Synthesis, Morphology, and Luminescent Properties of Nanocrystalline KYF₄:Eu³⁺ Phosphors

Abstract

The study of crystalline nanosized phosphors KYF₄:Eu³⁺ was carried out for the first time in a range of europium concentrations. The particles were obtained by a modified hydrothermal method. The formation of two different cubic phases (α-KY₃F₁₀ and KYF₄) was detected depending on the ratio of reagents under synthesis conditions. The sizes and shapes of the synthesized particles were determined. For obtained particles, excitation and emission spectra were recorded, luminescence lifetimes and quantum yields were determined, and the mechanism of concentration quenching in these concentration series was disclosed.

1. Introduction

Nanoscale luminescent materials based on solid solutions of rare earth fluorides are promising materials due to a wide range of applications, including bioimaging, image-guided tumor surgery, photocatalysis, luminescent thermometry, anticounterfeiting, sensors, photovoltaics, etc. [1,2,3,4,5,6,7,8,9,10,11]. The synthesis conditions, the nature of rare earth elements (REE), and their relative content usually significantly affect the particle morphology, crystalline phase, and the optical properties of such materials. Despite the wide variety of such compounds containing various REE ions obtained by different methods, which was previously reported in the literature [12,13,14,15,16,17,18,19,20,21,22,23,24], the effect of the material composition and synthesis conditions on the structure and functional properties is often unclear, especially fundamental reasons that could explain such behavior and predict properties are almost miserable.

It is known that the phase composition of the host matrix, which can be doped with other rare earth ions, plays a significant role in the luminescent properties of the final materials. β-NaYF₄ is among the most popular and well-studied matrices to date, mainly due to its low phonon energy, which allows the compounds to exhibit remarkable luminescent properties among inorganic phosphors [25]. Another similar compound that has attracted the attention of researchers is α-KY₃F₁₀ [26]. At the same time, compounds based on this matrix have been studied to a much smaller extent and contain significant gaps that require attention. This is confirmed, for example, by a recent remarkable review of upconversion compounds based on α-KY₃F₁₀ [27]. Among the luminescent ions used for doping in such solid solutions, trivalent europium is of particular importance. The emission spectrum of europium(III) exhibits intense and narrow lines in the visible spectrum, mainly in the red region (570–840 nm, corresponding to the ⁵D₀→⁷F_j transitions (j = 0–6)) [28]. Compounds containing europium(III) ions possess a high quantum yield, a large Stokes shift, and a relatively long emission lifetime, which allows their use in bioimaging [29] and is sensitive to the local environment, making it suitable for use as a structural probe [30,31]. The synthesis and study of some KY_xF_3x+1 (x = 1; 3) compounds doped with europium have been undertaken many times before, and fragmentary information on these compounds can be found in the literature [32,33,34,35,36,37,38,39,40]. In these works, the luminescent and morphological properties of KY_xF_3x+1 (x = 1; 3) doped with Eu³⁺ (only at a low concentration of europium, up to 15 at.%), synthesized by different methods, were studied. It should also be especially noted that the results of these works demonstrate that the synthesis conditions determine the crystalline phase of the final compounds: orthorhombic YF₃, tetragonal KY₃F₁₀, cubic KYF₄, KY₃F_10, or KY₃F₁₀·xH₂O.

To the best of our knowledge, the study of the entire line of solid solutions in the KF-YF₃-EuF₃ system obtained by hydrothermal synthesis has not been carried out. This work was performed to conduct such a study and to track the changes in morphological and luminescent properties over the entire range of possible europium concentrations in the compounds.

2. Materials and Methods

The following reagents were used for the synthesis of fluorescent nanoparticles: YCl₃∙6H₂O (99.9%, Chemcraft, Kaliningrad, Russia), EuCl₃∙6H₂O (99.9%, Chemcraft, Kaliningrad, Russia), KF∙2H₂O, NH₃∙H₂O, citric acid, and ethanol (Chemcraft, Kaliningrad, Russia). All reagents were used without additional purification.

