Novel Sol-Gel Route to Prepare Eu3+-Doped 80SiO2-20NaGdF4 Oxyfluoride Glass-Ceramic for Photonic Device Applications

Oxyfluoride glass-ceramics (OxGCs) with the molar composition 80SiO2-20(1.5Eu3+: NaGdF4) were prepared with sol-gel following the “pre-crystallised nanoparticles route” with promising optical results. The preparation of 1.5 mol % Eu3+-doped NaGdF4 nanoparticles, named 1.5Eu3+: NaGdF4, was optimised and characterised using XRD, FTIR and HRTEM. The structural characterisation of 80SiO2-20(1.5Eu3+: NaGdF4) OxGCs prepared from these nanoparticles’ suspension was performed by XRD and FTIR revealing the presence of hexagonal and orthorhombic NaGdF4 crystalline phases. The optical properties of both nanoparticles’ phases and the related OxGCs were studied by measuring the emission and excitation spectra together with the lifetimes of the 5D0 state. The emission spectra obtained by exciting the Eu3+-O2− charge transfer band showed similar features in both cases corresponding the higher emission intensity to the 5D0→7F2 transition that indicates a non-centrosymmetric site for Eu3+ ions. Moreover, time-resolved fluorescence line-narrowed emission spectra were performed at a low temperature in OxGCs to obtain information about the site symmetry of Eu3+ in this matrix. The results show that this processing method is promising for preparing transparent OxGCs coatings for photonic applications.


Introduction
The development of transparent glass-ceramics (GC) has drawn the attention of numerous researchers, and they have been extensively studied due to their important optical applications [1]. GCs can be used for light-emitting diodes, solar cells, sensing catalysis or in biomedical materials [2,3]. Particularly, when fluoride nanocrystals smaller than 40 nm are dispersed in an oxide glass matrix, the resulting materials are transparent GCs, called oxyfluoride glass-ceramics (OxGCs), which have attractive properties [4]. These materials are transparent from the visible to near-infrared region and are compatible with new optical devices for communications or sensors. Furthermore, OxGCs retain the most relevant properties of glasses-good mechanical and thermal stability-and present properties characteristic of fluoride crystals as an effective optical media for light propagation and luminescence enhancement, and a high linear and nonlinear refractive index [5][6][7][8]. The crystal structure of fluorides as hosts of RE ions reduces the probabilities of multiphonon relaxation, resulting in high luminescence efficiencies [9][10][11][12]. Thus, the efficiency of the emission, optical transmission and spectra profile can be modified depending on the fluoride phase with the composition Nd 3+ -doped 80SiO 2 -20LaF 3 , obtaining a bulk-like lifetime value of 440 µs [33].
A key point of the "pre-crystallised nanoparticles route" is the processing of the fluoride nanoparticle suspensions. Some authors reported the preparation of NaGdF 4 nanoparticle suspensions [34][35][36][37]. In 2004, Mech et al. [38] stated for the first time the preparation via hydrothermal synthesis of Eu 3+ -doped NaGdF 4 and KGdF 4 nanoparticles [38,39]. They prepared the nanoparticles by mixing the rare-earth oxides in a hydrochloric acid solution with further incorporation of sodium fluoride. The results were auspicious, showing good optical properties, with sharp and well-defined peaks for Eu 3+ emissions through the excitation of Gd 3+ . Later, Sudheendra, L. et al. [40] described the preparation of Eu 3+doped NaGdF 4 using a sodium citrate solution, indicating that the Gd 3+ -Eu 3+ host-dopant couple is the best system for down-conversion after UV excitation as the emission energy transition within Gd 3+ can resonantly couple to the excited state of Eu 3+ ions. The authors demonstrated that in Eu 3+ -doped NaGdF 4 nanoparticles, the emission of the hexagonal phase is 25% more intense than those of the cubic structure. Although different papers reported the preparation of powdered NaGdF 4 nanoparticles via hydrothermal synthesis in an autoclave controlling the pressure and temperature [40][41][42], only a few papers have reported the use of this process to obtain aqueous nanoparticle suspensions to control the morphology of the nanoparticles.
The objective of this work was the preparation and characterisation of optically active oxyfluoride glass-ceramics through the incorporation of Eu 3+ -doped NaGdF 4 nanoparticle aqueous suspensions into a silica sol. In this work, powdered glass-ceramics were prepared, and the suitability of this processing method to prepare transparent OxGCs was demonstrated. This is a promising method for preparing transparent coatings and overcoming the drawbacks of the traditional melting-quenching method. Moreover, the selection of Eu 3+ ions as a dopant is due to its excellent properties as a local probe, providing important information on the environmental structure where it is located [43].

Synthesis of 1.5Eu 3+ : NaGdF 4 Aqueous Suspensions
Nanoparticle suspensions of NaGdF 4 undoped and doped with 1.5 mol. % Eu 3+ were prepared by mixing gadolinium nitrate (Gd (NO 3 ) 3 , Merk) with europium acetate (Eu(CH 3 CO 2 ) 3 , Merk) in a molar ratio of 1Gd(NO 3 ) 3 : 0.015Eu(CH 3 CO 2 ) 3 with 20 mL of water at room temperature to obtain a homogeneous solution. After 30 min of stirring, 1 g of sodium fluoride (NaF, Merk) was incorporated into the solution, before stirring for another 15 min at room temperature. The final solution was transferred to a Teflon cup and put into a stainless-steel autoclave, heated up to 180 • C and maintained for 20 h and 24 h. The suspensions were collected and used directly. The nanoparticles were labelled according to the reaction time: 1.5Eu 3+ : NaGdF 4 -20 or 1.5Eu 3+ : NaGdF 4 -24.

