A New Ultrafine Luminescent La2O3:Eu3+ Aerogel

This paper reports on the synthesis and characterization of La2O3:Eu3+ luminescent aerogels fabricated by the sol–gel method and the supercritical drying technique. The % mol concentration of the Eu3+ ion was varied to study the effects on the luminescent properties of the aerogels. XRD and TEM analysis showed that the La2O3:Eu3+ aerogels exhibited a semi-crystalline behavior regardless of whether the concentration of europium was increased. SEM micrographs revealed a porous structure in the aerogels, which were composed of quasi-spherical nanoparticles that were interconnected and formed coral-shaped agglomerates. Photoluminescence spectroscopy characterization showed that the aerogels had an infrared emission located at λ = 793 nm, and the maximum photoluminescence emission intensity was observed for the aerogel with 50% Eu3+. The results demonstrate that, without heat treatment, it is possible to manufacture luminescent aerogels of rare-earth oxides that can be used in opto-electronic devices.


Introduction
Due to the properties of aerogels, such as a large surface area and small particle size [1,2], in addition to the fact that they can be made from metal oxides (for example, rare-earth oxides) [3], they have aroused great interest for use in photoluminescent applications [4,5], mainly as nanophosphors. Phosphors (inorganic luminescent materials) have been extensively studied for their use in displays and are currently used in high-definition television displays (HDTV), plasma displays (PDP), cathode ray tubes (CRT), and field emission displays (FED) [6,7]. As regards the synthesis of phosphors, three groups of variables are studied and controlled for: (a) morphology and particle size, (b) stoichiometry and composition, and (c) surface chemistry [8,9]. Because phosphors can be used in high-resolution applications, nanometric particles with a spherical morphology and homogeneous composition are highly prized [10,11]. Shea et al. [12] observed that the luminescence intensity of Y 2 O 3 :Eu 3+ phosphors increased if the reaction temperature was increased, which in turn led to an increase in the crystallite size of the material. Jung et al. [13] found that Y 2 O 3 :Eu 3+ particles with a small surface area showed higher luminescence than those with a large surface area and that the luminescence intensity was directly proportional to the crystallite size. This trend has also been observed in other phosphors, such as Gd 2 O 3 :Eu 3+ [11]. Wang et al. [9] demonstrated that a spherical morphology is required if one wishes to improve the emission intensity of phosphors prepared by the spray pyrolysis route. On the other hand, Zhang et al. [14] were able to synthesize Gd 2 O 3 aerogels using the sol-gel method and the CO 2 supercritical drying technique to obtain transparent aerogels. The XRD results showed that the aerogels were amorphous, and the nitrogen adsorption/desorption analysis showed a surface area of 223 m 2 /g, an average pore diameter of 42 nm, and a large pore volume of 1.83 mL/g. Similarly, Worsley et al. [15] Gels 2023, 9, 615 2 of 11 fabricated chlorine-free rare earth oxide aerogels from the lanthanide series using a modified epoxide-assisted sol-gel method and a CO 2 critical point dryer. All the aerogels were amorphous but became nanocrystalline after calcination at 923 K in air. The aerogels had high surface areas (up to 150 m 2 /g) and low densities (40-225 mg/cm 3 ), and the Eu 2 O 3 , Tb 2 O 3 , Sm 2 O 3 , and Nd 2 O 3 aerogels were photoluminescent. Cabrera et al. [16] synthesized TTA/Er 2 O 3 /Eu 2 O 3 aerogels using the sol-gel method and supercritical drying with CO 2 . The SEM analysis showed that the TTA/Er 2 O 3 /Eu 2 O 3 aerogels consisted of agglomerates of irregularly shaped particles ranging from 100 nm to 1 µm in size. These aerogels showed emissions when excited at λ = 613 nm.
A wide variety of methods are employed in the synthesis of phosphors, such as the hydrothermal method [17], spray pyrolysis [18], precipitation [19], and the sol-gel method, with a variation of the sol-gel method known as epoxide-assisted gelation [20], and, along with a low-temperature supercritical drying technique [3], aerogels can be made. One of the advantages of the sol-gel method is that it is possible to control the size and morphology of the particles, as well as their chemical composition [21], so that it is feasible to manufacture a luminescent rare earth oxide aerogel that is a nanophosphor.
