Method for synthesising monodisperse titanium oxide particles

Abstract

The present invention belongs to the field of materials production, and more specifically to a method for obtaining titanium oxide (TiO2) particles having a size that may preferably be selected between 0.5 and 3 μm and a low size dispersion (less than approximately 5% with respect to the average size). The synthesis route consists of controlled hydrolysis with an aqueous medium of hydrolysable titanium oxide precursors dissolved in solvents that are able to form complexes with the titanium oxide precursors and which have been pre-treated to remove wastewater and thus be able to access a wide range of experimental conditions. This synthesis route makes it possible to obtain monodisperse TiO2 microparticles at room temperature, easily, quickly, with low energy consumption and more efficiently. Furthermore, at an industrial level, the method is easily scalable.

Description

[0001] DESCRIPTION

[0002] METHOD FOR SYNTHESIS OF MONODISPERSE TITANIUM OXIDE PARTICLES

[0003] The present invention pertains to the field of materials manufacturing, and more specifically, to a method for obtaining titanium oxide (TiO2) particles of a selectable size, preferably between 0.5 and 3 µm, with low dispersion in this size, estimated by the statistical coefficient of variation (less than approximately 5% relative to the average size). The synthesis route consists of the controlled aqueous hydrolysis of hydrolyzable titanium oxide precursors dissolved in solvents capable of forming complexes with the titanium oxide precursors. These precursors have been previously treated to remove residual water, thus allowing access to a wide range of experimental conditions. This synthetic route allows for the simple, rapid, energy-efficient, and low-energy production of TiO2 monodisperse particles at room temperature. Furthermore, the method is easily scalable to an industrial scale.

[0004] BACKGROUND OF THE INVENTION

[0005] Titanium oxides (T₂O) are abundant, low-cost, chemically stable, and environmentally safe. These materials are of interest in materials science and technology because of their optical, catalytic, photovoltaic, and electrochemical properties, which provide broad applicability [K.-T Kim, et al., Adv. Funct. Maten, 24, 5464-5481 (2014); X. Chen et al., ACS Energy Lett., 6, 3332-3341 (2021)].

[0006] Optically, their high refractive index (2.6) combined with a sufficiently wide band gap (3.2 eV, or 390 nm), makes them ideal for use in light diffusion phenomena (not surprisingly, they are the main component in white paints) and in stochastic lasers [L. Pattelli et al., Optica, 5, 1037-1045 (2018); M. Leonetti et al., Nature Photon. 615-617 (2011)].

[0007] Among the various morphologies of titania, spherical and monodisperse particles are advantageous for building uniform and densely packed particle networks, exhibiting outstanding performance in environmental and energy applications, these characteristics of the microparticles being a necessary condition for forming regular arrays of photonic crystals that can be modulated by controlled alteration of size [A. Blanco et al., Nature (2000), 405, 437].

[0008] In the field of radiative cooling, theoretical predictions suggest the benefit of working with a wide library of monodisperse particle sizes that can give rise to Mie resonances [B. Mishra et al., Appl. Phys. Lett. (2022), 121, 202204]. Furthermore, optimization in Mie scattering processes is the basis of the latest advances in the field, for example, in smart textile materials [Wu et al., Science (2024), 384, 1203].

[0009] Regarding known methods for the formation of TiC>2 microparticles, some use titanium alkoxides as precursors, which, through hydrolysis in an aqueous medium, lead to the formation of the corresponding particulate titanium oxide. However, these methods result in a wide dispersion in the sizes of the TiO2 microparticles obtained, especially when the goal is to obtain microparticles larger than 0.5 pm [T. Sugimoto, T. Kojima, J. Phys. Chem. C, 112, 18760-18771 (2008); T. Kojima, T. Sugimoto, J. Phys. Chem. C, 112, 18445-18454 (2008)].