Synthesis of OxGCs with Composition 80SiO 2 -20(1.5Eu 3+ : NaGdF 4 )
OxGCs with the composition 80SiO 2 -20(1.5 Eu 3+ : NaGdF 4 ) were prepared following the "pre-crystallised nanoparticles route" [30]. First, tetraethyl orthosilicate (TEOS, Sigma Aldrich) and methyl-triethoxysilane (MTES, ABCR) were mixed with a molar ratio of 1TEOS:1MTES, followed by the incorporation of the previously prepared aqueous nanoparticle suspensions, 1.5Eu 3+ : NaGdF 4 -24, to reach a final molar relation of 80SiO 2 -20(1.5 Eu 3+ : NaGdF 4 ). After the incorporation of the nanoparticle suspensions, concentrated hydrochloric acid (HCl, Sigma Aldrich) was added under vigorous stirring to catalyse the hydrolysis and condensation reactions. The solution was immersed in an ice bath for 2 min to stop the reaction. After that, the sol was stirred for 15 min at room temperature. Finally, absolute ethanol was used to dilute the final sol up to a final concentration of 171 g L −1 .
2.3. Characterisation of 1.5Eu 3+ : NaGdF 4 Aqueous Suspensions 1.5Eu 3+ : NaGdF 4 -20 and 1.5Eu 3+ : NaGdF 4 -24 suspensions were centrifuged at 6000 rpm for 5 min, and the resulting powders were rinsed with deionised water; the rinsing process was repeated three times. The powders were dried at 75 • C overnight, then heat-treated at 450 • C and 550 • C for 5 h in an oven in an air atmosphere and further characterised.
X-ray diffractions were used to characterise the powders of both compositions Eu 3+ : NaGdF 4 -20 heat-treated at 450 • C for 5 h and Eu 3+ : NaGdF 4 -24 heat-treated at 450 • C and 550 • C for 5 h using an X-ray powder diffractometer (D8 advance, Bruker) with CuK α radiation (λ = 1.5406 A). The diffraction patterns were acquired in the range 10 • < 2θ< 70 • with a step of 0.03 • . The nano-crystalline sizes were calculated by using the Scherrer equation shown in Equation (1).
where D hkl is the calculated crystallite size, k = 0.94 for spherical crystals, θ is the Bragg angle, β is the full width of the diffraction peak at half maximum intensity (FWHM) and b is the correction of the instrument. The fits were performed using Origin software and the pseudo-Voigt function.
The size was also calculated using the Williamson-Hall (W-H) plot from which the strain broadening was estimated. To perform the W-H plot, the peak width was studied as a function of 2θ degree. The strain (E ) was calculated using Equation (2) for each peak of the XRD pattern. By using the calculated E and the D hkl taken from the Scherrer equation (Equation (1)) for all peaks, a linear regression was generated with its corresponding slope and y-intercept. Considering that the linear regression corresponds to Equation (3), the strain component was calculated from the slope and the crystallite size from the yintercept [33,34,44].
High-Resolution Transmission Electron Microscopy (HRTEM) was used to characterise 1.5Eu 3+ : NaGdF 4 NPs heat-treated at 450 • C for 5 h. The powder was re-dispersed in ethanol and then dripped onto a carbon-coated copper grid (Lacey Carbon, LC-200-Cu 25/pk). HRTEM images were taken with a HRTEM-JEO 2100 microscope, and the particle size distribution was determined through ImageJ ® software, using a maximum of 10 images. The lattice parameters of NaGdF 4 were determined using the Image-J source. The OxGCs with the composition 80SiO 2 -20(1.5 Eu 3+ : NaGdF 4 ) was dried at 75 • C overnight and then heat-treated at 450 • C for 5 h.
Structural properties of OxGCs powders were studied using X-ray diffraction (XRD) following the procedure described in Section 2.3. In addition, FTIR spectra of 80SiO 2 -20(1.5Eu 3+ : NaGdF 4 ) OxGCs before and after the heat treatment were also performed following the setup described in Section 2.3.

Optical Characterisation
Room temperature emission and excitation spectra, as well as luminescence decays of 1.5Eu 3+ : NaGdF 4 NPs and OxGCs with the composition 80SiO 2 -20(1.5Eu 3+ : NaGdF 4 ), with Nanomaterials 2023, 13, 940 5 of 16 both powders heat-treated at 450 • C for 5 h, were recorded by using a FS5 fluorescence spectrometer (Edinburg Instruments Ltd., UK) equipped with a 150 W xenon lamp. The lifetime value estimation was performed by fitting the exponential decay. The emission was detected using a Hamamatsu R928P photomultiplier.
The absolute photoluminescence quantum yield (PL QY) measurements were studied by using Hamamatsu Quantaurus-QY C11347-11, utilising an integrated sphere to measure all luminous flux. The excitation source was a 150 W xenon light source. The absolute photoluminescence quantum yield was measured under excitation wavelength λexc = 270 ± 10 nm, and the considered PL range for the QY calculation was from 400 nm to 700 nm. The procedure of each measurement was as follows: (i) measurement of a quartz Petri dish with cap, (ii) measurement of the quartz Petri disk containing the powder with cap, and (iii) calculation of the PL QY, following Equation (4).

PL QY =
Number of photons emitted as PL from the sample Number of photons absorbed by the sample (4) This measurement was repeated 10 times, and the final value of absolute photoluminescence quantum yield was obtained as the average of the 10 measurements.