Among luminescent materials, doped rare earth phosphors-for example-phosphors doped with Eu 3+ ions, which emit a red color, are of technological importance because they are widely used in color displays and fluorescent lamps [6][7][8][9][10][11]. Another example, Eu 3+doped yttrium oxide (Y 2 O 3 :Eu 3+ ), is considered one of the best red phosphors currently available [22]. One of the most common ions used in luminescent applications is the Eu ion, mainly because its spectroscopy is very well described. In its II and III oxidation states, this element produces blue (448 nm) light emissions in the case of Eu 2+ and red (613 nm) in the case of Eu 3+ [23]. These properties make europium compounds suitable for use in many devices and fields of application, for example, in optical fibers [24], photo-storage [25], lasers [26], biological markers [27], and inorganic light-emitting diodes (OLED) [28]. Both cations (Eu 2+ and Eu 3+ ) have a weak luminescence intensity, although it has been observed that this intensity increases when the particles are nano-sized [11]. One way to increase the light intensity of these ions is by inserting them into inorganic compounds (such as La 2 O 3 ), which promotes a transfer of energy to the metal ions and a breaking up of their structural symmetry, leading to an increase in emission intensity [29]. Also, lanthanum oxide has numerous industrial and technological applications [30,31], including use in luminescent displays and light-emitting diodes (LED) [32], and La 3+ ions can be easily replaced with luminescence-active Ln 3+ ions over a wide range of concentrations [33], due to their ionic radius, electronegativity [34], and electron structure [35], much like the other lanthanide ions. La 2 O 3 is recognized as an excellent host material for lanthanides in luminescence-related applications [36], and compared to other lanthanide host matrices, such as Gd 2 O 3 and Lu 2 O 3 , it is cheaper and, therefore, lends itself to more industrial and technological applications [37,38].
In this study, a series of novel luminescent La 2 O 3 aerogels doped with Eu 3+ were synthesized by the sol-gel method via epoxide-assisted gelation and the low-temperature supercritical drying technique (with CO 2 ). To obtain the highest luminescence intensity, the concentrations of the Eu 3+ ion were varied at 2%, 5%, 8%, 10%, 20%, 30%, 40%, and 50% mol within the La 2 O 3 host matrix. This could be performed because the spectroscopic properties of the Eu 3+ ion are well known. The aerogels were characterized by XRD, TEM, SEM, FT-IR, and PL analyses to determine their spectroscopical, structural, and morphological properties. Photoluminescent emission (PL) characterization was performed at room temperature using an excitation of 394 nm. Figure 1 shows the X-ray diffraction (XRD) patterns of the La 2 O 3 aerogels. It can be seen that the aerogels are amorphous. However, a slight peak was observed around 43 • , so a HRTEM analysis was performed.  Figure 1 shows the X-ray diffraction (XRD) patterns of the La2O3 aerogels. It can be seen that the aerogels are amorphous. However, a slight peak was observed around 43°, so a HRTEM analysis was performed.  Figure 2a shows the HRTEM image for the 50% Eu 3+ aerogel, whose lattice fringes show the crystallographic plane [002] with interplanar spacing of 0.321 nm, according to the ICDD 01-083-13-54 file of La2O3 with a hexagonal structure. It can be seen that the aerogels exhibit semi-crystalline behavior. A material can become crystalline if the temperature and pressure conditions are adequate, so the semi-crystalline nature of the aerogels is likely due to the pressure used in the supercritical drying. Figure 2b, an image edited with DigitalMicrograph software, shows an ordered region made up of multiple hexagons that repeat and form larger hexagons, something never seen before in an aerogel. Figure 3 shows SEM images of the 50% Eu 3+ monolithic aerogel. The micrographs of the La2O3 aerogel with 50% Eu 3+ reveal a non-ordered porous material composed of nanoparticles that form coral-shaped agglomerates; these nanoparticles are of varied sizes and are interconnected, forming a 3D network.  