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The influence of chemical prehistory on the structure, photoluminescent properties, surface and biological characteristics of Zro.98Euo.02O1.99 nanophosphors
A.N. Bugrov1'2, R.Yu. Smyslov1'3, A. Yu. Zavialova2'4, G.P. Kopitsa5'6
institute of Macromolecular Compounds of RAS, Bolshoy pr. 31, 199004 St. Petersburg, Russia
2St. Petersburg Electrotechnical University "LETI", ul. Professora Popova 5, 197376 St. Petersburg, Russia 3Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaya 29, 195251 St. Petersburg, Russia 4 St. Petersburg State Institute of Technology (Technical University), Moskovsky prospect 26,
St. Petersburg, 190013, Russia 5 St. Petersburg Nuclear Physics Institute, NRC KI, Orlova roscha mcr. 1, Gatchina, Leningrad region, 188300, Russia 6Grebenshchikov Institute of Silicate Chemistry RAS, Makarova nab. 2., letter B, 199034 St. Petersburg, Russia
alexander.n.bugrov@gmail.com, urs1968@gmail.com, zavialova.a.y@gmail.com, kopitsa_gp@pnpi.nrcki.ru PACS 78.67. n; 78.67.Bf DOI 10.17586/2220-8054-2019-10-2-164-175
ZrO2 nanoparticles doped with 2 mol.% of EuOi.5 were obtained from solutions of inorganic salts, zirconium alkoxide and chelating compounds under hydro and solvothermal conditions. The phase compositions of the synthesized nanophosphors were determined using the methods of X-ray diffraction, photoluminescence and Raman spectroscopy. The changes in a particle size, the value of the specific surface area and its charge depending on the conditions of preparation (the type of solvent, isothermal exposure time) and the precursor nature used in the synthesis were considered. It was found that Zr0.98Eu0.02O1.99 nanoparticles with a high content of the monoclinic phase, synthesized from zirconium and europium acetylacetonates, have the highest luminescence efficiency. At the same time, the maximum photoluminescence lifetime and the least cytotoxicity were characteristic of crystal phosphors with a more symmetrical crystal lattice of the host matrix, as well as a high surface area/volume ratio.
Keywords: solvothermal synthesis, zirconia, europium, phase transitions, nanoparticles, photoluminescence, fluorescence lifetime, quantum yield, cytotoxicity.
Received: 27 March 2019
Revised: 10 April 2019
1. Introduction
The rapid development of solid-state lighting technology has led to an increased demand for high-performance alternative luminescent sources of radiation [1]. It is known that Ln3+-doped wide-bandgap semiconductor oxides can exhibit unique optical properties such as single or multicolor radiation, large Stokes shift, narrow spectral transition bands, long lifetime, and high photoluminescence quantum yield [2, 3]. The optical properties of oxide nanoparticles doped with Ln3+ largely depend on the type and structure of the host matrix [4,5]. Therefore, the correct selection of the absorption oscillator is significant for achieving the desired photoluminescence characteristics of lanthanide ions. Nanocrystalline zirconia is the most suitable host matrix for the creation of high-performance solid-state light-emitting devices based on Ln3+ ions due to its high refractive index, the band gap in the range from 4 to 6 eV, transparency in the visible and infrared spectral regions, as well as low frequency of phonons (470 cm-1) [6,7]. To date, nanophosphors based on ZrO2 have been used in LEDs [8], fuel cells [9], solar panels [10], gas sensors [11], and photocatalytic systems [12,13]. The photostability and high lifetime of Ln3+ (the range of milliseconds) ion radiation in the ZrO2 biologically-inert matrix made it possible to use such luminescent markers for medical purposes for detecting, visualizing, diagnosing and treating diseases [14-17].
For increasing the luminescence efficiency of lanthanide ions, as a rule, uniformity of their distribution in the ZrO2 crystal lattice is required [18]. From this point of view, the methods of "soft chemistry" such as hydro and solvothermal synthesis are preferred [19-21]. They make it possible to control the processes of nucleation and crystallite growth by changing the treatment parameters, which leads to the formation of weakly aggregated particles of a given morphology with a narrow size distribution [22-24]. In turn, the different nature of the precursor and the composition of the reaction medium (e.g. water, an organic solvent, surfactant, mineralizer) predetermine the presence of the required functional groups on the surface of the nanoparticles being synthesized, which can then be used for conjugation with biomolecules [16,25].