Resonant time-resolved fluorescence line-narrowed (TRFLN) spectra of the 5 D 0 → 7 F 0,1,2 transitions of Eu 3+ were performed by exciting the OxGCs with the composition 80SiO 2 -20(1.5Eu 3+ : NaGdF 4 ) heat-treated at 450 • C for 5 h into the 7 F 0 → 5 D 0 transition with a pulsed frequency doubled Nd: YAG pumped tuneable dye laser of 9 ns pulsed width and 0.08 cm −1 linewidth and detected by an EGG&PAR Optical Multichannel Analyzer. The measurements were carried out by keeping the sample temperature at 9 K in a closed cycle helium cryostat.

Structural Characterisation of 1.5Eu 3+ : NaGdF 4 NPs
Stable aqueous suspensions of NaGdF 4 nanoparticles, undoped and doped with 1.5Eu 3+ , were successfully prepared at two reaction times (20 h and 24 h). Then, 1.5Eu 3+ : NaGdF 4 -20 and 1.5Eu 3+ : NaGdF 4 -24 nanoparticle suspensions were centrifuged and dried at 75 • C overnight to obtain the powders. Both powders were heat-treated at 450 • C for 5 h and characterised by X-ray diffraction in the range 10 • < 2θ< 70 • . The XRD diffractograms, shown in Figure 1a, confirm the crystallisation of NaGdF 4 in the hexagonal phase (JCPDS 28-1085) as the only appearing phase. Nevertheless, the definition of the peaks increases when increasing the reaction time from 20 to 24 h, suggesting bigger and better crystallised nanoparticles. The differences observed in the intensities of the peaks of XRD patterns are associated with a preferential orientation of the nanoparticles. This phenomenon can occur mainly due to the method of preparing the XRD samples. In the present work, the XRD samples were prepared by pressing the powder on a glass substrate with a small amount of Vaseline. Thus, the pressing procedure on the glass substrate could generate the preferential orientation of the nanoparticles. However, this procedure was chosen because the preparation of samples using other routes, such as putting the powder directly on the sample holder, could lead to the contamination of the XRD equipment [45].
The particle size, calculated using the Scherrer equation, increased from 30 nm to 70 nm for a reaction time of 20 to 24 h, respectively. He, F. et al. [46] described the preparation of hexagonal NaGdF 4 nanoparticles via hydrothermal synthesis where the crystal size increased from 170 nm to 1 µm when the reaction time increased from 2 h to 24 h. The growth of NaGdF 4 nanoparticles during hydrothermal synthesis is associated with a nucleation process followed by a crystal growth. Thus, the nucleation rate determines the growing crystal rate, and both processes are in competition [47].   Figure 2a shows nanoparticles with irregular shapes and sizes around 30 nm, around double of that calculated with the W-H plot, which is associated with the tendency of nanoparticles to agglomerate during synthesis and further centrifugation to obtain powders. The hexagonal shape is not clearly observed, probably due to the low amount of phase present in the nanoparticles. Wu, Y. et al. [48] reported the hydrothermal preparation of hexagonal NaGdF4 NPs using the same temperature and reaction times, observing that when NaF is used as a fluoride precursor, and for pH = 7, nanoparticles tend to have irregular shapes, as shown in this work. The size distribution is shown in Figure 2b, and a broad peak is observed with NP sizes between 20 nm and 42 nm with an average size of 34 nm. Figure 2c shows an amplified image of Figure 2a, revealing the existence of pores of around 2.5-3 nm inside the NP. The presence of pores can be attributed to a rapid formation of the nanoparticles followed by a long time for growing during which pores can be formed, as mentioned before [49]. The corresponding fast Fourier transform (FFT) pattern from the marked area in Figure 2d displays an interplanar distance of nearly 0.36 nm associated with the (001) plane of the NaGdF4 hexagonal phase, confirming the XRD results. To avoid a scattering process associated with large nanoparticle size or their agglomerations, a reaction time of 20 h was selected for the rest of the studies, to ensure particle sizes below 30 nm.
Furthermore, 1.5Eu 3+ : NaGdF 4 -20 nanoparticle suspension was sintered and heattreated at 450 • C or 550 • C for 5 h. Figure 1b shows the XRD pattern of 1.5Eu 3+ : NaGdF 4 -20 NPs heat-treated at 450 • C and 550 • C for 5 h. In the case of 1.5Eu 3+ : NaGdF 4 -20 nanoparticles heat-treated at 550 • C, a partial transformation of hexagonal to cubic (JCPDS 27-0697) and orthorhombic (JCPDS 33-1007) phases is observed, associated with the presence of peaks at 2θ = 31 • and 56 • , while only the hexagonal phase is detected at 450 • C. For this reason, a sintering condition of 450 • C for 5 h was chosen to ensure the presence of the hexagonal phase as unique. Figure 1c shows the W-H plot corresponding to the 1.5Eu 3+ : NaGdF 4 -20 and 1.5Eu 3+ : NaGdF 4 -24, both heat-treated at 450 • C for 5 h. The strain of the crystallites was calculated from the slope of each linear regression, and the strain decreases from 0.0986 to 0.0561 for 1.5Eu 3+ : NaGdF 4 -20 to 1.5Eu 3+ : NaGdF 4 -24, respectively. Moreover, the particle size, calculated from the y-intercept, increases from 16 nm for the sample 1.5Eu 3+ : NaGdF 4 -20 to 60 nm for 1.5Eu 3+ : NaGdF 4 -24. The fact that the crystal size calculated using the W-H plot for Eu 3+ : NaGdF 4 -20 is around half that calculated using the Scherrer equation likely indicates that a great part of the width of the XRD peaks corresponds to the intrinsic strain of the nanoparticles. By contrast, nanoparticles obtained with a high reaction time (1.5Eu 3+ : NaGdF 4 -24) present lower differences between the crystallite size calculated using the W-H and Scherrer equations, evidencing smaller strain contributions, probably due to nanoparticles with a bigger size and higher molecular order. Figure 2 shows the HR-TEM images corresponding to 1.5Eu 3+ : NaGdF 4 -20 nanoparticles