Luminescence of SiO₂-BaF₂:Tb³⁺, Eu³⁺ Nano-Glass-Ceramics Made from Sol–Gel Method at Low Temperature

Abstract

The synthesis and characterization of multicolor light-emitting nanomaterials based on rare earths (RE³⁺) are of great importance due to their possible use in optoelectronic devices, such as LEDs or displays. In the present work, oxyfluoride glass-ceramics containing BaF₂ nanocrystals co-doped with Tb³⁺, Eu³⁺ ions were fabricated from amorphous xerogels at 350 °C. The analysis of the thermal behavior of fabricated xerogels was performed using TG/DSC measurements (thermogravimetry (TG), differential scanning calorimetry (DSC)). The crystallization of BaF₂ phase at the nanoscale was confirmed by X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM), and the changes in silicate sol–gel host were determined by attenuated total reflectance infrared (ATR-IR) spectroscopy. The luminescent characterization of prepared sol–gel materials was carried out by excitation and emission spectra along with decay analysis from the ⁵D₄ level of Tb³⁺. As a result, the visible light according to the electronic transitions of Tb³⁺ (⁵D₄ → ⁷F_J (J = 6–3)) and Eu³⁺ (⁵D₀ → ⁷F_J (J = 0–4)) was recorded. It was also observed that co-doping with Eu³⁺ caused the shortening in decay times of the ⁵D₄ state from 1.11 ms to 0.88 ms (for xerogels) and from 6.56 ms to 4.06 ms (for glass-ceramics). Thus, based on lifetime values, the Tb³⁺/Eu³⁺ energy transfer (ET) efficiencies were estimated to be almost 21% for xerogels and 38% for nano-glass-ceramics. Therefore, such materials could be successfully predisposed for laser technologies, spectral converters, and three-dimensional displays.

1. Introduction

Barium fluoride, BaF₂, belongs to the group of attractive nanoparticles, produced using different preparation methods and applied in numerous multifunctional applications. Nd³⁺:BaF₂ nanocrystals synthesized by the reverse microemulsion technique present interesting luminescence properties [1]. Indeed, the quenching of fluorescence intensity (λ_em = 1052 nm) in nanosized Nd³⁺:BaF₂ domains was not observed even under very high dopant levels (~45 mol.% of Nd³⁺). Further experiments revealed the crystallization of cubic and orthorhombic BaF₂ nanoparticles, and it was proven that such fluoride crystals could be quite easily transformed from the orthorhombic phase to the more thermodynamically stable cubic phase under certain preparation conditions. This effect was confirmed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) used for self-assembled monodisperse BaF₂ nanocrystals accomplished by the liquid–solid-solution (LSS) approach [2]. BaF₂ nanocrystals were also fabricated from precursor Na₂O-K₂O-BaF₂-Al₂O₃-SiO₂ glasses via their controlled heat treatment. Their self-organized nanocrystallization processes [3] and size distribution [4] have been presented and discussed in detail. Luminescence properties of nanosized Eu³⁺-doped BaF₂ synthesized via an ionic liquid-assisted solvothermal method in different solvents (e.g., DMSO, water, or water with PVP solution) confirmed that these fluoride nanoparticles can be effectively used for bioimaging applications [5].

From the accumulated experience and literature data, it is known that RE³⁺ ions can be introduced into the fluoride nanocrystals dispersed within the transparent glassy host. Indeed, several precursor glasses doped with RE³⁺ ions were heat treated to fabricate RE³⁺:BaF₂ nanocrystals and obtain transparent glass-ceramics with enhanced luminescence properties. Nano-glass-ceramics with RE³⁺:BaF₂ have been examined for visible [6] and near-infrared [7,8] luminescence as well as white up-conversion applications [9]. Special attention has been devoted to the structure and luminescent properties of BaF₂ nanocrystals in glass-ceramics singly doped with Er³⁺ [10,11] and co-doped with Er³⁺/Yb³⁺ [12,13]. Among RE³⁺, trivalent europium ions are commonly used as a spectroscopic probe, indicating structural changes around the optically active ions and their surrounding environment [14]. Additionally, europium ions in a divalent oxidation state can also exist, thus, the silicate glasses containing EuF₃ synthesized by melt-quenching in reducing atmosphere tend to form Eu²⁺-doped glass-ceramics after the heat-treatment process. The prepared glass-ceramic system with Eu²⁺:BaF₂ nanocrystals could be potentially utilized as a blue phosphor for UV–LED applications [15]. Divalent europium ions in fluorosilicate glass-ceramics can be well stabilized via lattice site substitution [16]. In the field of preparation of the RE³⁺-doped glass-ceramics containing BaF₂ nanocrystals, particular attention should also be focused on the sol–gel method. The first synthesis of 95SiO₂−5BaF₂ (mol.%) nano-glass-ceramics via the sol–gel technique was reported and described in work by D. Chen et al. [17]. The authors proved that the size of precipitated BaF₂ nanocrystals (2–15 nm) and the luminescence of Er³⁺ ions are strictly dependent on heat-treatment conditions of initially obtained xerogels. C.E. Secu et al. [18,19] presented the fabrication, structure, and luminescence of 95SiO₂–5BaF₂ (mol.%) nano-glass-ceramics singly doped with Pr³⁺, Ho³⁺, Dy³⁺, Sm³⁺, and Eu³⁺ ions. Except for Eu³⁺-doped samples, the emission bands of other active dopants were revealed after controlled heat treatment of precursor xerogels, which was explained by incorporating RE³⁺ into BaF₂ crystals (3–7 nm) and removing residual OH groups from the silicate sol–gel host. The recently published work by M. Hu et al. [20] was concentrated on properties of 95SiO₂–5BaF₂ (mol.%) glass-ceramics singly and doubly doped by Tb³⁺, Eu³⁺, and Dy³⁺ ions, containing fluoride nanocrystals with an average size of ~5 nm. The authors verified the thermal stability of generated luminescence in a range from 30 °C to 290 °C, proving that synthesized sol–gel nano-glass-ceramics could be utilized as color and white light emitters. The properties of RE³⁺-doped sol–gel glass-ceramics containing BaF₂ nanocrystals were compared with other oxyfluoride systems in an extensive review published recently by Secu et al. [21]. This class of RE³⁺-doped materials is widely considered as a promising candidate for selected applications, e.g., three-dimensional displays, flat color screens, spectral converters, light-emitting diodes (LEDs), etc. [21].