[0010] In order to obtain monodisperse populations (i.e., those with a low coefficient of variation in TiO2 microparticle size), it is necessary to use additives to control the nucleation and growth rate of the microparticles. For example, carboxylic acids with variable hydrophobic chain lengths have been used to control the particle size of titanium oxides [MC Tsai et al., J. Phys. Chem. C, 112, 7, 2697-2702 (2008)], thus enabling the synthesis of a wide range of sizes (from 15 nm to 2 pm); however, even in this study, the coefficient of variation exceeds 5% and is especially high at large sizes (around 50%).

[0011] Another example of a hydrolysis method using additives (alkylamines) is patent US9676634B2, although in this study the hydrolysis reaction is carried out at sub-ambient temperatures (down to -30 °C), a low temperature that helps control polydispersity. However, at room temperature, the data suggest that polydispersity increases, and the microparticle size would in any case be less than 1 pm. A fundamental characteristic of glycols is their strong interaction with transition elements [T. Gaudisson et al., CrystEngComm (2021), 23, 1756-1764], and in extreme cases, this can lead to the formation of deprotonated glycolates [J. Teichert et al., Inorg. Chem. (2019), 3820-3831], Jian and collaborators discovered that at room temperature titanium alkoxides in the presence of ethylene glycol, acetone and water gave rise to the formation of monodisperse spherical particles [X. Jiang et al, Adv. Mater.15 (2003), 1205-1209], By adjusting the precursor concentration to relatively low values ​​within the range of 0.68 to 1.2 mM, the diameter of these colloids could be adjusted between 200 and 500 nm. However, attempts to increase the size beyond 500 nm have not been successful, possibly due to the fact that polydispersity is multiplied when trying to access sizes greater than 0.5-1 pm [T. Sugimoto et al., J. Phys. Chem. C, 112, 18445-18454], and therefore require rigorous control of the water content of the solvent or solvents. Modifications of the method based on the use of other organic solvents other than acetone and the use of a solvothermal method have not been satisfactory, since the spheres obtained again reach a coefficient of variation of around 10% for sizes of 1 pm [Al-M Alam et al., ACS Appl. Mater. Interf. (2017), 9, 23941-23948].

[0012] In summary, known methods for obtaining TiO2 microparticles, such as those described above, are limited in terms of the maximum size of the microparticles that can be obtained (0.5 pm), by the high dispersion or standard deviation in sizes of the population of microparticles obtained with respect to the desired size, by the use of temperatures other than ambient temperature, and by the use of additives (such as carboxylic acids or alkylamines).

[0013] In order to overcome the limitations of prior art methods for obtaining titanium oxides, the present invention provides an alternative method for obtaining titanium oxide microparticles, based on a modified hydrolysis method, the advantages of which will be disclosed throughout the description.

[0014] DESCRIPTION OF THE INVENTION

[0015] The invention relates to a chemical synthesis method for obtaining monodisperse titanium oxide (TiO2) microparticles, specifically a hydrolysis method, comprising the use of a hydrolysis reagent, a titanium oxide precursor, and solvents, at least a glycol and a ketone, previously treated to remove any residual water they may contain. The method includes preparing the solutions, mixing and stirring them, and then separating the resulting solid TiC2 microparticles.

[0016] Specifically, the present invention provides an alternative method for obtaining spherical, monodisperse titanium oxide microparticles of selectable (but not limited) size between 0.5 and 3 pm. The synthesis route consists of the controlled hydrolysis (in aqueous medium) of hydrolyzable titanium oxide precursors dissolved in solvents (previously treated to remove residual water) capable of forming complexes with the titanium oxide precursors. This synthetic route allows for the simple, rapid, and energy-efficient production of TiO₂ monodisperse microparticles at room temperature, as disclosed below.

