From Nanoparticle Assembly to Monolithic Aerogels of YAG, Rare

Article Cite This: Chem. Mater. 2018, 30, 5460−5467

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From Nanoparticle Assembly to Monolithic Aerogels of YAG, Rare Earth Fluorides, and Composites Mateusz Odziomek,†,‡ Frederic Chaput,*,† Frederic Lerouge,† Christophe Dujardin,§ Maciej Sitarz,‡ Szilvia Karpati,† and Stephane Parola*,†

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†

Laboratoire de Chimie, CNRS UMR 5182, Universite de Lyon, Ecole Normale Superieure de Lyon, 46 allée d’Italie, F69364 Lyon, France ‡ Faculty of Materials Science and Ceramics, University of Science and Technology AGH in Cracow, Al. Mickiewicza 30, 30-059 Kraków, Poland § Institute Lumiere Matiere, UMR 5306, University Claude Bernard Lyon 1-CNRS, batiment Kastler, 10 rue Ada Byron, 69622 Villeurbanne, France S Supporting Information *

ABSTRACT: In the past few years, the efforts of scientists in the field of nanoparticle design have focused on building 2D and 3D macroscopic objects from controlled assembly processes. Among the few proposed methods, the preparation of aerogels allows production of 3D monolithic materials of several centimeters in size with desired porosity. The use of nanocrystals as building blocks provides access to highly crystalline macroscopic structures, imitating porous crystals. Herein, a general and versatile approach to aerogels development starting from aqueous colloidal solutions is introduced. A strong and abrupt change of the dielectric constant of the nanoparticle dispersion solvent induces formation of homogeneous and stable gels. The drying of these gels, under supercritical conditions, leads to transparent aerogels. By this approach, Y3Al5O12:Ce and GdF3 monolithic aerogels are prepared for the first time, opening the route to a broad range of chemical compositions with metal oxides and fluorides. Additionally, the method is suitable for composite aerogels with sophisticated structures made of several different nanobuilding blocks.

1. INTRODUCTION In the last decades, many synthetic routes toward nanoparticles (NPs) with controlled size, shape, and crystallinity were developed. Currently, the research focuses on using them as nanobuilding blocks for designed crystalline functional materials for applications in various fields such as optics (sensors,1 filters2), catalysis (heterogeneous catalysis,3 photocatalysis4), energy (batteries,5 photovoltaics,6 lightning7,8), or environment.9 Among several possible approaches toward formation of functional materials from colloids, gelation of colloids is particularly interesting. Hitherto, it is practically the only method allowing the preparation of macroscopic materials (of several centimeters in size) in a bottom-up approach, starting from NPs solutions.10 Therefore, colloidal gelation has focused the scientific attention in recent years, resulting in theoretical11,12 and experimental research.13,14 The conventional drying of NP-based gels generates large shrinkage and their fragmentation, even if carefully performed with a slow removal of solvents. Alternatively, the solvent from a gel can be removed using supercritical drying, preventing material © 2018 American Chemical Society

shrinkage and cracking, thus preserving its monolithic character.15,16 The significant advantage of NP-based aerogels over aerogels prepared by a molecular approach is their crystallinity, which allows exploiting the full potential of their functional properties. Although considerable advances in the field have been achieved, still the composition and quality of synthesized NPbased aerogels are rather limited.10,14 Especially, in the large and important family of metal oxides, wherein only few aerogels were prepared mainly by Niederberger and coworkers.17−19 Moreover, such aerogels suffer from low mechanical strength, cracks, and low transparency. The key issues for their synthesis from NPs are, on the one hand, the highly stable colloidal solutions with a high solid content and, on the other hand, the development of appropriate techniques for destabilization of NPs leading to a gel instead of precipitate. Most of the processes reported use Received: June 10, 2018 Revised: July 17, 2018 Published: July 17, 2018 5460

