Photoluminescent properties of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles prepared by co-precipitation method

JOURNAL OF RARE EARTHS, Vol. 27, No. 4, Aug. 2009, p. 588

Photoluminescent properties of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles prepared by co-precipitation method T. Grzyb, S. Lis (Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland) Received 9 October 2008; revised 26 January 2009

Abstract: LaF3:Eu3+ and GdF3:Eu3+ nanoparticles were prepared by a co-precipitation method in the presence of the chelating agent, citric acid. The structural properties of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The average crystallite size was estimated from the full-width at half-maximum (FWHM) of the diffraction peaks by the Scherrer equation. The sizes of the nanoparticles were 12 nm for LaF3:Eu3+ and 17 nm for GdF3:Eu3+. The luminescent properties of the nanoparticles were investigated by excitation and emission spectra. Energy transfer from Gd3+ to Eu3+ was observed. Keywords: nanofluorides; phosphors; luminescence; energy transfer; rare earths

Rare earth fluorides have been the subject of intense studies in recent years. Materials based on doped fluoride nanoparticles have interesting optical properties different from those having the identical bulk structure, and therefore they have wide possibilities for applications. These nanoparticles can be used in light emitting devices which require efficient phosphors, like plasma panel displays, field emission displays or solid state lasers and optoelectronic devices[1–5]. Surface modified luminescent nanoparticles based on lanthanide fluorides also play an important role in biology and biotechnology[6–8]. Their sharp emission bands cover the whole spectral window in the visible and the near-infrared. The lifetimes of the excited states, range from micro- to milli-seconds. These optical properties indicate that lanthanide doped nanoparticles can be considered as a good alternative to heavy metal containing quantum dots[7]. Their photo-stability is better and their luminescence yields are higher than the respective properties of organic dyes[6]. Lanthanum fluoride is well known to be a good host material for luminescent lanthanide Ln3+ ions. Because of the low-energy phonons (~350 cm–1) of LaF3, quenching of the excited states of the guest Ln3+ ions is minimal[9]. Eu3+ ions can easily be substituted for the La3+ sites in the lattices, and solid solutions can be obtained. GdF3 is a good alternative for LaF3. Because of a 4f energy-level overlap between the 6 PJ states of Gd3+ and the 5HJ states of Eu3+, energy transfer from Gd3+ to Eu3+ is possible[2]. Additionally the occurrence of efficient quantum cutting phenomena (emission of two Corresponding author: S. Lis (E-mail: [email protected]) DOI: 10.1016/S1002-0721(08)60294-X

photons of visible light per absorbed vacuum ultraviolet) have been investigated in Eu3+ doped LiGdF4 and GdF3[1,10]. Gd3+ ions have a large magnetic moment and a relaxation time on the nanosecond scale. GdF3 nanoparticles can be used as contrast agents in magnetic resonance imaging (MRI) because of their paramagnetic properties[8]. There are many ways for obtaining nanomaterials. To synthesize lanthanide nanofluorides the best results were given by their precipitation in ethanol[3], co-precipitation with organic ligands as stabilizing agents[6–8,11], decomposition of acetates in trioctylphosphine oxide[12] hydrothermal[10] and microemulsion methods[4,14]. The co-precipitation method with citric acid as a stabilizing ligand was chosen for this work. The addition of organic compounds as coatings to the nanoparticles should prevent the growth of the precipitated nanoparticles by capping their surface.

1 Experimental 1.1 Synthesis All reagents were analytical grade and were purchased from Sigma Aldrich. LaF3:Eu3+ and GdF3:Eu3+ nanoparticles were prepared by the method described previously[8]. Both of the fluorides were doped to 5mol.% Eu3+. 1.1.1 LaF3:Eu3+ synthesis A solution of citric acid (0.81 g, 4.2 mmol) and NH4F (0.20 g, 5.45 mmol) in 45 ml of water was neutralized by adding NH3(aq). The solution was heated