[0017] Therefore, a first aspect of the invention relates to the method of obtaining spherical, monodisperse titanium oxide (TiO2) microparticles comprising the steps of: a) preparing at least one solution comprising a ketone, a glycol, a hydrolysis reagent selected from water, ammonium hydroxide, tetraalkyl (C1-C5) ammonium hydroxide, and any combinations thereof, and a titanium alkoxide, b) optionally stirring the solution obtained in step (a), preferably at 200-1000 rpm for a time frame preferably greater than 5 min, and c) separating the solid obtained in step (a) or (b), wherein: the ketone and the glycol are pre-treated to step (a) to remove the residual water they contain.

[0018] According to the invention, “microparticles” are defined as those particles with a size between 0.3 pm and 3 pm, preferably between 0.5 pm and 3 pm, more preferably between 0.6 pm and 3 pm, and even more preferably between 1 pm and 3 pm.

[0019] The “size” of said microparticles refers to the size of the microparticles as measured by transmission electron microscopy (“TEM”).

[0020] According to the invention, “monodisperse particles” are defined as those populations of microparticles whose coefficient of variation does not exceed 5%.

[0021] The “coefficient of variation (CV)” is defined in its usual way, that is, as the ratio between the standard deviation (SD) and the mean size (X) of the microparticle population. The CV is usually expressed as a percentage. In the present invention, the CV is equal to or less than 5%. This CV has been calculated according to the following formula:

[0022] CV=SD / X (1)

[0023] Where the average size (X) of the microparticle population refers to the sum of the diameters of each of the microparticles observed, for example, in a TEM (Transmission Electron Microscopy) image, divided by the number of microparticles observed.

[0024] Where the standard deviation (SD) of formula 1 has been measured according to formula (2)

[0025] Where X refers to the diameter of a particular microparticle, X is the average diameter or size explained above, and yn refers to the number of microparticles observed (e.g., in a TEM image).

[0026] The advantages of this method are that it allows the production of monodisperse microparticles (CV below or around 5%, a value considered necessary / optimal for numerous applications) at a single temperature (room temperature), with a wide size range (from 0.3 µm to 3.0 µm). Furthermore, the versatility of the method allows for highly precise control of the particle size. Particle size, particularly between 0.5 and 3 µm, is precisely controlled by systematically adjusting the addition protocols for the four reagents, the concentration of all reagents, and the stirring times and speeds (specifically the optional stirring in step b). This is highly relevant because it allows for the preparation of TiC>2 microparticles of any desired size with great precision, which is important in applications where the function depends on particle size.On the other hand, the steps of the method are simple and quick, and therefore easily scalable to an industrial level.

[0027] The method of the invention requires that the ketone and glycol be free of residual water. On the one hand, water inhibits the formation of a complex between the ketone and Ti, which helps control the size of the TiO2 microparticles. Therefore, its presence inhibits the formation of this complex, favoring uncontrolled hydrolysis that would result in uncontrolled sizes of the obtained microparticles. On the other hand, the presence of water in the glycols (even though they may be able to dissolve the alkoxides in the presence of residual water) alters the formation of TiO2 microparticles and hinders the production of large microparticles, especially those larger than 1 pm.

[0028] Therefore, to carry out the method of the invention, ketones and / or glycols that do not contain residual water will be used, for example, commercial ketones and / or glycols that have been rigorously treated to remove their residual water. Therefore, a commercial ketone and / or glycol that initially does not contain residual water without the need for further treatment is considered equivalent to such a ketone and / or glycol that has been previously treated to remove the residual water.

[0029] If ketones and glycols containing wastewater are used, the wastewater must be removed beforehand. Water is a hydrolysis reagent, and its presence in the ketone and glycol, as previously mentioned, would hinder the control of the TiO2 microparticle formation reaction.

[0030] The removal of ketones and glycols from wastewater can be achieved using any known prior art wastewater treatment method, such as the use of desiccants or alternative technologies like pervaporation, or the use of molecular sieves (zeolites) as wastewater absorbents. The preferred method for water removal according to the invention is the use of zeolites, given their easy availability and versatility in treating both ketones and glycols.