Selectivity for the excitation wavelength, energy transfer efficiency, lifetime and quantum yield of photoluminescence are determined mainly by the structure and size of ZrO2 crystals, as well as the concentration and localization of Ln3+ active centers in them [26-29]. The distribution of trivalent lanthanide ions between the amorphous, monoclinic and more symmetrical tetragonal and cubic phases of ZrO2 can be adequately distinguished using Eu3+ as a probe. Luminescent properties of europium ions are susceptible to changes in the first coordination sphere, which is manifested in the emission/excitation spectra and the dynamics of the excited state of ZrO2:Ln3+ nanoparticles [26,30].
In this connection, this work aimed to study the influence of chemical prehistory on the structure of Zr0.98Eu0.02Oi.99 nanoparticles, formed in the conditions of hydrothermal and solvothermal synthesis, using the europium ions as local probes. In addition, we sought to determine the interrelation of structural and dimensional parameters with the efficiency of luminescence of the obtained crystalline phosphors, as well as the analysis of their cytotoxicity depending on the evolution of surface characteristics concerning dermal fibroblasts to establish the possibility of their further biomedical application.
2. Experimental methods
2.1. Materials
The following reagents were used in the hydro and solvothermal synthesis of luminescent nanoparticles studied in this work: zirconium (IV) oxychloride octahydrate (98.5 %, Neva-Reactive, Saint Petersburg, Russia, CAS: 7699-43-6); zirconium (IV) acetylacetonate (Zr(acac)4; 97 %, Vecton, Saint Petersburg, Russia, CAS: 17501-44-9); zirconium (IV) n-butoxide solution (Zr(OBun)4; 80 wt.% in 1-butanol, Sigma-Aldrich, St. Louis, MO, USA, CAS: 1071-76-7); europium (III) chloride hexahydrate (99.9 % trace metals basis, Sigma-Aldrich, St. Louis, MO, USA, CAS: 13759-92-7); europium (III) acetylacetonate hydrate (Eu(acac)3 x nH2O; 99.9 % trace metals basis, Sigma-Aldrich, St. Louis, MO, USA, CAS: 62667-64-5); ammonium hydroxide solution (25 %, Neva-Reactive, Saint Petersburg, Russia, CAS: 1336-21-6); ethanol (95 %, Vecton, Saint Petersburg, Russia, CAS: 64-17-5). Toluene (99.5 %, Vecton, Saint Petersburg, Russia, CAS: 108-88-3) was distilled before use.
2.2. Hydrothermal synthesis of nanoparticles
ZrO2 nanoparticles containing 2 mol.% of EuO1.5 were obtained by co-precipitating zirconium and europium hydroxides from 0.5 M solutions of their chlorides at pH = 9, using ammonia water (25 %). The precipitate was washed with distilled water until a negative reaction to chloride ions by repeated stirring with followed decantation and then dried in air at 100 °C. The obtained ZrO(OH)2-Eu(OH)3 powder was redispersed in distilled water (pH = 4-5), placed in a steel autoclave and treated under hydrothermal conditions during 4 hours at 250 °C.
2.3. Solvothermal synthesis of nanoparticles
The Eu(acac)3 x nH2O with a mass of 0.04 g was previously dissolved in anhydrous toluene for 3 hours with constant stirring. Zirconium acetylacetonate or butoxide in the amount of 1.87 g and 1.35 ml, respectively, were added to the obtained solutions. The resulting reaction mixtures were then magnetically stirred for 24 hours. The thermal treatment of solutions of organometallic compounds in toluene with pH = 4 - 5 was carried out in steel autoclaves at a temperature of 250 °C for 72 hours. The obtained powders of Zr0.98Eu0.02O1.99 nanoparticles were repeatedly washed in ethanol, followed by centrifugation to remove organic impurities. After that, they were dried at 100 °C to constant weight and annealed in air at 500 °C for 2 hours.
