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Synthesis of NaYF4:Yb, Er up-conversion luminophore from nitrate flux
P. P. Fedorov1*, M. N. Mayakova1, A. A. Alexandrov1, V. V. Voronov1, D. V. Pominova1, E. V. Chernova1, V. K. Ivanov2
1Prokhorov General Physics Institute of the Russian Academy of Sciences, 119991, Moscow, Russia 2Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences,
119991, Moscow, Russia
PACS 81.07.Bc, 76.30.Kg DOI 10.17586/2220-8054-2020-11-4-417-423
The behavior of nanoparticle ensembles was studied using of NaRF4 hexagonal phases. The evolution of particles in the process of rapid and productive synthesis from flux as a result of a chemical reaction was investigated. A low-temperature synthesis process in the medium of sodium nitrate was used. Synthesis of the samples of up-conversion phosphor NaY0.7sYb0.2Er0.02F4 was performed from rare-earth nitrates at 350 -430 °C for 15 - 500 min. NaF was used as the fluorinating agent. Powder X-ray phase analysis and scanning electron microscopy revealed a rapid transformation of the cubic alpha modification into a hexagonal phase, followed by the transformation of nanoparticles into hexagonal prisms of micron sizes. The up-conversion luminescence energy yield increased as the reaction time increased.
Keywords: sodium yttrium fluoride, fluorite crystal structure, gagarinite crystal structure, flux, sodium nitrate.
Received: 15 April 2020
Revised: 30 April 2020
1. Introduction
The hexagonal phases formed in NaF-RF3 (R - rare earth elements, REE) systems are effective matrices for anti-Stokes luminescence. They are phases of variable composition of the general formula Na3xR2-xF6, and their composition is close to the ratio 50:50, according to which they are usually attributed the formula ,0-NaRF4. These phases are low-temperature ones, as compare with cubic fluorite-type a-NaRF4 phases. (0-NaRF4 phases, when co-doped with Yb-Er, Yb-Tm, or Yb-Ho ions, are well known as effective up-conversion phosphors that convert IR radiation to the visible spectral range [1,2]. The number of publications devoted to these materials is huge, see for example [3-13]. The main areas of their potential application are biomedical applications and lighting sources [11-13]. The NaYo.78 Ybo.2Ero.o2F4 is the optimized composition. Functional characteristics of these materials depend strongly on the granulometry of the resulting nano- and micro-powders, and the mechanisms of phase formation, growth and agglomeration of particles are of crucial importance.
(0-NaRF4 powders can be prepared by a broad variety of methods [11,13-16]. Until now, several techniques have been employed for the synthesis of hexagonal NaRF4' phases, both individual and co-doped ones, including solidstate synthesis [14,15,17], co-precipitation from water [18-22] and organic solvents [23], hydrothermal [12,24,25], solvothermal [26-30], mechanochemical [31], ionic liquid-assisted [32,33] and molten salt/flux methods [18,33-43], etc.
Among the various methods of synthesis, much attention is drawn to the first melt-solution method proposed by Batsanova [18,38,44], which consists in the reaction of REE nitrates with sodium fluoride, which acts as a fluorinating agent. As a flux, low-melting sodium nitrate is used. The method is characterized by simple hardware design, high performance, environmental friendliness and low cost. We have shown [45] that in this way it is easy to obtain powders of equilibrium hexagonal phases formed in systems of sodium fluoride with REE fluorides RF3 (R = Pr-Lu,Y).
The purpose of this work is to use this synthesis technique to prepare an up-conversion phosphor NaYF4:Yb,Er and test its effectiveness.
2. Experimental details 2.1. Sample preparation
The samples were synthesized by spontaneous crystallization from a solution in a melt. The synthesis was based on the following equation:
R(NO3)3 + 4NaF ^ NaRF4 | +3NaNO3.
