Luminescence properties of nanocrystalline YVO4:Eu3+

under UV and VUV excitation

Yuhua Wang *, Yinyan Zuo, Hui Gao

Department of Materials Science, Lanzhou University, Lanzhou 730000, PR China

Received 6 January 2006; received in revised form 20 March 2006; accepted 29 March 2006

Available online 5 May 2006

Abstract

Single phase of Eu3+-doped YVO4 nanophosphors at different pH values were synthesized by a mild hydrothermal method.

Their photoluminescence were evaluated under UVand VUV region, respectively. Monitoring by 619 nm emission, broad bands at

around 143 nm, 200 nm, 260 nm were observed in the excitation spectrum of YVO4:5 mol%Eu3+. These peaks could be assigned to

host absorption, the overlap of the VO43� host absorption and charge transfer transition between Eu3+ and O2�, respectively. Both

254 nm and 147 nm excitations, the emission spectra were identical, they were all composed of Eu3+ emission transitions arising

mainly from the 5D0 level to the 7FJ (J = 1, 2, 3, 4) manifolds. With the pH values ranging from 7 to 11, the relative intensity of the

emission spectra were decreasing, and the position of the predominant peak (5D0! 7F2) was changed from 619 nm to 615 nm when

the pH values changed from 7 to 11.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; B. Chemical synthesis; C. X-ray diffraction; D. Luminescence

1. Introduction

As well known, grain size, morphology, agglomeration or surface passivation are indeed known to have an impact

on the phosphor efficiency [1]. The synthesis of conventional phosphor powders involves high-temperature solid-state

reactions which provide agglomerated powders with a grain size in the 5–20 mm range. Such conditions do not allow

one to easily change the structural characteristics of the obtained powders. Now, the development of new methods for

the production of nanoparticles might increase the possibilities for better control of the structural parameters. To this

extent, the preparation of nanoparticle is necessary to meet the requirement of the host of the phosphors with

decreasing size and narrow size distribution, demanded by recent development of displays towards the high quality,

efficiency and miniaturization. Meanwhile, the reduction of the particle size will also lead to the decrease of the

optimum screening weight, which results in the abatement of the overall thickness of the coating and elimination of

pinholes and mottling of the screen [2]. Wet chemistry methods have been shown to be very powerful technique to

prepare nanoparticles, such as sol–gel process [3], solution combustion process [4], microemulsion-mediated

synthetic process [5], by co-precipitation reaction [6] and hydrothermal method [7].

Hydrothermal method is one of the most promising solution chemical methods. The hydrothermal method, which

uses autogenous pressure developed at temperatures above the boiling point of water, has been used especially in the

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Materials Research Bulletin 41 (2006) 2147–2153

* Corresponding author. Tel.: +86 931 891 2772; fax: +86 931 8913554.

E-mail address: wyh@lzu.edu.cn (Y. Wang).

0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2006.03.034

synthesis of various fluorescent materials. The advantage of the hydrothermal method is that the fluorescent materials

can be synthesized at relatively low temperatures (100–300 8C) without milling or calcination [8]. The particle size

and shape can also be controlled by various processing variables such as temperature, pH value, and the addition of

surfactants.

Yttrium orthovanadate (YVO4), having a zircon structure, belongs to the space group D194hI41/amd

(a = b = 0.7118 nm, c = 0.6289 nm), with four oxygen atoms occupying every vanadium atom, eight oxygen atoms

occupying every yttrium atom, and eight oxygen atoms forming two distorted tetrahedron. It has been known to be a

very attractive host lattice for several rear earth ions. For example, when doped with trivalent Eu3+, red-emitting

YVO4:Eu3+ phosphor is a strongly luminescent material showing high color purity which is resulted from the

noncentrosymmetric site of Eu3+ [9]. It can be utilized as a red phosphor in cathode ray tubes (CRTs) [10], plasma

display panels (PDPs) [11], field emission displays (FEDs) [12], light emitting diodes (LED) [13] and color television

monitors [14]. YVO4 is usually prepared by solid-state reaction. However, it easily happens