heat-treated at 450 • C for 5 h. Figure 2a shows nanoparticles with irregular shapes and sizes around 30 nm, around double of that calculated with the W-H plot, which is associated with the tendency of nanoparticles to agglomerate during synthesis and further centrifugation to obtain powders. The hexagonal shape is not clearly observed, probably due to the low amount of phase present in the nanoparticles. Wu, Y. et al. [48] reported the hydrothermal preparation of hexagonal NaGdF 4 NPs using the same temperature and reaction times, observing that when NaF is used as a fluoride precursor, and for pH = 7, nanoparticles tend to have irregular shapes, as shown in this work. The size distribution is shown in Figure 2b, and a broad peak is observed with NP sizes between 20 nm and 42 nm with an average size of 34 nm. Figure 2c shows an amplified image of Figure 2a, revealing the existence of pores of around 2.5-3 nm inside the NP. The presence of pores can be attributed to a rapid formation of the nanoparticles followed by a long time for growing during which pores can be formed, as mentioned before [49]. The corresponding fast Fourier transform (FFT) pattern from the marked area in Figure 2d displays an interplanar distance of nearly 0.36 nm associated with the (001) plane of the NaGdF 4 hexagonal phase, confirming the XRD results. The structural characterisation of the 1.5Eu 3+ : NaGdF4 nanoparticles reveals that, even though 1.5Eu 3+ : NaGdF4-24 nanoparticles show a higher crystallisation degree, those synthesised for 20 h are more suitable for being incorporated into silica sol due to their smaller particle size. Moreover, a heat treatment of 450 °C for 5 h was chosen for these nanoparticles to avoid undesired crystallisation together with the hexagonal NaGdF4. For these reasons, 20 h was selected as the best reaction time to prepare NaGdF4 nanoparticle suspensions to carry out the further preparation of the OxGCs, and a heat treatment of 450 °C for 5 h as the best condition for sintering and performing the further characterisations.
3.2. Structural Characterisation of 80SiO2-20(1.5 Eu 3+ : NaGdF4) OxGCs Powders OxGCs with the composition 80SiO2-20(1.5Eu 3+ -NaGdF4) were prepared through the incorporation of 1.5Eu 3+ : NaGdF4-20 nanoparticles in aqueous suspension in the silica sol precursors. For XRD characterisation, the 80SiO2-20(1.5Eu 3+ : NaGdF4) sol was dried and The structural characterisation of the 1.5Eu 3+ : NaGdF 4 nanoparticles reveals that, even though 1.5Eu 3+ : NaGdF 4 -24 nanoparticles show a higher crystallisation degree, those synthesised for 20 h are more suitable for being incorporated into silica sol due to their smaller particle size. Moreover, a heat treatment of 450 • C for 5 h was chosen for these nanoparticles to avoid undesired crystallisation together with the hexagonal NaGdF 4 . For these reasons, 20 h was selected as the best reaction time to prepare NaGdF 4 nanoparticle suspensions to carry out the further preparation of the OxGCs, and a heat treatment of 450 • C for 5 h as the best condition for sintering and performing the further characterisations.

) OxGCs Powders
OxGCs with the composition 80SiO 2 -20(1.5Eu 3+ -NaGdF 4 ) were prepared through the incorporation of 1.5Eu 3+ : NaGdF 4 -20 nanoparticles in aqueous suspension in the silica sol precursors. For XRD characterisation, the 80SiO 2 -20(1.5Eu 3+ : NaGdF 4 ) sol was dried and heat-treated at 450 • C for 5 h. Figure 3 shows the presence of hexagonal (JCPDS 28-1085) and orthorhombic (JCPDS 33-1007) phases. The partial phase transformation can be attributed to the change in pH during the preparation of the sol, from pH 7 of NP suspension to pH 2 of the final sol. Some authors reported that the nanoparticles with the formula NaLnF 4 (Ln = Gd, Y, Li) are very sensitive to pH changes [46,50].  Figure 4a shows the FTIR spectra obtained for undoped and 1.5Eu 3+ -doped NaGdF4 nanoparticles heat-treated at 450 °C for 5 h (violet and blue line, respectively) and 80SiO2-20(1.5 Eu 3+ : NaGdF4) heat-treated at 450 °C for 5 h and dried at 75 °C overnight (red and green line, respectively) in the range of 4000 cm −1 to 400 cm −1 . In all cases, no clear peaks were detected in the range of 3000 cm −1 -4000 cm −1 , indicating that the materials were free of water (-OH bonds should appear around 3000-3500 cm −1 ). Figure 4b shows the amplified spectra between 1500 cm −1 and 400 cm −1 . In the 1.5Eu 3+ -doped NaGdF4 nanoparticles, at around 500 cm −1 , a rising peak is identified associated with the tetrafluoride bond vibration [51] and confirming the formation of NaGdF4 nanoparticles. This peak is not completely observed due to the range limitation of the equipment. A small peak at 807 cm −1 and a broad double peak around 1114 cm −1 are also observed, associated with traces of Gd-O bonds, indicating the presence of oxygen defects in the nanoparticle lattice [52,53]. These defects could be due to the reactions of acetates with nitrate precursors, since these peaks appear for both doped and undoped NP samples. These traces can affect the luminescence properties [54]. On the other hand, in the spectrum corresponding to the OxGC dried at 75 °C (green plot), a sharp band is identified at about 1260 cm −1 together with a small band at 600 cm −1 associated with Si-CH3 groups from the silica precursor. The band at 1260 cm −1 remains after the heat treatment (red plot), indicating the presence of Si-CH3 groups. In both OxGCs spectra (green and red plots), a broad band between 1200 and 1000 cm  Figure 4a shows the FTIR spectra obtained for undoped and 1.5Eu 3+ -doped NaGdF 4 nanoparticles heat-treated at 450 • C for 5 h (violet and blue line, respectively) and 80SiO 2 -20(1.5 Eu 3+ : NaGdF 4 ) heat-treated at 450 • C for 5 h and dried at 75 • C overnight (red and green line, respectively) in the range of 4000 cm −1 to 400 cm −1 . In all cases, no clear peaks were detected in the range