Synthesis of luminescent KY3F10 nanopowder multi-doped with lanthanide ions by a co-precipitation method

JOURNAL OF RARE EARTHS, Vol. 34, No. 8, Aug. 2016, P. 808

Synthesis of luminescent KY3F10 nanopowder multi-doped with lanthanide ions by a co-precipitation method Szymon Goderski, Marcin Runowski, Stefan Lis* (Department of Rare Earths, Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan, Poland) Received 28 December 2015; revised 14 March 2016

Abstract: A series of KY3F10 nanophosphors doped with Gd3+, Ce3+ and Eu3+ ions were obtained with the use of a co-precipitation method. The resulting products were white precipitates, consisting of spherical particles with diameter about 150–200 nm, which was confirmed using transmission electron microscopy (TEM) technique. Powder X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX) measurements confirmed appropriate structures of the nanoparticles obtained. Spectroscopic properties of the products were examined on the basis of the measured excitation/emission spectra and luminescence decay curves. The synthesized samples showed orange-red luminescence, characteristic for Eu3+ ions. The reaction process was performed in required alkaline pH adjusted with the use of ethylenediaminetetraacetic acid (EDTA) and potassium hydroxide. The samples containing large amounts of Gd3+ dopant ions exhibited a tendency to form products with different morphologies. Keywords: fluorides; co-precipitation; nanophosphors; luminescence; nanopowders; Ce3+/Gd3+/Eu3+ doping; rare earths

Luminescent properties of lanthanide ions, Ln(III), make them a subject of many studies and possible applications, including inorganic luminescent nanomaterials[1–8]. Inorganic luminophores doped with Ln(III) show intense, multicolour emission, long luminescence lifetime and narrow emission bands resulting from the forbidden 4f-4f transitions[9–11]. Inorganic character of nanophosphors results in their high resistance against photobleaching, thermal degradation, high-energy radiation, etc. This leads to stable and efficient luminescence, which is especially important for their bioanalytical applications. Other benefits for the use of such lanthanide doped luminescent nanomaterials are their biocompatibility and possibility of formation of more complex, functional materials[12–18]. One of the simplest and characterized by very intense luminescence group of nanomaterials are matrices based on fluorides, e.g., LaF3 YF3, CeF3, GdF3, NaYF4, KY3F10, Sr2LnF7, etc.[19–22]. In general, lanthanide fluorides reveal very low phonon energy of crystal lattice (≈350 cm–1 for LaF3[23]), resulting in relatively high quantum yields of luminescence, related to negligible nonradiative relaxation of their excited states[24]. The un-doped or doped KY3F10 can be obtained as single crystals and micro-/nanocrystals[25,26]. However, preparation methods are usually quite problematic in their use because of complexity and difficulties in reproducibility of the product, they require hydrothermal conditions or high temperature calcination as well [25,27,28]. In this study we focused on the synthesis, structural,

morphological and spectroscopic characterization of luminescent KY3F10 nanopowders doped with lanthanide ions (Ce3+/Gd3+/Eu3+). The method used for the synthesis was based on the co-precipitation of components in the presence of ethylenediaminetetraacetic acid (EDTA), acting as an anti-agglomerating and complexing agent. EDTA worked also as a buffer, which maintained an alkaline environment during the reaction.

1 Experimental 1.1 Synthesis Materials: Aqueous solutions of Gd(NO3)3, Y(NO3)3 and Eu(NO3)3 were obtained by dissolving respectively Gd2O3, Y2O3 and Eu2O3 oxides (Stanford Materials, 99.99%) in a concentrated HNO3 (POCh S.A., pure, p.a.). CeCl3 solution was obtained by dissolving CeCl3·7H2O salt in deionized water. As a source of fluoride ions potassium tetrafluoroborate, KBF4 (Sigma-Aldrich, ≥96%) was used. KOH (pure p.a., 85%) was purchased from POCh S.A., whereas ethylenediaminetetraacetic acidEDTA (ACS reagent, 99.4%–100%) was from SigmaAldrich. Deionized water was used for all experiments. Syntheses were performed to get each time 0.5 g of the product. Steps of the synthesis procedure were as follows: 0.5 g of ethylenediaminetetraacetic acid was added to 40 mL of deionized water and heated to 60 ºC. Next, to this solution, mixed with a magnetic stirrer was added dropwise

Foundation item: Project supported by the Polish National Science Centre (2015/17/N/ST5/01947) * Corresponding author: Stefan Lis (E-mail: [email protected]; Tel.: +48 61 829 1679) DOI: 10.1016/S1002-0721(16)60098-4

Szymon Goderski et al., Synthesis of luminescent KY3F10 nanopowder multi-doped with lanthanide ions by a…

with a solution of KOH until pH 8 was reached, after that stoichiometric amount of KBF4 (relative to Ln3+ ions) was added. Subsequently, the solution of lanthanide ions (mixed at the desired molar ratio) in 40 mL of water was prepared. Both of the solutions were mixed together and heated to 60 ºC. Afterwards the pH was raised to 8.5 with KOH (except of the synthesis with 97% of Gd3+, where the pH was adjusted to 9.5). The reaction was continued for 20 h with stirring and heating at 60 ºC. The obtained white precipitates were collected and dried in an oven overnight, at 80 ºC. In the whole article the percentage values (x) are molar percentage (mol.%). It has been observed, that with an increasing amount of the Gd3+ ions, the yield of the synthesis decreased. This observation was the reason for adjusting of higher reaction pH (approx. 9.5) for the sample with 97% of the Gd3+ ions (at pH=8.5 no product has precipitated). This also suggests that synthesis of the KLn3F10 nanopowders is strongly dependent on the reaction pH.