[0031] This treatment involves adding or immersing zeolites (solids) within the ketone and / or liquid glycol itself, with the aim of absorbing any residual water they may contain. As mentioned, the residual water acts as a hydrolysis reagent, which would alter the formation of the TiO2 microparticles, both in terms of size and size variability. Therefore, this pre-treatment stage with zeolites is crucial for controlling the microparticle size and ensuring they are monodisperse (with a coefficient of variation CV of 5% or less). Once the treatment time has elapsed, the solid zeolites are extracted from the solvent and / or co-solvent (liquid) in order to use the solvent and / or co-solvent to form the respective solution(s).

[0032] In another preferred embodiment, at least 40% by weight of zeolites is immersed in the ketone and / or glycol, relative to the weight of the glycol and / or ketone, respectively.

[0033] In another preferred embodiment, the ketone and / or glycol in which the zeolites are immersed is stirred for at least 1 minute (preferably for 1 to 2 minutes, preferably manually) before being allowed to stand for at least 2 days, and then the zeolite is separated from the ketone and / or glycol. This achieves complete removal of any excess residual water that may be present in the ketone or glycol.

[0034] Step a) consists of preparing a solution comprising 4 components, specifically a ketone, a glycol, a hydrolysis reagent, and a titanium alkoxide.

[0035] Note that preparing several solutions comprising one or more of any of these 4 components in any order, and their subsequent mixing into a single final solution, is considered equivalent to preparing a solution comprising 4 components according to step a).

[0036] The ketone according to the invention can be any ketone, although preferably selected from acetone, acetyl ketone, and any combination thereof, as non-limiting examples. The use of these particular ketones, compared to the use of other ketones, provides the advantage that they readily form complexes with Ti alkoxides at room temperature, which facilitates control of both the size of the Ti₂ microparticles formed according to the method of the invention and their size variability (CV less than 5%).

[0037] The glycol according to the invention can be any glycol, although preferably selected from ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, and any combination thereof, as non-limiting examples. In another preferred embodiment, the glycol is selected from ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, and combinations thereof.

[0038] The use of glycol(s) as a co-solvent is particularly advantageous, given their strong interaction with transition elements such as titanium (from titanium alkoxide). This interaction helps control the hydrolysis reaction between the titanium alkoxide and the hydrolysis reagent, thus contributing to the control of the size and degree of monodispersity (CV <5%) of the titanium oxide microparticles that form. Additionally, removing residual water from the glycol provides the advantage that the wastewater does not interfere with the formation of the titanium microparticles, allowing for the production of TiO₂ microparticles larger than 0.5 pm, which is not possible if the glycol contributes wastewater during microparticle formation.

[0039] The hydrolysis reagent is selected from water, ammonium hydroxide, tetraalkyl(C1-C5) ammonium hydroxide, and any combination thereof. In a preferred embodiment, the tetraalkyl(C1-C5) ammonium hydroxide is selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide (CH3CH2)4N(OH), tetrapropylammonium hydroxide (CH3CH2CH2)4N(OH), tetrabutylammonium hydroxide (CH3CH2CH2CH2)4N(OH), and combinations thereof. These particular ammonium hydroxides offer the advantage of readily dissolving in polar media and increasing the basicity of the aqueous medium, thereby affecting the rate of hydrolysis.

[0040] Titanium alkoxide can be any hydrolyzable alkoxide, although preferably selected from titanium butoxide, titanium isopropoxide, and combinations thereof, as non-limiting examples.

[0041] Alternatively, prior to step a), a solution 1 can be prepared comprising, in addition to the hydrolysis reagent, a ketone and / or glycol as a solvent and / or co-solvent, solution 1 which would then be mixed in step a), and therefore without the need to add the hydrolysis reagent, and the ketone and / or glycol in step a).

[0042] To ensure a homogeneous mixture of the components of this solution 1 (at least the hydrolysis reagent and the ketone and / or glycol), mixing is carried out at at least 500 rpm for a short time (not exceeding 10 minutes) in order to mix the reagents, but not to progress in the formation of the complex between the ketone and the titanium alkoxide that would correspond to another protocol mentioned later.