of 3000 cm −1 -4000 cm −1 , indicating that the materials were free of water (-OH bonds should appear around 3000-3500 cm −1 ). Figure 4b shows the amplified spectra between 1500 cm −1 and 400 cm −1 . In the 1.5Eu 3+ -doped NaGdF 4 nanoparticles, at around 500 cm −1 , a rising peak is identified associated with the tetrafluoride bond vibration [51] and confirming the formation of NaGdF 4 nanoparticles. This peak is not completely observed due to the range limitation of the equipment. A small peak at 807 cm −1 and a broad double peak around 1114 cm −1 are also observed, associated with traces of Gd-O bonds, indicating the presence of oxygen defects in the nanoparticle lattice [52,53]. These defects could be due to the reactions of acetates with nitrate precursors, since these peaks appear for both doped and undoped NP samples. These traces can affect the luminescence properties [54]. On the other hand, in the spectrum corresponding to the OxGC dried at 75 • C (green plot), a sharp band is identified at about 1260 cm −1 together with a small band at 600 cm −1 associated with Si-CH 3 groups from the silica precursor. The band at 1260 cm −1 remains after the heat treatment (red plot), indicating the presence of Si-CH 3 groups. In both OxGCs spectra (green and red plots), a broad band between 1200 and 1000 cm −1 is assigned to Si-O-Si bonds, with overlapping bands such as the 1040 cm , which becomes better defined after the heat treatment of OxGC (red plot). Finally, in both OxGCs spectra, the band around 400 cm −1 associated with crystalline tetrafluoride, also identified in the NP spectrum (blue line), is difficult to assign due to the proximity of the silicate tetrahedrons band and the limitation of the equipment [13]. These results confirm the successful incorporation of the 1.5Eu 3+ : NaGdF4 nanoparticles into the silica sol as their presence was evidenced both in the XRD pattern and FTIR spectra. However, the partial crystal transformation that takes place during the formation of the sol, from hexagonal to orthorhombic, should affect the optical properties of the Ox-GCs due to a reduction in the crystal symmetry. Nevertheless, molecular characteristics of the nanoparticles, such as the oxygen defect revealed in the FTIR spectra, suggest that the hexagonal nanoparticles that remain keep their properties throughout the sol formation.
3.3. Luminescence Properties of 1.5Eu 3+ : NaGdF4 NPs Figure 5a shows the excitation spectrum of the heat-treated 1.5 Eu 3+ : NaGdF4 NPs recorded by monitoring the 5 D0→ 7 F2 emission of Eu 3+ at 615nm. The spectrum shows a broad band centred around 245 nm attributed to the Eu 3+ -O2-charge transfer band (CTB). The CTB reflects the accommodation of oxygen into a lattice of the NaGdF4 NPs, which has already been reported for fluoride nanoparticles prepared via wet chemistry [38], and it is in concordance with the FTIR spectra shown in Section 3.2, where oxygen bonds have been identified. In addition to the CTB, the spectrum shows a peak at 272 nm attributed to the 8 S7/2→ 6 IJ transition of Gd 3+ and the 7 F0→ 5 L6 peak of Eu 3+ at 395 nm. The presence of the Gd 3+ peak in the excitation spectrum obtained by monitoring the Eu 3+ luminescence confirms the Gd-Eu energy transfer. The other two small peaks at 423 and 501 nm are from the xenon lamp used for excitation. These results confirm the successful incorporation of the 1.5Eu 3+ : NaGdF 4 nanoparticles into the silica sol as their presence was evidenced both in the XRD pattern and FTIR spectra. However, the partial crystal transformation that takes place during the formation of the sol, from hexagonal to orthorhombic, should affect the optical properties of the OxGCs due to a reduction in the crystal symmetry. Nevertheless, molecular characteristics of the nanoparticles, such as the oxygen defect revealed in the FTIR spectra, suggest that the hexagonal nanoparticles that remain keep their properties throughout the sol formation. Figure 5a shows the excitation spectrum of the heat-treated 1.5 Eu 3+ : NaGdF 4 NPs recorded by monitoring the 5 D 0 → 7 F 2 emission of Eu 3+ at 615nm. The spectrum shows a broad band centred around 245 nm attributed to the Eu 3+ -O 2 -charge transfer band (CTB). The CTB reflects the accommodation of oxygen into a lattice of the NaGdF 4 NPs, which has already been reported for fluoride nanoparticles prepared via wet chemistry [38], and it is in concordance with the FTIR spectra shown in Section 3.2, where oxygen bonds have been identified. In addition to the CTB, the spectrum shows a peak at 272 nm attributed to the 8 S 7/2 → 6 I J transition of Gd 3+ and the 7 F 0 → 5 L 6 peak of Eu 3+ at 395 nm. The presence of the Gd 3+ peak in the excitation spectrum obtained by monitoring the Eu 3+ luminescence confirms the Gd-Eu energy transfer. The other two small peaks at 423 and 501 nm are from the xenon lamp used for excitation. anomaterials 2023, 13, x FOR PEER REVIEW 11 of 18 The emission spectra of the 1.5 Eu 3+ : NaGdF4 NPs, shown in Figure 5b, were obtained under excitation at 245 nm (CTB) and 272 nm ( 8 S7/2→ 6 IJ) (Gd 3+ ), respectively. The spectra show the Gd 3+ emission ( 6 P0→ 8 S7/2) around 311 nm together with the Eu 3+ emissions. The Gd 3+ emission appears superimposed to a weak broad band centred at around 360 nm, likely associated with the 4f 6 5d-4f 7 transition of Eu 2+ . For both excitations, the main emissions correspond to the 5 D0→ 7 FJ transitions of Eu 3+ . A small contribution of the 5 D1,2,3→ 7 FJ transition is also observed. The observed wavelength emissions, shown in Table 1, are similar to those published in the literature for other fluoride phases, such as GdF3 or NaLaF4. Moreover, the emission spectrum obtained after excitation through the charge transfer band (λexc = 245 nm) is much higher in intensity than that obtained after Gd 3+ excitation at 272 nm, in accordance with the most efficient energy transfer.