Our previously published work [22] was concerned with sol–gel SiO₂-BaF₂ nano-glass-ceramic systems doped with europium ions in a trivalent oxidation state. Their structural and optical properties have been studied using various experimental techniques, such as differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM) coupled with the energy-dispersive X-ray spectroscopy (EDS), infrared (ATR-IR), and luminescence spectroscopy. The properties of Tb³⁺, Eu³⁺ co-doped glass-ceramic systems containing BaF₂ nanocrystals made from the sol–gel method at a low temperature are communicated here. To the best of our knowledge, these aspects for SiO₂-BaF₂:Tb³⁺, Eu³⁺ nano-glass-ceramics have not yet been examined.

2. Materials and Methods

The xerogels singly doped with Tb³⁺ and co-activated with Tb³⁺, Eu³⁺ ions were prepared via the previously described sol–gel synthesis [22]. The reagents from Sigma-Aldrich (St. Louis, MO, USA) were applied for the fabrication the samples. In the first step of preparation, precursor (TEOS), ethanol, deionized water, and acetic acid (AcOH) were mixed (in a molar ratio 1:4:10:0.5) in round-bottom flasks for 30 min. TEOS, Si(OC₂H₅)₄, was used as a precursor for creating the SiO₂ silicate host, water was necessary to perform the hydrolysis reaction of TEOS, and AcOH played a role as a catalyst. Due to the significantly limited solubility of TEOS in water, ethyl alcohol was introduced into the reaction systems, enabling the hydrolysis reaction by increasing the TEOS–water contact surface. The hydrolysis could be expressed by the following reaction:

Si(OC₂H₅)₄ + nH₂O ⇄ Si(OH)_n(OC₂H₅)_(4-n) + nC₂H₅OH,

in which n ≤ 4. Simultaneously with the hydrolysis reaction, the condensation begins, which allows for the creation of a silicate network through the formation of siloxane bridges, Si–O–Si. The homocondensation could be given by the following chemical reaction:

(OC₂H₅)_n(OH)_(3-n)Si–OH + HO–Si(OC₂H₅)_n(OH)_(3-n) ⇄
(OC₂H₅)_n(OH)_(3-n)Si–O–Si(OC₂H₅)_n(OH)_(3–n) + H₂O,

and the heterocondensation could be expressed by:

(OC₂H₅)_n(OH)_(3–n)Si–OH + C₂H₅O–Si(OC₂H₅)_n(OH)_(3–n) ⇄
(OC₂H₅)_n(OH)_(3–n)Si–O–Si(OC₂H₅)_n(OH)_(3–n) + C₂H₅OH,

in which n ≤ 3. The mechanisms of hydrolysis and condensation reactions of alkoxides were discussed in detail in the paper [23].

After pre-hydrolysis and pre-condensation, the solutions of Ba(AcO)₂ and RE(AcO)₃ (RE = Tb or Tb/Eu) in trifluoroacetic acid (CF₃COOH, TFA) and deionized water were added dropwise, and the obtained mixtures were stirred for the next 60 min. Since the electrolytic dissociation of TFA acid (K_a = 5.9 × 10⁻¹) is greater than for AcOH (K_a = 1.8 × 10⁻⁵), TFA is a much stronger acid than AcOH, and the following reaction occurs:

Ba(AcO)₂ + 2TFA → Ba(TFA)₂ + 2AcOH.