Figure 2a shows the HRTEM image for the 50% Eu 3+ aerogel, whose lattice fringes show the crystallographic plane [002] with interplanar spacing of 0.321 nm, according to the ICDD 01-083-13-54 file of La 2 O 3 with a hexagonal structure. It can be seen that the aerogels exhibit semi-crystalline behavior. A material can become crystalline if the temperature and pressure conditions are adequate, so the semi-crystalline nature of the aerogels is likely due to the pressure used in the supercritical drying. Figure 2b, an image edited with DigitalMicrograph software, shows an ordered region made up of multiple hexagons that repeat and form larger hexagons, something never seen before in an aerogel.   Figure 3 shows SEM images of the 50% Eu 3+ monolithic aerogel. The micrographs of the La 2 O 3 aerogel with 50% Eu 3+ reveal a non-ordered porous material composed of nanoparticles that form coral-shaped agglomerates; these nanoparticles are of varied sizes and are interconnected, forming a 3D network. The nitrogen adsorption/desorption technique was used to measure the pore volume, the average pore diameter, and the surface area of the La2O3:Eu 3+ aerogel. According to Brunauer et al., it is a type IV isotherm with a H4 hysteresis loop [14,39]. The results are shown in Figure 4. Its surface area is 109.235 m 2 ·g −1 , as the average pore diameter and pore volume are 31 nm and 0.750 mL·g −1 , respectively ( Figure 5). The nitrogen adsorption/desorption technique was used to measure the pore volume, the average pore diameter, and the surface area of the La 2 O 3 :Eu 3+ aerogel. According to Brunauer et al., it is a type IV isotherm with a H4 hysteresis loop [14,39]. The results are shown in Figure 4. Its surface area is 109.235 m 2 ·g −1 , as the average pore diameter and pore volume are 31 nm and 0.750 mL·g −1 , respectively ( Figure 5).      Figure 7 shows the excitation spectra of the La 2 O 3 aerogels at different concentrations of Eu 3+ . The excitation spectra were measured in the range from 200 to 560 nm with λ em = 613 nm. Maximum absorption can be seen at 394 nm, corresponding to the 5 L 6 ← 7 F 0 transition. Other absorption bands are located at 468 nm and 502 nm, corresponding to the 5 D 2 ← 7 F 0 and 5 D 1 ← 7 F 0 transitions, respectively. Note that the intensity of the charge transfer band (CTB) is lower than the maximum absorption peak located at 394 nm. This lower intensity of the CTB with respect to the transition 5 L 6 ← 7 F 0 was due to the lack of oxygen linked to La 2 O 3 and to the weak interaction between the Eu 3+ and O 2 -orbitals [42]. Even at high concentrations of Eu, no displacement of CTB was observed, which suggests that the crystalline environment around the Eu ions in the La 2 O 3 :Eu 3+ aerogels was not substantially affected [42], since the CTB is normally displaced toward longer wavelengths due to changes in the crystalline field around the Eu dopant [43]. The electric dipole 5 L 6 ← 7 F 0 transition is the strongest transition in the excitation spectrum of the Eu 3+ compounds, except when a 7 F 6 ← 7 F 0 transition in the near infrared region is observed [44]. This transition is commonly used to excite Eu 3+ and induce photoluminescence if excitation through ligands is not possible due to a lack of efficient energy transfer. The excitation at the 5 L 6 level allows the 4f levels to be populated directly. It is worth noting that the transition 5 D 0 → 7 F 4 in yttria aerogels is particularly strong and not split [41]. The 5 D 1 ← 7 F 0 transition is a magnetic dipole transition, while the 5 D 2 ← 7 F 0 transition is a hypersensitive electric dipole transition (∆J = 2) [43].   Figure 7 shows the excitation spectra of the La2O3 aerogels at different concentrations of Eu 3+ . The excitation spectra were measured in the range from 200 to 560 nm with λem = 613 nm. Maximum absorption can be seen at 394 nm, corresponding to the 5 L6 ← 7 F0 transition. Other absorption bands are located at 468 nm and 502 nm, corresponding to the 5 D2 ← 7 F0 and 5 D1 ← 7 F0 transitions, respectively. Note that the intensity of the charge transfer band (CTB) is lower than the maximum absorption peak located at 394 nm. This lower intensity of the CTB with respect to the transition 5 L6 ← 7 F0 was due to the lack of oxygen linked to La2O3 and to the weak interaction between the Eu 3+ and O 2-orbitals [42]. Even at high concentrations of Eu, no displacement of CTB was observed, which suggests that the crystalline environment around the Eu ions in the La2O3:Eu 3+ aerogels was not substantially affected [42], since the CTB is normally displaced toward longer wavelengths due to changes