Luminescence Properties of 1.5Eu 3+ : NaGdF 4 NPs
The most intense peak of the spectra corresponds to the electric dipole 5 D0→ 7 F2 transition, indicating that Eu 3+ ions are in a non-centrosymmetric site; the intensity of this hypersensitive transition is strongly affected by the local field [55]. The asymmetry ratio, R, defined as the ratio of the 5 D0→ 7 F2 and 5 D0→ 7 F1 emission intensities, was calculated to describe this behaviour. The lower this value, the closer the local symmetry is to the one with an inversion centre [55]. The obtained value of 1.46 confirms the non-centrosymmetric site, which can be attributed to the substitution of a F − ion by an O 2− ion in the coordination environment of an Eu 3+ which lowers the site symmetry of the Eu 3+ in the oxygenfree NaGdF4 crystal [54].
In addition, the quantum yield calculated under excitation at λexc = 270 nm for the 1.5 Eu 3+ : NaGdF4 NPs gives a value of 43.1%. The quantum yield of Eu 3+ -doped fluorides with Gd 3+ ions has been calculated by many authors, most of them reporting a theoretical value of 200% due to the energy transfer between Gd 3+ and Eu 3+ with exciting Gd 3+ [56][57][58]. The low emission efficiency after Gd 3+ excitation, evidenced through the quantum yield value, could be due to the presence of oxygen impurities in the NaGdF4 nanoparticles, as it is known that even a small amount of oxygen in the vicinity of Eu 3+ ions can lead to a reduction in the photon emission [9].
The experimental decay curve from the 5 D0 level was obtained by exciting at 245 nm through the CTB and collecting the luminescence of the 5 D0→ 7 F2 transition at 615 nm. The decay curve, displayed in Figure 6, shows a rise time of 0.44 ms, due to the population of the 5 D0 level from higher energy levels, followed by a single exponential decay with a lifetime of 9.6 ms. This lifetime value is longer than some of those reported for hexagonal nanoparticles with the composition Eu 3+ : NaYF4 or Eu 3+ : NaGdF4 [59][60][61] and is similar to that found by Karbowiak et al. for Eu 3+ : NaGdF4 nanocrystals [62].  The emission spectra of the 1.5 Eu 3+ : NaGdF 4 NPs, shown in Figure 5b, were obtained under excitation at 245 nm (CTB) and 272 nm ( 8 S 7/2 → 6 I J ) (Gd 3+ ), respectively. The spectra show the Gd 3+ emission ( 6 P 0 → 8 S 7/2 ) around 311 nm together with the Eu 3+ emissions. The Gd 3+ emission appears superimposed to a weak broad band centred at around 360 nm, likely associated with the 4f 6 5d-4f 7 transition of Eu 2+ . For both excitations, the main emissions correspond to the 5 D 0 → 7 F J transitions of Eu 3+ . A small contribution of the 5 D 1,2,3 → 7 F J transition is also observed. The observed wavelength emissions, shown in Table 1, are similar to those published in the literature for other fluoride phases, such as GdF 3 or NaLaF 4 . Moreover, the emission spectrum obtained after excitation through the charge transfer band (λ exc = 245 nm) is much higher in intensity than that obtained after Gd 3+ excitation at 272 nm, in accordance with the most efficient energy transfer.
The most intense peak of the spectra corresponds to the electric dipole 5 D 0 → 7 F 2 transition, indicating that Eu 3+ ions are in a non-centrosymmetric site; the intensity of this hypersensitive transition is strongly affected by the local field [55]. The asymmetry ratio, R, defined as the ratio of the 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 emission intensities, was calculated to describe this behaviour. The lower this value, the closer the local symmetry is to the one with an inversion centre [55]. The obtained value of 1.46 confirms the non-centrosymmetric site, which can be attributed to the substitution of a F − ion by an O 2− ion in the coordination environment of an Eu 3+ which lowers the site symmetry of the Eu 3+ in the oxygen-free NaGdF 4 crystal [54].
In addition, the quantum yield calculated under excitation at λ exc = 270 nm for the 1.5 Eu 3+ : NaGdF 4 NPs gives a value of 43.1%. The quantum yield of Eu 3+ -doped fluorides with Gd 3+ ions has been calculated by many authors, most of them reporting a theoretical value of 200% due to the energy transfer between Gd 3+ and Eu 3+ with exciting Gd 3+ [56][57][58]. The low emission efficiency after Gd 3+ excitation, evidenced through the quantum yield value, could be due to the presence of oxygen impurities in the NaGdF 4 nanoparticles, as it is known that even a small amount of oxygen in the vicinity of Eu 3+ ions can lead to a reduction in the photon emission [9].
