The High-Energy Milling Preparation and Spectroscopic Characterization of Rare-Earth Ions Doped BaY₂F₈ Nanoparticles

Abstract

BaY₂F₈ nanoparticles (NPs), doped with Yb³⁺ and Er³⁺ ions, were successfully produced by high-energy ball-milling. High-quality monoclinic single crystals (sp. gr. C₂/m, a = 0.6969(3), b = 1.0502(1), c = 0.4254(1) nm, β = 99.676°) grown from the melt by the Bridgman technique were used as raw materials. The prepared nanocrystals were comprehensively studied by X-ray diffraction analysis, transmission electron microscopy, and optical spectroscopy. The fabrication possibility of single-phase irregular shaped Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs in the size range of 20–100 nm with a milling duration of 10 h at 600 rpm is demonstrated. Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs show intense luminescence by both up- (λ = 540 and 650 nm) and down-conversion (λ = 1540 nm) mechanisms upon IR excitation (λ = 980 nm). A qualitative comparison of the spectroscopic characteristics of the produced Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs with the initial bulk crystal and the widely used up-conversion β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ NPs is presented. Experimental data offer great opportunities of the Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs applications in nanophotonics and biotechnology. High-energy ball-milling has potential as a versatile method for the scalable production of fluoride nanoparticles.

1. Introduction

Photoluminescent (PL) nanomaterials doped with rare-earth elements (REE) have become an important basis for various photonic applications: biomedicine, photocatalysis, nanosensors, anti-counterfeiting, etc. [1,2,3,4,5,6,7]. Among the variety of the inorganic materials, the development of REE-doped fluoride NPs attracts special attention due to high chemical and thermal stability, low phonon frequencies, wide optical transparency, high isomorphic capacity, and a large selection of different dopants [7,8]. These unique features provide the ability to generate luminescence in a wide spectral range. Currently, approaches for the chemical synthesis (hydro-, solvothermal methods, coprecipitation, etc.) of fluoride NPs, based on different REE-doped crystal hosts, such as MF₂ (M = Ca, Sr, Ba), LiRF₄, NaRF₄, BaRF₅, KR₃F₁₀, and RF₃, (R = Y, La-Lu) have been successfully developed [9,10,11,12,13,14,15,16], and hexagonal β-NaRF₄ NPs demonstrate excellent up-conversion emission upon IR excitation [8,17]. However, the active development of new types of highly efficient nanophosphors continues at present. The preparation of new prospective fluoride hosts is limited by the technological capabilities of existing chemical techniques, which explains the active interest in the search for alternative physical “top-down” approaches, such as laser ablation [18,19] or mechanical milling (nanodispergation) of initial bulk materials [20,21].

Among fluoride PL nanocrystals, host compounds formed in the BaF₂-YF₃ binary system attract significant interest, and their potential for photonic and biological applications is being actively studied. There are three crystal matrices in the BaF₂-YF₃ system [22]: a fluorite solid solution Ba_1-xY_xF_2+x (x < 0.36); a trigonal phase (sp. gr. R-3) with nominal composition Ba_4±xY_3±xF₁₇, existing in the middle concentration range [23]; and a monoclinic BaY₂F₈ phase (sp. gr. C₂/m) [24]. Effective luminescence generation in BaYF₅ and BaY₂F₈ bulk crystal hosts doped with various REE combinations was first demonstrated in 1969 [25]. REE-doped BaY₂F₈ single crystals have been studied as active laser media in a wide spectral range both up- and down-conversion excitation schemes [26,27,28,29,30]. These crystals are considered as promising scintillation materials in the vacuum UV and visible regions due to high ionizing radiation resistance [31,32,33].

The modern trend in the search for new efficient PL inorganic materials is focused on the nanoscale. Wet chemical methods for the synthesis of BaYF₅ NPs with different crystal structures (cubic [14,34,35] or tetragonal [36,37,38]) were developed in detail, and the applications of these nanocrystals in bioimaging, optical thermometry, and white light emitting devices have been described. However, the stoichiometric composition of these particles needs to be specified due to the nonequilibrium state of the system during the synthesis. These BaYF₅ compounds are probably the metastable fluorite-type solid solutions [39,40]. At the same time, the synthesis strategies for Ba₄Y₃F₁₇ and BaY₂F₈ NPs are quite limited, what is associated with the significant technological complexity of preparing single-phase nanosized particles via chemical synthesis [41,42].

