Fluorescence Study of Eu-YVO₄ Nano-Phosphors as a Function of Calcination Temperature and Excitation Wavelengths

Abstract

Nano phosphors have been prepared with 1 mol % and 5 mol % doping of Eu in vanadate by sol-gel method. X-ray pattern indicated that nano phosphors did not lead to phase change and has tetragonal crystal structure. Excitation spectra revealed that the band at 308 nm arises due to the charge transfer transitions among Eu³⁺ and VO₄ ^3- ions. The peak position of fluorescence shifted from 518 to 638 nm on varying the excitation wavelength from 257 nm to 318 nm. It is also noticed that the fluorescence intensity increases with the rise of calcination temperatures of the samples.

Introduction

Extensive studies of yttrium orthovanadate (YVO₄) have been carried out since past three decades in both bulk as well as nano-crystalline form because of its high sensitivity in lanthanide ions [1–7]. The red light emitting rare earth vanadates doped with europium ion (Eu³⁺) show intense absorption bands in the near UV and blue wavelength region based on charge transfer (CT) absorption between O^2- and Eu³⁺ groups and 4f-4f transitions. Yttrium orthovanadate (YVO₄) is one of the most promising inorganic luminescent materials which possess zirconia type tetragonal structure. The lanthanide ion substitute for yttrium in YVO₄ lattice (site symmetry D_2d). The introduction of the rare earth ion in the crystal results into the splitting of the degenerate levels and shift of the gravity centre of the level. This last effect is related to nephelauxetic effect [8], which is a measure of metal ligand covalency. For nano-crystals doped with the rare earth ions, a valuable tool in the investigation of the morphology changes produced by various thermal treatments is the optical spectroscopy. In case of europium ion, the luminescence spectrum is relatively easy to analyse, since strongest transitions originate on energy state (⁵D₀) not split by crystal field. Consequently, the europium ion has been extensively used to probe the local environment of dopant sites [9–11]. There are few studies regarding the effect of the thermal treatments on the YVO₄: Eu nano-crystals. Evolution of particle size as a function of calcination temperature for YVO₄: Eu was presented [12]. Generally the sizes of the nano-crystals increase with the calcination temperature, though the coherence domain does not increase linearly with the calcination temperature [15]. As a consequence of the increase of the particle size, the Eu³⁺ emission lines become narrower and asymmetry ratio decreases. This behaviour denotes an increase of local symmetry at Eu³⁺ sites (improvement of crystallinity). The improved crystallinity and removal of various impurities adsorbed at the crystallite surface lead generally, to better emission efficiency.

In this paper Y_(0.001-x)Eu_xVO₄ (x = 0.0001, 0.0005) nano phosphors were synthesized by sol-gel method. Samples were calcined at different temperatures. Meanwhile, the temperature dependence of luminescence intensities and energy transfer mechanism of Eu³⁺ ions in YVO₄ host are discussed in details. In addition to this the fluorescence behaviour of YVO₄: Eu phosphors have been studied as a function of excitation wavelength. During the excitation of YVO₄: Eu phosphor by UV light, red emission takes place due to the intra-configurational f-f transitions of Europium ions. On the other hand when the same phosphor is excited by far UV light ranging from 257 nm to 298 nm, the emission of different transition at different excitation takes place by dominating the characterstic red emission (⁵D₀-⁷F_j, j = 1,2,3 and 4) of YVO₄:Eu phosphor.

Materials and Methods

Material Used

Ammonium metavanadate (NH₄VO₃), yttrium acetate [Y(CH₃CO₂)] and europium acetate[Eu(CH₃CO₂)₃] materials with the purity 99.99 % were supplied from Aldrich chemical company. All the procedure was carried out in dry atmosphere of nitrogen. Besides this ethylene glycol (C₆H₆O₂), citric Acid (C₆H₈O₇) and deionised water were also used.