DOI: 10.1021/acs.chemmater.8b02443 Chem. Mater. 2018, 30, 5460−5467

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Chemistry of Materials

Figure 1. Destabilization process of YAG:Ce colloidal solution. (a) Highly concentrated YAG:Ce NCs of 4−5 nm in aqueous solution (45 wt %). (b) TEM image of nonaggregated YAG:Ce NCs. (c) Scheme of the NCs gelation mechanism. (d) TEM image of aggregated YAG:Ce NCs. (e) YAG:Ce NCs-based gel. pyrrolidinone and ethylene glycol (1:1 v/v). The clear obtained solution was added on a mixture of 100 mL of pyrrolidinone and 50% HF (5.5 mL). The final mixture was then heated up to 170 °C for 1 h in an autoclave. The obtained suspension was precipitated in acetone, centrifuged, and redispersed in methanol. The washing step with methanol was repeated 2 times to give a colorless gel-like precipitate which was redispersed in water. After 30 h of dialysis, the particle suspension could be concentrated under vacuum. 2.3. Synthesis of NP-Based Gels. Due to the high tunability of gels compositions, specific examples are given below for each system. Gelation of YAG:Ce. 600 mg of a colloidal suspension of YAG:Ce NCs (40% w/w in water) was mixed with ethanol (800 mg) under mild stirring, followed by dropwise addition of 1,4-dioxane (1,4-DO, 700 mg for 30 s). Gelation was observed after approximately 10 min. The obtained gel was then aged 2−3 days at 50 °C in an atmosphere of 1,4-DO. Gelation of GdF3. The procedure was the same as that described for YAG:Ce NCs, with 700 mg of GdF3:Tb or GdF3:Eu (30% in water), 500 mg of ethanol, and 800 mg of 1,4-DO. In that case, gels were observed after 30 min. Gelation of YAG + GdF3. The procedure was the same as that described for YAG:Ce NCs, and the composition could be tuned by varying the amount of each NPs. For example, for the gel corresponding to a mixture of YAG:Ce/GdF3:Tb/GdF3:Eu 50/25/ 25, the procedure was as follows: aqueous solutions of YAG:Ce (250 mg), GdF3:Tb (190 mg), and GdF3:Eu (190 mg) were mixed with 600 mg of ethanol and 800 mg of 1,4-DO. The gelation was observed after 20 min. 2.4. Supercritical Drying. The procedure was the same for all the systems whatever their composition. Impregnation of the gels with acetone was performed stepwise by successive transfer of the gels into beakers filled with acetone and 1,4-DO at various ratios (first 40% acetone v/v, then 50%, 60%, 75%, 90%, and 100%). Before being transferred into acooled autoclave (Critical Point Dryer E3100, Quorum Technologies), the gels were kept in pure acetone for 24 h. The acetone was then replaced by liquid CO2 after 6 cycles of partial liquid removal and filling with CO2. The whole procedure was repeated 3 times. The temperature and the pressure of the autoclave were increased up to 45 °C and 95 bar. These conditions were kept for 1 h before complete evacuation of the CO2 within 3h. 2.5. Characterization. Scanning electron microscope (SEM) observations were conducted on a Zeiss Supra 55VP SEM. Transmission electron microscopy (TEM) was performed on a Tecnai Osiris TEM. Dynamic light scattering measurements were done on a Malvern Zetasizer device. X-ray diffraction on powders was conducted with an X-ray diffractometer using Cu Ka radiation (XRD, model: X’Pert Pro from Panalytical). FTIR analysis was collected on a PerkinElmer Fourier IR Spectrum 65.

either destabilization through surface ligand removal by heating or through pH changes. We propose a general procedure for monolithic aerogels preparation from concentrated aqueous colloidal solutions using abrupt change of solvent dielectrical constant, which allows controlled reactions and aggregations between NPs, without the need of thermal treatment for particle destabilization and gelation. Typically, the NPs aqueous solutions are destabilized by addition of a solvent miscible with water but possessing a much lower dielectric constant. The procedure is evaluated on YAG:Ce materials, well-known for their high luminescence yield commonly used in white LED, as well as sensors for ionizing radiation detection. In this way, pure YAG:Ce gels are obtained and supercritically dried to form aerogels. The same procedure is applied to rare earth (RE) fluorides, resulting in fluoride aerogels, demonstrating the versatility of the approach. Both of these compounds have never been reported in the form of aerogels to the best of our knowledge. In addition, homogeneous composite aerogels made of a mixture of oxide and fluoride NPs are obtained by the same approach.