For Tb³⁺-doped samples, the molar ratio of TFA:Ba(AcO)₂:Tb(AcO)₃ was equal to 5:0.95:0.05, and for Tb³⁺, Eu³⁺ co-doped materials, the molar ratio of TFA:Ba(AcO)₂:Tb(AcO)₃:Eu(AcO)₃ was equal to 5:0.9:0.05:0.05. The mass of TEOS, ethanol, deionized water, and acetic acid reached 90 wt.% of each sample, and the mass of the remaining part containing TFA, Ba(AcO)₂, and RE(AcO)₃ (RE = Tb or Tb/Eu) equaled 10 wt.%. The liquid sols were dried at 35 °C for several weeks and then heat treated at 350 °C per 10 h in a muffle furnace (Czylok, Jastrzębie-Zdrój, Poland). The thermal treatment of xerogels at 350 °C aims to transform them into SiO₂-BaF₂ nano-glass-ceramics. Indeed, TFA was introduced as a fluorination reagent, allowing for successful crystallization of BaF₂ fraction inside the silicate sol–gel host. The fabricated xerogels were denoted as XG_Tb and XG_Tb/Eu (for singly and doubly doped xerogels), as well as nGC_Tb and nGC_Tb/Eu (for singly and co-doped nano-glass-ceramics).

The thermogravimetry and differential scanning calorimetry (TG/DSC) were carried out using a Labsys Evo system with a heating rate of 10 °C/min in argon atmosphere (SETARAM Instrumentation, Caluire, France). To verify the formation of fluoride nanocrystals within the silicate sol–gel host at 350 °C, the X-ray diffraction was performed using an X’Pert Pro diffractometer equipped by PANalytical with CuK_α radiation (Almelo, the Netherlands). Additionally, the fluoride nanocrystals were observed by a JEOL JEM 3010 transmission electron microscope operated at 300 kV (JEOL, Tokyo, Japan). The structural characterization was supplemented by infrared spectroscopy (IR). The experiment was performed with the use of the Nicolet iS50 ATR spectrometer (Thermo Fisher Scientific Instruments, Waltham, MA, USA), and the spectra were collected in attenuated total reflectance (ATR) configuration within the 4000–400 cm⁻¹ as well as 500–200 cm⁻¹ ranges (64 scans, 4 cm⁻¹ resolution).

The luminescence measurements were performed on a Photon Technology International (PTI) Quanta-Master 40 (QM40) UV/VIS Steady State Spectrofluorometer (Photon Technology International, Birmingham, NJ, USA) supplied with a tunable pulsed optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser (Opotek Opolette 355 LD, OPOTEK, Carlsbad, CA, USA). The laser system was coupled with a 75 W xenon lamp, a double 200 mm monochromator, and a multimode UV/VIS PMT (R928) (PTI Model 914) detector. The excitation and emission spectra were recorded with a resolution of 0.5 nm. The luminescence decay curves were recorded by a PTI ASOC-10 (USB-2500) oscilloscope. All structural and optical measurements were carried out at room temperature.

3. Results and Discussion

3.1. Thermal Behavior of Synthesized Xerogels

Figure 1 presents the TG/DSC curves recorded for fabricated xerogels in an inert gas atmosphere in a temperature range from 45 °C to 475 °C (the heating rate during measurement was 10 °C/min). According to TG curves, there are two distinguishable degradation steps for both fabricated samples: first, identified at 45–(~205) °C, and second, observed in the temperature range (~205) °C–(~320) °C. A slight weight loss, about 2.75% (XG_Tb) and 3.56% (XG_Tb/Eu), is associated with the elimination of residual solvents (ethyl alcohol, acetic acid) and water from the porous sol–gel host. At higher temperatures, strong exothermic peaks with maxima at 305 °C (XG_Tb) and 306 °C (XG_Tb/Eu) were identified, which appear along with ~17.55% weight loss. Generally, trifluoroacetates tend to decompose at temperatures near ~300 °C, which is well documented and described in the current literature [24,25,26]. Thus, recorded exothermic DSC peaks are clearly correlated with thermal decomposition of Ba(TFA)₂ and crystallization of BaF₂, which could be given by the chemical reaction:

$$\mathrm{Ba}(\mathrm{TFA}{)}_2\stackrel{\mathrm{T}}{\to }{\mathrm{BaF}}_2+{\mathrm{CF}}_3\mathrm{CFO}+{\mathrm{CO}}_2+\mathrm{CO}.$$

The thermolysis led to cleavage of C–F bonds from −CF₃ groups, and the resultant fluorine ions (F⁻) tend to react with Ba–O bonds, forming BaF₂ phase [27]. The heat exchanged during degradation of Ba(TFA)₂ in studied sol–gel materials is close to −118 J/g. J. Farjas et al. [28] pointed out that the denoted heat exchange during the degradation of trifluoroacetates depends on atmosphere (air or ambient gas) and the presence of vapored water. Our obtained value is comparable with DSC results obtained for pure Ba(TFA)₂ salt in argon atmosphere [28]. The data obtained from TG/DSC analysis for the studied sol–gel samples are shown in

Table 1. The parameters from TG analysis for studied sol–gel materials.

Sample	Number of Degradation Steps	Temperature Range (°C)	Weight Loss (%)
XGTb	1st	45–208	2.75
	2nd	208–322	17.56
XGTb/Eu	1st	45–204	3.56
	2nd	204–321	17.54

and

Table 2. The parameters from DCS curves recorded for fabricated silicate sol–gel samples.