(1)
The starting chemicals were sodium fluoride (NaF, Lanhit®, purity mark "P.A."), yttrium nitrate hexahydrate (Y(NO3)3 • 6H2O, Lanhit®®, purity 99.99 %), ytterbium nitrate hexahydrate (Yb(NO3)3 • 6H2O, Lanhit®®, purity 99.99 %), erbium nitrate pentahydrate (Er(NO3)3 • 5H2O, Lanhit®®purity 99.99 %) and sodium nitrate (NaNO3, Chimmed Group, purity mark "CP") as a solvent. The raw materials were carefully mixed, transferred to a porcelain-glazed crucible, and placed in a furnace for synthesis. The ratio of components was: a seven-fold excess of the fluorinating agent from the stoichiometric composition (eq. 1), the mass fraction of the solvent (NaNO3) was 35 %. The temperature range was from 320 to 350 °C (see Table 1). The heating speed was 10 0C/min for all samples. The sample holding time at the maximum temperature (t, min) was from 15 to 500 minutes. The resulting residue was extracted from the crucible and washed several times with doubly distilled water.
Table 1. Synthesis conditions and hexagonal unit cell parameters of NaYo.78 Ybo.2Ero.o2F4.o powders
No. T, °C t , min Unit cell parameters, A
a c
1 320 15 5.975(1) 3.5111(7)
2 330 30 5.9757(4) 3.5137(3)
3 340 45 5.9726(2) 3.5165(2)
4 350 60 5.9713(2) 3.5154(1)
5 350 180 5.9705(2) 3.5181(1)
6 350 500 5.9731(1) 3.5167(2)
2.2. Sample characterization
X-ray diffraction analysis (XRD) was performed on a Bruker D8 Advance diffractometer with Cu (Ka) radiation. The obtained data were processed by the TOPAS software package. When calculating the lattice parameters, the space group P63/m was set.
The Carl Zeiss NVision 40 electron microscope (Zeiss, Germany) was used for scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) in high vacuum mode, at an accelerating voltage of 20 kV. Images were generated using a backscattered electron detector (BSE) and a secondary electron detector (SE).
The spectroscopic analysis included registering the spectra of up-conversion luminescence and diffusely scattered laser radiation within the range of 300 - 1000 nm, and calculating the energy yield of up-conversion luminescence. Luminescence studies were performed using a laser-induced luminescent spectroscopy unit that includes a fiber-optic spectroanalyzer LESA-01-BIOSPEC [46]. The values of the energy yield of up-conversion luminescence in the visible range of the spectrum were measured using a setup in which the test sample was placed in a modified integrating sphere produced by Avantes. Excitation of up-conversion luminescence was performed by a laser with a wavelength of 974 nm ("Biospek", Moscow, Russia) and a power of 1 W. Spectral data were analyzed using the UnoMomento spectrum processing program, taking into account the hardware function of the spectrometer and the integrating sphere.
3. Results and discussion
The results of XRD analysis are presented in Fig. 1. For a short synthesis time, a mixture of cubic and hexagonal phases is obtained. A half-hour exposure of the reaction mixture in the melt leads to the formation of a pure hexagonal phase. Further increase in the heat treatment duration does not lead to a change in the phase composition of the samples. The values of lattice parameters of hexagonal phase are presented in Table 1.
Electron microscopy data are shown on Figs. 2 and 3. The initially formed ensemble of nanoparticles (Fig. 2(a,b)) generates the formation of elongated particles of micron sizes. The grain size distribution of the samples is not uniform. Elongated particles have an approximately hexagonal faceting corresponding to their crystallographic symmetry. Trigonal prisms are distinguishable. However, microcrystals that are approximately characterized by these simple habit forms have a huge number of rough defects and are splices of smaller crystals. These results are similar to what we previously observed in the synthesis of NaRF4 phases [45].
EDX data show that the actual composition of synthesized samples corresponds to the nominal one. Admixtures of other elements were not recorded.
* * 5 1 * 1 ? A * *ji* **
: ' ' * ' 1 ' 1 ' 1 ' 1 i t A 1 t Á* **
' 1 I I 1 1 1 1 1 1 1 1 * * * 3 il A Jit. *ki t*
1 1 1 * J 1 1 1 1 1 1 1 1 f Î ** 2 ]\ * A A * * **
i I 1 1 1 1 1 1 1 1 r î * 1 lAjr » t kj, *JAJ
i i i i i i i i i i i i
10 20 30 40 50 60 70
20, grad.