T. Grzyb et al., Photoluminescent properties of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles prepared by co-precipitation method

to 75 °C followed by the addition of La(NO3)3·6H2O (1.046 g, 2.42 mmol) and Eu(NO3)3·6H2O (0.057 g, 0.13 mmol) in 3.6 ml of water. The 3.6 ml solution was added dropwise and stirred at 75 °C for 4 h. 1.1.2 GdF3:Eu3+ synthesis A solution of citric acid (0.75 g, 3.9 mmol) and NH4F (0.19 g, 5.13 mmol) in 42 ml of water was neutralized by adding NH3(aq). The solution was heated to 75 °C. Next, Gd(NO3)3·6H2O (1.002 g, 2.22 mmol) and Eu(NO3)3·6H2O (0.052 g, 0.12 mmol) in 3.4 ml of water were added dropwise. The resulting solution was stirred for 4 h keeping the temperature at 75 °C. Isolation of the nanoparticles was achieved by adding ~100 ml of methanol followed by centrifugation. The resulting product was dried at 80 °C for 24 h, and calcined at 350 and 500 °C for 3 h in order to remove the organic impurities (in an air atmosphere). All procedures were done under an air atmosphere. The 5% molar concentration of the dopant was chosen because of the probability for cross-relaxation quenching between Eu3+ at higher concentrations[3].

The estimated sizes of the nanocrystals were (12±5) nm for samples dried at 80 °C, (13.5±2.5) nm after calcination at 350 °C and (27±2) nm for samples calcined at 500 °C. All samples were white after drying and calcination. Fig. 2 gives the XRD patterns of GdF3:Eu3+ nanoparticles and a reference pattern from the JCPDS card (12-0788) with orthorhombic structure (space group Pnma(62)). Calcinations of the product obtained at 350 °C were not effective enough to remove the organic impurities. The sample was dark, and its luminescent properties could not be measured. The sample annealed at 500 °C was composed mainly of GdF3, but small amounts of the oxyfluoride were found. The average crystal size was (17±8) nm for the sample dried at 80 °C, (20.5±4) nm after calcination at 350 °C and (52±3) nm after calcination at 500 °C. The TEM image of a particular sample showed in Fig. 3 indicates aggregation of the nanoparticles. The observed sizes of the crystallites are comparable with those calculated from the XRD pattern.

1.2 Characterization XRD patterns were measured with a Bruker AXS D8 Advance diffractometer using Cu KĮ radiation (Ȝ= 0.1541874 nm) in the 2ș ranges from 20° to 60°. The TEM images were measured on JEM 1200 EXII, JOEL transmission electron microscope, using an accelerating voltage of 80 kV. Average crystallites sizes were calculated from the Scherrer equation D=0.9Ȝ/ȕcosș, where D is the average grain size, the factor 0.9 is characteristic of spherical objects, Ȝ is the X-ray wavelength, ș and ȕ are the diffraction angle and full-width at half-maximum of an observed peak[15]. The excitation and emission spectra were performed on a MPF-3 spectrofluorimeter at room temperature.

Fig. 1 XRD patterns of the LaF3:Eu3+ nanoparticles calcined at different temperatures

2 Results and discussion Fig. 1 shows the XRD patterns of the LaF3:Eu3+ nanoparticle and the data from the JCPDS card (32-0483) as a reference pattern with hexagonal structure (space group P-3c1 (165)). The formation of LaF3 hexagonal crystals with no second phase was observed in samples dried at 80 °C for 24 h and calcined at 350 °C for 3 h. All the diffraction peaks were shifted toward smaller lattice parameters. The observed decrease in lattice parameters indicates that the Eu3+ ions have substituted for the La3+ sites (the radius of Eu3+ is smaller than La3+)[3]. Annealing at a temperature about 500 °C for 3 h results in the formation of a second phase composed from oxyfluoride LaOF. At 600 °C LaOF is the only phase[16].