Nanocrystalline samples of solid solutions were synthesized using a modified hydrothermal method with citric acid as a stabilizer. This modification enabled the synthesis from potassium fluoride. Rare earth chlorides, taken in stoichiometric quantities (total amount of 0.375 mmol), were dissolved with 2.4 mmol of citric acid in distilled water to yield a 2 mL solution. Then, 2 mL of an aqueous solution containing 10 mmol NH₃ was added to the flask with the previous solution. After intense stirring for 30 min, 12.5 mL of an aqueous solution containing 10 mmol KF was added. The mixture was stirred intensively for another 30 min at room temperature, then transferred to a Teflon-lined autoclave with an internal volume of 20 mL and heated for 17 h at 180 °C. The resulting colorless precipitate was separated from the reaction mixture by centrifugation, washed with ethanol and deionized water, and dried at 60 °C for 24 h.

The ratio of REE contents in the synthesized compounds was confirmed using energy dispersive X-ray spectroscopy (EDX) (EDX-spectrometer EDX-800P, Shimadzu, Kyoto, Japan)). The morphology and size of particles of the synthesized compounds were characterized using scanning electron microscopy (SEM) on a Zeiss Merlin electron microscope (Zeiss, Jena, Germany) equipped with an EDX module (Oxford Instruments INCAx-act, Oxford, UK). Powder X-ray diffraction (PXRD) measurements were carried out on a D2 Phaser X-ray diffractometer (Bruker, USA Warwic, RI, USA) using Cu Kα radiation (λ = 1.54056 Å). For quantitative photoluminescence studies, synthesized samples were compressed into pellets (20 mg of sample per 300 mg of potassium bromide). Luminescence spectra were recorded on a modular Fluorolog-3 fluorescence spectrometer (Horiba Jobin Yvon, Kyoto, Japan). Lifetime measurements were conducted using the same spectrometer with a Xe pulse lamp (pulse duration 3 ms). Photoluminescence quantum yield was measured by an absolute method using Fluorolog-3 equipped with a 6-inch integrating sphere Quanta-φ.

3. Results

3.1. Crystal Structure and Morphology

The powder X-ray diffraction (PXRD) patterns for the synthesized compounds are shown in Figure 1. Phase analysis shows the formation of two non-identical crystalline phases in the studied system. For the undoped composition (without europium ions), the analysis revealed the formation of the cubic α-KY₃F₁₀ phase (JCPDS No. 27-0462). In the range of 5–10 at.% of europium added, a mixture of two phases is formed, previously defined as cubic α-KY₃F₁₀ and cubic α-NaYF₄ (JCPDS file No. 06-0342). Some of the low-intensity peaks of α-KY₃F₁₀ disappear when the fraction of europium is increased up to 20 at.%, and within the range of compositions from 20 at.% up to 100 at.%, the particles crystallize in the single α-NaYF₄ phase. Comparison of experimental diffraction patterns and calculated one for α-NaYF₄ in Figure 1 shows a slight shift in experimental diffraction peaks towards a smaller 2θ angle, which may be explained by ion replacement of smaller ionic radii of Na⁺ and Y³⁺ by larger ionic radii of K⁺ and Eu³⁺ and formation of the cubic KYF₄ phase [32,34], which is assumed to be isostructural with KEuF₄. Based on this assumption, a cubic unit cell for the simulated phase KEuF₄ with a cell constant of 5.7636(2) Å (space group Fm-3m) was obtained by using TOPAS software (version 5). The calculated positions of the peaks for this compound are presented as blue bars with asterisks in Figure 1 and show an excellent match with the experimental one for the sample with 100 at.% of europium. More accurate analysis of PXRD patterns reveals a monotonic shift in peaks within the range 0–5 at.% of europium (Figure 2), which is caused by a change in phase composition. In the region from 20 to 100 at.% of Eu³⁺, a uniform shift in the peaks towards smaller angles is observed, which can be easily explained by the homogeneous substitution of yttrium ions by larger europium ions in the KEuF₄ phase. To clarify the phase composition for solid solution samples in the range of 5 at.% and 10 at.% compositions, the Rietveld refinement of PXRD was performed (see Tables S1 and S2 in Supplementary Material), which showed that the proportion of the KEuF₄ phase for both obtained samples is around 70 mass%, making it the dominant phase. These simulated powder diffractograms were further used to calculate the unit cell parameters of the obtained nanocrystals with 5–100 at.% Eu³⁺. The dependence of the refined unit cell volumes on the sample composition is shown in Figure 3. Unit cell volumes linearly depend on the concentration of europium(III) ions, which confirms that this series of solid solutions obeys Vegard’s law. The increase in Eu³⁺ content leads to an increase in unit cell volumes due to the larger ionic radius of Eu³⁺ ions (1.120 Å) compared to Y³⁺ ions (1.075 Å) [41,42]. Therefore, the dominant phase (5–10 at.% Eu³⁺) or single phase (20–100 at.% Eu³⁺) corresponds to the cubic KYF₄:Eu³⁺ structure across the entire composition range.