Sample	Peak Maximum (°C)	Exchanged Heat (J/g)
XGTb	305	−118.3
XGTb/Eu	306	−117.9

.

3.2. Structural Characterization by XRD, TEM, and ATR-IR

Figure 2 presents the X-ray diffraction (XRD) patterns of the xerogels and nano-glass-ceramics fabricated at 350 °C. The diffractograms collected for Tb³⁺ singly doped samples are depicted in Figure 2a, meanwhile, the data for Tb³⁺, Eu³⁺ co-doped materials are shown in Figure 2b. The XRD patterns of the precursor xerogels revealed any sharp diffraction lines, but only a broad hump with a maximum at ~25°, indicating their amorphous nature without long-range order [29]. Conversely, the intense diffraction lines were observed only after thermal treatment of xerogels at 350 °C for 10 h. The XRD patterns of prepared glass-ceramics are in accordance with the standard diffraction lines of cubic BaF₂ crystallized in the Fm3m space group (ICDD card no. 00-004-0452), confirming the precipitation of fluoride crystals inside the silicate sol–gel matrix. The crystalline size of BaF₂ in fabricated glass-ceramics was evaluated by calculations with the Scherrer formula given below [30]:

$$\mathrm{D}=\frac{\mathrm{K}\mathsf{\lambda }}{\mathrm{B}\cos \mathsf{\theta }}$$

(1)

in which K is a shape factor (in our calculations, K = 1 was taken), λ is a wavelength of X-rays (0.154056 nm, Kα line of Cu), Β is a broadening of the diffraction peak at half the maximum intensity, and θ is Bragg’s angle. The average crystal sizes of BaF₂ were calculated to be 5 nm ± 0.1 nm for both nGC_Tb and nGC_Tb/Eu samples. The average size of BaF₂ nanocrystallites was also calculated from a Williamson–Hall plot as follows [31]:

$$\mathrm{D}=\frac{\mathrm{K}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}$$

(2)

where β is half of the width of the diffraction line, whereas (Δa/a) refers to the lattice deformation.

The Scherrer method makes the half-width of the diffraction line dependent only on the size of the crystallites. On the other hand, in the Williamson–Hall analysis, the internal stresses reflected by the lattice deformation are additionally taken into account in the broadening of the diffraction line. The mean crystal sizes, calculated with the Williamson–Hall method, were estimated to be 4.3 nm ± 0.1 nm for nGC_Tb, and 4.8 nm ± 0.1 nm for nGC_Tb/Eu. Moreover, the lattice deformation was negligible and less than 0.1%. The obtained results of the average crystallite size, from both methods, reveal good agreement. They proves the lack of internal stresses in the formed BaF₂ particles. The crystal lattice parameters for BaF₂ phase were determined to be 6.188 (8) Å (Tb³⁺-doped sample) and 6.169 (7) Å (Tb³⁺, Eu³⁺ co-doped sample), which are slightly smaller than the lattice parameter for undoped barium fluoride (a₀ = 6.2001 Å). Indeed, both Tb³⁺ (1.04 Å) and Eu³⁺ (1.07 Å) ions [32] with smaller ionic radii could substitute Ba²⁺ (1.35 Å) [33] cations in BaF₂ crystal lattice, resulting in a decrease in the unit cell volume. The indicated changes in the lattice parameter are also noticeable as a slight shift of the recorded diffraction lines towards higher values of the 2θ angle (an enlargement within a 22–28° angle, in which the (111) diffraction line was detected; shown in Figure 1). Additionally, it was observed that the shift of diffraction lines is more clearly visible for the nGC_Tb/Eu sample, which evidences that the total incorporation of RE³⁺ ions inside BaF₂ nanocrystals is higher than for nGC_Tb. Similar results from XRD measurements were described in the literature for other oxyfluoride optical systems, e.g., SiO₂-LaF₃:Er³⁺ sol–gel nano-glass-ceramics [34] and germano-gallate glass-ceramics containing BaF₂:Er³⁺ nanocrystals [11]. Figure 2c,d display the TEM images of prepared nano-glass-ceramic samples singly doped with Tb³⁺ and co-doped with Tb³⁺, Eu³⁺ ions, respectively. The size of BaF₂ nanocrystals was average, estimated from the Scherrer equation and the Williamson–Hall method.

