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12Condensed Matter and Interphases (Kondensirovannye sredy i mezhfaznye granitsy)
Original articles
DOI: https://doi.org/10.17308/kcmf.2020.22/2524 elSSN 2687-0711
Received 26 January 2020 Accepted: 15 March 2020 Published online 25 March 2020
Synthesis of Upconversion Luminophores Based on Calcium Fluoride
©2020 A. A. AlexandrovHab, M. N. Mayakovab, V. V. Voronovb, D. V. Pominovab, S. V. Kuznetsovb, A. E. Baranchikovc, V. K. Ivanovc, E. I. Lysakovaa, P. P. Fedorovb
aMIREA - Russian Technological University,
86 Prospekt Vernadskogo, Moscow 119571, Russian Federation
bProkhorov General Physics Institute of the Russian Academy of Sciences, 38 Vavilov str., Moscow 119991, Russian Federation cKurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, 31 Leninsky prospekt, Moscow 119991, Russian Federation
Abstract
The aim of our study was to synthesize a luminophore based on calcium fluoride doped with rare-earth elements: 5 % Yb and 1 % Er, using the molten salt synthesis method.
NaNO3 was used as a solvent and sodium fluoride NaF served as the fluorinating agent. The obtained samples were analysed and described using X-ray powder diffraction analysis, energy dispersive X-ray spectroscopy, scanning electron microscopy, and luminescence spectroscopy.
During the study we also investigated the effect of the synthesis conditions on the phase composition and the particles morphology. It was determined that single-phase samples (solid solutions based on calcium fluoride) can only be obtained at a temperature of at least 400 °C, with the optimal exposure time being 3 hours. The composition of the obtained samples was determined. It differs from the nominal composition and can be described as Ca0 88(Yb, Er)0 06Na0 06F2. It was demonstrated that the parallel insertion of sodium and rare-earth element ions increases the solubility limit of sodium fluoride in calcium fluoride. The luminescence efficiency was 1.21 %.
As a result of this study we obtained a new material with upconversion properties.
Keywords: luminophores, molten salt synthesis, inorganic fluorides, upconversion, nanopowders, rare-earth elements. For citation: Alexandrov A. A., Mayakova M. N., Voronov V. V., Pominova D. V., Kuznetsov S. V., Baranchikov A. E., Ivanov V. K., Lysakova E. I., Fedorov P. P. Synthesis of upconversion luminophores based on calcium fluoride. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2020; 22(1): 3-10. DOI: https://doi.org/10.17308/ kcmf.2020.22/2524
1. Introduction
Anti-Stokes luminescence, or upconversion, occurs when a luminophore, excited by electromagnetic radiation at a certain wavelength, emits radiation with a shorter wavelength and thus with more energy. This phenomenon was independently discovered by V. Ovsyankin and P. Feofilov [1] and F. Auzel [2] in the mid-1960s. Since then, quite
El Alexander A. Alexandrov,
e-mail: alexandrov1996@yandex.ru
The content is available under Creative Commons Attribution 4.0 License.
a number of upconversion luminophores have been synthesized. Fluorides are among the most promising classes of compounds used for synthesizing upconversion luminophores. Due to low-energy phonons, their mechanical and optical properties, as well as high isomorphic capacity, fluorides are good host matrices for doping with rare-earth ions. Due to the high upconversion efficiency of luminescence the most commonly used fluoride matrices include hexagonal modifications NaYF4 [3] and
NaGdF4 [4], as well as fluorides of alkaline earth techniques, doped with Yb3+, Er3+ [3,5-6].
At the moment, there are several common methods for the synthesis of fluorides, including coprecipitation from aqueous solutions [7-8], hydrothermal synthesis [9-10], solvothermal synthesis [5], the sol-gel method [11], mecha-nochemical synthesis [12], molten salt synthesis [13], combustion synthesis (CS) [6], and thermal decomposition of the precursors [14]. There are also methods which use ionic liquids as fluorinating agents, templates, and reaction media [15-16].