[0043] Alternatively, prior to step a), a solution 2 can be prepared comprising, in addition to titanium alkoxide, also a ketone and / or a glycol as a solvent and / or co-solvent, solution 2 which would then be mixed in step a), and therefore without the need to add the titanium alkoxide, and the ketone and / or glycol in step a).

[0044] The solution prepared in step a) comprises the 4 components (ketone, glycol, hydrolysis reagent, and titanium alkoxide), where the mixing of said 4 components can be carried out in any order.

[0045] Step b) is optional and consists of stirring the solution resulting from step a), preferably at a moderate stirring speed between 200 and 1000 rpm, and preferably for at least 5 minutes. The stirring speed and time can be selected based on the desired characteristics (size and monodispersity) of the TiC>2 microparticles. Performing this optional stirring step b) helps to obtain larger particle sizes compared to simply mixing the four components according to step a) without performing step b).

[0046] Furthermore, this step b) provides the time and agitation necessary for the hydrolysis reaction between the titanium alkoxide and the hydrolysis reagent to occur, thus forming titanium oxide (TiO2) microparticles of desired size and dispersion, depending on the time and speed of agitation. The size and dispersion of the microparticles are adjustable according to the characteristics of the reagents mixed in step a) (for example, their concentrations, or the degree of basicity determined by the water / hydroxide ratio), as well as the mixing times and speeds. This is particularly relevant if, prior to step a) where all the reagents are mixed, intermediate solutions are generated, as is the case with solutions 1 and 2. These intermediate solutions can be agitated at different speeds and times before being finally mixed together in step a), which comprises all four components.

[0047] The parameters of the previous stage of wastewater removal from the solvents and / or co-solvents (i.e., ketones and / or glycols) also help to control the sizes and dispersion of the TiC>2 microparticles obtained by the method of the invention.

[0048] Finally, step c) is carried out where the solid TiC>2 microparticles obtained after step b), which are the entire solid fraction present in the mixture, are separated from the rest of the liquids present in the mixture from step c). The separation of the solid (TiO2 microparticles) from the rest of the liquid fraction can be carried out using any known solid-liquid separation technique in the state of the art, for example, by filtration or centrifugation, without being limited to these techniques.

[0049] In one embodiment of the method of the present invention, the dissolution of step (a) comprises: i. preparing a solution 1 comprising the hydrolysis reagent and a co-solvent selected from glycol and ketone, and / or

[0050] i. Prepare a solution 2 comprising titanium alkoxide and a solvent selected from glycol and ketone. Where: if the solvent is glycol or ketone, the co-solvent is ketone or glycol, respectively, and solutions 1 or 2 are mixed with the remaining components of step a).

[0051] In a preferred embodiment, solutions 1 and 2, prior to being mixed with the other components in step a), are stirred at at least 500–2000 rpm, preferably for 5 to 10 minutes, and even more preferably for 10 minutes. This pre-homogenizes solutions 1 and 2 before mixing all four components in step a).

[0052] In a preferred embodiment, the method of the invention comprises preparing a solution 2 comprising ketone together with titanium alkoxide. This mixture is particularly advantageous because a complex is formed between the ketone and the titanium alkoxide, which is visually observable (as the initially clear solution gradually acquires a yellow coloration, the intensity of which increases with stirring time due to the progressive formation of the complex). This complex has a significantly slower hydrolysis rate compared to the initial alkoxide, which is very helpful in controlling the size and degree of monodispersity (CV <5%) of the titanium oxide microparticles formed. It is important to note that this ketone-alkoxide complex does not form if the ketone contains residual water; hence the importance of removing residual water from the ketone before use.

[0053] In fact, the formation of complexes between the ketone and titanium oxide increases the amount of solid (TiO2 microparticles) obtained after performing the method of the invention, since it allows adding a higher initial concentration of titanium alkoxide to obtain a wide variety of monodisperse titania microparticles.