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2 Results and discussion 2.1 Structure and morphology Comparison of the measured XRD patterns (Fig. 1) with the reference pattern (ICSD 108778) confirms a successful preparation of KY3F10. Synthesized samples crystallized in a cubic system, Fm-3m space group. Moreover, all of the reflexes are broadened because of the nanocrystallinity of the products synthesized. During the synthesis of the products highly doped with Gd3+ ions, the isostructural KGd3F10 nanocrystals are formed, having similar structural and luminescent properties. This effect is due to the replacement of Y3+ ions by larger Gd3+ ones. TEM images of the three samples of KY3F10:2%Ce3+, 1%Eu3+,xGd3+, where x=0% (a, b), 30% (c, d), 97% (e, f) are shown in Fig. 2. All of the synthesized KY3F10 prod

1.2 Characterization TEM measurements were performed with a Philips CM200FEG electron microscope operating at 200 kV, equipped with an EDAX analyzer. Luminescence measurements, i.e., excitation/emission spectra and luminescence decay curves were conducted on a Hitachi F-7000 spectrofluorometer. The recorded spectra were corrected for the apparatus response. XRD patterns were collected with a Bruker AXS D8 Advance diffractometer, using Cu Kα radiation (λ=0.15406 nm).

Fig. 1 XRD patterns of KY3F10:2%Ce3+,1%Eu3+,xGd3+, x=0%, 30%, 97% Gd3+ ions

Fig. 2 TEM images of KY3F10:2%Ce3+,1%Eu3+,xGd3+, where x=0% (a, b), 30% (c, d), 97% (e, f)

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ucts are composed of sub-microspheres with diameter approx. 150–200 nm. Each of these sub-microspheres consists of smaller nanoparticles with sizes of ~10– 20 nm. The obtained microspheres are uniform and they are well separated (non-agglomerated) from each other. In the case of the sample with 97% of Gd3+ ions there are also present other smaller structures next to the spheres – this may be caused by the high content of the Gd3+ ions, which leads to the formation of isostructural KGd3F10. In Table 1 are shown the sizes, together with their standard deviations, of particles estimated from Scherrer formula[29] for KY3F10 doped with different amounts of Gd3+ ions. One can observe that the size of the particles increases with higher amounts of Gd3+ ions in KY3F10, except the samples containing 0 and 97% of gadolinium ions. Calculated sizes correspond to the small particles inside the microspheres. EDX spectra (Fig. 3) confirm the composition of the obtained grains of the products. For all samples the signals from potassium, fluorine and cerium ions are observed. No signal from gadolinium is observed for the sample without Gd3+ (Fig. 3(c)), and similarly for the product without Y3+—no signal from yttrium ions is observed (Fig. 3(a)). Due to the low content of Eu3+, the signals from europium recorded in all samples are very low and they overlap with the Gd3+ signals (a, b). 2.2 Luminescence properties Fig. 4 shows normalized excitation spectra (a) of the same samples recorded using λem=594 nm. In the excitation spectra one can observe a very intense and broad peak in the region 200–300 nm, due to the 4f→5d transition of Ce3+, related to energy transfer from Ce3+ to Eu3+ ions (via Gd3+ ions), and narrow peaks in the region of 300–400 nm, resulting from internal 4f-4f transitions in Eu3+ ions. For the samples with Gd3+ ions one can also observe two bands at 272 and 310 nm, that correspond to the 8S7/2→6IJ and 8S7/2→6PJ transitions of the Gd3+ ion. The presence of these transitions is related to the direct excitation of Gd3+ ions and subsequent energy transfer to Eu3+ ions. Table 1 Average size of particles estimated from Scherrer formula for KY3F10 doped with 2% Ce3+, 1% Eu3+ and different amounts of Gd3+ ions Amount of Gd3+ ions/% in KY3F10: 2%Ce3+,1%Eu3+

Diameter of particle/nm

97

17.08±3.82

70

31.30±5.79

50

19.31±0.97

40

13.57±1.49

30

15.04±1.85

20

12.45±0.88

10

12.57±1.98

0

17.00±3.63

Fig. 3 EDX spectra of KY3F10:2%Ce3+,1%Eu3+,xGd3+, where x equals to 97% (a), 30% (b), 0% (c) Gd3+ ions