It is still important to search for new upconversion luminophores and develop new synthesis methods. Upconversion luminophores are applied in various areas of science and technology, including biomedicine [17], solar cells production [18], and thermometry [19]. Other applications of upconversion materials include the visualisation of infrared radiation [20] and synthesis of white light luminophores [21].
The aim of this study was to synthesise an upconversion luminophore based on a calcium fluoride matrix with a fluorite structure, doped with rare-earth elements (REE). Since the method of coprecipitation from aqueous solutions has certain drawbacks [6], our aim was also to determine the optimal conditions for the molten salt synthesis of single-phase calcium fluoride powders, doped with ytterbium and erbium. The choice of the concentrations of the doping rare-earth additives (5 mol% of Yb3+ and 1 mol% of Er3+) was based on the fact that the highest upconversion efficiency of a similar fluorite matrix of SrF2 was observed, when the concentration range of Yb3+ was between 2 mol% and 12 mol%, and of Er3+ between 0.25 mol% and 2.25 mol% [3,22].
2. Experimental
The starting materials were: calcium nitrate tetrahydrate Ca(NO3)2- 4H2O (Lanhit®, 99.99 %), yttrium(III) nitrate hexahydrate Yb(NO3)3- 6H2O (Lanhit®, 99.9%), erbium nitrate pentahydrate Er(NO3)3- 5H2O (Lanhit®, 99.99 %), sodium nitrate NaNO3 (Chimmed Group, CP), and sodium fluoride NaF (Lanhit®, P.A.). All the substances were used without additional purification.
The samples were obtained by molten salt synthesis [13]. First, the weighed quantities of hydrates of calcium nitrate and REEs were
homogenized in an agate mortar. Then, sodium nitrate was added to the mixture. It acted both as a solvent and the medium for the chemical reaction. Next, sodium fluoride was added, which acted as a fluorinating agent. The mixture was homogenized, put into a glazed porcelain crucible, covered, and annealed at a temperature of 300 or 400 °C for 1 or 3 hours. After the crucible cooled down, the reaction mass was taken out and put in a polypropylene reactor, where the nitrates were washed off the samples. The reactor is filled with 900 ml of double distilled water. Next, the anchor of the magnetic stirrer was put inside the reactor, and the mixture was stirred for 30 minutes. The presence of nitrate-ions was determined by the qualitative reaction with diphenylamine. It was usually enough to wash the samples three times to remove the nitrates. After the last washing the samples were dried in air at the temperature of 60 °C for 4 hours.
The characterisation of the obtained samples was performed using the X-ray powder diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and luminescence spectroscopy. The XRD analysis was performed using a Bruker D8 Advanced diffractometer (Germany) with CuKa radiation. The obtained diffraction pattern were analysed using DifWin and Powder2.0 software packages (AO < 10). The size of the particles and the morphology of the samples were studied by means of SEM using a Carl Zeiss NVision 40 scanning electron microscope (Germany) with an Oxford Instruments XMAX microprobe analyser (UK) (80 mm2) for energy dispersive X-ray spectroscopy. The spectroscopic analysis included registering the spectra of upconversion luminescence and diffusely scattered laser radiation within the range of 300-1000 nm, and calculating the upconversion efficiency of upconversion luminescence. In order to conduct the measurements we used a combination of the fibre optic spectrometer LESA-01-BIOSPEC (BIOSPEC, Russia) with the UnoMomento software package, and a modified integrating sphere (Avantes, the Netherlands) connected by fibre-optic light carriers [23].
To carry out the measurements, the powder sample placed between two cover slips was put inside the integrating sphere. The radiation
of the diode laser (974 nm wavelength) was focused on the sample so that the power density on the surface of the sample was 1 W/cm2. The integrating sphere had been preliminary calibrated with LEDs of various wavelengths and known power, measured with a LabMax®-TO (Coherent, USA) [24]. The scattered laser radiation and upconversion luminescence were captured by optic fiber and transmitted to the spectrometer. The upconversion luminescence efficiency was calculated using the formula
BO =
pR _ pS
974 sc 974 sc
(1)
where P/ is the radiation power of the sample in the visible range, P974 ab is the laser power absorbed by the sample.