8YVO4 ! Y8V2O17þ 3V2O5 (1)

It will make the obtained samples yellow so much as gray. Recently, as far as the preparation methods we are

concerned, there are a few papers, such as Wu et al. synthesized well-defined YVO4 nanocrystals with clear face in

strongly acid media and with dimensions of 5–50 nm in basic media [15]. Riwotzki and Haase [16] reported on a

hydrothermal synthesis at 200 8C of YVO4:Eu3+ powders and investigated its luminescent properties under UV

excitation. Huignard and Gacoin [17] and Chen [18] also synthesized nanocrystals using hydrothermal method. But

most of them adjusted pH value using sodium hydroxide, this process can easily blend sodium ion which possibly

influence the luminescent intensity of the samples. Also, NH3 have the function to disperse the granularity of the

sample. Moreover, until now, no literature has reported to investigate its luminescent properties under VUVexcitation.

In this paper, hydrothermal method is employed to synthesize YVO4:Eu3+ at different pH values. We use ammonia

to adjust pH value of the solution instead of sodium hydroxide, and investigate their luminescent properties under UV

and VUV excitations.

2. Experimental

High-purity Y2O3 (99.99%), Eu2O3 (99.99%), and V2O5 (99%) were taken as the starting materials. V2O5

(0.020 mol) was dissolved in hydrochloric acid in order to become solution, Y2O3 (0.019 mol), Eu2O3 (0.001 mol)

were dissolved in nitric acid for turning into clear solutions, respectively. A clear starting mixed solution was prepared

by mixing the two solutions of V2O5 and Y2O3, Eu2O3. Then the mixed solution was slowly added into ammonia under

vigorous stirring, through controlling the quantity of ammonia to make the final pH value of solution equal to 7–11.

The obtained solution was taken into autoclave for 10 h at 200 8C. The autoclave was cooled naturally and the product

was filtered, and dried in air.

The phase of powder samples were identified by using Rigaku D/max-2400 X-ray diffractometer (XRD) with Ni-

filtered Cu Ka radiation. The morphology of obtained samples was studied with a JEOL JEM-1200 transmission electron

microscope (TEM) operated at 200 kV. Excitation and emission spectra in the VUV range were measured by Edinburgh

Instruments FLS-920T with VM-504 vacuum monochromator using a deuterium lamp as the lighting source. The

excitation spectra were corrected with sodium salicylate. All the spectra were recorded at room temperature.

3. Results and discussion

Since that the form of vanadium ions were extremely sensitive to the pH of the solution, as reported by Ropp and

Carroll [19], the vanadium ions existed as VO2+ when the solution was in strong acidity. When the pH rose to 2,

vanadium ions were in the form of V10O286� principally; while the pH further increased to 5, vanadium ions were

liable to the form of V3O93�. VO4

3� existed in the solution when the pH values was 7–11. In our case, since the pH of

the solution ranges from 7 to 11, the following reactions might occur:

3OH� þV3O93� ! 3VO4

3� þ 3Hþ (2)

VO43� þY3þ ! YVO4 (3)

Y. Wang et al. / Materials Research Bulletin 41 (2006) 2147–21532148

based on the above-mentioned mechanisms. With increased pH value of solution, the reaction favors the formation

of VO43� in Eq. (2), thus leading to a favorable product of YVO4 nanoparticles according to Eq. (3). This was the

reason we adjusted the pH value above 7.

The XRD patterns of YVO4:5% Eu3+ at different pH values are shown in Fig. 1. They were all single phases and all

white color inbody, which could be indexed as the tetragonal structured (JCPDS No. 17-0341). The particle sizes were

calculated by half-widths of (2 0 0) peaks with Scherrer Formula, it could be calculated that when adjusting the pH

from 7, 8, 9, 10 to 11, the mean particle size of YVO4:Eu3+ varies from 46.1 nm, 36.8 nm, 32.1 nm, 32.0 nm to

26.3 nm, respectively. With the increasing of the pH values of the solution, the diffraction peaks were broad gradually

and the intensities of peaks decrease gradually.