2.4. Characterization techniques
The elemental composition of the Zr1-xEuxO2-0.5X nanopowders obtained in the study was determined using energy-dispersive X-ray spectroscopy (EDX; SUPRA 55VP Carl Zeiss AG with an INCA microanalysis system, Germany) and X-ray fluorescence analysis (XRF; Spectroscan Max-GF2E spectrometer Spectron, Russia Q5). ZrO2 nanoparticles doped with 2 mol.% Eu3+ ions were characterized using an X-ray diffraction method (XRD; Rigaku SmartLab diffractometer) with CuKa radiation (40 kV and 40 mA), scan rate of 0.5°/min in a range of 29 angles from 10 to 80°. The PD-Win 4.0 [31] and ReX [32] software complexes, as well as ASTM and COD databases, were used to determine the phase composition and calculate the unit cell parameters of the nanoparticles. Transmission electron microscopy (TEM; JEOL JEM-2100F) with an accelerating voltage of 200 kV was used to determine the shape and size of the Zr0.98Eu0.02O1.99 nanoparticles. The phase composition of the synthesized nanophosphors was analyzed using Raman spectroscopy (RS; LabRAM HR800, Horiba Jobin-Yvon) at room temperature with 488 nm of Ar+ laser. A specific surface area (SBET) and porosity of synthesized nanopowders were determined by low-temperature nitrogen adsorption (QuantaChrome Nova 4200B analyzer,
Quantachrome Instruments, USA). Zeta potential was determined using an electrophoretic light scattering method (Malvern Zetasizer Nano ZS laser analyzer, Malvern Instruments, UK) at 25 °C, pH value of 6.4 with a background electrolyte concentration (NaCl) of 10-3 M. The optical band gap of nanocrystals based on zirconia was determined using diffuse reflectance spectra (Shimadzu UV-2550 spectrophotometer equipped with an ISR-2200 integrating sphere). MTT assay of ZrO2-2 mol.% EuOi.s nanoparticles were carried out on dermal fibroblasts in accordance with the procedure described in detail in [33]. The photoluminescence (PL) quantum yield of nanophosphors was measured by the modified de Mello's method using a Fluorolog-3 fluorescence spectrometer equipped with a Quanta-j integrating sphere [34]. The excitation and emission spectra of nanoparticle powders were studied using a luminescence spectrophotometer LS-100 (PTI®, Canada). The luminescence lifetimes of Zr0.98Eu0.02O1.99 nanoparticles were determined from the emission intensity decay using a pulse xenon lamp.
3. Results and discussion
The effects of the chemical prehistory of ZrO2-2 mol.% EuO1.5 nanoparticles on their phase composition, specific surface area, porosity, electrokinetic potential, photoluminescence, and biological activity were considered.
The elemental analysis of Zr1-xEuxO2-0.5X nanoparticle samples is carried out under the conditions of hydro and solvothermal treatment corresponded to the 98 mol.% ZrO2/2 mol.% EuO1.5 ratio, according to the EDX and XRF methods (Table 1). The data obtained coincide within the error with the composition preset by the synthesis.
Table 1. Elemental analysis of synthesized nanoparticles based on ZrO2 doped with EuOi.s
Chemical prehistory Data of fluorescence analysis calculated as oxides, mol.% Data of EDX-analysis calculated as oxides, mol.%
ZrO2 EuOi.5 ZrO2 EuOi.5
ZrOCl2 x 8H2O/EuCl3 x 6H2O 97.8 ± 2.9 2.2 ± 0.1 97.1 ± 4.9 2.9 ± 0.2
Zr(acac)4/Eu(acac)3 x nH2O 98.2 ± 3.0 1.8 ± 0.1 98.1 ± 4.9 1.9 ± 0.1
Zr(OBun)4/Eu(acac)3 x «H2O 98.1 ± 2.9 1.9 ± 0.1 97.7 ± 4.9 2.3 ± 0.1
The PD-Win 4.0 and ReX software complexes, along with ASTM and COD databases [35-37] were used to determine the phase composition, calculate the average size of coherent scattering regions, as well as the lattice parameters for the unit cell of Zr0.9sEu0.02O1.99 nanoparticles. Nanophosphors obtained from 0.5 M solutions of ZrOCl2 x 8H2O and EuCl3 x 6H2O by coprecipitating with following hydrothermal treatment are a mixture of tetragonal (t), cubic (c) and monoclinic (m) zirconia phases in the 74.8/12.2/13 ratio (Fig. 1, pattern 1). The phase composition of nanoparticles synthesized by the treatment of toluene solutions of Zr(acac)4 and Eu(acac)3 x nH2O at an elevated temperature and pressure includes 46.9 vol.% tetragonal, 23.5 vol.% cubic and 29.6 vol.% monoclinic zirconia (Fig. 1, pattern 2), according to results of XRD pattern analysis. A monoclinic phase is not observed in the sample, but t- and c-ZrO2 are present in the ratio 33.8/66.2 if Zr(OBun)4 is used as a precursor (Fig. 1, pattern 3, and Table 2). The parameters and the volume of the unit cell in the Cl —y C5H7O- —у C4H9O series decrease for t-ZrO2, and grow for its cubic phase. For nanopowders obtained from precursors of different nature, the average sizes of coherent scattering regions calculated by the Debye-Scherrer formula are comparable with the particle diameter fixed by the TEM (Table 2, Fig. 2). According to microphotographs, the average diameter of Zr0.98Eu0.02O1.99 quasi-spherical particles obtained by the solvothermal synthesis from zirconium and europium acetylacetonates was 8 nm, and in the case of a mixture of Zr(OBun)4/Eu(acac)3 x nH2O - 4 nm (Fig. 2, micrograph 2 and 3). The dehydration under hydrothermal conditions of ZrO(OH)2-Eu(OH)3 mixture co-precipitated from inorganic salts leads to the crystallization of nanophosphors larger size of about 15 nm (Fig. 2, micrograph 1).