The latter is calculated as the difference between P9R4 sc - the power of the scattered radiation of the non-absorbing reference sample and P9S74 sc - the power of the scattered radiation of the studied sample.
3. Results and discussion
In our study the synthesis was performed several times. The nominal composition of sample No. 1 was pure calcium fluoride. The other experiments were conducted in order to synthesise samples with the nominal composition
C^Y^osEW^.o^ using the equation:
0.94Ca(NO3)2 • 4H2O + 0.05Yb(NO3)3 • 6H2O + + 0.01Er(NO3)3 • 5H2O + 2.06NaF = [2]
= Ca0.94Yb0.05Er0.01F2.06 + 4.HH2O + 2.06NaNO3.
Table 1. Synthesis conditions and actual yield
The conditions of the synthesis process and the actual yield are given in Table 1.
The X-ray diffraction patterns of the synthesised samples are shown in Fig.1. The calculated lattice parameters and the size of coherent scattering regions (CSR) are given in Table 2.
The peaks of the cubic phase were indexed on all the XRD patterns. They correspond to the cubic fluorite phase of calcium fluoride (JCPDS card No. 35-0816). Samples No. 2 and 3, in addition to the cubic phase, demonstrated the peaks of the hexagonal phase. The XRD patterns of samples No. 4 and 5 demonstrated five peaks. A (200) peak appeared, which was suppressed in case of pure calcium fluoride. A broadening of the cubic phase peaks was observed. The size of the CSRs was calculated according to the Scherrer equation.
The lattice parameters of the cubic phase of samples No. 2 and 3 coincide with the parameters of pure calcium fluoride a = 5.463Á. Samples No. 4 and 5 are single-phase. The lattice parameters of their cubic phases should increase, since their unit cells contained ions of rare-earth elements [25]. However, the lattice parameters of samples No. 4 and 5 are lower than the lattice parameters of pure CaF2. This means that the crystal lattice was intercalated with ions with smaller ionic radii, which is quite possible, since there was a lot of sodium in the system. According to the EDX results presented in Table 3, there actually was sodium in the samples, and its amount is comparable to the number of rare-earth ions. The concentration of erbium is within the margin of error for the EDX method.
Sample No. Sample code Annealing temperature, °C Annealing time, hours Concentrations of the starting materials, mol. (M, Ln)(NO3)x: NaF:NaNO3 Actual yield, %
1 F1804 300 1 1:3:2 3 87.0
2 F1814 300 1 1:3:2 86.2
3 F1826 300 3 1:3:2 91.2
4 F1699 400 1 1:3:10 77.2
5 F1836 400 3 1:3:2 76.0
Table 2. The results of the XRD analysis
Sample No. Lattice parameter a (cubic phase), Â CSR size, nm
1 5.460(1) 32
2 5.463(2) 24
3 5.464(1) 24
4 5.452(1) 23
5 5.455(1) 41
S
S
974 ab
1 "I
01 -
0 -1 -
3
d
0 -
01 -
I> (2: - Î0) 1 (311) I A <r>
' 1 (HI) T ?j i 1 i ■ (2; a* • ? J 1 ■ i ■ 1 ■ •0) 2 (311) I . A
1 1 1 (111) • 1 J 1 ' 1 ' (2; IÎÎ* f J 1 1 1 * 1 1 !0) 3 (311) I (T
' r ' (in) J 1 ' 1 ' (2 M200) 1 1 1 i ■ 1 ■ 20) 4 (311) I A <r
■ r ■ (111) J 1 ' 1 1 (2; i (200) j 1 1 1 ■ 1 ■ m 5 (311) I__L W
1 1 1 1 1 1 . 1 1 T 1 I .
10
20
30
40 50
20, deg.