Because the TEM images were similar from 8 to 11, Fig. 2 showed the typical TEM images of samples prepared at

pH 7 and 8. From the figures we could see that the particle size estimated was from 30 nm to 50 nm at pH 7 and the

width of the particle size estimated was from 28 nm to 42 nm, respectively. The primary crystal sizes were consistent

with the results from XRD. It was clearly shown that when pH values were 7 and 8, they were sphere-like and nanobar-

like, respectively. The electron diffraction pattern was shown in Fig. 2(b) and (d), respectively.

With pH values ranging from 7 to 11, the excitation spectra was similar to each other under 254 nm. Fig. 3 gave the

UVexcitation spectrum of YVO4:5 mol%Eu3+ at pH value was 7. Monitoring by 619 nm emission, a broad band with a

maximum at about 260 nm was observed in the excitation spectrum. Also, the 7F0! 5L6 excitation line of Eu3+ at

394 nm was observed with very weak intensity. The peak at 260 nm was attributed to the charge transfer from the

oxygen ligands to the central vanadium atom inside the VO43� ion in ref. [17]. On the other hand, as shown in ref. [20],

the broad band around 260 nm was assigned to the charge transfer (CT) transition between Eu3+ andO2�, i.e., an

electron transfers from O2� (2p6) orbital to the empty orbital of 4f6 for Eu3+. It could be concluded that the broad band

around 260 nm was assigned to the overlap of VO43� absorption and charge transfer transition between Eu3+ and O2�.

Fig. 4 showed the emission spectra of YVO4:5 mol%Eu3+ at different pH values under 254 nm excitation. The

spectra were dominated by the emission from the trivalent europium ions, and mainly the 5D0! 7F2,4 forced electric-

dipole transitions for which high intensities were at 619 nm and 700 nm. Other contributions of weaker importance

were the 5D0! 7F1,3 magnetic dipole transitions at 594 nm and 649 nm. Although the major peak positions in the

emission spectra were identical to each other, the intensity patterns and each position were different. Obviously, the

position of the predominant peak was changed from 619 nm to 615 nm when the pH values changed from 7 to 11. It

was well known that the 5D0! 7F2 transition is highly sensitive to structure change and environment effects [21]. The

difference of 5D0! 7F2 emission peak in positions was due to the difference of the effects of the crystal field

perturbation on the individual f–f transitions [22]. The intensity of emission spectra was gradually decreasing with the

Y. Wang et al. / Materials Research Bulletin 41 (2006) 2147–2153 2149

Fig. 1. XRD patterns of YVO4:5%Eu3+ at different pH values.

pH values varied from 7 to 11. This phenomenon could be explained by two aspects: (1) As the particles were

dispersed in water, their surface is covered by a large number of OH groups either chemically bonded to the surface or

just adsorbed as water molecules. Such hydroxyl groups were well-known to be very efficient quenchers of the

luminescence of lanthanide elements through multiphonon relaxation [23]. The concentration of OH was increasing

with the pH value increased, so the intensity of the emission spectra is reducing. (2) The better of the crystallization

and the bigger of the particles, less defects were existed on the surface, which could increase the luminescent intensity.

This result was identical with result of XRD.

Y. Wang et al. / Materials Research Bulletin 41 (2006) 2147–21532150

Fig. 2. TEM images (a) and (c), and electron diffraction pattern (b) and (d) of YVO4:5 mol%Eu3+ (pH 7, 8).

