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Optical Properties of Pure and Europium-doped YVO₄ Phosphor

Rajendra H. J*, C. Pandurangappa

Department of Physics, RNS Institute of Technology, Channasandra, Bangalore-560098, India

Abstract: Nanocrystals of pure and Eu-doped YVO4 phosphor is synthesized by co-precipitation method and characterized by X-ray diffraction (XRD), Fourier transform infrared red Spectroscopy (FTIR), Scanning electron microscopy (SEM) and Photoluminescence (PL). The XRD patterns showed the tetragonal phase. SEM showed that the particles are small and agglomerated. FTIR spectrum showed the characteristics of the V-O bonds ranging from 761 to 861 cm-1. The PL spectra of pure YVO4 sample showed blue emission band in the range 406-510 nm and it is attributed to the de-excitation of electrons from the filled oxygen 2p levels in the valence band to the empty vanadium 3d levels of the conduction band. The Eu-doped YVO4 sample showed strong emission at 618nm (red) at an excitation of 340nm.

Keywords: precipitation, yttrium vanadate, europium, x-ray diffraction, photoluminescence

Citation: Rajendra H. J, C. Pandurangappa, Optical Properties of Pure and Europium-doped YVO₄ Phosphor. Nanoscience and Nanotechnology Research, 4 (2). 43-48. January 2017. doi:10.12691/nnr-4-2-2

1. Introduction

Nanomaterials are extensively studied because of their optical properties differ from those of bulk materials [1]. Due to excellent luminescence and potential applications in luminescent devices and display equipments vanadates are widely studied [2].

Phosphor materials in the Nanorange exhibit enhanced optical properties compared to the bulk counterpart. It is due to quantum confinement effect and increased surface to volume ratio. Among the various materials much attention has given to the Yttrium orthovanadate because of its excellent electro-optical and acoustical properties [3]. The zircon type YVO₄ is built from the chains of alternating edge sharing of VO₄ and YO₈. The YVO₄ tetragonal crystal belongs to D4h space group, with D₂d point symmetry of trivalent yttrium ion and is surrounded by eight oxygen ions at the vertices of a tetragonal dodecahedron [4,5]. Among rare-earth doped materials, YVO₄: Eu is a significant red-emitting phosphor. It is widely used in color television, high-pressure mercury lamps and as a scintillator in medical image detectors [6]. The first scientific report on Europium doped YVO₄ as a new red phosphor for cathode ray tubes (CRT) was in 1964. Over the last few years, much attention has been paid to the luminescent properties of Eu-ropium doped material due to the flat display applications.

The Yttrium vanadate (YVO₄) has been shown to be a useful host lattice for rare earth ions to produce phosphors emitting a variety of colors as high luminescence quantum yields are observed in its f–f transitions [7]. Recent studies on different luminescent nanomaterials have showed potential applications in dosimeter of ionizing radiations for the measurements of high doses using the TL technique [8].

There are several methods used to synthesize nanomaterial. Co-precipitation method with the advantages of mild reaction conditions, low cost and simple process, compared with the other methods has been widely used in the synthesis of pure and rare-earth doped YVO₄ materials.

In the present work pure and Europium doped YVO4 is synthesized by co-precipitation method. The synthesized samples are characterized by the by XRD, FTIR, SEM and PL.

2. Experimental

2.1. Material Used

Ammonium Metavanadate (NH4VO3), Yttrium nitrate [YNO₃]₃ and Europium oxide [Eu₂O₃], Acetone, materials are obtained from chemical company and all the procedures are carried out.

2.2. Synthesis

Stoichiometric amount of NH4VO3 is dissolved in hot water, Yttrium nitrate is dissolved in distilled water and Europium oxide is dissolved in 3M HNO3. Solution of Yttrium nitrate and nitric acid are mixed and stirred and ammonium Meta vanadate is added to this solution. Then PH was raised with the addition of NH4OH. The obtained precipitate is centrifuged with distilled water and acetone, and dried at 50°C. Finally the obtained product is calcined at 800°C.

3. Characterization

The XRD Measurements of synthesized samples was carried out using Philips X-pert PRO Powder diffractometer with Cu-Kα radiation. In the scan range 10-79°. The morphology of syn- thesized sample was studied using scanning electron microscope. Fourier transform infrared (FTIR) spectra were recorded using Nicolet Magna 550 spectrometer Frontier with KBr pellets. PL emissions spectra of the samples were recorded using spectrofluoremeter with 450W xenon lamp as the excitation source.

4. Results and Discussion

4.1. Powder X-ray Diffraction (XRD)

Figure 1 shows the XRD patterns of the pure and Eu-doped YVO₄ Phosphor powders synthesized by using co-precipitation method. All the peaks are indexed to tetragonal phase of YVO₄ (JCPDS74-1172) with space group I41/amd (141). The XRD pattern shows prominent peaks at angle of 25°, 33° and 50°. Using <hkl> values of different peaks, the lattice constant of the sample is calculated. The average value of lattice constant a=7.126Å which is in agreement with lit- erature value (JCPDS 72-0861) [9]. The crystallite size was calculated using the Debye-Scherer formula D= (kλ/βcosθ). The average crystallite size calculated is found to be 7-22nm.