The ATR-IR spectrum within the 4000–400 cm⁻¹ range for a representative XG_Tb/Eu sample is shown in Figure 3a, and the assignment of individual IR peaks was carried out based on the literature [35,36]. The recorded infrared signals confirmed the formation of a polycondensed silicate network created by Q² (949 cm⁻¹), Q³ (1045 cm⁻¹), and Q⁴ (1134 cm⁻¹) units of SiO₂ tetrahedrons as well as Si–O–Si siloxane bridges (1192 cm⁻¹, 801 cm⁻¹). On the other hand, the signals recorded at ~3665 cm⁻¹ and ~3400 cm⁻¹, according to vicinal/geminal and hydrogen-bonded Si–OH groups, respectively, clearly pointed to the presence of unreacted silanol groups. Indeed, xerogels are highly porous materials [37], hence, the IR bands originating from Si–OH groups are expected. Indeed, the next recorded infrared band, 1659 cm⁻¹, revealed the vibrations of Si–OH groups, but also oscillations within C=O carbonyl groups (from residual AcOH and unreacted TFA), as well as molecular water. A signal at ~3200 cm⁻¹ was interpreted as vibrations from hydrogen-bonded OH groups in organic compounds and water, and may confirm that pores inside the silicate network are filled with liquids. It should be noted that peaks located near ~1134 cm⁻¹ and ~1192 cm⁻¹ could be assigned, despite Q⁴ units and Si–O–Si bridges, to oscillations of C–F bonds in Ba(TFA)₂ and unreacted TFA. Indeed, a comparison of ATR-IR spectra in this region for XG_Tb/Eu xerogel and an analogous sample prepared without the addition of Ba(AcO)₂ and TFA revealed that the signals are more intense for XG_Tb/Eu (inset of Figure 3a). This could point to the presence of additional oscillators that contribute to overall signals recorded at ~1134 cm⁻¹ and ~1192 cm⁻¹. The signal recorded at ~420 cm⁻¹ was assigned to the O–Si–O bending vibration.

The ATR-IR spectrum within the 4000–400 cm⁻¹ region registered for a representative nGC_Tb/Eu sample obtained at 350 °C is shown in Figure 3b. Compared with the ATR-IR spectrum for XG_Tb/Eu, the intensities of signals at >3000 cm⁻¹ and 1659 cm⁻¹ weakened significantly, which allows us to make conclusions about evaporation of volatile chemical components from the sol–gel host and progressive reactions between unreacted Si–OH groups. Additionally, it was also observed that for the nGC_Tb/Eu nano-glass-ceramic sample, the intensity of the IR signal near ~949 cm⁻¹ is weaker than for the XG_Tb/Eu xerogel sample. An indicated effect may also suggest a continuation of polycondensation because Q² units probably transformed into Q³ and Q⁴ ones, which favor the creation of a more cross-linked sol–gel host. It was also observed that intensities of IR signals near ~1134 cm⁻¹ and ~1192 cm⁻¹ weakened compared with those recorded for the xerogel. This effect could be explained by thermal decomposition of Ba(TFA)₂ compound into BaF₂ crystals within the prepared silicate sol–gel host in the proposed heat-treatment conditions. Indeed, the Ba–F vibrations might be observed at lower frequencies (inset of Figure 3b), which agrees with the IR spectrum recorded for pure BaF₂ [38]. The peak with a maximum at ~440 cm⁻¹ was recorded for both nGC_Tb/Eu nano-glass-ceramics and an analogous sample prepared without the addition of Ba(AcO)₂ and TFA, confirming that such a band is not related to the fluoride fraction but to the oscillations within the silicate host (O–Si–O vibration). It should be noticed that this band shifts toward a higher frequency for nano-glass-ceramics in comparison with the xerogel. The reason for such spectral behavior could be explained by differences in the inter-tetrahedra angle of SiO₄ units in xerogels and glass-ceramics, as was stated in the literature [39].

3.3. Luminescence of Amorphous Silicate Xerogels

Figure 4a shows the excitation spectra of the prepared XG_Tb/Eu samples. The spectra were registered within the 340–520 nm spectral range on collecting the luminescence at 541 nm and 612 nm wavelengths. The excitation spectrum, while monitoring the green emission line at 541 nm, revealed the characteristic bands for Tb³⁺ ions according to the following transitions within the near-UV and VIS scope: ⁷F₆ → ⁵L₉ (352 nm), ⁷F₆ → ⁵L₁₀ (370 nm), ⁷F₆ → ⁵D₃ (379 nm), and ⁷F₆ → ⁵D₄ (488 nm). Meanwhile, the spectrum recorded by collecting the red luminescence at 612 nm showed the excitation lines of Eu³⁺ related to the electronic transitions from the ⁷F₀ ground level into the following excited states: ⁵G_J (376 nm), ⁵L₇ (384 nm), ⁵L₆ (394 nm), ⁵D₃ (418 nm), and ⁵D₂ (464 nm). However, it was observed that the spectrum recorded for XG_Tb/Eu contains some additional weak bands, which did not appear for the sample singly doped with Eu³⁺ (for better visibility, an enlargement of the 340–390 nm scope is presented in the inset of Figure 2a, and the bands are marked by asterisks). It should be noted that the recorded additional bands correspond to the contribution of excitation lines originating from Tb³⁺ ions (⁷F₆ → ⁵L₉ (352 nm), ⁷F₆ → ⁵L₁₀ (379 nm), and ⁷F₆ → ⁵D₄ (488 nm)). Moreover, a slight shift of the ⁷F₀ → ⁵L₇ band (from 384 nm to 382 nm) was also denoted, which could be related to its overlapping with the ⁷F₆ → ⁵D₃ excitation line originating from Tb³⁺ co-dopant. Hence, the obtained results could suggest the occurrence of Tb³⁺ → Eu³⁺ ET. A similar interpretation of excitation spectra was described for lead borate glasses co-doped with Tb³⁺ and Eu³⁺ ions [40].