Fig. 1. X-ray powder diffraction patterns of the samples. Stars denote the peaks of the hexagonal NaRF4 phase, and squares denote the cubic one. Sample numbers are the same as in the Table 1
Fig. 2. SEM images of samples 1-3. (a) and (b) - the sample 1; (c) and (d) - the sample 2; (e) and (f) - the sample 3
Fig. 3. SEM images of samples 4-6. (a) and (b) - the sample 4; (c) and (d) - the sample 5; (e) and (f) - the sample 6
It should be noted that the cubic a-phase is a high-temperature phase in NaF-RF3 systems, and it is non-equilibrium one under our synthesis conditions. Cubic-hexagonal transitions (a — ft transitions) in NaRF4 phases have been reported in the numerous publications describing NaRF4 phase syntheses by a variety of techniques [4-11]. Temperature increase and/or extension of the synthesis duration promote the aforementioned a — ft phase transition. This transformation, which is also observed in our experiment when the thermal treatment time is increased to 30 min (see Fig. 1), is an actual realization of the Ostwald's step rule [47,48].
Bard et al. [12] attempted to interpret this transformation as a manifestation of non-classical crystal growth by the oriented attachment of nanoparticles. This assumption should be considered untenable [49]. In view of the different structures of a and ft polymorphs, one should realize that beta-phase cannot be built from the pieces of the alpha-phase. A change in the coordination number (CN) of the rare earth ions (CN 8 for cubic a phase and CN 9 for hexagonal ft phase) unequivocally requires complete recrystallization of the formed cubic nanoparticles.
On the other hand, the enlargement of beta phase particles during thermal annealing fits well into the scheme of non-classical crystal growth by agglomeration of particles [50,51]. The poor faceting of the formed crystallites is a confirmation of this [52].
The results of the luminescent studies are presented on the Fig. 4 and in the Table 2.
The luminescence spectra of the samples are typical for this luminophore and show green (510 - 575 nm) and red (625 - 670 nm) bands, corresponding to the radiative transitions of the erbium ions: 2Hh/2, 4S3/2 —^ 4Ii5/2 and 4Fq/2 — 4Ii5/2, respectively.
Changes in the synthesis duration had a tangible effect on luminescence efficiency of the samples. As the exposure time increased, the energy yield of up-conversion luminescence increased monotonously. At the same time, the ratio of red and green luminescence bands changed slightly.
The results shown in the Table 2 allow the following conclusions. The difference in the values of energy yield may be accounted for by the larger size of the particles (resulting from the longer synthesis), which leads to an increase in the surface area-to-volume ratio. Similar phenomena (surface quenching effect) have been observed for upconversion NaGdF4:Yb,Tm nanoparticles [53] and for the intrinsic exciton luminescence in MF2 nanocrystals also [54,55].
Fig. 4. Luminescence spectra of samples 1-6. The wavelength of the pumping laser is 974 nm
Table 2. Estimated external energy yield (EQ) of up-conversion luminescence of Er3+ ions of NaYo.7sYbo.2oEro.o2F4.o powder samples measured in integrating sphere, %
No. EQ,% EQi %, red band EQ2 %, green band EQi/EQ2
1 0.73 0.0033 0.0040 0.80
2 0.85 0.0044 0.0041 1.06
3 0.50 0.0032 0.0019 1.70
4 0.68 0.0041 0.0027 1.54
5 0.57 0.0034 0.0023 1.50
6 2.25 1.08 1.17 0.92
4. Conclusion
Our studies have shown the possibility of synthesizing an effective up-conversion phosphor NaYF4:Yb, Er by a chemical reaction in a sodium nitrate flux. The target hexagonal phase is formed from the primary non-equilibrium cubic nanophase in accordance with the Ostwald's step rule. The resulting particles quickly grow in size with the formation of micron-sized particles that have a rough hexagonal habit with strong symmetry violations and multiple defects. The implementation of a non-classical mechanism of crystal growth by directed agglomeration of particles is assumed. An increase in the particle size results in an increase in the efficiency of up-conversion luminescence.
Acknowledgements
The work was preformed under the RFBR grant 18-29-12050mk. The authors are grateful to Dr. A. E. Baranchikov for his assistance in conducting research and to Dr. S. V. Kuznetsov for helpful discussions. All experiments were performed on the equipment kindly provided by the Centre for Collective Use of Prokhorov General Physics Institute of the Russian Academy of Sciences and the Centre for Collective Use of Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences.
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