Fig. 4 shows emission spectra (a) of three samples of KY3F10:2%Ce3+,1%Eu3+,xGd3+ (where x=0, 30%, 97%) for λex=250 nm. Excitation wavelength has been chosen based on most intense peaks in the excitation spectra (Fig. 4(a)). All the obtained products show orange-red luminescence, resulting from the 4f-4f transitions characteristic of Eu3+ ions[10]. One can observe low intense peaks resulting from the 5D1→7F1, 2 and 5D0→7F0, 3 transitions, and three high intense peaks from the 5D0→7F1, 2, 4 transitions. The sample with 0% of the Gd3+ ions is characterized by very low intensity of luminescence, due to absence of Gd3+ ions which play a role as energy mediators, decreasing the energy gap between the lowest excited states of Ce3+ and Eu3+ and facilitating energy transfer between these ions[24]. The intensity of luminescence for the samples 30% and 97% is similar, but the shape of the spectra is different, what is probably connected with the formation of KGd3F10 of different morphologies obtained in different pH of the reaction. Emission lifetime of the obtained nanophosphors were determined by analysis of the recorded lumines-

Szymon Goderski et al., Synthesis of luminescent KY3F10 nanopowder multi-doped with lanthanide ions by a…

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Fig. 4 Emission (a) and excitation (b) spectra of KY3F10:2%Ce3+, 1%Eu3+,xGd3+, x=0%, 30%, 97% Gd3+ ions

Fig. 5 Luminescence decay curves and calculated lifetime of KY3F10:2%Ce3+,1%Eu,xGd3+, x=5%, 10%, 20%, 30%, 50%, 70% Gd3+ ions

cence decay curves (Fig. 5). All of the decay profiles, except the sample with 97% of Gd3+ ions, have a monoexponential character, and fit well to the mathematic function y=A*exp(–x/τ)+y0, with R2>0.999, where y and y0 are the luminescence intensities at time x and 0; A represents a constant; x is the time; τ is a decay constant of luminescence; and R2 is a coefficient of determination. Up to 20% of the Gd3+ ions content, lifetime of luminescence grows almost to 9 ms. Above this point with increasing amount of gadolinium ions, the emission lifetime is shortened to ~7.5 ms for 70% of Gd3+. The calculated relatively long luminescence are typical of the

Eu3+ ions embedded in structure of crystalline rare earths fluorides[10,22,24]. The biexponential character of decay curve of the sample with 97% of Gd3+ ions (fitted to mathematic function y=A1*exp(–x/τ1)+A2*exp(–x/τ2)+y0), where y and y0 are the luminescence intensities at time x and 0; A1 and A2 represent constants; x is the time; τ1 and τ2 are decay constants of luminescence. It is probably related to different coordination environments of the Eu3+ ions, due to non-homogenous morphology/structure of the product (see TEM image Fig. 2(e, f)). Fig. 6 shows the dependency of luminescence intensities on the amounts of each dopants. It can be observed, that with the increasing amount of the Gd3+ ions up to 50% (a), the luminescence intensity of KY3F10 grows up. In the case of Ce3+ (b) the optimum amount of dopants is approx. 4% and for Eu3+ (c) is approx. 1.5%, respectively. Fast decreases of the luminescence intensity above the optimal amounts of the dopants is mostly due to cross-relaxation processes, that may occur in highly doped systems. The CIE 1964 color coordinates were calculated to characterize the color of luminescence emitted by the sample KY3F10:2%Ce3+,30%Gd3+,1%Eu3+. As shown in Fig. 7, the resultant orange-red color in the CIE chart is consistent with the luminescence of the sample shown in Fig. 4 (left inset).

Fig. 6 Dependency of luminescence intensity as a function of different amounts of dopants (a) KY3F10:2%Ce3+,1%Eu3+,xGd3+; (b) KY3F10:xCe3+,1%Eu3+,30%Gd3+; (c) KY3F10:2%Ce3+,xEu3+,30%Gd3+

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Fig. 7 CIE chromaticity diagram of the KY3F10:2%Ce3+, 30%Gd3+, 1%Eu3+ nanophosphor obtained using a co-precipitation method

3 Conclusions A series of KY3F10 samples containing different amounts of Gd3+, Ce3+ and Eu3+ dopant ions were synthesised with the use of a co-precipitation method. The synthesised nanophosphors in the form of a white powder consisted of spherical particles with diameter approx. 150–200 nm. XRD pattern and EDX spectra confirmed obtaining of the desired products. The samples exhibited relatively strong orange-red luminescence, characteristic for the Eu3+ ions, due to energy transfer Ce3+→Gd3+→ Eu3+. The measured luminescence lifetime of the Ln3+ doped KY3F10 nanocrystals was relatively long (≈7.5–9 ms). The presence of Gd3+ dopant was the key issue to obtain intense luminescence of these systems. As a result of the stable, multicolour luminescence, narrow 4f-4f emission bands and long emission lifetime, the synthesised lanthanide based nanophosphors can be used in a wide range of specific applications, such as laser materials, bioanalytical markers, lighting sources, etc. Acknowledgement: M.R. is a recipient of the scholarship supported by the Foundation for Polish Science (FNP).

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