The fluorescence spectra of prepared sol–gel specimens are displayed in Figure 4b. The emission spectrum recorded for the XG_Tb/Eu sample under excitation at 394 nm (presented as a red line) consisted of several emission lines at 574 nm (⁵D₀ → ⁷F₀), 590 nm (⁵D₀ → ⁷F₁), 612 nm (⁵D₀ → ⁷F₂), 648 nm (⁵D₀ → ⁷F₃), and 696 nm (⁵D₀ → ⁷F₄) within the reddish-orange light area. It was observed that the ⁵D₀ → ⁷F₂ red emission band is the most prominent luminescence line, and the spectrum is similar to other Eu³⁺-doped typical glassy-like optical materials described in the literature [41,42]. Based on the collected spectrum, the R/O ratio (red-to-orange) was calculated using the areas of the ⁵D₀ → ⁷F₂ (R) and the ⁵D₀ → ⁷F₁ (O) bands. The R/O ratio value estimated for precursor silicate xerogel is relatively high and equals 3.92. It indicates that Eu³⁺ ions are far from an inversion center, which is characteristic for amorphous materials. In the luminescence spectrum of the XG_Tb sample (marked as a green line), the bands centered at 486 nm, 541 nm, 580 nm, and 618 nm were attributed to the ⁵D₄ → ⁷F_J (J = 6–3) electronic transitions, respectively.

To verify the occurrence of ET between Tb³⁺ and Eu³⁺ ions in the studied silicate xerogels, the emission spectrum for the XG_Tb/Eu sample was recorded upon excitation at a 352 nm wavelength (shown as a blue line). The spectrum consisted of the following emission bands in the VIS spectral range: blue (486 nm), an intense green (541 nm), yellowish-orange (584 nm), and red (616 nm). The same bands within the blue–green light area were detected for the XG_Tb xerogel, and the mentioned emission lines were ascribed to the ⁵D₄ → ⁷F₆ and the ⁵D₄ → ⁷F₅ electronic transitions, respectively. Although the positions of these emission bands are the same, their intensity is slightly lower for the co-doped XG_Tb/Eu sample than for the singly doped XG_Tb one. Simultaneously, an increase in luminescence intensity within the yellowish-orange as well as red ranges was observed, and—compared with emissions recorded for the XG_Tb xerogel—the maxima of these bands were slightly shifted (from 580 nm to 584 nm, and from 618 nm to 616 nm). Thus, based on this observation, we could conclude that the indicated shift is a result of the superimposition of the yellow (⁵D₄ → ⁷F₄, 580 nm) and red band (⁵D₄ → ⁷F₃, 618 nm) of Tb³⁺ ions with orange (⁵D₀ → ⁷F₁, 590 nm) and red (⁵D₀ → ⁷F₂, 612 nm) luminescence originating from Eu³⁺.

Hence, our experimental results indicate the occurrence of Tb³⁺/Eu³⁺ ET upon excitation at a 352 nm wavelength when Tb³⁺ ions are excited from the ⁷F₆ ground state. Then, the electrons at the ⁵L₉ level decay rapidly through the ⁵G₅, ⁵L₁₀, and ⁵D₃ states by the multiphonon relaxation process until the ⁵D₄ level is populated. Since there is the energetical resemblance of the ⁵D₄ (Tb³⁺) and the ⁵D₁/⁵D₀ (Eu³⁺) levels, the Tb³⁺/Eu³⁺ energy migration is feasible, and the excitation energy is transferred from Tb³⁺ to the adjacent Eu³⁺ ion. The acceptor ions relax from the ⁵D₀ state to the ⁷F_J levels, promoting the light emission within the reddish-orange spectral region [43]. The ET is schematized in the level diagram presented in Figure 5.

The decay curves were registered for the green light at 541 nm, upon excitation at 352 nm from the near-UV range (inset in Figure 4b). For xerogels, a mono-exponential fit was used to evaluate the lifetimes of Tb³⁺, and the fitted curves are marked with a black line, while the collected experimental data are shown as green and blue lines for XG_Tb and XG_Tb/Eu, respectively. A slight shortening in the decay time of the ⁵D₄ (Tb³⁺) state from 1.11 ms (XG_Tb) to 0.88 ms (XG_Tb/Eu) was identified. An indicated decline in a lifetime for co-doped xerogel could be explained by introducing an additional decay pathway via Eu³⁺ ions. Indeed, the ET from Tb³⁺ to Eu³⁺ enhances the decay rate of the excited Tb³⁺ ions, resulting in the shortening of the ⁵D₄ (Tb³⁺) lifetime. Hence, the analysis of luminescence decay curves also enables calculation of the efficiency of Tb³⁺/Eu³⁺ ET, based on the following equation [44]:

$${\mathsf{\eta }}_{\mathrm{ET}}=\left(1-\frac{\mathsf{\tau }}{{\mathsf{\tau }}_0}\right)·100\%.$$

(3)

where τ₀ and τ are the lifetimes of the ⁵D₄ (Tb³⁺) state for sample singly doped with Tb³⁺, and the sample co-doped with Tb³⁺, Eu³⁺ ions, respectively. In the case of the studied xerogels, the efficiency of Tb³⁺/Eu³⁺ ET was estimated to be about 21%, and the comparable values were denoted for, e.g., fluoroborate glass (η_ET = 20%) [45].