The scanning electron microscopy (SEM) images of the synthesized materials are presented in Figure 4. Particle diameters were determined from these SEM images, with the size distribution displayed in the insets of Figure 4. The mean particle diameter was derived from this distribution and is provided in the figure legends. The particles exhibit spherical nanostructures, with sizes ranging from 23 ± 7 nm (for 0% Eu) to 54 ± 6 nm (for 100% Eu), depending on the composition. The nanoparticle size increases in a linear fashion with the increasing concentration of Eu³⁺.

3.2. Luminescent Properties

Excitation spectra (Figure 5a) of solid solution samples monitored at the ⁵D₀-⁷F₂ (615 nm) transition were measured in the spectral range of 355–420 nm. One can see that the spectra consist of sharp peaks attributed to the electron transitions of Eu³⁺ inside the 4f-shell: ⁷F₀-⁵D₄ (360 nm), ⁷F₀-⁵G₃ (375 nm), ⁷F₀-⁵L₇ (382 nm), ⁷F₀-⁵D₄ (360 nm), ⁷F₁-⁵L₆ (393 nm), ⁷F₀-⁵D₃ (415 nm). All obtained spectra have the same shape and are dominated by the ⁷F₀-⁵L₆ (393 nm) transition.

Emission spectra (Figure 5b) of KYF₄:Eu³⁺ samples upon 393 nm excitation (⁷F₀-⁵L₆ transition) were measured in the spectral range of 550–720 nm. Spectra consist of sharp peaks attributed to ⁵D₀-⁷F_J transitions: ⁵D₀-⁷F₀ (577 nm), ⁵D₀-⁷F₁ (586 and 591 nm), ⁵D₀-⁷F₂ (608, 611, 615, 618, and 625 nm), ⁵D₀-⁷F₃ (650 nm), and ⁵D₀-⁷F₄ (690 and 700 nm).

Analysis of luminescence spectra has shown that europium concentration does not affect the shape of the spectra.

Local symmetry of europium ions in the solid solutions was evaluated by calculating the asymmetry coefficient from the integrated intensity ratio of the ⁵D₀-⁷F₁ (structure-independent) and ⁵D₀-⁷F₂ (hypersensitive) transitions [43]. The obtained values ranged from 0.7 to 0.9 across all compositions, confirming the consistent Eu³⁺ coordination symmetry in all synthesized compounds.

The dependence of the integral intensity on the europium concentration is shown in Figure 6a. The integral intensity grows rapidly from 0 to 20% of the europium concentration, reaching a maximum at 20% because the number of luminescence sites increases. The competitive effect of concentration quenching results in a slow decrease in integral intensity moving from 20 to 100%. This type of concentration dependence can be explained by the two competitive effects in phosphors upon Eu³⁺ concentration rise [44]. Thus, the rise of Eu³⁺ concentration results in an increase in the number of luminescent sites and, as a result, an increase in emission intensity. At the same time, as the Eu³⁺ concentration increases, the distance between Eu³⁺ ions decreases, resulting in the probability of non-radiative processes increasing, leading to the emission quenching.