2. EXPERIMENTAL SECTION 2.1. Synthesis of YAG:Ce NCs. In a mixture of 170 mL of 1,4butanediol (1,4-BD) (Alfa Aesar) and 30 mL of diethylene glycol (Alfra Aesar) were added: 10.2 g of preliminary distilled aluminum isopropoxide (Sigma-Aldrich), 7.83 g of dry yttrium acetate (SigmaAldrich), and 41 mg of dry cerium acetate (Sigma-Aldrich). The suspension was homogenized by stirring for 15 min with an UltraTurrax disperser. The milky mixture was then heated to 300 °C for 2 h in an autoclave equipped with mechanical stirring. The resulting yellowish suspension was added in a mixture of THF and diethyl ether (2:1 v/v), followed by centrifugation to give a white precipitate (450 mg, boehmite based-mixture) and a clear light yellow supernatant. The yellow solution was evaporated until only 1,4-butandiol remained. The slightly viscous suspension was then dialyzed against water for 2 days. Finally, the suspension was concentrated under vacuum (total amount of finally prepared YAG:Ce NCs = 4.5 g). The potential remaining acetates on the NPs surface were removed by treating the particles suspension with hydrochloric acid 1 M (final pH around 3). The mixture was then dialyzed and concentrated under vacuum. 2.2. Synthesis of GdF3:Tb and GdF3:Eu NPs. 23.75 g of GdCl3· 6H2O and 1.25 g of the corresponding doping rare earth (TbCl3· 6H2O or EuCl3·6H2O) were mixed in a solution of 20 mL of 5461


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Chemistry of Materials Photoluminescence was measured using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer, equipped with a three-slit doublegrating excitation and emission monochromator.

level. The potential energy of attraction depends on the nature of the material and the dispersion medium, the particle size, and the interparticle distance. Electrostatic forces result from the interaction of EDLs on the surface of particles. They are repulsive between particles of the same charge. The total energy of interaction is the sum of the energies of attraction and repulsion. The variation of the total potential energy as a function of the interparticle distance generally has one maximum and two minima. If the energy barrier corresponding to the maximum is high (a few tens of kT), collisions between particles cannot provide enough energy to overcome it and the suspension is stable. For a given size of particles, the height of the barrier depends mainly on the following factors influencing the thickness of the EDL.

3. RESULTS 3.1. Preparation of YAG:Ce Aerogels. The preparation method for NP-based aerogels requires 3 essential steps: (i) the synthesis of crystalline, monodispersed NPs with a size below a few tens of nanometers, dispersible in high concentration; (ii) destabilization of colloidal solution to induce the gelation; (iii) supercritical drying of the gel. YAG:Ce nanocrystals (NCs) are synthesized by the previously reported modified glycothermal approach (Supporting Information (SI)).20,21 High synthesis temperature (300 °C) ensures high crystallinity of the NCs, and the solvent composition allows controling their size, shape, and aggregation.22 The synthesis leads to pure YAG phase, confirmed by X-ray diffraction (XRD) data (Figure S1a). The diffraction peaks are consistent with the pure cubic structure of Y3Al5O12 (JCPDS: 33-0040) in the space group Ia3̅d (230). X-ray diffraction peak profiles are broadened due to the nanosized character of crystallites. Organic residues from the synthetic process remain on NCs surface. The partial elimination of these groups, followed by IR spectroscopy, is carried out by washing under mild acidic conditions. The intensity of the absorption bands between 1600 and 1400 cm−1, assigned to COO− asymmetric and symmetric stretching vibrations of carboxylate complexing surface cations, decrease sharply after the acid treatment (Figure S1b). To remove unbound molecules and to lower the ionic strength of the final product, colloidal solutions are subjected to dialysis against deionized water for 30 h. Such obtained NCs exhibit a zeta potential value of 50 mV (Figure S1c), thus ensuring a significant electrostatic stabilization. They are very stable in aqueous solution and can be easily concentrated up to 50% by weight (Figure 1a). Transmission electron microscopy (TEM) images show homogeneous NCs with a narrow size distribution of 4−5 nm and high crystallinity (Figure 1b and Figure S2). YAG:Ce NCs are further used as nanobuilding blocks for the preparation of 3D porous materials. To trigger the aggregation of NPs, the starting point of gelation, there is a need to evaluate the total interaction potential resulting from the contribution of all interparticle interactions. As a general feature, particles in a solution display a wellknown surface charge organization called an electrical double layer (EDL), which ensures the electrostatic stabilization of colloids.23 In this model, the ions are arranged around NPs in two layers: (i) a Stern layer of chemically bonded ions to the surface and (ii) a diffusive layer of loosely associated ions with the surface, that can be rapidly exchanged in the fluid under influence of Coulombic interactions and thermal motions. This ionic atmosphere near a charged surface has a characteristic thickness known as a Debye length that is proportional to the square root of the dielectric constant of the solvent and inversely proportional to the square root of the ionic strength of the solution. Derjaguin, Landau, Vervey, and Overbeek (DLVO) have developed a theory of colloidal stability, which allows an understanding of interactions between colloidal particles and their aggregation behavior. It establishes the balance of the forces acting on the particles. van der Waals forces are attractive and result from dipolar interactions at the molecular