In the Raman spectra of the nanoparticles obtained from precursors of different nature, there are apparent differences in the phase composition (Fig. 3). The use of zirconium and europium acetylacetonates as the starting materials in the synthesis of ZrO2-2 mol.% EuO1.5 nanoparticles leads to a significant increase in the content of the monoclonal phase (peaks at 183, 335, 533, 556, 613 cm-1) and a decrease in the volume fraction of the
m-Zr03 I II I I ill tlllll III lllll III! Ill I III llll III! IIII li mill
t-Zr02 I llll
C-ZIOZ
10 20 30 40 50 60 70 80
26,°
Fig. 1. X-ray diffraction patterns of Zr0.9sEu0.02O1.99 nanoparticles synthesized from ZrOCl2 and EuCl3 crystalline hydrates (1), Zr(acac)4 and Eu(acac)3 x nH2O (2), as well as Zr(OBun)4 and Eu(acac)3 x nH2O (5)
Table 2. Structural parameters of Zr0.98Eu0.02O1.99 nanoparticles synthesized in hydro- and solvothermal conditions from inorganic salts, metal alkoxide and chelating agents
Chemical prehistory Phase composition Unit cell parameters Average crystallite size (XRD), nm
ZrOCl2 x 8H2O/ EuCl3 x 6H2O 13 vol.% m-ZrO2 a _ 5.1783; b _ 5.2099; c _ 5.3171; a _ y _ 90 °; £ _ 99.25 V _ 141.6 A3 15 ± 3
74.8 vol.% t-ZrO2 a _ b _ 3.6032; c _ 5.1827; a _ £ _ y _ 90 °; V _ 67.3 A3 15 ± 3
12.2 vol.% c-ZrO2 a _ b _ c _ 5.1272; a _ £ _ y _ 90 °; V _ 134.8 A3 15 ± 3
Zr(acac)4/ Eu(acac)3 x nH2O 23.5 vol.% m-ZrO2 a _ 5.1578; b _ 5.1975; c _ 5.3194; a _ y _ 90 °; £ _ 99.14 °; V _ 140.8 A3 8 ± 2
46.9 vol.% t-ZrO2 a _ b _ 3.5964; c _ 5.1872; a _ £ _ y _ 90 °; V _ 67.1 A3 6 ± 2
29.6 vol.% c-ZrO2 a _ b _ c _ 5.1313; a _ £ _ y _ 90 °; V _ 135.1 A3 6 ± 2
Zr(OBun)4/ Eu(acac)3 x nH2O 33.8 vol.% t-ZrO2 a _ b _ 3.5885; c _ 5.1552; a _ £ _ y _ 90 °; V _ 66.4 A3 4 ± 1
66.2 vol.% c-ZrO2 a _ b _ c _ 5.1567; a _ £ _ y _ 90 °; V _ 137.1 A3 4 ± 1
tetragonal polymorphic modification (peaks at 149, 224, 292, 324, 407, 456 and 636 cm-1) in comparison with analogs obtained from solutions of chlorides. Solvothermal treatment of Zr(OBun)4/Eu(acac)3 x nH2O mixture in toluene makes it possible to synthesize nanoparticles mainly with the c-ZrO2 structure characterized by a wide band of 530 - 670 cm-1 in the Raman spectra, as well as a unique phonon band F2g of ca. 625 cm-1 [7]. RS data confirm the phase composition determined by XRD (Table 2).