Figure 1. X-ray diffraction spectrum of pure and Eu- doped YVO₄ Nanoparticles

4.2. Scanning Electron Microscopy (SEM)

The morphology and size of the prepared materials were determined from SEM images.

Figure 2(a) - Figure 2(E). Shows the SEM images of the pure and Eu doped YVO₄ Phosphor. The synthesised particles show spherical perturbances and certain agglomeration of smaller particles. The agglomeration is ranging from 3-6 microns. The morphology of the samples also showed the voids in the agglomerated nanoparticles. This may be due to the large surface to volume ratio of Nanoparticles. SEM pictures of Eu-doped Nanocrystals showed greater agglomeration compared to undoped YVO₄ Nanocrystals.

Figure 2. (a) SEM image of pure YVO₄ Nanoparticles

Figure 2. (b) SEM image of 1%molEu-doped YVO₄ Nanoparticles

Figure 2. (c) SEM image of 3%mol Eu-doped YVO₄ Nanoparticles

Figure 2. (d) SEM image of 5%mol Eu-doped YVO₄ Nanoparticles

Figure 2. (e) SEM image of 7%mol Eu-doped YVO₄ Nanoparticles

4.3. Fourier Transform Infrared Spectrum (FTIR)

The purity and functional groups of all the samples are confirmed by the FTIR spectra. The FTIR spectra of undoped and Eu-doped YVO₄ Phosphor are shown in the Figure 3. The spectrum of YVO₄ showed strong bands at 825, 1384, 3145cm⁻¹. The peak at 825cm⁻¹ is due to the stretching frequency of V-O. The peak observed at 1384cm⁻¹ is due to the asymmetrical vibration of the carbonyl groups [10]. The weak absorption peak at 435cm⁻¹ is attributed to stretching frequency of Y-O. The peak observed in the range 3125-3300cm⁻¹ is due to O-H absorption.

The Europium doped YVO₄ spectrum shows strong absorption peaks at 1613 and 2388cm⁻¹. The peak at 1613 cm⁻¹ was attributed to the Characteristic bending vibration of water molecule and asymmetrical stretching mode of CO2 molecule. The peak at 1635cm⁻¹ is due to the N-H bending vibration [11]. The peak at 498 cm⁻¹ is due to the Y-O stretching vibration. Further we have observed the additional bands (around3700cm⁻¹). These are due to inorganic materials present in the final product.

Figure 3. FTIR spectrum of pure and Eu-doped YVO₄ Nanoparticles

4.4. Photoluminescence (PL)

The PL emission spectra of pure and Eu-doped of YVO₄ Nanocrystals of different moles are shown in the Figure 4. Pure YVO₄ showed a prominent emission peak at 406nm with an excita- tion wavelength 340 nm. The PL spectra showed blue emission band in the range 406-510nm. This is due to the de-excitation of electrons from the filled oxygen 2p levels in the valence band to the empty vanadium 3d levels of the conduction band [12]. This reveals the energy transfer of VO⁴₃- groups.

Figure 4. Photoluminescence spectrum of pure and Eu-doped YVO₄ Nanoparticles

The PL spectra of Europium doped YVO₄ consists of bands ranging from 340 to 620nm. The intense peak at 620nm is due to 5D0-7F2 [13] electric dipole transition with energy 2.0154eV [14]. It means that its intensity is much more influenced by the local symmetry of the Eu³⁺ ion and the nature of the ligands than that of the other electric dipole transitions. The peak observed at 702nm is due to the 5D0-7F4 electric dipole transition [15].

In the emission Process from Europium ions, first UV light is absorbed by a vanadate group and then the excited energy is transferred to the higher lying excited states of Europium ions. Finally the emission from Europium occurs in the visible and NIR regions. The emission intensity of Europium depends on the UV-absorption by the YVO₄ host and the efficiency of energy transfer from host to the Europium ion. The peak position of the emission lines for all concentrations are almost same, but the intensity is maximum at 3% mol of Europium ion (Figure 5).

Figure 5. Relative intensity of Eu-doped YVO₄ Nanoparticles

The PL intensity is found to be enhanced in the Eu-doped Nanocrystals when compared with un-doped YVO₄ Nanocrystals.

5. Conclusions

The pure and doped Europium YVO₄ phosphor was prepared by co-precipitation method. The XRD patterns showed the tetragonal phase of YVO₄. SEM images indicated that the particles are small and agglomerated. FTIR spectrum showed the characteristics of the V-O bonds ranging from 761 to 861 cm-1. The PL study shows intense peak at 620nm corresponding to 5D₀-7F₂ transition under excitation wavelength of 340nm. This shows sufficient energy transfer from vandate to dopant Europium. This phosphor shows strong emission in the visible region and weak in NIR.

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