1. The concentration and the charge of the ions of the electrolyte (ionic strength of the solvent). Their increase causes the lowering of the potential barrier by decreasing the Debye length. 2. The dielectric constant of the medium. Using a lower dielectric constant liquid will also lower the potential barrier and decrease the Debye length. 3. Zeta potential and surface charge. This represents a simplified image of particles interactions, especially for systems with NPs of only few nanometers in diameter where more complex interactions and additional phenomena, such as non-DLVO forces (i.e., hydration, solvation, and hydrophobic forces), have to be taken into account24,25 as well as the particle concentration.26 The synthesized YAG:Ce NCs are electrostatically stabilized as shown by their high zeta potential (50 mV, Figure S1c). Therefore, they are stable in solvents with high dielectric constant such as water (ε = 80.4 at 20 °C). The solution of NCs is dialyzed, ensuring very low ionic strength and consequently a higher stability. The NCs remain well dispersed in the solution as long as the repulsive interactions between NCs are dominant. In the opposite case, when decreasing the dielectric constant of the solvent, and for the reasons mentioned above, the particles aggregate due to the collisions induced by Brownian motions (Figure 1c,d). Depending on the interaction forces and the density of the aggregates, they can then flocculate, precipitate, or span over the entire sample, forming a gel (Figure 1e). The concentrated YAG:Ce NCs are efficiently destabilized, leading to gelation, by addition of 1,4-DO which, despite its very low dielectric constant (ε = 2.2 at 20 °C), is still miscible with water. The experiments show that best-quality gels are obtained when the concentrated aqueous solution of YAG:Ce is first diluted with ethanol (ε = 24.5 at 20 °C) and then mixed with 1,4-DO. Depending on the ratio between these 3 components, the gelation time is easily tuned from instantaneous to several days at ambient temperature (Table S1). The gelation is quicker, when heating (20−90 °C) is applied. This procedure provides the control over the density of the gel and the resulting aerogel by the simple adjustment of the colloid/solvents ratio (Figure S3). The gels obtained by NPs aggregation are clear and transparent, showing their high quality (Figure 1e). It is relevant to note that the gelation of YAG:Ce NCs from highly concentrated colloidal solution (45% w/w) without ethanol is poorly controlled. The gels are less transparent and full of bubbles, suggesting too fast aggregation process. Moreover, the colloids diluted with water 5462


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Figure 2. YAG:Ce NC-based aerogels. (a) Picture of monolithic and transparent aerogel; inset: aerogel irradiated with UV lamp (365 nm). (b) SEM image of highly porous and homogeneous aerogel structure. (c, d) STEM and TEM images demonstrating randomly aggregated NCs building the porous network. (e) Image of two pieces of the same aerogel before (left) and after (right) calcination at 750 °C. (f−h) SEM, STEM, and HRTEM images of preserved homogeneous and porous structure of calcined aerogel. (i) Nitrogen adsorption and desorption isotherms of asprepared and calcined aerogel.