Monodisperse “nanobelts” BaY₂F₈:Yb³⁺/Er³⁺ with dimensions of 1000 × 75 nm² were synthesized by the liquid–solid solution strategy [43]. BaY₂F₈:Yb³⁺/Ho³⁺ compounds were produced by solid-phase fluorination of the corresponding electrospun oxide compositions such as nanofiber [44] or submicrosized particles via co-precipitation in aqueous nitrate solutions [45]. Therefore, the development of a facile production method for BaY₂F₈ NPs is extremely essential.

In this regard, a promising approach for the BaY₂F₈ NPs fabrication is nanonization of raw single-phase bulk crystals via high-energy milling. The advantages of this method are technological simplicity, low cost, scalability, and ecological safety. However, this method is not widely used due to the possibility of pollution with milling set materials and the high polydispersity of the resulting product, which requires additional sedimentation procedures to isolate the nanofraction from larger particles. This technique was previously applied to prepare BaY₂F₈ NPs, doped with Pr³⁺ and Yb³⁺/Tm³⁺ ions [46,47], but the milling procedure is not described in detail. Utilization of such a “top-down” approach to produce new types of REE doped NPs and a comprehensive characterization of their luminescent properties is desirable to extend of photonics and biotechnology possibilities.

In this study, bulk BaY₂F₈ single crystals doped with Yb³⁺, Er³⁺ ions were grown from the melt by the Bridgman technique, and single-phase NPs were prepared via high-energy milling of these compounds. The size, morphology, and composition of fabricated nanocrystals were characterized by X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), and optical and fluorescent spectroscopy in a wide wavelength range. Structural and spectroscopic features of Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs were compared with ones for bulk materials and commonly used up-conversion β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ nanocrystals, prepared by solvothermal method.

2. Materials and Methods

2.1. Crystal Growth Process

Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ single crystals were successfully grown by the Bridgman technique in a double-zone resistive setup in a fluorinating atmosphere. The graphite heating unit and crucible were utilized in the growth process. Anhydrous BaF₂ (99.98%, Angarsk Electrolysis Chemical Plant Ltd., Angarsk, Russia), YF₃, YbF₃, and ErF₃ (99.99%, Lanhit Ltd., Moscow, Russia) powders were used as initial reagents. The powders were preliminarily annealed in vacuum at 500 K and melted in a fluorinating atmosphere for purification from oxygen impurities. Preliminary evacuation was carried out to the level of 5 × 10⁻³ Pa. A high-purity He + CF₄ mixture was used to create a fluorinating growth atmosphere. The temperature gradient in the growth zone was 80 K/cm. W-Re thermocouples were used for continuous temperature control. The crucible pulling rate was about 3.5 mm/h. After growth, the crystals were cooled down to room temperature (RT) at a rate of 50 K/h. The losses of the substance due to evaporation during the crystallization process were less than 0.5 wt. %.

2.2. Optical Characterization

Transmission spectra of the crystals were recorded at RT using a Varian Cary 5000 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) in the spectral region λ = 190–2000 nm.

2.3. Nanodispergation of Bulk Crystals

High-energy milling of the grown crystals in the Retsch PM-200 planetary ball mill (Idar-Oberstein, Germany) was carried out for the fabrication of Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs. Crystal pieces with volume of about 0.1 cm³ were preliminarily pestled in an agate mortar and placed in stainless steel beakers filled with 10 milling balls with a diameter of 10 mm. Trioctylphosphine oxide was used as a surface stabilizer to prevent NPs agglomeration. The crystals were milled in cycles of 5 × 5 min at 600 rpm for 10 h. Separation of the nanosized fraction was performed through centrifugation at 6000 rpm for 15 min.

