Photoluminescence Characterization of Zn- and Cs-Vanadate

Phosphors

Tingting Li, Zentaro Honda, Takeshi Fukuda, Jiaolian Luo and Norihiko Kamata

Graduate School of Science and Engineering, Saitama University, 255 Shimo-Ohkubo,

Sakura-ku, Saitama 338-8570, Japan

Keywords: Zn

Csvo

, Sol-gel Process, Quantum Yield.

Abstract: We synthesized Zn

, CsVO

and Cs

by sol-gel process and studied their crystalline and

luminescent properties. By optimizing the sintering conditions, pure phases of aim samples including

were obtained. The annealing temperatures of 450℃ for CsVO

, 600℃ for Cs

, respectively,

are lower than that of 750℃ for Zn

at the same duration of 12h. The Cs

showed quantum yield of

90% with the half-width of 120nm. It became clear that the Cs-V-O system, especially Cs

, is

promising for white LED applications.

1 INTRODUCTION

White light-emitting diodes (W-LEDs) are replacing

traditional incandescent and fluorescent lamps due

to their superior efficiency, lifetime, and

controllability of lighting environment (Li et al.,

2013). However, recent widespread W-LEDs,

realized by blue LED and yellow phosphor of Ce-

doped yttrium aluminum garnet, lack red emission

component and show a low color rendering index

(CRI) (Kim et al., 2009). It is necessary, therefore,

to find phosphor materials with higher CRI together

with keeping efficiency and reliability. As a kind of

efficient phosphor materials, a family of vanadates

has been widely investigated for various types of W-

LEDs and flat-panel displays due to their better

chromaticity (Huang et al., 2012); (Nakajima et al.,

2009).

The VO4 unit of a central vanadium ion and

coordinating four oxygen ions in a tetrahedral (Td)

symmetry is known as the luminescent center of the

vanadate group. Unlike sharp emission lines due to

4f transitions of rare earth ions as Eu3+, Tb3+ etc., a

family of zinc and cesium vanadates, Zn3V2O8,

CsVO3 and Cs3VO4, shows efficient and broad

emission spectra in a visible wavelength region.

Each VO4 tetrahedron in Zn3V2O8 is isolated in an

orthorhombic structure, while that in CsVO3 is two-

dimensionally arrayed as the VO4 sheet in an

orthorhombic pyroxene structure (Nakajima et al.,

2010).

In the present work, we synthesized Zn3V2O8,

CsVO3 and Cs3VO4 by a sol-gel method and

studied their X-ray diffraction (XRD) patterns,

surface images, photoluminescence (PL), PL

excitation (PLE) spectra and the value of PL-

quantum yield (QY). The synthesized Cs3VO4 was

most efficient with the PL-QY of 90%. Different

luminescence properties and synthesis conditions

among three phosphors were discussed.

2 EXPERIMENTAL

2.1 Preparation

We chose Zn(CH3COO)2•2H2O, Cs2CO3 and

NH4VO3 as starting materials of Zn3V2O8, CsVO3

and Cs3VO4 by a sol-gel process. First, we weighed

these starting materials, and dissolved them in

aqueous ammonia, respectively. The first stirring

step was performed for two hours with 250 rpm at

room temperature (RT) in order to prevent the

evaporation of aqueous ammonia and make sure that

materials fully react with each other in ionic states.

Then the temperature was increased to 80ºC with

keeping 250 rpm stirring until the solution became a

semitransparent gel. Second, the semitransparent gel

was dried at 120ºC about 4h, then ground thoroughly

by a mortar, and transferred into a crucible which

Li T., Honda Z., Fukuda T., Luo J. and Kamata N..

Photoluminescence Characterization of Zn- and Cs-Vanadate Phosphors.

DOI: 10.5220/0004704800630065

In Proceedings of 2nd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2014), pages 63-65

ISBN: 978-989-758-008-6

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

was put into a sintering oven. After the sintering

process, a yellowish powder of CsVO3 (450ºC, 12h)

or a white powder of Cs3VO4 (600ºC, 12h) were

obtained.

In case of Zn3V2O8 (750ºC, 24 h), it also

contained the phase of Zn2V2O7 or Zn4V2O9. (Li

et al., 2013) In order to obtain a uniform crystalline

phase, the inchoate sample was placed into ethanol

solution again. It was stirred first at 250 rpm for 2h

at RT, then raised the temperature to 80ºC until the

ethanol solution was completely removed. After

that, we sintered it again at the temperature of 750ºC

during 24h, and at last obtained a yellowish powder

of pure Zn3V2O8.