in the crystalline field around the Eu dopant [43]. The electric dipole 5 L6 ← 7 F0 transition is the strongest transition in the excitation spectrum of the Eu 3+ compounds, except when a 7 F6 ← 7 F0 transition in the near infrared region is observed [44]. This transition is commonly used to excite Eu 3+ and induce photoluminescence if excitation through ligands is not possible due to a lack of efficient energy transfer. The excitation at the 5 L6 level allows the 4f levels to be populated directly. It is worth noting that the transition 5 D0 → 7 F4 in yttria aerogels is particularly strong and not split [41]. The 5 D1 ← 7 F0 transition is a magnetic dipole transition, while the 5 D2 ← 7 F0 transition is a hypersensitive electric dipole transition (∆J = 2) [43].   The down-conversion (DC) luminescence spectra of La2O3 aerogels at different concentrations of Eu 3+ are shown in Figure 8. The emission spectra were measured in the range from 450 to 900 nm, with λex = 394 nm. An infrared emission band can be observed at 793 nm, corresponding to the 5 D0 ← 7 F4 transition. This transition is sometimes considered hypersensitive because it does not obey the selection rules for quadrupole transitions (∆J ≠ 2); the intensity of this transition is determined by symmetry factors and by the chemical composition of the host matrix [44]. The luminescence spectra of compounds with D4d The down-conversion (DC) luminescence spectra of La 2 O 3 aerogels at different concentrations of Eu 3+ are shown in Figure 8. The emission spectra were measured in the range Gels 2023, 9, 615 7 of 11 from 450 to 900 nm, with λex = 394 nm. An infrared emission band can be observed at 793 nm, corresponding to the 5 D 0 ← 7 F 4 transition. This transition is sometimes considered hypersensitive because it does not obey the selection rules for quadrupole transitions (∆J = 2); the intensity of this transition is determined by symmetry factors and by the chemical composition of the host matrix [44]. The luminescence spectra of compounds with D 4d symmetry are often dominated by the 5 D 0 → 7 F 4 transition since, with this symmetry, the 5 D 0 → 7 F 2 transition is forbidden, and the 5 D 0 → 7 F 4 transition is intense, because there is no center of symmetry [39,44]. Since the La 2 O 3 :Eu 3+ aerogels have a strong emission only at 793 nm, we can assume that the Eu ions might be situated at a symmetrical site. The down-conversion (DC) luminescence spectra of La2O3 aerogels at different concentrations of Eu 3+ are shown in Figure 8. The emission spectra were measured in the range from 450 to 900 nm, with λex = 394 nm. An infrared emission band can be observed at 793 nm, corresponding to the 5 D0 ← 7 F4 transition. This transition is sometimes considered hypersensitive because it does not obey the selection rules for quadrupole transitions (∆J ≠ 2); the intensity of this transition is determined by symmetry factors and by the chemical composition of the host matrix [44]. The luminescence spectra of compounds with D4d symmetry are often dominated by the 5 D0 → 7 F4 transition since, with this symmetry, the 5 D0 → 7 F2 transition is forbidden, and the 5 D0 → 7 F4 transition is intense, because there is no center of symmetry [39,44]. Since the La2O3:Eu 3+ aerogels have a strong emission only at 793 nm, we can assume that the Eu ions might be situated at a symmetrical site.   An increase in the luminescent center concentration should be accompanied by an increase in the intensity of emitted light due to higher absorption efficiency [33]. In Figure 8, it can be observed that the aerogel with 50% mol of Eu 3+ has the highest luminescence intensity. This suggests that the Eu 3+ ion does not act as a luminescence sensitizer in a La 2 O 3 matrix; however, the La 3+ ion does act as a luminescence sensitizer of the Eu 3+ ion. An ordinary photograph of the La 2 O 3 :Eu 3+ aerogel is show in Figure 9. An increase in the luminescent center concentration should be accompanied by an increase in the intensity of emitted light due to higher absorption efficiency [33]. In Figure  8, it can be observed that the aerogel with 50% mol of Eu 3+ has the highest luminescence intensity. This suggests that the Eu 3+ ion does not act as a luminescence sensitizer in a La2O3 matrix; however, the La 3+ ion does act as a luminescence sensitizer of the Eu 3+ ion. An ordinary photograph of the La2O3:Eu 3+ aerogel is show in Figure 9.