2.4. Synthesis of Reference β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ Nanoparticles

Hexagonal β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ NPs were synthesized by the standard precursor thermolysis method in a high-boiling solvents medium [12,48]. Yttrium, ytterbium, and erbium trifluoroacetates were prepared by dissolving the corresponding rare-earth oxides in a 50 wt.% solution of trifluoroacetic acid and then used as fluoride precursors. For the synthesis of β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ NPs, 0.78 mmol (CF₃COO)₃Y, 0.20 mmol (CF₃COO)₃Yb, 0.02 mmol (CF₃COO)₃Er, and 1.00 mmol CF₃COONa were added to a 100 mL three-necked flask containing 15 mL of oleic acid and 15 mL of 1-octadecene. The resulting mixture was heated to 600 K with a heating rate of 20 K/min under Ar flow and kept at this temperature for 40 min. After cooling the flask to RT, NPs were collected by centrifugation of the resulting mixture at 6000 rpm for 15 min and washed three times with 2-propanol. The prepared particles were dispersed in 10 mL of hexane and used for further analysis.

2.5. XRD Analysis

XRD patterns of the analyzed samples were carried out on an X-ray powder diffractometer MiniFlex 600 (Rigaku, Tokyo, Japan) with CuKα radiation. Diffraction peaks were recorded within the angle range 2θ from 10° to 100°. The phases were identified using the ICDD PDF-2 (2014). The unit-cell parameters were calculated by the Le Bail full-profile fitting (the Jana2006 software).

2.6. Electron Microscopy Characterization

TEM, including selected area electron diffraction (SAED), high-angle annular dark-field scanning TEM (HAADF-STEM), and simultaneously acquired energy-dispersive X-ray spectroscopy (EDS) elemental mapping analysis were performed on a Tecnai Osiris (FEI, Hillsboro, OR, USA) electron microscope operated at 200 kV and equipped with a Super-X SDD high-sensitivity EDX spectrometer. TEM samples were prepared by dropping nanoparticle hexane dispersion onto the carbon-coated Cu TEM grids. The experimental images were processed using the Digital Micrograph software 2.31 (Gatan Inc., Pleasanto, CA, USA). Interpretation of the electron diffraction patterns and HRTEM diffractograms were carried out using the Java Electron Microscopy Software (JEMS) [49].

2.7. Fluorescent Spectroscopy Analysis

The photoluminescence characteristics of the produced NPs were performed on a Fluorolog-3 fluorometer (HJY, Longjumeau, France) equipped with the Hamamatsu photomultiplier R929P and H10330A-75 tubes (Hamamatsu Photonics, Shizuoka, Japan) for registration of luminescent radiation in the visible (350–850 nm) and IR (1100–1700 nm) optical ranges, respectively. A semiconductor ATC IR laser λ = 975 nm (pump power density was 25 W/cm²) was used as an excitation light source. The luminescence decay curves were recorded on a WaveRunner 9000-MS high-speed oscilloscope (Teledyne LeCroy, Chestnut Ridge, NY, USA) under excitation by a semiconductor ATC IR laser (λ = 975 nm) in a pulsed mode. The pump power density was 115 and 55 W/cm²; the pulse repetition rate was 200 and 50 Hz for recording the decay curves in the visible and IR optical ranges, respectively. The laser pulse duration was 1000 μs.

3. Results

3.1. Growth and Optical Properties of Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ Crystal

Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ crystals with a diameter of 10 mm and a length of up to 40 mm were successfully grown (Figure 1a). Crystal boules were optically transparent, no light-scattering inclusions and cracks were observed. The crystals demonstrate bright green luminescence under IR excitation (Figure 1b).

In the optical transmission spectrum (Figure 1c), the absorption bands in the UV (380 nm), visible (487, 522, 650 nm), and IR (1480–1530 nm) regions are assigned to electric dipole intra-configurational 4f-4f transitions in Er³⁺ ions, while the intense band in the region of 960–980 nm is attributed to the absorption of the Yb³⁺ ions. Weak additional bands registered at 246 and 300 nm are associated with uncontrolled contamination of a Ce³⁺ ions impurity [50].