2.2 Characterization

Synthesized samples were ground again and then

transferred onto a glass plate, and the X-ray

diffraction (XRD) patterns were measured by using

a RINTUltimaIII (RIGAKU) diffractometer with Cu

Ka radiation (40 kV 9 40 mA). The samples were

identified by Joint Committee on Powder Diffraction

Standards (JCPDS) files, and the relative

percentages of each phase were estimated from the

total area under the most intense diffraction peaks

(Francisco et al., 2007). The surface morphology

was observed by using a S-4100 (HITACHI)

scanning electron microscopy (SEM).

The photoluminescence (PL) and PL excitation

(PLE) spectra of each sample, pressed inside a

circular dip of 4 mm diameter with 1 mm depth on a

Cu-holder, were measured by a FluoroMax-3

(Horiba Jovin-Yvon) spectrophotometer. The PL-

QY was determined by a QEMS-2000 (Systems

Engineering) by an excitation light of an LED with

the peak wavelength of 375 nm. The PL-QY was

obtained by a comparison between the PL spectrum

of the sample and the scattered excitation light when

a standard diffuser was placed at the sample position

(Li et al., 2013). All measurements were performed

at room temperature.

3 RESULTS AND DISCUSSION

3.1 Crystalline Phase Formations

Figure 1 shows the XRD patterns of synthesized

samples with corresponding homologous databases.

We can see that our experimental data of Zn3V2O8

and CsVO3 agreed well with the standard PDF cards

of No.34-0378 (Zn3V2O8) and No.033-0381

(CsVO3), respectively. No PDF data were found as

for the case of Cs3VO4 phosphor, but the principal

patterns of our synthesis were consistent with those

of a commercial powder from Alfa.

Figure 1: XRD patterns for vanadates.

Figure 2: SEM images for vanadate phosphors: (a)

Zn3V2O8 750Ԩ12h, (b) CsVO3 450Ԩ12h, (c) Cs3VO4

600Ԩ12h, and (d) Cs3VO4 450Ԩ12h.

Surface morphologies of synthesized samples

were observed by SEM. Resultant images of

synthesized vanadates were shown in Figure 2(a)-

(d). In case of Zn3V2O8 (750ºC 12h), each block

throughout the observing volume seems to be

homogeneously crystallized with a typical size of

several µm as shown in Fig. 2(a).

Fairly irregular particles both on size and shape

were observed in CsVO3 (450ºC 12h) as shown in

Figure 2 (b). In contrast, the morphology of Cs3VO4

(600ºC 12h) was relatively homogeneous with

particle sizes about 1-3µm as shown in Figure 2 (c).

At 450 ºC 12h, however, the dregs-like particles

were found at the surface of the Cs3VO4 as shown

in Figure 2(d).

PHOTOPTICS2014-InternationalConferenceonPhotonics,OpticsandLaserTechnology

3.2 The Excitation and Emission

Spectra

(a)

(b)

Figure 3: The PLE (a) and PL (b) spectra of the vanadate

phosphors.

Figure 3(a) and (b) show the PLE and PL spectra

of these vanadate phosphors, respectively. The broad

PLE band ranging from 300nm to 400nm enables

these vanadates to be combined with UV or blue

LEDs. Meanwhile, Zn3V2O8, CsVO3 and Cs3VO4

produce intense emission from yellowish white to

mid white band with the FWHM of 180, 110 and

120nm, respectively. Meaningful spectral shifts were

observed, though these originate from the same

luminescence center VO4 at transitions among 3T2,

3T1 and 1A1 levels. Compared with the PLE and PL

peaks of Zn3V2O8, those of CsVO3 shift about 20

and 40nm, and those of Cs3VO4 about 30 and

60nm, respectively.

3.3 The PL-Quantum Yield (PL-QY)

The PL-QY at 375nm excitation is listed in Table 1

together with the synthesis condition, the FWHM,

the peak wavelengths of the PLE and PL spectra.

The QY value of 90% was obtained in the case of

Cs3VO4 which is higher than that of 76% obtained

by commercial powder from Alfa. We consider that

the difference between Cs3VO4 and CsVO3 is

important for their application to W-LEDs.

4 CONCLUSIONS

In conclusion, vanadate phosphors of Zn3V2O8,

CsVO3 and Cs3VO4 were synthesized by the sol-gel

method. By comparing the difference of crystalline

structure and luminescence properties among the

three, the Cs3VO4 with the heating process at 600ºC

during 12h showed highest PL-QY of 90% and is

promising for the application to the W-LEDs.

Table 1: Comparison between the three vanadate

phosphors.

Samples

CsVO

Synthesis

conditions

750 ºC

48h

450 ºC

12h

600 ºC

12h

FWHM (nm)

180 110 120

λex (nm)

348 365 374

λem (nm)

580 540 518

PL-QY (%)

52 81 90

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PhotoluminescenceCharacterizationofZn-andCs-VanadatePhosphors