3.4. Luminescence of SiO₂-BaF₂ Nano-Glass-Ceramics

The excitation spectra recorded for the nGC_Tb/Eu sample are shown in Figure 6a. The spectra emerged by monitoring the green luminescence characteristic for Tb³⁺ (541 nm), and the red emission originating from Eu³⁺ ions (612 nm). The luminescence of Tb³⁺ ions (541 nm) could be efficiently excited by the following wavelengths from the near-UV scope: 352 nm (⁷F₆ → ⁵L₉), 369 nm (⁷F₆ → ⁵L₁₀), and 377 nm (⁷F₆ → ⁵D₃), as well as from the VIS range: 485 nm (⁷F₆ → ⁵D₄). In the case of the excitation spectrum recorded at a 612 nm emission wavelength, an intense line appeared at 394 nm (⁷F₀ → ⁵L₆, Eu³⁺), but a few weaker bands at 376 nm (⁷F₀ → ⁵G_J), 384 nm (⁷F₀ → ⁵L₇), 418 nm (⁷F₀ → ⁵D₃), and 465 nm (⁷F₀ → ⁵D₂) were also detected. Similarly, as for xerogels, the recorded additional excitation lines at 352 nm, 369 nm, and 485 nm—marked in Figure 6a by asterisks—are typical for the ⁷F₆ → ⁵L_9,10, ⁵D₄ transitions of Tb³⁺ ions, which could suggest the occurrence of Tb³⁺/Eu³⁺ ET in the studied nano-glass-ceramic samples. Similar results were found for other Tb³⁺, Eu³⁺ co-doped fluoride-based optical systems, e.g., pure CaF₂ nanocrystals [46], and glass-ceramics containing SrF₂ [47], as well as NaYF₄ nanocrystals [48].

Figure 6b depicts the emission spectra collected for nGC_Tb/Eu and nGC_Tb samples, recorded upon excitation at 352 nm (a blue line for nGC_Tb/Eu, and a green line for nGC_Tb) and 394 nm (a red line) wavelengths. An excitation of the nGC_Tb sample using 352 nm results in registration of the visible emissions ascribed to the ⁵D₄ → ⁷F₆ (487 nm), ⁵D₄ → ⁷F₅ (541 nm), ⁵D₄ → ⁷F₄ (580 nm, 587 nm), and ⁵D₄ → ⁷F₃ (619 nm) transitions characteristic for Tb³⁺ ions. Subsequently, when the nGC_Tb/Eu co-doped sample was excited by a 394 nm wavelength, the luminescence bands originating from Eu³⁺ ions centered at 589 nm (⁵D₀ → ⁷F₁), 611 nm/614 nm (⁵D₀ → ⁷F₂), 648 nm (⁵D₀ → ⁷F₃), and 688 nm/696 nm (⁵D₀ → ⁷F₄) were observed. One can see that, in contrast to xerogel, the ⁵D₀ → ⁷F₁ magnetic dipole transition dominates the spectrum, which indicates that Eu³⁺ ions are placed at sites close to an inversion symmetry [49]. According to the calculated R/O ratio value (3.92 for XG_Tb/Eu and 0.51 for nGC_Tb/Eu) and the literature [18], the observed change in emission profile clearly suggests that Eu³⁺ ions tend to embed into the BaF₂ fluoride nanocrystal lattice by substituting Ba²⁺ cations. The decrease in the R/O ratio value was denoted for other Eu³⁺-doped oxyfluoride glass-ceramic systems described in the literature [50,51,52].

The emission spectrum of the nGC_Tb/Eu sample, collected upon 352 nm excitation, revealed an intense orange (589 nm) and red (611 nm/615 nm, and 647 nm) luminescence corresponding to the transitions of Eu³⁺ from the ⁵D₀ level. Along with those bands, two emission lines with relatively low intensity were found within the blue–green scope and were assigned to the emissions originating from the ⁵D₄ state of Tb³⁺ ions. Therefore, compared with XG_Tb/Eu, the luminescence in the reddish-orange spectral range is particularly enhanced for nGC_Tb/Eu. Based on this observation, we could conclude that the distance between interacting Tb³⁺ and Eu³⁺ ions in the prepared nano-glass-ceramics might be significantly shorter than in xerogels. Such shortening in the inter-ionic distance, strictly related to the segregation of rare earths inside BaF₂ nanocrystals precipitated at 350 °C, could be responsible for a more efficient transfer of excitation energy from Tb³⁺ to Eu³⁺ ions.