Based on the diffuse reflectance spectra R(A) in the visible and ultraviolet range from 2.5 to 6 eV, one calculated the Schuster-Kubelka-Munk or remission function [38]:
F (R (A))_ (1 - Ro(A))2 _ K _ 2.303eC
F (A)) _ 2R00(A) _ S _ (1)
where (A) is the value of reflectance when it is already not changed in increasing system thickness; K is the absorption coefficient (twice the Beer's law absorption coefficient); S is twice the scattering coefficient of the sample; e is the absorptivity, and C is the analyte concentration.
Using the plot of (F (R0) hv)1/n versus energy, hv, one can estimate the optical bandgap, Eg, between the conduction band and the valence one in Zr0.98Eu0.02O1.99 nanoparticles (Fig. 4). In the region of the highest
(1) (2) (3)
Fig. 2. TEM micrographs of Zr0.9sEu0.02O1.99 nanoparticles synthesized from ZrOCl2 and EUCI3 crystalline hydrates (1), Zr(acac)4 and Eu(acac)3 x nH2O (2), as well as Zr(OBun)4 and Eu(acac)3 x nH2O (3)
i
1 1 1 1 1 1 1 1 1 1 ■ 1
200 300 400 500 600 700 Raman shift, cm 1
Fig. 3. Raman spectra of Zr0.9sEu0.02O1.99 nanoparticles synthesized from ZrOCl2 x 8H2O and EuCl3 x 6H2O (1), Zr(acac)4 and Eu(acac)3 x nH2O (2), as well as Zr(OBun)4 and Eu(acac)3 x nH2O (3). Spectra were normalized at 474 cm-1
obtained energy of 5 - 6 eV, the allowed indirect absorption transitions turned out to manifest themselves because there are linear approximations at n = 2 for all the cases under investigation [39]. At that, the Eg values of the studied nanoparticles are limited from 4 to 6 eV, which is consistent with prior research [6]. The bandgap of ZrO2-2 mol.% EuO1.5 nanoparticles synthesized from inorganic salts (Eg1) in comparison with Eg1/ of undoped zirconia increases from 4.85 to 5.12 eV. The introduction of 2 mol.% of EuO1.5 into zirconia crystal lattice leads to decreasing the value of (F hv)1/2 as opposed to it (Note curves 1 and 1'). This result can mean the higher absorption ability of m-ZrO2, which lessens in doping zirconia by Eu3+, relative to one of t/c-phases in the region of 4 - 5.3 eV. For nanoparticles obtained from Zr(OBun)4 and Zr(acac)4, one observed the shifts from 4.85 eV of Eg1 to 4.72 and 4.6 eV, respectively (Fig. 4). It was found that in the region of 3 - 4 eV absorption is less
for Zr0.98Eu0.02O1.99 nanoparticles formed from Zr(OBun)4 and Zr(acac)4 as compared to those obtained from ZrOCl2 x 8H2O. Here, the influence of t-phase is already reflected; the absorption ability of nanoparticles could decrease with the lower content of it. Moreover, the c-phase happens to be the least light-absorbing (Note curve 3, Fig. 4). It should be noted that the smaller additional Eg values for the systems synthesized from ZrOCl2 x 8H2O with 74.8 vol.% of t-ZrO2, from Zr(acac)4 with 23.5 vol.% of m-ZrO2, and from Zr(OBun)4 with 66.2 vol.% of c-ZrO2 are determined from the plot in Fig. 4, as well. They are equal to 4.5, 3.6, and 3.5 eV, respectively. These data are consistent with the theoretical minimum Eg values obtained by the DFT method for the three different ZrO2 modifications: 4.0 eV for t-ZrO2, 3.6 eV for m-ZrO2, and 3.3 eV for c-ZrO2 [40]. Moreover, the shoulder in the range of 3.5 - 3.9 eV (curve 1, Fig. 4) may be attributed to intersitial oxygen states or oxygen vacancy states [40].