The experimental decay curve from the 5 D 0 level was obtained by exciting at 245 nm through the CTB and collecting the luminescence of the 5 D 0 → 7 F 2 transition at 615 nm. The decay curve, displayed in Figure 6, shows a rise time of 0.44 ms, due to the population of the 5 D 0 level from higher energy levels, followed by a single exponential decay with a lifetime of 9.6 ms. This lifetime value is longer than some of those reported for hexagonal nanoparticles with the composition Eu 3+ : NaYF 4 or Eu 3+ : NaGdF 4 [59][60][61] and is similar to that found by Karbowiak et al. for Eu 3+ : NaGdF 4 nanocrystals [62]. The luminescent properties of the OxGCs powders with the composition 80SiO2-20(1.5Eu 3+ -20NaGdF4) heat-treated at 450 °C for 5 h are presented in Figure 7a, which shows the emission spectrum recorded under excitation at 245 nm. The spectral features are similar to those obtained for 1.5Eu 3+ : NaGdF4 NPs, with the peaks revealing the transition 6 P0→ 8 S7/2 of Gd 3+ and the 5 D0→ 7 FJ transition from the Eu 3+ ion. In addition to the nanoparticles study, the most intense peak was found to be corresponding to the 5 D0→ 7 F2 transition, in agreement with a non-centrosymmetric site for the Eu 3+ ion. In this case, the calculated R-value is also 1.46, which is similar to that of the NPs. Furthermore, the wavelength of the emissions is the same as that detected for the nanoparticles reported in the previous section. Figure 7b shows the luminescence decay curve from the 5 D0 level recorded while collecting the emission at 615 nm and exciting the CTB at 245 nm. The wavelength emissions are listed in Table 1. The lifetime value obtained was 7.9 ms, which was lower than the one obtained for the nanoparticles (9.6 ms). The decrease in the lifetime value could be due to the presence of the remaining organics in the silica matrix from the sol-gel precursor, as shown in the FTIR spectra of Section 3.2, where a peak of CH3 from the MTES was identified. Moreover, the rising time was 0.4 ms, which was similar to that from the NPs.

Luminescence Properties of 80SiO 2 -20(1.5Eu 3+ NaGdF 4 ) OxGCs
The luminescent properties of the OxGCs powders with the composition 80SiO 2 -20(1.5Eu 3+ -20NaGdF 4 ) heat-treated at 450 • C for 5 h are presented in Figure 7a, which shows the emission spectrum recorded under excitation at 245 nm. The spectral features are similar to those obtained for 1.5Eu 3+ : NaGdF 4 NPs, with the peaks revealing the transition 6 P 0 → 8 S 7/2 of Gd 3+ and the 5 D 0 → 7 F J transition from the Eu 3+ ion. In addition to the nanoparticles study, the most intense peak was found to be corresponding to the 5 D 0 → 7 F 2 transition, in agreement with a non-centrosymmetric site for the Eu 3+ ion. In this case, the calculated Rvalue is also 1.46, which is similar to that of the NPs. Furthermore, the wavelength of the emissions is the same as that detected for the nanoparticles reported in the previous section. The luminescent properties of the OxGCs powders with the composition 80SiO2-20(1.5Eu 3+ -20NaGdF4) heat-treated at 450 °C for 5 h are presented in Figure 7a, which shows the emission spectrum recorded under excitation at 245 nm. The spectral features are similar to those obtained for 1.5Eu 3+ : NaGdF4 NPs, with the peaks revealing the transition 6 P0→ 8 S7/2 of Gd 3+ and the 5 D0→ 7 FJ transition from the Eu 3+ ion. In addition to the nanoparticles study, the most intense peak was found to be corresponding to the 5 D0→ 7 F2 transition, in agreement with a non-centrosymmetric site for the Eu 3+ ion. In this case, the calculated R-value is also 1.46, which is similar to that of the NPs. Furthermore, the wavelength of the emissions is the same as that detected for the nanoparticles reported in the previous section. Figure 7b shows the luminescence decay curve from the 5 D0 level recorded while collecting the emission at 615 nm and exciting the CTB at 245 nm. The wavelength emissions are listed in Table 1. The lifetime value obtained was 7.9 ms, which was lower than the one obtained for the nanoparticles (9.6 ms). The decrease in the lifetime value could be due to the presence of the remaining organics in the silica matrix from the sol-gel precursor, as shown in the FTIR spectra of Section 3.2, where a peak of CH3 from the MTES was identified. Moreover, the rising time was 0.4 ms, which was similar to that from the NPs.    Table 1. The lifetime value obtained was 7.9 ms, which was lower than the one obtained for the nanoparticles (9.6 ms). The decrease in the lifetime value could be due to the presence of the remaining organics in the silica matrix from the sol-gel precursor, as shown in the FTIR spectra of Section 3.2, where a peak of CH 3 from the MTES was identified. Moreover, the rising time was 0.4 ms, which was similar to that from the NPs. As mentioned before, the optical properties of trivalent Eu are highly sensitive to the local environment. Since the 5 D 0 state is non-degenerative under any symmetry, the structure of the 5 D 0 → 7 F J emissions is only determined by the splitting of the terminal levels caused by the local crystal field. Moreover, as the 7 F 0 level is also non-degenerative, site-selective excitation within the 7 F 0 → 5 D 0 absorption band can be performed using fluorescence line narrowing (FLN) spectroscopy, useful for distinguishing between different local environments around the Eu 3+ .