Conclusions
In this study, the monolithic aerogels synthesized by the sol-gel method via epoxideassisted gelation exhibited a non-ordered 3D porous structure composed of interconnected nanoparticle agglomerates. Although no heat treatment was applied to the aerogels, they exhibited semi-crystalline behavior due to the high pressure used during super-

Conclusions
In this study, the monolithic aerogels synthesized by the sol-gel method via epoxideassisted gelation exhibited a non-ordered 3D porous structure composed of interconnected nanoparticle agglomerates. Although no heat treatment was applied to the aerogels, they exhibited semi-crystalline behavior due to the high pressure used during supercritical drying. On the other hand, a change in the luminescence intensity was seen as the concentration of the dopant ion was increased, and the highest photoluminescence emission intensity was observed for the La 2 O 3 aerogel with 50% mol Eu 3+ ; this increase in luminescence intensity was probably due to a higher concentration of luminescent Eu 3+ ions, which made them more likely to be directly excited at the wavelength used.

Materials and Method
The La 2 O 3 :Eu 3+ aerogels were synthesized using the sol-gel method with epoxideassisted gelation and the low-temperature supercritical drying technique. This approach offers many advantages in the preparation of metal oxide aerogels. First, this technique utilizes simple metal salts (e.g., metal nitrates or halides) as precursors in the sol-gel reaction, eliminating the need for organometallic precursors, such as metal alkoxides (more expensive, difficult to obtain, and very unstable). In addition, the process is flexible and allows for control over the microstructure of the gel network through modification of the synthetic parameters. Moreover, one of the advantages of epoxide-initiated gelation is that this approach provides a versatile and relatively straightforward route to the preparation of binary or ternary oxides. Like single-component systems, mixed metal oxide gels can be readily prepared through the addition of epoxides to solutions containing two or more metal salts. When synthesizing these mixed-metal oxide aerogels, hydrolysis and condensation reactions can yield a variety of different network architectures within the aerogel framework. For example, the condensed phase can be comprised of separate interpenetrating networks of the two metal oxides, -M1-O-M1-and -M2-O-M2-, or mixed phases of the two materials, -M1-O-M2-. Alternatively, one of the metal oxides can exist as discrete entities (i.e., nanoparticles) supported by the primary oxide structure. In general, the composition and bonding motif of the gel structure are primarily functions of the reaction stoichiometry of the inorganic precursors and the relative rates of hydrolysis of the metal ions [3,45]. In this work, the concentrations (molar percentages) of Eu 3+ ions were varied to study the effects on the luminescent properties of a La 2 O 3 aerogel.
The oxide precursors were changed into their respective chloride forms by reacting with hydrochloric acid, while being subjected to agitation until the solution became transparent. Once this occurred, 34.6 mmol of ethanol was added, and the mixture was stirred (300 rpm) for 5 min to homogenize it. After that, 7.5 mmol of propylene oxide was added to the solution and stirred (300 rpm) for another 5 min. Subsequently, 18.1 mmol of an alcoholic solution of citric acid (0.1 molar) was added, stirred for 2 min, and poured into cylindrical plastic containers until the mixture gelled. Once the wet gel was obtained, enough ethanol (EtOH, CH 3 CH 2 OH, 99.9%, Fermont) to cover the wet gel was poured into the container to begin the gel aging process, which lasted 24 h. After this, the wet gels were placed for 24 h (what is the time required to produce an exchange between the ethanol into the wet gels and the CO 2 ) inside an E3100 critical point dryer filled with liquid CO 2 and the CO 2 brought to supercritical condition (32 • C and 74 bars). Later, the dryer was heated to 40 • C and pressurized to 83 bars, and it was kept at those settings for 1 h. After that, the temperature was raised to 50 • C, and the dryer was pressurized to 97 bars for 30 min. Finally, it was depressurized over the course of 1 h. The fluorescence emission was analyzed using Hitachi model F-7000 equipment with a 150 W xenon lamp and a R928F photomultiplier tube. The output slit was set at 5, as was the input slit, and the wavelength scanning speed was 1200 nm·min −1 . The system was controlled by PC, using FL-Solutions software. The equipment used for the FT-IR analysis was a Perkin-Elmer model Spectrum 65 in the range from 4000 to 400 cm −1 with a speed of 5 scans per minute. The system was controlled by PC using PerkinElmer Spectrum software. For this test, the KBr filling method was also used. The crystalline structure of the powders was identified with a Bruker model D8 Advance Eco powder diffractometer with a Cu tube (1.5418 Å), and a model SS160 high-speed detector. The 2θ analysis interval was established from 20

Data Availability Statement:
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.