(,F(RJhv)v2
-'-1-—-1—-'-r
3 4 5 E (eV) 6
Fig. 4. Absorption spectra of Zr0.9sEu0.02O1.99 phosphors synthesized from ZrOCl2 and EUCI3 crystalline hydrates (1), Zr(acac)4 and Eu(acac)3 x nH2O (2), Zr(OBun)4 and Eu(acac)3 x nH2O (3), as well as pure ZrO2 nanoparticles prepared from ZrOCl2 x 8H2O (1')
The influence of the precursor nature on the photoluminescence of ZrO2-2 mol.% EuO1.5 nanoparticles synthesized by hydro- and solvothermal synthesis was also studied (Fig. 5). Peaks observed at 591, 607, 614, 628, 634, 653, 659, 702 and 749 nm, as well as the shoulder at 580 nm can be compared with the spectral terms of the Stark splitting for Eu3+ and the radiative transitions in the Dieke diagram for the 4fn configuration of Eu3+ 7 FJ ^ 5D0 (J = 0 - 5) [41,42]. The excitation spectra (Fig. 6) for these nanoparticles cover the range of intraconfiguration 4fn—4fn transitions in the region of 270 - 490 nm. In addition, the peaks at 227, 240, 256, and 265 nm are correlated with the interconfiguration 4fn—5dn transitions and charge transfer states. According to the excitation spectra, ones used the PL excitation wavelength of 255, 270, and 397 nm.
At the excitation of 247 - 255 nm in PL spectra (Fig. 5a) one observed a weak shoulder or a small peak at 614 nm. At this wavelength, the m-ZrO2 polymorph manifests itself in photoluminescence. Thus, the contribution of the m-ZrO2 increases in the series of Zr(OBun)4/ Eu(acac)3 x nH2O (3) ^ ZrOCl2 x 8H2O /EuCl3 x 6H2O (1) ^ Zr(acac)4/Eu(acac)3 x nH2O (2). The latter fact is reflected in the XRD and RS data (Table 2, Fig. 3) as the growth of the monoclinic phase. To that end, the ratio between the magnetic dipole transfer contribution and the electric dipole transfer one decreases, which indicates the lessening of crystal field symmetry around Eu3+. It means that the pseudocubic (c/t) phase is lowered in the series. Also, it was found out that upon excitation at 270 and 397 nm of the 4fn—4fn transitions (Fig. 5b and c), the contribution of the c-ZrO2 in the PL spectra (bands at 606 and 633 nm) of ZrO2-2 mol.% EuO1.5 nanoparticles increases in the series ZrOCl2 x 8H2O/EuCl3 x 6H2O (1) ^ Zr(acac)4/Eu(acac2)3 x nH2O (2) ^ Zr(OBun)4/Eu(acac)3 x nH2O (3), again consistent with the XRD data (Table 2, Fig. 3). Thus, there is the PL sensitivity of different ZrO2-polymorph modifications to the excitation wavelengths [26]; in a short-wavelength region, the m-ZrO2 determines luminescence, while in the long-wavelength region, the cubic phase is decisive.
600 650 , 700
ппч (c)
Fig. 5. The luminescence spectra (a - c), normalized at 606 (a) and 614 nm (c), for Zr0.98Eu0.02O1.99 nanoparticles obtained by hydro and solvothermal synthesis from different precursors: ZrOCl2 and EuCb crystalline hydrates (1), Zr(acac)4 and Eu(acac)3 x nH2 O (2), Zr(OBun)4 and Eu(acac)3 x «H2O (3). Excitation at 247 - 255 (a), 270 (b) and 397 nm (c)
Table 3. Photoluminescence lifetimes1) of Zr0 98Eu0.02O1.99 nanoparticles formed from different precursors
Chemical prehistory A1 tpl1, ms A = 1 - A1 tpl2, ms Xred
ZrOCl2 x 8H2O/EuCl3 x 6H2O 0.687 0.865 0.313 2.21 0.751
Zr(acac)4/Eu(acac)3 x nH2O 0.628 0.768 0.372 2.02 1.518
Zr(OBun)4/Eu(acac)3 x «H2O 0.436 1.160 0.564 3.22 2.668
^PL lifetimes were fitted on the basis of the dual exponent decay (see Eq. 2) in minimizing a reduced Xred-statistics.
2)Note the mean-weighted PL lifetimes, tPLw, calculated upon Eq. 3 and shown in Fig. 7.
