from 88 mA to100 mA when the accelerating voltage is kept at 3.5 kV, as shownin Fig. 10. The reason is also similar to the previous analysis. Theelectron penetration depth can be estimated by the empirical for-mula L[Å] = 250(A/q)(E/Z1/2)n, in which n = 1.2/(1 � .29 log10Z), A isthe atomic weight, q is the density, Z is the atomic number, and E isthe accelerating voltage (kV) [36]. The increase of filament currentcan result in the larger electron beam current density. As a result,both the raise of the accelerating voltage and filament current canbring deeper penetration of electrons into the phosphor’s body.Accordingly, the deeper the electron penetration depth, the moreplasma will be produced, thus the more Eu3+ ions can be excited,and the cathodoluminescence intensity also increases [15]. From

0 1 2 3 4

Inte

nsi

ty (

a. u

.)

Decay time (ms)

Blank circles: experimental data Solid line: fitted by I = A

1exp(-t/τ

1) +A

2exp(-t/τ

2)

Average lifetime τ = 0.79 ms

(a)λem

= 612 nm 5D0-7F

2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Inte

nsi

ty (

a. u

.)

Decay time (ms)

Blank circles: experimental data Solid line: fitted by I = A

1exp(-t/τ

1) +A

2exp(-t/τ

2)

Average lifetime τ = 0.19 ms

(b)λem

= 573 nm 4F9/2

-6H13/2

0 1 2 3 4

Inte

nsi

ty (

a. u

.)

Decay time (ms)

Blank circles: experimental data Solid line: fitted by I = A

1exp(-t/τ

1) +A

2exp(-t/τ

2)

Average lifetime τ = 0.52 ms

(c)λem

= 602 nm 4G5/2

-6H7/2

Fig. 7. The decay curves of YVO4:Eu3+ (a), YVO4:Dy3+ (b), and YVO4:Sm3+ (c) samples prepared at 180 �C for 24 h with DMF/H2O volume ratio of 3/1 and 0.4 g/L PVPconcentration.

400 500 600 700 800 900

0

2000

4000

6000

ed

cb

PL In

tens

ity (a

. u.)

Wavelength (nm)

a

Fig. 8. PL emission spectra of the as-prepared YVO4:Eu3+ (a) and annealed at 600 �C(b), 700 �C (c), 800 �C (d), and 900 �C (e).

300 400 500 600 700 800 900

0

200

400

600

800

1000

Filament current: 100 mA

ed

cb

CL

Inte

nsi

ty (

a. u

.)

Wavelength (nm)

a

Fig. 9. CL intensities of the YVO4:Eu3+ sample prepared at 180 �C for 24 h withDMF/H2O volume ratio of 3/1 and 0.4 g/L PVP concentration accelerated at 1.5 kV(a), 2.0 kV (b), 2.5 kV (c), 3 kV (d), and 3.5 kV (e).

F. He et al. / Journal of Colloid and Interface Science 343 (2010) 71–78 77

the above results and analysis we can conclude that the as-pre-pared YVO4:Eu3+ sample possess strong low-voltage cathodolumi-nescence intensity and excellent dispersing properties, whichmake them show potential applications in the field of emission dis-play devices.

4. Conclusions

In summary, rare-earth ion (Eu3+, Dy3+, and Sm3+) doped YVO4

microspheres have been successfully synthesized by a facile hydro-thermal process which use N,N-dimethylformamide (DMF) as

solvent and polyvinylpyrrolidone (PVP) as protective agent. Thecharacteristics of as-synthesized phosphors highly depended onthe DMF/H2O volume ratio, concentration of PVP additive, andcalcination temperature. The product obtained at the optimizedconditions of 180 �C for 24 h with DMF/H2O volume ratio of 3/1and 0.4 g/L PVP concentration exhibits well-defined spherical mor-phology with relatively narrow size distribution. The YVO4:Ln3+

(Ln = Eu, Dy, and Sm) phosphors exhibit the characteristic emissionlines of Eu3+, Dy3+ and Sm3+, respectively. The decay curves of allthe samples fit well into a double-exponential function. Thesephosphors demonstrate a potential application in the displayfields based on their special size, morphology and luminescentproperties.

300 400 500 600 700 800 900

0

400

800

1200

1600Accelerating voltage: 3.5 kV

fe

cd

b

CL

Inte

nsi

ty (

a. u

.)

Wavelength (nm)

a

Fig. 10. CL intensities of the YVO4:Eu3+ sample prepared at 180 �C for 24 h withDMF/H2O volume ratio of 3/1 and 0.4 g/L PVP concentration with filament currentof 88 mA (a), 90 mA (b), 93 mA (c), 95 mA (d), 97 mA (e), and 100 (f).

78 F. He et al. / Journal of Colloid and Interface Science 343 (2010) 71–78

Acknowledgment

This project is financially supported by the National Basic Re-search Program of China (2007CB935502), the National NaturalScience Foundation of China (NSFC 20871035, 50702057,50872131, 00610227), China Postdoctoral Special Science Founda-tion (200808281), and Harbin Sci.-Tech. Innovation Foundation(No. 2009RFQXG045).

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