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Fig. 2 XRD patterns of the GdF3:Eu3+ nanoparticles calcined at different temperatures

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Figs. 4 and 5 show the room-temperature excitation spectra (monitored at 619 nm) of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles obtained under different conditions. Both of the spectra present a broad and intense band between 260–275 nm, which increased with increasing temperature of calcination. This broad excitation band is attributed to O2–-Eu3+ charge transfer (CT), indicating the formation of LnOF:Eu3+ nanocrystals[16,17]. The sharp band in the GdF3:Eu3+ excitation spectra with a maximum at 274 nm is connected with the energy transfer (ET) from the Gd3+ to the Eu3+ ions. The Gd3+ ions in GdF3 can be excited by 274 nm UV light from a very stable 8S7/2 ground state to the 6IJ excited states. The 6IJ states can decay nonradiatively to the energetically lower 6PJ states. Energy transfer can then occur between Gd3+ and Eu3+ because the Gd3+ 6PJ and Eu3+ 5HJ states are energetically close to each other[2]. Figs. 6 and 7 show the emission spectra of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles which consist of the characteristic emission bands of the Eu3+ ion. The sharp lines ranging from 580 nm to 720 nm are associated with the Eu3+ transitions from the excited 5D0 level to the 7FJ levels.

Fig. 3 TEM picture of the LaF3:Eu3+ calcined at 350 °C for 3 h

Fig. 4 Excitation spectra of LaF3:Eu3+ obtained at different temperatures

JOURNAL OF RARE EARTHS, Vol. 27, No. 4, Aug. 2009

No emission from higher Eu3+ levels (5D1 and 5D2) was observed because of water content or citric acid presence, which can quench these excited states. It is well known that the Eu3+ ion can be used as a sensitive structural probe to detect the lattice symmetry of host materials[16]. The relative intensity of the electric-dipole 5 D0–7F2 transition depends on the local symmetry of the

Fig. 5 Excitation spectra of GdF3:Eu3+ obtained at different temperatures

Fig. 6 Emission spectra of LaF3:Eu3+ dried at 80 °C

Fig. 7 Emission spectra of GdF3:Eu3+ dried at 80 °C

T. Grzyb et al., Photoluminescent properties of LaF3:Eu3+ and GdF3:Eu3+ nanoparticles prepared by co-precipitation method

Eu3+ ions. In the hexagonal matrix of LaF3, the Ln3+ ion is at a site of C2 symmetry. Electric and magnetic dipole transitions are allowed[3,18]. The emission corresponding to 5 D0–7F1 is a dominating transition what was previously reported[3,13]. In the orthorhombic GdF3 structure, Eu3+ present in Cs symmetry and the 5D0–7F1 transition has a higher probability than the electric dipole transitions 5D0–7F2[2]. The performed luminescence spectra enable to observe the above-mentioned facts. Oxidizing of the fluorides lowers the local symmetry of the Eu3+ ions. The hypersensitive 5D0-7F2 emission became dominant in the spectra what is showed in Figs. 8 and 9. The observed luminescence increased with annealing temperature.

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firmed that the samples dried at 80 °C and calcined at 350 °C had the desired morphology. Calcination of the samples at temperatures above 500 °C resulted in the formation of oxyfluorides. The prepared fluorides had intense luminescence without any purification. Energy transfer from Gd3+ to Eu3+ in GdF3: Eu3+ nanoparticles was observed. This fact contributed to the potential use of the obtained product in applications where far UV was used as an excitation source. The presence of citric acid prevented the growth above ~20 nm of the precipitated nanoparticles, however, their aggregation was observed. Only in the case of LaF3:Eu3+ annealing at 350 °C gave positive results. Acknowledgment: Financial support from the Polish Ministry of Science and Higher Education; Grant N N204 329736 is gratefully acknowledged.

References:

Fig. 8 Emission spectra of LnF3:Eu3+ calcined at different temperatures

Fig. 9 Emission spectra of GdF3:Eu3+ calcined at different temperatures

3 Conclusion LaF3 and GdF3 Eu3+ doped nanoparticles could be prepared by the co-precipitation method under an air atmosphere. Additional calcination processes were used to remove organic impurities from the precipitate. The prepared products had nanosized dimensions in the form of solid state solutions. XRD patterns, excitation and emission spectra con-

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