Powder Preparation

Two solutions A and B were prepared for YVO₄-Eu synthesis. Solution A is comprised of 0.01 mole of ammonium metavanadate, 20 ml of ammonium hydroxide and 100 ml of deionised water. Solution B is prepared by mixing yttrium acetate (0.01-x) mole, europium acetate (x = 0.0001 and 0.0005 mol) and 100 ml of deionised water. The prepared solutions were mixed together, result in the change of colour from colourless to pure white. The solution is stirred magnetically for half an hour. After this, added 0.02 mole citric acid and 1.1 ml of ethylene glycol. The solution turned into the dark pale yellow. Further stirring for 20 minutes resulted light, transparent pale yellow solution. All this procedure was carried out through the slow flow of nitrogen [16]. Finally solution was kept at 120 °C for one day. Dried precursor was obtained, which was given thermal treatments for 3 h.

Characterizations

The synthesized samples were characterized by a X-ray diffractometer (XRD, Philips). Fluorescence spectra of the phosphor were studied with the Fluor meter at different excitation wavelengths.

Results and Discussion

X-Ray Diffraction

To check the phase purity of the samples XRD measurements were performed. The XRD pattern of the synthesized phosphors at 800 °C of 1 mol % and 5 mol % is presented in the Fig. 1. It is found that both the samples have tetragonal structure of YVO_4. The samples also show crystallinity of YVO₄-Eu corresponding to JCPDS-17-0341 showing the prominent peaks (200), (112) and (312). Among the materials with different amount of Eu ions there was no significant change in the different profiles indicating no change in the crystal structure. The absence of the significant changes in the XRD pattern with increasing europium concentration may have strict relationship with the ionic radii between the Y³⁺ ions of matrix (0.900 Å) and Eu³⁺ ion (0.947 Å) which acts as a dopant. Gasparatto et al., (2009) reported that there is a direct relationship between lattice parameters and occupation fraction of rare earth ions in a crystalline structure. In fact, for the samples with higher thermodynamic perturbation (i e 800 °C) the changes in the atomic coordinates and unit cell were almost negligible. The volume cell was 130.759 Å³ for both the samples. One of the favourable conditions for doping of the europium ion in the host lattice is that the difference in the ionic radii between both Y³⁺ and Eu³⁺ ions is less than 5 %. This behaviour is strong indication that Eu³⁺ ion is easily accommodated into the crystalline structure of host matrix. No reflection assigned to secondary phases was observed in diffractograms.

Fig. 1

X-ray Diffraction pattern

The lattice parameters were calculated using the following relationship between Bragg diffraction spacing and tetragonal cell parameter, which were given in the Table 1.

Table 1 Lattice parameters of YVO₄:Eu prepared by sol-gel method at different concentrations

$$ \frac{1}{d_{hkl}^2}=\frac{h^2+{k}^2}{a^2}+\frac{l^2}{c^2} $$

(1)

where d_hkl is the Bragg diffraction spacing for (hkl) plane and h,k,l are miller indices, a and c are cell parameters.

Fluorescence study

The excitation spectrum obtained by monitoring the ⁵D₀-⁷ F₂ transition of Eu³⁺ at 617 nm for the sample YVO₄: Eu (x = 0.0001, 0.0005) is as shown in the Fig. 2. The broad excitation band of the YVO₄: Eu nano-phosphors at approximately 308 nm arose from a charge transfer transition between Eu³⁺ and VO₄ ^3- anions. With the addition of europium to the YVO₄ host lattice, the compound exhibits the charge transfer transitions causing the transfer of electrons from 2p orbital of oxygen to the 4f orbital of europium [17–20].

Fig. 2

Excitation spectra of YVO₄ doped europium at 1 mol %

The emission spectrum was composed of sharp lines ranging from 400 to 750 nm that were associated with the ⁵D_J-⁷F_J transitions in europium ion. ⁵D₀-⁷ F₂ transition is the strongest red light emission assigned to be at the wavelength of 618 nm. The deep red emission line at 698 nm was due to ⁵D₀-⁷ F₄ transition.

The emission and excitation spectra in Figs. 2, 3 and 4 indicate that highest excitation and emission intensities were obtained at 5 mol% due to increase in the optically active centres at higher concentrations.