60
70
Fig. 1. Results of the XRD analysis. Dots denote the peaks of the hexagonal NaYF4 phase. Sample numbers are the same as in Table 1
Table 3. The EDX results, in atomic percent
Sample No. Na, at.% Ca, at.% Yb, at.%
4 6 88 6
5 6 88 6
Thus, the composition of the single-phase samples is different from the nominal composition. Certain lattices contained sodium, which was inserted together with REEs. Isomorphic substitutions can be represented as
2Ca2+^ Na+ + Ln3+, (3),
where Ln = Yb, Er. This kind of joint intercalation means that the sodium ions had an in-
creased solubility limit. The maximal solubility in the system NaF - CaF2 is 2.2 mol.% [26]. Het-erovalent isomorphism of this type is common for NaF - CaF2 - LnF3 systems [27-28].
SEM photographs of samples No. 3, 4, and 5 and their resolutions are given in Figures 2-4.
Fig. 2 shows a rod-like particle, several microns in size, with a hexagonal cross-section surrounded by an agglomeration of particles (a few dozen nanometres) with no definite faceting, which implicitly confirms that sample No. 3 is two-phase. Microphotogaphs in Fig. 3 and 4 demonstrate the morphological homogeneity
Fig. 2. SEM photograph of sample No. 3
Fig. 4. SEM photograph of sample No. 5
and high dispersion of particles in samples No. 4 and 5. The particles in sample No. 5 are larger than those in sample No. 4, which complies with the calculations of CSR size. The average size of the particles in both samples is below 120 nm.
The luminescence spectra of samples No. 4 and 5 are given in Fig. 5.
The luminescence spectra of the samples show green (510-575 nm) and red (625670 nm) bands, corresponding to the radiative transitions of the erbium ions 2H11/2,4S3/2 ^ 4I15/2 and 4F9/2 ^ 4I15/2. The upconversion efficiency was 0.02 % for sample No. 4, and 1.21 % for sample No. 5. The difference in the values of upconversion efficiency may be accounted for by the larger size
Fig. 3. SEM photograph of sample No. 4
of the particles in sample No. 5 (resulting from the longer synthesis), which leads to the decrease in the surface area to volume ratio.
The obtained results lead to several conclusions. First, when the temperature is 300 °C, two-phase samples are formed. At higher temperatures the hexagonal phase NaLnF4 disappears from the reaction system. According to the XRD results, the samples obtained at 400 °C are single-phase. Also, the calculated lattice parameters are lower than those of calcium fluoride, even though their crystal lattices were intercalated with REE ions. Our assumption that the lattice contained sodium ions was confirmed by the EDX results. The concentration of sodium is comparable to the concentration of Yb3+ and Er3+. Thus, while developing the technology, we obtained a new material CaF2:Na+, Yb3+, Er3+ with upconversion properties.
Second, the SEM analysis allowed us to describe the morphology of the particles of singlephase powders. The particles are spherical, highly homogeneous, and have particle size distribution within a narrow range. When the samples are annealed for a longer time, the particles continue growing, and their size reaches 60-120 nm. This results in a higher volume to surface area ratio, and hence increased luminescence.
The luminescence spectra confirm the upconversion properties of the material. When excited with IR laser at a wavelength of 974 nm, the material demonstrates two luminescence bands in the visible range: in the green and
Wavelength, rim
Fig. 5. Luminescence spectra of samples No. 4 and 5. The wavelength of the pumping laser is 974 nm
red spectra. The upconversion luminescence efficiency of the samples is lower than that of a similar material obtained by coprecipitation from aqueous solutions: 1.21 % as compared to 3.11 % [8]. This is partly accounted for by the insertion of sodium in the crystal lattice. Nevertheless, the molten salt synthesis method has a number of advantages. For instance, the pyrohydrolysis process is slower, and the EDX results confirm that the samples do not contain oxygen. The molten salt synthesis is also easy to perform, does not involve dangerous reagents, such as hydrofluoric acid, and there is no need to reconstruct the precise synthesis conditions to reproduce the results. Also, the adsorbed water can be removed without further thermal treatment.