For the SiO₂-BaF₂ nano-glass-ceramics, the luminescence decay from the ⁵D₄ level follows a double-exponential function with two different decay lifetimes. It results from the distribution of RE³⁺ ions between either the sol–gel host (described by faster τ₁ component) and BaF₂ nanocrystals (described by longer τ₂ lifetime). The results are presented in the inset of Figure 6b, and the fitted decay curves are labeled with a black line, whereas the experimental data are tagged as green and blue lines for nGC_Tb and nGC_Tb/Eu, respectively. For the sample singly doped with Tb³⁺ ions, the lifetime components are equal to τ₁ = 2.51 ms and τ₂ = 6.97 ms, while for the sample co-doped with Tb³⁺, Eu³⁺ the decay times are equal to τ₁ = 1.05 ms and τ₂ = 4.53 ms. Based on lifetime components, the average decay times, τ_avg, were calculated from the following formula [53]:

$${\mathsf{\tau }}_{\mathrm{avg}}=\frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}.$$

(4)

Thus, the average luminescence lifetime of the ⁵D₄ (Tb³⁺) state for nGC_Tb/Eu was determined to be τ_avg = 4.06 ms, and for nGC_Tb it equaled τ_avg = 6.56 ms. The analysis of luminescence decay curves showed a noticeable prolongation in lifetimes for SiO₂-BaF₂ nano-glass-ceramics compared with xerogels. It suggests that the amount of OH groups characterized by high vibrational energy (>3000 cm⁻¹) should be significantly reduced in glass-ceramics. Moreover, the dopant ions tend to enter into the BaF₂ nanocrystals characterized by low phonon energy (~319 cm⁻¹ [54]), making the radiative relaxation from the ⁵D₄ level more prominent compared with xerogels.

Additionally, based on luminescence lifetimes, the calculated ET efficiency for prepared SiO₂-BaF₂ nano-glass-ceramic exceeds 38%. In such a case, the distance between interacting RE³⁺ ions entering into BaF₂ nanocrystals decreased, resulting in a reinforced transfer of energy absorbed by Tb³⁺ to Eu³⁺. Indeed, it is related to creating an energy transfer net among the donor and acceptor ions, causing the ET to become more frequent. Comparable values of ET efficiency were described for glass-ceramics containing YF₃:1Tb³⁺, 0.5Eu³⁺ (mol.%) nanophase (η_ET ≈ 39%) [55].

Summarizing, due to the unique properties of BaF₂, e.g., a broad region of transparency from 0.14 μm up to 14 μm, wide bandgap (11 eV), and low maximum phonon energy (~319 cm⁻¹), the oxyfluoride glass-ceramics containing BaF₂ nanophase are extensively applied to generate an efficient up- [11] and down-conversion luminescence [9], or white light emission [20]. Therefore, such materials could be successfully used for laser technologies, spectral converters, and three-dimensional displays [10]. Since Eu³⁺ ions emit within the red or reddish-orange light area, and Tb³⁺ ions are well known as green emitters, the fabricated SiO₂-BaF₂:Tb³⁺, Eu³⁺ nano-glass-ceramics are able to generate multicolor luminescence. Thus, sol–gel materials might be considered for use as optical elements in RGB lighting optoelectronic devices operating upon near-UV excitation.

4. Conclusions

This work presented the fabrication of Tb³⁺, Eu³⁺ co-doped oxyfluoride glass-ceramics at 350 °C from xerogels prepared via the sol–gel technique. The analysis of the thermal behavior of xerogels was performed using TG/DSC measurements, and the structural properties were determined based on ATR-IR spectroscopy. The crystallization of BaF₂ at the nanoscale was confirmed by XRD and TEM measurements. The characterization of sol–gel samples involved an excitation of the prepared sol–gel materials upon near-UV irradiation at 352 nm which showed the Tb³⁺/Eu³⁺ energy transfer, resulting in strengthening the luminescence within the reddish-orange light scope due to additional emission from Eu³⁺ ions. Nevertheless, for xerogels, the blue–green luminescence (⁵D₄ → ⁷F_5,6 of Tb³⁺) dominated, meanwhile, the reddish-orange emission (⁵D₀ → ⁷F_0–4 of Eu³⁺ overlapped with ⁵D₄ → ⁷F_4,3 bands of Tb³⁺) was particularly enhanced for SiO₂-BaF₂ nano-glass-ceramics. The luminescence decay kinetics showed that in the co-doped sol–gel materials, the energy transfer from Tb³⁺ to Eu³⁺ ions occurred with an efficiency that varied from 21% for xerogels to 38% for nano-glass-ceramics. An indicated increase in energy transfer efficiency for prepared nano-glass-ceramics could be explained by shortening the distance between interacting Tb³⁺ and Eu³⁺ ions embedded into the BaF₂ nanocrystal lattice. The obtained results suggest that the fabricated SiO₂-BaF₂:Tb³⁺, Eu³⁺ nano-glass-ceramics could be predisposed to application in selected technologies, e.g., three-dimensional displays and color screens.