Fig. 5 showed the VUVexcitation spectrum of YVO4:5 mol%Eu3+ at pH value was 7. The excitation spectrum was

identical when the pH values were varying. Below 200 nm, the excitation spectrum of YVO4:5 mol%Eu3+ consisted of

a broad band with the maxima at 143 nm and 200 nm, respectively. The broad band at 143 nm was assigned to host

absorption which concluded two parts. The one was that ref. [24] reported charge transfer from the oxygen ligands to

the central vanadium atom inside the VO43� at about 150 nm. On the other hand, in ref. [25], Wang et al. reported the

charge transfer transition between Y3+ and O2– at about 150 nm in YAlO3:Eu3+ and proved by doped with other

trivalent rear earth ions (Ce3+,Tb3+, etc.), also they reported the band should be assigned to the electronic transition of

O2�:2P6 to Y3+:4P6 (4d + 5s)[26]. The two parts were overlapped well. Since in the system of Y2O3:Eu3+, Berkowitz

et al. referred that the charge transfer transition between Y3+ and O2� at 200 nm [27], in YVO4:5%Eu3+, the 200 nm

Y. Wang et al. / Materials Research Bulletin 41 (2006) 2147–2153 2151

Fig. 3. UV excitation spectrum of YVO4:5 mol%Eu3+ (pH 7).

Fig. 4. Emission spectra of YVO4:5 mol%Eu3+ under 254 nm excitation at different pH values.

band was related to the charge transfer transition between Y3+ and O2�. The excitation spectrum was similar with the

excitation spectra of UV when the wavelength was above 200 nm.

Under 147 nm excitation, the emission spectra of YVO4:5%Eu3+ shown in Fig. 6 present the characteristics of Eu3+

emission transitions arising mainly from the 5D0 level to the 7FJ (J = 1, 2, 3, 4) manifolds. Among these transitions, the

structurally sensitive electric dipole 5D0! 7F2 transition was the most dominant. The position of the emission peaks

and relative intensity were similar with the emission spectra excited under 254 nm. When pH value was 7, CEI color

chromaticity x and y were calculated using the spectrum date under 147 nm excitation. We could get x and y were

0.673 and 0.327, nevertheless, to inquire into the reliability of our calculation, the CEI color chromaticity x and y

values of commercial (Y, Gd)BO3:Eu3+ phosphor were estimated to be 0.646 and 0.354.

Y. Wang et al. / Materials Research Bulletin 41 (2006) 2147–21532152

Fig. 5. VUV excitation spectrum of YVO4:5 mol%Eu3+ (pH 7).

Fig. 6. Emission spectra of YVO4:5 mol%Eu3+ under 147 nm excitation at different pH values.

4. Conclusions

Single phase of Eu3+-doped YVO4 nanophosphors at different pH values were synthesized via a low-temperature

hydrothermal route without further calcining treatment. Their photoluminescence was investigated under UV and

VUV region, respectively. Monitoring by 619 nm emission, broad bands at around 143 nm, 200 nm, 260 nm and sharp

band around 394 nm were observed in the excitation spectrum of YVO4:5 mol%Eu3+. These peaks could be assigned

to host absorption, the overlap of charge transfer from the oxygen ligands to the central vanadium atom inside the

VO43� and charge transfer transition between Eu3+ and O2�, the 7F0! 5L6 excitation line of Eu3+, respectively. Under

254 and 147 nm excitations, the emission spectra were identical, they were all composed of Eu3+ emission transitions

arising mainly from the 5D0 level to the 7FJ (J = 1, 2, 3, 4) manifolds. With the pH values ranging from 7 to 11, the

relative intensity of the emission spectra were decreasing, and the position of the predominant peak (5D0! 7F2) was

changed from 619 nm to 615 nm when the pH value changed from 7 to 11. The reason could be attributed to the

different of sizes, crystallization and OH group on the surface.

Acknowledgments

This work was supported by Program for New Century Excellent Talents in University of China (NCET, 04..C0978),

the Key Science Research Project of Ministry of Education of China (105170) and Specialized Research Fund for the

Doctoral Program of Higher Education of China (SRFDP, 20040730019).

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