4. Conclusions
During our study, we determined the conditions for the synthesis of calcium fluoride powder doped with ytterbium and erbium ions with upconversion properties. We obtained a single-phase sample with the composition Ca0 88(Yb, Er)0 06Na0 06F2, which differs from the nominal. It was determined, that the crystal lattice of the obtained solid solution included sodium ions. The joint intercalation of sodium and REE ions increases the solubility limit of sodium fluoride in calcium fluoride up
to 6 mol. % against 2.2 mol. %, as determined by previous studies. The study demonstrated that the synthesis conditions influence the morphology and phase composition of the particles. Two-phase samples are obtained at 300 °C (CaF2 + NaLnF4), and single-phase samples at 400 °C. The size of the particles increases with longer annealing time. The obtained sample demonstrates upconversion properties. When pumped with 974 nm wavelength laser, green and red luminescence bands are detected. Due to its upconversion luminescence properties, the obtained material can find various applications in biomedicine.
Acknowledgements
All the experiments were performed on the equipment kindly provided by the Joint Research Centre for the Physical Methods of Research of Prokhorov General Physics Institute of the Russian Academy of Sciences and the Joint Research Centre for the Physical Methods of Research of Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences
Conflict of interests
The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.
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Information about the authors
Alexander A. Alexandrov, MSc student, MIREA -Russian Technological University, Institute of Fine Chemical Technologies named after Lomonosov, Moscow, Russian Federation; research technician, Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russian Federation; e-mail: alexandrov1996@yandex.ru. ORCID iD: https://orcid.org/0000-0001-7874-7284.
Mariya N. Mayakova, PhD in Chemistry, Researcher, Prokhorov General Physics Institute of the Russian Academy of Science, Moscow, Russian Federation; e-mail: mn.mayakova@gmail.com. ORCID iD: https:// orcid.org/0000-0003-0713-5357.
Valery V. Voronov, PhD in Physics and Mathematics, Head of the Laboratory, Prokhorov General Physics Institute of the Russian Academy of Science, Moscow, Russian Federation; e-mail: voronov@lst.gpi.ru. ORCID iD: https://orcid.org/0000-0001-5029-8560.
Daria V. Pominova, PhD in Physics and Mathematics, Researcher, Prokhorov General Physics Institute of the Russian Academy of Science, Moscow, Russian Federation; e-mail: pominovadv@gmail.com. ORCID iD: https://orcid.org/0000-0003-0713-5357.
Sergey V. Kuznetsov, PhD in Chemistry, Leading Researcher, Prokhorov General Physics Institute of the Russian Academy of Science, Moscow, Russian Federation; e-mail: kouznetzovsv@gmail.com. ORCID iD: https://orcid.org/0000-0002-7669-1106.
AlexanderE. Baranchikov, PhD in Chemistry, Head of the Laboratory, Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Moscow, Russian Federation; e-mail: a.baranchikov@yandex.ru. ORCID iD: https://orcid. org/0000-0002-2378-7446.
Vladimir K. Ivanov, DSc in Chemistry, Associate Member of the Russian Academy of Sciences, Director of the Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Moscow, Russian Federation; e-mail: van@igic.ras.ru. ORCID iD: https://orcid.org/0000-0003-2343-2140.
Elena I. Lysakova, PhD in Chemistry, Associate Professor, MIREA - Russian Technological University, Institute of Fine Chemical Technologies named after Lomonosov, Moscow, Russian Federation; e-mail: elenalysakova@mail.ru. ORCID iD: https://orcid. org/0000-0001-6298-5712.
Pavel P. Fedorov, DSc in Chemistry, Head of the Department, Prokhorov General Physics Institute of the Russian Academy of Science, Moscow, Russian Federation; e-mail: ppfedorov@yandex.ru. ORCID iD: https://orcid.org/0000-0003-0713-5357.
All authors have read and approved the final manuscript.
Translated by Yulia Dymant.
Edited and proofread by Simon Cox.