3.2. XRD Analysis of a Bulk Crystal, Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ and β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ Nanoparticles

The phase purity and the assignment of grown crystals and fabricated NPs to a monoclinic (sp. gr. C₂/m) BaTm₂F₈-type structure [51] were confirmed by XRD analysis (Figure 2). Comparison of the calculated structural characteristics of bulk and milled Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ samples, shown in

Table 1. Calculated unit cell parameters for a bulk Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ crystal and nanodispergated particles and β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ NPs, synthesized by thermolysis.

	Space Group	Unit Cell Parameters, Å
bulk Ba(Y0.964Yb0.030Er0.006)2F8	C2/m	a = 6.971(1), b = 10.506(1), c = 4.257(1), β = 99.676°
milled Ba(Y0.964Yb0.030Er0.006)2F8	C2/m	a = 6.969(3), b = 10.502(1), c = 4.254(1), β = 99.676°
β-Na1.5(Y1.17Yb0.3Er0.03)F6	P63/m	a = 5.972(1), c = 3.505(1)

, confirms the absence of changes in the crystal structure during the milling process. A minor reducing in the lattice parameters for the nanosized crystal sample is probably attributed to the originating mechanical compression strains during such high energy treatment. A significant broadening and a decrease in the intensity patterns for fabricated NPs indirectly indicate the crystallite sizes reducing and a transition to nanoscale. No impurity phases were observed. The reference β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ particles synthesized by thermolysis are single-phase and crystallize in a gagarinite β-Na_1.5Y_1.5F₆ structure type (sp. gr. P6₃/m) [52].

3.3. TEM Analysis of Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ and β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ Nanoparticles

TEM data demonstrate that the high energy milling of the initial bulk Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ crystal for 10 h leads to the formation of irregular NPs. The HAADF-STEM image of the agglomerate of NPs without any prominent morphological features in a wide size range from 10 to 70 nm is presented in Figure 3a. The electron diffraction pattern (Figure 3b) taken from an agglomerate corresponds to the structure of the initial bulk monoclinic crystal and confirms the above XRD results. No other phases were found. The EDS analysis results also show no changes in the sample elemental composition during the milling process (Figure 3c).

HRTEM image of the Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs produced by high-energy milling of a bulk crystal and the corresponding diffractograms of nanocrystals A and B are presented in Figure 4. Interpretation of FFT diffractograms using JEMS shows that A and B NPs are oriented along the [311] and [$\overline{3}$12] directions.

TEM image of β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ nanophosphors is shown in Figure 5. According to TEM data (Figure 5a), the thermolysis method leads to the near-monodisperse β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ NPs in the size range of 21 ± 5 nm. SAED pattern of β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ particles, shown in Figure 5c, corresponds to the hexagonal crystal structure of β-Na_1.5Y_1.5F₆.

3.4. Photoluminescence Properties of Bulk and Nanocrystals

Nanosized and bulk Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ samples demonstrate intense visible and IR PL upon excitation by λ = 975 nm radiation at RT. Intense characteristic luminescence bands, corresponding to 4f-4f energy transitions in Er³⁺ ions, namely transitions from the ⁴F_7/2 (408 nm), ⁴H_11/2 (522 nm), ⁴S_3/2 (540 nm), ⁴F_9/2 (651 nm), and ⁴I_13/2 (1535 nm) multiplet manifolds to the ⁴I_15/2 ground state due to up- and down-conversion energy transfer processes, were observed in the PL spectra (Figure 6a). Comparison the PL spectra of nanosized and bulk Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ samples did not reveal any qualitative differences; no shifts or broadening of the emission bands were detected.

The energy transfer mechanism in BaY₂F₈ crystals co-doped with Yb³⁺ and Er³⁺ ions is shown in Figure 7. Upon absorption of 975 nm photon, Yb³⁺ ions are excited from the ²F_7/2 ground state to ²F_5/2 state, and then non-radiatively transfer energy to neighboring Er³⁺ ions, populating their intermediate ⁴I_11/2 state. As a result of the successive absorption of two photons, Er³⁺ ions are excited to the ⁴F_7/2 state, which can non-radiatively decay to the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 electronic states, followed by radiative green ⁴S_3/2 → ⁴I_15/2 (522 nm), ²H_11/2 → ⁴I_15/2 (540 nm), and red ⁴F_9/2 → ⁴I_15/2 (650 nm) characteristic luminescent transitions. Additionally, Er³⁺ ions can be excited to the ²H_9/2 state by a three-photon absorption process, followed by a ²H_9/2 → ⁴I_15/2 radiative transition at a wavelength λ = 408 nm. The IR luminescence (1535 nm) results from a radiative decay of the long-lived intermediate ⁴I_11/2 state (⁴H_11/2 → ⁴I_15/2).