procedure requires that the solvent in the pores is miscible with liquid CO2. Thus, the pore liquor is gradually exchanged to acetone prior to supercritical drying. One example of obtained luminescent YAG:Ce aerogel is shown in the Figure 2a. Supercritical drying does not induce shrinkage of the aerogels if the volume fraction of NPs is high enough (at least 2%). The aerogels are noncracked and transparent which is highly beneficial for prospective applications as for instance optical sensors. A transmittance spectrum on a 8 mm thick aerogel is displayed in Figure S5a, showing 30−60% transmission in the visible wavelengths range. In the literature, a repeatedly reported problem of NP-based aerogels is their easy cracking and difficulties in handling, thus inhibiting their application potential. In our case, aerogels easy to manipulate without any specific caution are obtained (Figure S5b). Their densities vary depending on the starting composition, between 0.092 and 0.209 g cm−3 (Figure S3). Higher densities are accompanied by higher transparency of the materials because of lower pore sizes.

instead of ethanol remain too stable to achieve gelation even at elevated temperatures. This peculiar sol−gel transition is also successfully achieved with other cosolvents such as propylene oxide (ε = 16 at 25 °C) and less toxic ones like acetone (ε = 20.7 at 20 °C) or isopropanol (ε = 17.9 at 20 °C). However, the gels obtained with isopropanol are turbid while the others are transparent. For the same volume ratio of solvents, the gelation time is lower concomitantly with the decrease of the solvent dielectric constant. The gelled samples are aged for at least 2 days at 50 °C under an atmosphere of the cosolvent vapors. This step is crucial to facilitate the handling of the gel, which is necessary prior to the next steps leading to the final aerogel structure. As mentioned above, the Debye length can also be reduced by increasing the ionic strength of the solvent. For example, the gelation is induced by addition of an aluminum chloride solution to the colloidal solution (Figure S4). The gels consist of a mineral skeleton impregnated with solvents. While conventional drying causes the network to collapse due to capillary forces, supercritical CO2 drying allows the liquid to be removed without affecting the solid structure. However, this 5463


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parallel adsorption and desorption isotherms in the calcined sample demonstrates the narrow pore size distribution. 3.3. Preparation of GdF3 and Composite Aerogels. Further, we investigate whether the same aerogel preparation method can be applied to other systems, not necessarily oxides. The attempt is made on fluoride NPs since these materials, in particular for RE elements, possess interesting optical properties for solar cells,29 optical amplifiers,30 or sensors,31 and no preparation method of aerogel has been reported so far. The GdF3 colloids are prepared according to the previously reported method.32 The synthesized GdF3 NPs are composed of pure orthorhombic phase with NPs size of 15 nm measured by TEM (Figures S7a, S8). They are not spherical but present an elongated shape; most of them are characterized by the aspect ratio of around 2. The surface of NPs is covered with residual organics most likely coming from ring opening decomposition of pyrrolidinone used as synthesis solvent. The FTIR spectra show peaks from both amines (3300 cm−1) and carboxylic groups (1530−1450 cm−1) (Figure S7b). The NPs are not aggregated in aqueous solution and possess high positive surface charge as shown by zeta potential measurements (Figure S7c). The colloidal solution with weight concentration of 30% is readily gelled at room temperature by the same protocol used for YAG:Ce (SI). The ratio between solvents is adapted to the GdF3 NPs, which present different particle size/morphology compared to YAG:Ce, thus different gelation kinetic. Analogous procedures of solvent exchange and supercritical drying lead to pure and transparent GdF3 aerogels (Figure 3a).