The decay of PL intensity (/;„m(t)) for nanoparticles can be approximated by a model of the dual exponential function:
Ii«m(t) = Ai exp(-i/TpLi) + A2 exp(-t/rpL2) + B, (2)
where tPl1 and tPL2 are the PL lifetime for the first and second emission processes; A1 and A2 are the first and second pre-exponential factors. Term B is a background contribution. Based on the obtained tPl and their contributions in the two-exponential approximation (Table 3), the weighted-average luminescence lifetimes, tPLw, are calculated according to the expression (Fig. 6):
Si=1
PLi
Ei=1 AiTPLi
(3)
1000
soo
600
400
200
Fig. 6. The normalized excitation spectra of the luminescence of Zr0.98Eu0.02O1.99 nanoparticles, obtained by hydro and solvothermal synthesis from different precursors: ZrOCl2 and EuCl3 crystalline hydrates (1), Zr(acac)4 and Eu(acac)3 x nH2O (2), Zr(OBun)4 and Eu(acac)3 x nH2O (3). Luminescence was observed at 606 nm (1) and 613 nm (2,3). The spectra are normalized at 240 nm (1) and 248 nm (2, 3)
According to the data shown in Fig. 7, PL lifetimes decrease from 2.8 to 1.5 ms upon increasing the m-ZrO2content. As a result, the quantum yield rises to 2.3 %, as it was fixed in [43].
The results of low-temperature nitrogen adsorption show that ZrO2-2 mol.% EuO1.5 nanoparticles synthesized from ZrOCl2 x 8H2O/EuCl3 x 6H2O, Zr(acac)4/Eu(acac)3 x nH2O and Zr(OBun)4/Eu(acac)3 x nH2O are characterized by SBET values of 80.2, 91.6 and 96.6 m2/g, respectively (Table 4). Sorption isotherms for these nanoparticles
2
Fig. 7. Dependence of the lifetime and photoluminescence quantum yields of Zr0.98Eu0.02O1.99 nanoparticles on the chemical prehistory ZrOCl2 x 8H2O, Zr(acac)3, and Zr(OBun)4. Excitation was at 262 nm. Observation was at 614 nm
depend significantly on the method of their preparation (Fig. 8). The isotherms presented for nanopowders obtained from acetylacetonate solutions of zirconium and europium, as well as their chlorides, clearly belong to the IV type characteristic of mesoporous materials. Capillary condensation of the adsorbent can occur in the pores of these materials, which in turn leads to the appearance of hysteresis between adsorption and desorption isotherms (Fig. 8a). The course of the capillary-condensation hysteresis loop for these nanopowders according to IUPAC classification can be attributed to the H2 type, which indicates the presence of bottle-shaped mesopores in it. At the same time, for ZrO2-2 mol.% EuO1.5 nanoparticles obtained from zirconium (IV) butoxide solution and europium acetylacetonate hydrate, the sorption isotherm is close to the I type, which is inherent mainly to microporous samples. However, the observed hysteresis between the adsorption and desorption branches makes it possible to classify them also as an IV type.
(a)
(b)
Fig. 8. Adsorption-desorption isotherms (a) and BJH distributions of pore size (b) Zr0.98Eu0.02O1.99 nanoparticles synthesized from ZrOCl2 x 8H2O and EuCl3 x 6H2O (1), Zr(acac)4 and Eu(acac)3 x nH2O (2), as well as Zr(OBun)4 and Eu(acac)3 x nH2O (3)
Table 4. The surface properties of Zr0.9sEu0.02O1.99 nanoparticles with a different chemical prehistory, measured by the method of low-temperature nitrogen adsorption and electrophoretic light scattering
Chemical prehistory Sbet, m2/g Dpore, nm Vpore*, cm3/g Z-potential, mV
ZrOCl2 x 8H2O/EuCl3 x 6H2 O 80.2 ± 2.4 8 0.24 -8.7
Zr(acac)4/Eu(acac)3 x nH2O 91.6 ± 1.8 — 0.20 -4.2
Zr(OBun)4/Eu(acac)3 x «H2O 96.6 ± 3.9 — 0.05 15.3
*The specific volume of pores is determined by the limiting filling (P/P0 = 0.99).
The pore size distribution in Zr0.98Eu0.02O1.99 nanopowders, obtained from different precursors, was calculated for the desorption branch according to the BJH algorithm and is presented in Fig. 8b. The nanoparticle powder obtained from a toluene solution of Zr(OBun)4/Eu(acac)3 x nH2O is characterized by a gradual pore size decrease in the range from 2 to 10 nm, which confirms the presence of both micro- and mesopores in it. A wide trapezoidal distribution of mesopores is observed in sizes ranging from ca. 3 to 20 nm, with a small number of micropores in the case of a sample synthesized from acetylacetonate hydrates of zirconium and europium. Moreover, only nanoparticles obtained by hydrothermal treatment of the ZrO(OH)2-Eu(OH)3 composition is characterized by a lognormal distribution of mesopores in the range from 5 to 20 nm with a maximum at ca. 8 nm.
