**2.3 Sol-gel syntheses**

A colloid is a suspension in which the dispersed phase particle's size is so small (�1–1000 nm) that gravitational forces are negligible and interactions are dominated by short-range forces, such as van der Waals attraction and surface charges. In the context of the sol-gel synthesis, the "sol" is formed by a colloidal suspension of solid particles in a liquid, while the "gel" is a suspension of a liquid phase in a continuous solid phase [23]. Basically, two sol-gel routes are used: the polymeric route using alkoxides and the colloidal route using metal salts.

In a typical polymeric sol-gel process, as the one used for low-temperature preparation of SiO2 monoliths from a tetraethoxisilane (TEOS) solution, a polymerized structure is formed by the condensation of alcohols proceeding from the TEOS hydrolysis. Another widely used sol-gel synthesis is the complexation by amines, known as internal gelation [24–27]. This synthesis is common to find in the nuclear field associated to the fabrication of UO2 microspheres formed by agglomerated nanoparticles as in the work of Daniels et al. [25]. In this case, an uranyl nitrate solution is mixed with urea (CO(NH2)2) and hexamethylenetetramine (HMTA) solution. Then, the HMTA is decomposed at low temperature (90°C) causing an increase in pH and hydrolysis of uranium (Eqs. (7) and (8)), resulting in a solution gelation:

$$\text{Hydrolysis} : \text{UO}\_2^{\cdot 2} + 2\text{H}\_2\text{O} \leftrightarrow \text{UO}\_2(\text{OH})\_2 + 2\text{H}^+ \tag{7}$$

$$\text{Condensation} : \text{UO}\_2(\text{OH})\_2 + \text{H}\_2\text{O} \rightarrow \text{UO}\_3 \bullet \text{2H}\_2\text{O} \,\text{4} \tag{8}$$

This gel is washed with NH4OH and dried to obtain dry UO3. Later, thermal treatments at 800°C in air allow obtaining U3O8 powders that are further reduced to UO2 particles. With this method, UO2 millimeter-sized spheres with a nanometric substructure were obtained by different authors [25, 26]. The powder morphologies and particle sizes depend on the temperature and the calcination atmospheres used. The average particle size varies between 100 and 4000 nm. The samples obtained through the oxalic route and a single calcination (in neutral or reductive atmosphere) showed similar lattice parameters, close to the value of UO2 [24].

Recently Leblanc et al. presented another method that they called "advanced thermal denitration in presence of organic additives" that includes a gelation step of the uranyl nitrate solution [28]. In this process, an acidic uranyl nitrate solution is prepared, and urea is added to avoid uranium precipitation. Oxide synthesis was performed by adding two monomer types: acrylic acid (AA) and N,N<sup>0</sup> -methylene bis acrylamide (MBAM) in a molar ratio of 20:1 (AA:MBAM). A fully homogeneous solution was obtained, which when heated up to 100°C and after the addition of 25 mL of hydrogen peroxide (30 wt%) as initiator completely polymerized into a

gel. The entire solution is incorporated into the polymer network, ensuring that all the cations of the system are stripped into the obtained gel. Drying at 150°C, following an oxidative calcination of the organic part at 800°C, and finally reducing it in Ar:5%H2 at 800°C resulted in a nanostructured material with crystallite size below 100 nm, as determined by XRD diffraction.

## **2.4 Electrochemically assisted syntheses**

Recently, Rousseau et al. presented a wet chemical novel method to synthesize UO2 (and also UO2 doped with tetra- or trivalent elements), based on the electrochemical reduction of U6+ to U4+, followed by a precipitation in a reducing and anoxic condition, at constant pH [29]. The mother U4+ solution was made dissolving UNH in 1 M NaCl. The authors studied two methods for precipitating stable UO2+x nanoparticles of different sizes. In the pH range 2.5–4, the starting U6+ solution was added to the NaCl solution under reducing conditions, and U6+ cations were reduced electrochemically to U4+. The increment in pH was compensated with 0.1 M HCl. In the pH range 4–8, the mother U6+ solution was added drop by drop directly to the 1 M NaCl solution, balancing the pH change with 0.1 M NaOH. A redox potential of �300 mV/NHE was applied using Pt electrodes. The obtained products were filtered with a 0.22 μm filter, and the precipitates were washed two times with ultrapure water. The nanoparticles produced correspond to a single fluorite UO2.19 � 0.01 phase and average TEM coherent domain size of (12 � 2) nm for pH < 4 and UO2.11 � 0.02 of 4–6 nm for pH 6.5. The BET surface area for this nanomaterial was 10.3 � 0.1 m<sup>2</sup> /g, which the authors associated to a grain size of 53 nm, indicating a moderate agglomeration of the nanoparticles. XPS, in good agreement with the other analytical techniques, resulted in a U6+/U4+ ratio close to 0.1.

Moreover, an electrolytically reduced aqueous solution of 0.5 M uranyl nitrate was used as precursor, together with NaOH solution as alkalinization agent, to trigger the precipitation of UO2 nanoparticles near the U4+ solubility line. XRD and HR-TEM analyses showed that the nanoparticles obtained exhibit the typical slightly oxidized UO2+x fcc fluorite structure, with an average crystal size of 3.9 nm and a narrow size distribution [6].

In these cases, the reduction is mediated by the reactions occurring in the cathode (Eq. (9)) and in the anode (Eq. (10)), respectively [6]:

$$\text{U}\text{O}\_2^{2+} + 4\text{H}^+ + 2\text{e}^- \leftrightarrow \text{U}^{4+} + 2\text{H}\_2\text{O} \tag{9}$$

$$\text{2H}\_2\text{O} \leftrightarrow 4\text{H}^+ + 4\text{e}^- + \text{O}\_2 \tag{10}$$

To maintain the reducing environment, the oxygen must be eliminated with an oxygen-free gas such as pure Ar. In the work of Jovani-Abril et al. [6], for example, the starting pH was 0.5, and the solution was slowly alkalinized to allow the precipitation of the UO2 nanoparticles, following the equation:

$$\rm U^{4+} + 4\rm OH^{-} \leftrightarrow UO\_{2} \\ \downarrow + 2H\_{2}O \tag{11}$$

#### **2.5 Fluidized bed syntheses**

Thermal denitration in a fluidized bed is another way to indirectly obtain UO2 micro (and nano) particles. It involves spraying a concentrated solution of UNH on a bed of UO3 at moderated temperatures (240–450°C) and fluidizing it with air or steam. The UO3 produced nucleates on the existing UO3 particles of the bed,

enlarging their volume, or forming new particles. Afterward, thermal treatments can be used to convert the UO3 to U3O8 and UO2. This method uses less chemicals than the precipitation type of syntheses and allows the recuperation of the solvents but reported grain sizes are in the 100–500 μm [30], i.e., three orders of magnitude larger than the nanoparticles. However, it should be noted here that the equipment reviewed in most of the publications regarding fluidized beds are tuned to fabricate nuclear fuels. Under certain conditions of bed lengths, temperature, and solution feed speed, the authors reported the formation of "a very fine powder, not well suited to the subsequent powder handling" that is elutriated in the process [31]. This means that those grains smaller than some microns were separated by their different density, grain size and morphology in the vapor/gas stream, losing all information about the possible existence of nanoparticles. Thus, it is possible that nanoparticles would be obtained in fluidized bed denitration by tuning appropriate operative characteristic.