The morphology of the aerogels is investigated with electron microscopy. Scanning electron microscopy (SEM) proves the presence of a 3D highly porous network with the pore size up to 100 nm. No large and dense aggregates are detected, confirming high homogeneity (Figure 2b). Complementary analysis by STEM and TEM confirms that the porosity is a result of random aggregation of NCs (Figure 2c,d). Deeper high resolution TEM shows that the particles are rather randomly attached. However, crystallographic alignment between some neighboring NCs is observed (Figure S6) such as reported for TiO217 and SnO219 aerogels. 3.2. Thermal Treatment of the YAG:Ce Aerogels. Aerogel structure can be reinforced by calcination at 750 °C with low heating rate (1 °C min−1) in order to avoid cracking. The calcined samples are slightly shrunken (their volume is around 70% of initial aerogel) due to densification (Figure 2e) and thus more stable. Thermally treated aerogels preserve their large porosity and unique structuration. This is confirmed by SEM and STEM analyses (Figure 2f,g). Besides the presence of initial NC building blocks of 4−5 nm, the images show that a part of them grows up to the size of 10 nm (Figure 2h). In addition, the shape of the particles becomes more regular and spherical. Comparison between TEM pictures of calcined and noncalcined samples indicates that coalesced NCs merge into a single particle by the diffusion processes. Such treatment increases the contact area between NCs and induces their partial sintering. Consequently, the calcined aerogels possess improved stability, allowing easy handling. The yellowish color of the sample is lost after calcination, due to the oxidation of Ce3+ to Ce4+. The high porosity is confirmed by nitrogen adsorption measurements (Figure 2i). Special care was taken to ensure proper equilibration time at each step of gas dosing.27 The recorded isotherms are assigned as deformed isotherms of type IV according to the updated IUPAC classification scheme,28 which is typical for mesoporous materials. The flat part of the adsorption curve at low and medium relative pressures indicates the low amount of micro- and low mesoporosity. The adsorption strongly accelerates at the high relative pressure, confirming the existence of large mesopores and macropores. The total pore volume in micro- and mesoporous range (Vmicro+meso [cm3 g−1]) is calculated using the Gurvich rule which assumes that adsorbate is present in all pores as a liquid-like state. In this case, total pore volume is calculated from the volume of gas adsorbed (Vads [cm3(STP)g−1]) at the pressure of the plateau region (∼0.99) by the following expression:

Figure 3. Properties of GdF3 NP-based aerogel. (a) Picture of the transparent monolithic aerogel; inset picture shows three-dimensional character of the aerogel. (b) SEM image of homogeneous and highly porous microstructure of the aerogel. (c) STEM picture of selforganized NPs, creating the gel network. (d) TEM image of randomly stacked elongated NPs in the aerogel structure.

Vmicro + meso = Vads*c

Herein, c represents the constant converting volume of gas phase to liquid phase, and for N2, it is 0.001547 cm3 cm−3 (STP). Thus, the pore volume of prepared and calcined samples is calculated to 1.47 and 2.72 cm3 g−1, respectively, being much lower than the pore volume calculated on the basis of aerogel weight and diameter assigned with a balance and a caliper (5.5 and 4 cm3 g−1, respectively). It supports the observation from SEM that the sample contains pores with a diameter exceeding 100 nm, which cannot be detected by nitrogen adsorption. The lack of clear plateau, especially for a noncalcined sample, indicates that the fragile structure of the gel is strongly affected by compression stresses during nitrogen sorption.27 Therefore, the calculated pore size distribution is affected by partial deformation of the aerogel. On the contrary,

The transmittance spectrum is shown in Figure S9, with 40− 80% transmission in the visible wavelengths. The morphology characterized by electron microscopy is analogous to YAG:Ce aerogels. SEM and STEM images confirm the existence of 3D highly porous and open structure of aerogels (Figure 3b−d). Similarly to the previous YAG:Ce aerogel, interconnected NPs build the solid network. In addition, the STEM picture (Figure 3c) indicates that aggregates are less dense as a consequence of 5464