If Eu³⁺ ions occupy a single crystallographic position in the host, the energy transfer mechanism is determined by the critical energy transfer distance R_c. One can calculate R_c using Equation (1) [45]:

$${\mathrm{R}}_{\mathrm{c}}=2{\left({\displaystyle \frac{3\mathrm{V}}{4\mathsf{\pi }{\mathsf{\chi }}_{\mathrm{c}}\mathrm{N}}}\right)}^{{\displaystyle \frac{1}{3}}},$$

(1)

${\mathsf{\chi }}_{\mathrm{c}}$—critical concentration of europium (0.2), V—unit cell volume of sample with critical concentration (185.6 ų), N—number of cation sites in crystal structure (2 for cubic KEuF₄). The obtained value of R_c is 9.6 Å. It is known that if R_c > 5 Å, then non-radiative energy transfer occurs mainly by multipole-multipole interactions [45]. The mechanism of concentration quenching can be defined more precisely using Equation 2 [46]:

$${\displaystyle \frac{\mathrm{I}}{\mathsf{\chi }}}={\displaystyle \frac{\mathrm{k}}{1+\mathsf{\beta }{\mathsf{\chi }}^{\frac{\mathsf{\theta }}{3}}}},$$

(2)

I—integral intensity, χ—concentration of luminescent ion, θ—auxiliary coefficient for determining the mechanism of concentration quenching. Assuming that $\mathsf{\beta }{\mathsf{\chi }}^{{\displaystyle \frac{\mathsf{\theta }}{3}}}$ >> 1, one can build the linearized coordinates lg(I/χ) − lgχ (Figure b). The linear fitting of this function gives the value of θ/3 (slope of the fitting line). The dipole–dipole, dipole-quadrupole and quadrupole-quadrupole interactions correspond to θ = 6, 8 and 10, respectively. The fitting of the obtained dependence gives θ/3 -1.55, which gives the value for θ closer to 6, meaning that concentration quenching in KYF₄:Eu³⁺ is mainly caused by dipole–dipole interactions. The same concentration quenching mechanism is observed for NaYF₄:Eu³⁺ [21].

Luminescence decay curves of KYF₄:Eu³⁺ monitored at 615 nm upon 393 nm excitation are presented in Figure 7a. All of the obtained curves demonstrate biexponential behavior. This effect is explained by different decay times of luminescent ions on the surface and in the depth of nanoparticles. Luminescence intensity as a function of time is described by Equation 3 [47,48,49,50]:

$$\mathrm{I}\left(\mathrm{t}\right)={\mathrm{A}}_1{\mathrm{e}}^{{\displaystyle \frac{\mathrm{t}}{{\mathsf{\tau }}_1}}}+{\mathrm{A}}_2{\mathrm{e}}^{{\displaystyle \frac{\mathrm{t}}{{\mathsf{\tau }}_2}}},$$

(3)

A₁, A₂—pre-exponential constants, τ₁, τ₂—fitting lifetimes.

A lifetime of ⁵D₀ energy level (average lifetime) can be approximately calculated using Equation 4 [49,50]:

$$\mathsf{\tau }={\displaystyle \frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}}$$

(4)

The obtained values of τ₁ and τ₂ are given in Table S3. The dependence of the average lifetime on europium concentration is demonstrated in Figure 7b. The obtained curve demonstrates almost monotonous behavior (average lifetime decreases from 6.6 to 0.7 ms with europium concentration increase). The rise of europium concentration causes a decrease in the distance between Eu³⁺ ions, resulting in the growth of non-radiative transfer probability. This effect leads to an average lifetime decrease due to the increase in non-radiative decay.

Quantum yields of KY_(1−x)Eu_xF₄ (x = 0.05, 0.1, 0.2, 0.3, 0.5, 1) were obtained using an integrating sphere (Figure 8). The measured values for samples with 5 at.% and 10 at.% europium appear to be lower than those at 20 at.%, although one would expect a monotonic decrease in the quantum yield over the entire concentration range. We attribute this effect to the formation of a mixture of two different crystalline phases (α-KY₃F₁₀ and KYF₄), which occurs up to 20%. In the composition range of solid solutions with higher europium content, a smooth decrease in quantum yield is observed, which is consistent with an increase in the concentration of europium ions in the matrix and the appearance of concentration quenching.