Time-resolved fluorescence line-narrowed (TRFLN) spectra of the 5 D 0 → 7 F 0-2 transitions of 80SiO 2 -20(1.5Eu 3+ : NaGdF 4 ) powders heat-treated at 450 • C for 5 h were obtained at 9 K using different resonant excitation wavelengths throughout the 7 F 0 → 5 D 0 transition with a time delay of 10 µs. Depending on the excitation wavelength, the emission spectra present different characteristics, mainly related to the relative intensity and the level splitting of the transitions. Figure 8 shows the low temperature (9 K) 5 D 0 → 7 F 0-2 emission spectra, obtained under excitation at 578.6 nm and 579 nm, both with a time delay of 10 µs after the laser pulse. These excitation wavelengths correspond to the highest emission intensities, with the spectrum obtained at 578.6 nm being more intense. Under excitation at 578.6 nm (Figure 8a), the 5 D 0 → 7 F 1 transition displays three stark components, which is in agreement with an Eu 3+ occupying a site with orthorhombic symmetry, point group C 2v , or lower [68]. The spectrum obtained under 579 nm excitation (Figure 8b) shows two components for the 5 D 0 → 7 F 1 transition. The presence of two components for the splitting of the 7 F 1 level and three for the 7 F 2 level is compatible with trigonal site symmetry (C 3, C 3v ) for Eu 3+ ions. It is well known that the expected space group for β-NaGdF 4 is P6 where Gd 3+ occupy point symmetry C 3h sites [38]. In this point symmetry, the 5 D 0 → 7 F 0 transition is forbidden and the number of lines for the 5 D 0 → 7 F 1,2 transitions are two and one, respectively [68]. However, the spectrum in Figure 8b shows the presence of the resonant 5 D 0 → 7 F 0 line as well as two and three components for the 5 D 0 → 7 F 1,2 emissions, respectively. This implies that a breaking symmetry occurs produced by lattice distortions which may reduce the original C 3h symmetry to C 3 according to the branching rules of the 32 point groups [69]. The presence of two distinguishable sites for Eu 3+ ions is, therefore, compatible with the observed hexagonal and orthorhombic phases detected in the XRD patterns of the OxGCs powders. C3h symmetry to C3 according to the branching rules of the 32 point groups [69]. The presence of two distinguishable sites for Eu 3+ ions is, therefore, compatible with the observed hexagonal and orthorhombic phases detected in the XRD patterns of the OxGCs powders.
Nanoparticle synthesis is described as a process consisting of a first nucleation step, followed by a competition with the crystal growth.
XRD confirmed that hexagonal NaGdF4 is the only present phase after sintering at 450 °C for 5 h.
In addition, nanoparticles present pores, likely due to the long synthesis process during which the crystal growth needs a much longer time than the nucleation step.
Although it is still possible to fit synthesis parameters to obtain better crystallised and defect-free hexagonal NaGdF4 nanoparticles, these results are promising, opening a new route to prepare NaGdF4 nanoparticle aqueous suspensions without any addition of organic dispersant.
It was confirmed that the acid media in which the OxGCs were prepared affects the stability of NaGdF4, generating a partial crystal transformation from hexagonal to orthorhombic.
Emission and excitation spectra together with the lifetimes of the 5 D0 state were measured for NPs and OxGCs with 1.5% Eu 3+ . The excitation spectra showed the Eu 3+ -O 2charge transfer band in accordance with the presence of oxygen in the lattice of the NPs. The emission spectra obtained by exciting the charge transfer band show similar features in both NPs and OxGCs, corresponding the higher emission intensity to the 5 D0→ 7 F2 transition which indicates a non-centrosymmetric site for Eu 3+ . The reduction in the lifetime value of the 5 D0 level from 9.6 ms in the NPs to 7.9 ms in OxGCs could be attributed to the presence of the remaining organics in the silica matrix. TRFLN spectra obtained under selective excitation in the 7 F0→ 5 D0 absorption band at 9 K allow us to identify the existence of two distinguishable sites for Eu 3+ ions in the OxGCs powders with C2v and C3 symmetries, which are compatible with the observed hexagonal and orthorhombic phases detected in the XRD diffraction patterns.
Nanoparticle synthesis is described as a process consisting of a first nucleation step, followed by a competition with the crystal growth.
XRD confirmed that hexagonal NaGdF 4 is the only present phase after sintering at 450 • C for 5 h.
In addition, nanoparticles present pores, likely due to the long synthesis process during which the crystal growth needs a much longer time than the nucleation step.
Although it is still possible to fit synthesis parameters to obtain better crystallised and defect-free hexagonal NaGdF 4 nanoparticles, these results are promising, opening a new route to prepare NaGdF 4 nanoparticle aqueous suspensions without any addition of organic dispersant.
It was confirmed that the acid media in which the OxGCs were prepared affects the stability of NaGdF 4, generating a partial crystal transformation from hexagonal to orthorhombic.
Emission and excitation spectra together with the lifetimes of the 5 D 0 state were measured for NPs and OxGCs with 1.5% Eu 3+ . The excitation spectra showed the Eu 3+ -O 2− charge transfer band in accordance with the presence of oxygen in the lattice of the NPs. The emission spectra obtained by exciting the charge transfer band show similar features in both NPs and OxGCs, corresponding the higher emission intensity to the 5 D 0 → 7 F 2 transition which indicates a non-centrosymmetric site for Eu 3+ . The reduction in the lifetime value of the 5 D 0 level from 9.6 ms in the NPs to 7.9 ms in OxGCs could be attributed to the presence of the remaining organics in the silica matrix. TRFLN spectra obtained under selective excitation in the 7 F 0 → 5 D 0 absorption band at 9 K allow us to identify the existence of two distinguishable sites for Eu 3+ ions in the OxGCs powders with C 2v and C 3 symmetries, which are compatible with the observed hexagonal and orthorhombic phases detected in the XRD diffraction patterns.