Fig. 3

Excitation spectra of YVO₄ doped europium with 5 mol %

Fig. 4

Fluorescence spectra at λ_ex = 308 nm of YVO₄-Eu nano-phosphors prepared by sol-gel method

Under the 308 nm light excitation, both the samples show the strong red emission, which contains exclusively characterstic emission transition lines of Eu³⁺ ion listed in Table 2, including ⁵D₀-⁷ F₁ and ⁵D₀-⁷ F₂ transitions. We can also find the relative intensity of ⁵D₀-⁷ F₂ is higher than that of ⁵D₀-⁷ F₁, since Eu³⁺ ion occupies non-centro-symmetric site in the tetragonal YVO₄ crystalline phase.

Table 2 Emission wavelength positions corresponding to transitions at 308 nm excitation wavelength (Fig. 4) in YVO₄-Eu nano phosphors

UV laser irradiation results in the excitation of different optical centres at different excitation wavelength, which is onus for the emission in nano-phosphors. This resulted in the shift of emission peak with different excitation wavelengths.

The Raman shift is calculated by using the relation

$$ \frac{\varDelta E}{hc}\left(c{m}^{-1}\right)=\frac{1}{\lambda_{exc}}-\frac{1}{\lambda_{em}} $$

(2)

where λ_exc and λ_em are the excitation and emission wavelengths and h is the Planks constant. So the observation of emission and change in the emission peak with the excitation wavelength could be interpreted in context of myriad optical centres present in the YVO₄-Eu nano-phosphors.

Different excitation wavelengths relate to the dominance of emission peaks at different wavelengths, whose intensity might be decreasing with the rise in the excitation wavelengths. Figures 5 and 6 show the fluorescence spectra at 1and 5 mol % doping concentration of europium in the samples at different excitation wavengths.The intensity of ⁵D₀-

⁷ F₁ peak is maximum at the 296 nm. On the other hand at 308 nm the ⁵D₀-⁷ F₂ peak is in dominance. But it is found that no emission peak can be observed upon 397 nm excitation corresponding to the emission intensity of 617 nm. Different emission at different excitation has been tabulated in the Table 3. Besides this we have also calculated stokes shift in YVO₄-Eu as in the Table 3 [21].

Table 3 Emission wavelength and transitions related to different excitations (Figs. 5 and 6) for YVO₄-Eu at 1 mol% and 5 mol%

Fig. 5

Fluorescence spectra of YVO₄-Eu (1 %) for λ_em = 617 nm at different excitation wavelengths

Fig. 6

Fluorescence spectra of nano phosphors with mole (5 %) of Eu for λ_em = 617 nm at different excitation wavelengths

Figure 7 presents the fluorescence spectra of YVO₄:Eu³⁺ phosphor at different calcined temperatures under the excitation wavelength of 308 nm. The spectra consist of band ranging from 530 to 690 nm, which is attributed to the electronic transitions of europium ions [12–14]. The most intense peak at 619 nm is due to the ⁵D₀-⁷ F₂ electric dipole transition with energy 2.015 eV and is sensitive to the chemical bond in the vicinity of Eu³⁺ ions.

Fig. 7

Fluorescence spectra at different calcination temperatures

Peak at 594 nm is due to ⁵D₀-⁷ F₁ magnetic dipole transition with energy 2.086 eV and it hardly changes with the crystal field strength around the Eu³⁺. The intensity of the peak at 619 nm is stronger than the peak at 594 nm confirming that there is no inversion symmetry site for Eu³⁺ ion in the tetragonal structure. The other low intensity peak is at 538 nm due to ⁵D₁-⁷ F₁ transition having energy 2.30 eV. Figure 8 shows the variation of integral luminescence intensity with temperature. It reflects that integral intensity increases with the rise of calcination temperature of the sample.It is due evaporation of impurities (OH^-1, NO₃- etc.) which are adsorbed at the particle surface during synthesis. 

Fig. 8

Integral luminescence as a function of calcination temperatures

Conclusion

In summary, we have demonstrated synthesis of nano phosphors by sol-gel method. Samples were calcined up to 1,000 °C temperatures. XRD study concluded that the samples have single phase. Fluorescence study strongly reflects that the transitions depend on the excitation wavelength. It is also noticed that fluorescence intensity increases with the rise of the calcination temperatures.