The PL decay kinetics of the most intense spectral bands at 540, 650, and 1535 nm for Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ samples are shown in Figure 8. Each decay curve is well fitted in a single-exponential dependence:

$$I\left(t\right)=I_0\ast \exp \left(-\frac{t}{\tau } \right),$$

where I(t) is the PL intensity at time t; I₀ is the PL intensity at the end of the laser excitation pulse, τ is the average lifetime of the excited state.

The calculated PL lifetimes for the studied samples are presented in

Table 2. The PL lifetimes of Er³⁺ excited states in specified compositions.

	τ, ms
	λ1 = 540 nm	λ2 = 650 nm	λ3 = 1535 nm
bulk Ba(Y0.964Yb0.030Er0.006)2F8	0.873	1.200	11.420
milling Ba(Y0.964Yb0.030Er0.006)2F8	0.679	0.823	7.530
β-Na1.5(Y1.17Yb0.3Er0.03)F6	0.380	0.527	6.950

. A significant decrease in the lifetime for each PL band is observed for Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs with respect to that for a bulk crystal. This phenomenon is associated with an increase in the surface-to-volume ratio for the nanosized particles. Milling NPs have a high concentration of surface defects generated by strains in the crystallites during the mechanical milling process. The interaction of active Er³⁺ centers, lying on the particle surface, with these defects leads to a significant PL quenching.

A comparative analysis of the PL spectra of Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ and reference β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ NPs demonstrates the maxima position shifts and the broadening of the spectral lines for a milling sample (Figure 6a). A strong splitting of PL bands for Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs is observed due to a local symmetry lowering of the active centers. The local symmetry of REE ions in the BaY₂F₈ and β-NaYF₄ crystal hosts is C₂ and C_3h, respectively. Strong Stark splitting of Er³⁺ electronic levels in the more asymmetric crystal field of the monoclinic BaY₂F₈ matrix leads to a stronger degeneracy of existing metastable energy states, which is more clearly observed in the PL spectrum for Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ in the IR range. Such a degeneracy of states leads to an energy gap increasing between closely spaced 4f states of Er³⁺ ions and a reduction in the non-radiative relaxation processes probability and, as a consequence, to a rise in the ⁴S_3/2, ⁴F_9/2, and ⁴I_13/2 states’ lifetimes. Therefore, the PL lifetimes of Er³⁺ excited states for the BaY₂F₈ matrix are higher than β-Na_1.5(Y_1.17Yb_0.3Er_0.03)F₆ ones.

4. Conclusions

In this study, Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs in the size range of 30–70 nm were successfully produced by high-energy milling of the Bridgman-grown single crystals. This scalable and versatile method provides a feasible strategy for the mass production of novel fluorescent nanomaterials in a narrow size range that are currently difficult to prepare by available chemical approaches. Ba(Y_0.964Yb_0.030Er_0.006)₂F₈ NPs demonstrate a bright PL in a wide spectral range, which provides great opportunities for applications of these nanophosphors in photonics and biotechnology, for example, as visible and NIR luminescent labels. Further optimization of the milling procedure, namely the adjustment of nanodispersion technological parameters and suitable hydrophobic or hydrophilic surfactants for the nanosized fraction stabilizing, will provide the possibility to control NP sizes in a narrower range. Optimization of REE doping content and the particle surface passivation by coating a protective shell will significantly enhance the PL properties of studied NPs and are the object of future research.

We believe that BaY₂F₈-based NPs will occupy a rightful place in nanotechnology, the same as the commonly used β-NaREF₄ nanocrystals, which are the most efficient PL nanomaterials to date.