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and excitation spectrum of an aerogel. The example of aerogel composite presented here contains three types of separated RE ions in different NPs, which are mixed in the composite. Each element could be exclusively and selectively excited (Figure 5,

the NPs anisotropic shape. No specific crystallographic or shape related alignment of NPs is detected. The method turns out to be very flexible and adjustable for an aqueous colloidal solution, and both YAG and GdF3 aerogels are prepared. To move further on toward more complex structures and compositions, the composite aerogel made of both types of NPs is synthesized. Most of reported examples of multicomponent aerogels are in the form of doped materials, so that one phase is embedded in relatively low amount in the aerogel structure.33 Only few examples of composite structures of NP-based aerogel exist in the literature as for instance CdTe-ZnSe34 or metal-QD aerogels.35−37 To the best of our knowledge, the demonstrated crystalline fully inorganic monolithic composite aerogel made from NPs with totally different chemical nature evenly building the aerogel network is unique. Moreover, the sample does not lose macroscopic quality compared to pure YAG and GdF3 aerogels. The performed experiments unveil that the method is flexible and that a composite aerogel composition can be freely designed. An example of aerogel with the composition 50% YAG:Ce NCs - 25% GdF3:Tb NPs - 25% GdF3:Eu NPs, by weight, is presented here (Figure 4a). The procedure is

Figure 5. PL emission of composite YAG:Ce-GdF3:Tb-GdF3:Eu aerogel with selective excitations (top) and multiple emission of the composite (bottom).

top) or all of them could be excited at the same time with a proper wavelength (Figure 5, bottom). The excitation and emission spectra of the pure reference aerogels (YAG:Ce, GdF3:Tb, and GdF3:Eu) are given in Figure S11. Apart from listed RE metals, the GdF3 matrix also gives a broad emission band with a maximum around 400 nm. The very broad emission spectrum results in white-light emission upon excitation in UV (Figure 4a, inset). Such aerogel composites can thus be used as an interesting starting optical platform.

Figure 4. Structural and optical properties of YAG:Ce-GdF3:TbGdF3:Eu composite aerogel. (a) Picture of composite aerogel; inset shows white-light emission upon excitation by UV lamp. (b) SEM picture of porous and homogeneous structure. (c) STEM image demonstrating self-assembled particles of YAG and GdF3 creating 3D network. (d) Mapping of yttrium (red) and gadolinium (green) elements proving homogeneous distribution of NPs inside the aerogel structure.

4. CONCLUSION A versatile strategy for the preparation of high quality monolithic aerogels from NPs is reported. The method gives access to aerogels with broad compositions. The efficiency of the method is illustrated by the preparation of original oxide (YAG) and fluoride (GdF3) aerogels. Additionally, the method gives tools for combination of materials into mixed monolithic composites. A similar strategy can be now proposed to build 3D materials with complex structures and a broad range of properties and applications in various fields. The large and accessible porosity and surface area connected, in this case, with good optical properties is especially interesting for the photocatalysis38 and optical sensing. The example of the latter is conversion of ionizing radiation by scintillating YAG:Ce to

analogous to the previously prepared aerogels (Experimental Section 2.1−2.4). The microstructure is equivalent to the pure aerogels as shown by SEM and STEM images (Figure 4b,c). STEM mapping of Y and Gd elements demonstrates a good homogeneity of NPs repartition (Figure 4d), indicating that NPs segregation does not occur during the destabilization process. Similar results are obtained for larger scale mapping by SEM EDX (Figure S10). 3.4. Optical Properties of the RE Doped Aerogels. The use of doped NPs with RE ions provides additional photoluminescent (PL) properties to this material. In the case of emitting NPs, the synthetic route toward composite aerogels opens the opportunity for precise design of emission 5465


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visible light and their easy detection. Potential usefulness of such materials in LED or optical gain applications was also reported.34,39

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02443. Structural properties of YAG:Ce and GdF3 NPs, pictures of aerogels with varying density, PL spectra of colloids and aerogels (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.P.). *E-mail: [email protected] (F.C.). ORCID

Christophe Dujardin: 0000-0002-0205-9837 Stephane Parola: 0000-0001-7560-988X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.O. was supported primarily by the French Minister of Research through Ecole Normale Supérieure de Lyon and by the LABEX IMUST funding for the Ph.D. project.

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REFERENCES

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From Nanoparticle Assembly to Monolithic Aerogels of YAG, Rare

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