In a series of Zr0.9sEu0.02O1.99 nanoparticles obtained from different precursors, the highest survival of fibroblast cells was observed in a suspension of phosphors with the largest specific surface area and positive values of the zeta potential (Table 4). Moreover, the degree of cell culture survival in aqueous dispersions of ZrO2-2 mol.% EuO1.5 nanoparticles, obtained from Zr(OBun)4/Eu(acac)3 x nH2O, in the concentration range of 5 - 500 yg/ml varies quite weakly compared with other phosphors synthesized in this study (Fig. 9). Thus, the stabilization of the cubic zirconia modification by selecting of zirconium alkoxide as initial material in the synthesis allows the production of nanoparticles with a more developed surface and smallest cytotoxicity.
5 50 500
Fig. 9. MTT-test on human dermal fibroblasts in the dispersions of ZrO2-2 mol.% EuO1.5 nanoparticles obtained from different precursors: ZrOCl2 x 8H2O/EuCl3 x 6H2O (1), Zr(acac)4/Eu(acac)3 x nH2O (2), Zr(OBun)4/Eu(acac)3 x nH2O (3)
4. Conclusion
As a result of the performed studies it is established that at the same content of Eu (iii) in a reaction mixture crystallization of three polymorphic forms is observed, as in case of ZrO(OH)2-Eu(OH)3 dehydration under hydrothermal conditions, and for solvothermal treatment of zirconium and europium chelating compounds. In the latter case, the content of monoclinic and cubic phases of ZrO2 is higher. It has been noted that the solvothermal treatment of zirconium butoxide and europium acetylacetonate hydrate under similar conditions leads mainly to the crystallization of nanoparticles with the structure of ZrO2 metastable phases. It was possible based on measurements of quantum yields to connect the luminescence efficiency with the presence of monoclinic phase in the series of Zr(OBun)4/Eu(acac)3 x «H2O ^ ZrOCb x 8H2O/ EuCb x 6H2O ^ Zr(acac)4/ Eu(acac)3 x «H2O for Zr0.98Eu0.02O1.99 nanoparticles. It has been revealed that the more Zr0.98Eu0.02O1.99 crystallites with monoclinic structure are formed in hydrothermal and solvothermal conditions of synthesis, the higher the quantum yield of Eu3+ luminescence is, and the shorter emission times are fixed. According to MTT-test, the metabolic activity of cells is preserved after three days of human fibroblast incubation in an aqueous dispersion of ZrO2-2 mol.% EuO1.5 nanoparticles, which indicates that they practically no cytotoxicity. Stabilization of more symmetrical t, c-ZrO2 modifications in the process of solvothermal synthesis by selecting Zr(OBun)4 and Eu(acac)3 x nH2O as precursors made it possible to obtain nanoparticles with a high surface/volume ratio and a positive zeta potential value, whose dispersions showed the highest survival rate of fibroblasts cells. Thus, the selection of synthesis conditions (nature of the precursor, type of solvent, and isothermal holding time) allows for varying the phase composition of the formed Zr1-xEuxO2-0.5x nanoparticles. Moreover, the combination of the most effective monoclinic modification for luminescence with the least cytotoxic cubic phase of zirconia makes it possible to obtain phosphors with the required quantum yield and extinction coefficients, which have potential for application in biophotonics and medicine.
Acknowledgements
Alexander N. Bugrov acknowledges the Russian Foundation for Basic Research (grant No. 16-33-60227) for the financial support. The experimental work was facilitated by the Engineering Center equipment of the St. Petersburg State Technological Institute (Technical University). The quantum yield and Raman spectroscopy measurements were determined at the Center for Optical and Laser Materials Research, St. Petersburg State University. TEM studies were performed using equipment of the Federal Joint Research Center "Material science and characterization in advanced technology" supported by the Ministry of Education and Science of the Russian Federation (id RFMEFI62117X0018). The authors are grateful to Kirilenko D. A. from the Ioffe Institute for the study of samples obtained in this article by TEM. The authors are also appreciative to Kryukov A. E. from Institute of Macromolecular Compounds RAS for cytotoxicity studies and Rodionov I. A. from Institute of Chemistry, St. Petersburg State University for bandgap measurements.
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