Luminescence decay is a combination of radiative and non-radiative decay. Radiative decay rate of the independent ⁵D₀–⁷F₁ transition can be calculated using Equation (5) [51]:

$${\mathrm{A}}_{0-1}=14.65{\mathrm{n}}_0^3,$$

(5)

$n_0$—refraction index ≈ 1.49. The obtained value is 48.46. Radiative decay rates of other transitions can be calculated using Equation 6 [51]:

$${\mathrm{A}}_{0-\mathrm{j}}={\mathrm{A}}_{0-1}{\displaystyle \frac{{\mathsf{\nu }}_{0-1}{\mathrm{I}}_{\mathrm{o}-\mathrm{j}}}{{\mathsf{\nu }}_{0-\mathrm{j}}{\mathrm{I}}_{0-1}}},$$

(6)

${\mathrm{I}}_{\mathrm{o}-\mathrm{j}}$ and ${\mathrm{I}}_{\mathrm{o}-1}$ —integral intensities of ⁵D₀-⁷F_j and ⁵D₀-⁷F_j transitions, ${\mathsf{\nu }}_{0-\mathrm{j}}$ and ${\mathsf{\nu }}_{0-1}$ frequencies. The total radiative decay rate (A_rad) can be calculated as a sum of the radiative decay rates of ⁵D₀-⁷F_j transitions (j = 0–4). The total decay rate (A_total) equals τ⁻¹. The non-radiative decay rate (A_nrad) can be calculated as the difference between A_total and A_rad. The quantum efficiency of the ⁵D₀ level can be calculated using Equation (7):

$$\mathsf{\eta }={\displaystyle \frac{{\mathrm{A}}_{\mathrm{r}\mathrm{a}\mathrm{d}}}{{\mathrm{A}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}}}$$

(7)

The obtained values of A_total, A_rad, A_nrad, $\eta$ are given in

Table 1. Values of A_total, A_rad, A_nrad, $\eta$ of KYF₄:xEu³⁺ (x = 20, 30, 40, 50, 60, 80, 100 at.%).

ΧEu (at.%)	Atotal(s−1)	Arad(s−1)	Anrad(s−1)	η (%)
20	188	101	87	54
30	273	103	170	38
40	474	103	371	22
50	637	104	534	16
60	794	104	689	13
80	1369	106	1262	8
100	1504	109	1395	7

.

The concentration dependence of A_rad almost repeats the concentration dependence of the asymmetrical ratio, which can be explained by the Laporte selection rule relaxation. Obtained values of A_nrad are easily explained: an increase in those values with europium concentration increase is due to a decrease in the distance between Eu³⁺ ions (higher probability of non-radiative processes).

4. Conclusions

In the present work, a series of KF-EuF₃-YF₃ solid solutions was synthesized by a hydrothermal method at a temperature of 180 °C using citric acid as a stabilizing agent. Analysis of PXRD patterns demonstrated that the compound without added europium ions crystallizes in the cubic α-KY₃F₁₀ phase. In the range of 5–10 at.% of europium added, a mixture of two phases is formed, cubic α-KY₃F₁₀ without europium ions and cubic KYF₄:Eu³⁺. For compositions between 20 at.% and 100 at.% of europium, solid solutions crystallize in a single cubic phase KYF₄:Eu³⁺. Unit cell volumes linearly depend on the concentration of europium(III) ions in the KYF₄ phase, which demonstrates that Eu³⁺ ions isomorphically substitute Y³⁺ ions in the structure. According to SEM data, the particles of all synthesized compounds have a spherical form and sizes ranging from 23 to 58 nm, depending on the sample composition. The substitution of Y³⁺ by Eu³⁺ ions leads to an increase in particle size. All europium-doped synthesized compounds demonstrate photoluminescence under 393 nm excitation (⁷F₀-⁵L₆ transition in Eu³⁺). The experimental Eu³⁺ optimal doping concentration in the cubic KYF₄ host is 20 at.%. An increased concentration of Eu³⁺ leads to strong quenching due to dipole–dipole interactions between Eu³⁺ ions.