[0054] Therefore, given the formation of these complexes, in the case of preparing a solution of titanium alkoxide in acetone, the stirring time of this solution is another variable that allows control of the particle size.

[0055] In a preferred embodiment, the method of the invention comprises the preparation of a solution 2 comprising titanium alkoxide and a glycol (from which residual water has been previously removed). This preferred embodiment also allows for obtaining microparticles with the desired size and degree of monodispersity. Indeed, as explained above, the glycol exhibits a strong interaction with transition elements such as titanium, slowing down the reaction of titanium with the hydrolysis reagent.Hence, it is advantageous to prepare a prior solution 2 of glycol and titanium alkoxide that does not yet contain the hydrolysis reagent, thus providing time for the strong interaction between the glycol and the titanium, so that when it is mixed with the hydrolysis reagent of step a), the hydrolysis reaction between the titanium and the hydrolysis reagent is slowed down even further, thanks to the prior contact between the glycol and the titanium before being mixed with the hydrolysis reagent.

[0056] In a preferred embodiment, the method of the invention comprises:

[0057] (i) prepare solution 1 comprising a hydrolysis reagent and a glycol, preferably diethylene glycol; and optionally stir solution 1 at 200-1000 rpm for at least 10 minutes; (ii) prepare solution 2 comprising titanium alkoxide and ketone; and optionally stir solution 2 at 200-1000 rpm for at least 10 minutes;

[0058] (iii) mix solution 2 with solution 1, preferably by stirring at 200-1000 rpm for at least 5 minutes.

[0059] Note that the stirring time and speed of substages (i), (iii) and (iii) is variable depending on the characteristics of the microparticles to be obtained.

[0060] In another preferred embodiment, since wastewater negatively affects the control of the TiC>2 microparticle formation reaction, the material to be used according to the method (material that is not attacked or corroded by the reagents used, preferably glass or Teflon) is freed from wastewater or ambient water, preferably by drying it with a temperature prior to use (e.g., at 70 °C as an example of a non-limiting temperature), preferably by storing it in an oven to avoid contact with ambient humidity (e.g., at 70 °C).

[0061] In another preferred embodiment, step c) is carried out by centrifugation. That is, the solid (TiO2 microparticles) is separated from the remaining liquid by centrifugation. The use of centrifugation to separate the TiO2 microparticles offers the advantage that, for microparticles of this size (0.5 to 3 pm), it is a technique that achieves the separation of the solid microparticles from the remaining liquid very quickly (complete separation of the microparticles is ensured in less than 5 minutes).

[0062] In another preferred embodiment, after step c), a step d) is performed comprising washing the solid (TiO2 microparticles) at least once with a polar solvent, preferably selected from acetone, ethanol, isopropanol, etc., and any combination thereof. Washing the TiO2 microparticles with one of these liquids after separating them from the remaining liquid of the solution obtained in step c) provides the advantage of removing any unreacted reagents, thus purifying the sample and preventing future side reactions between these reagents that could alter the properties of the resulting TiO2 microparticles.

[0063] In another preferred embodiment, after step c) or d), a drying step is performed on the solid obtained from the solution, preferably by heating the solid in an oven at a moderate temperature of 50-70°C. This ensures thorough drying without thermally decomposing the solid. The drying time is not limited, although it is sufficiently long, for example, at least 10 hours, to ensure thorough drying. This step can be carried out using any drying technique known in the prior art, for example, by heating to a certain temperature, or in desiccant or vacuum chambers, or with the use of a dry gas under pressure, or by any other method known in the prior art.This drying stage, carried out after stage c) of separation of the solid microparticles, or after stage d) of washing, provides the advantage that it eliminates any possible washing liquid that might remain absorbed in the microparticles, a liquid that would contain the previously washed reagents, thus purifying the sample and preventing future secondary reactions between these reagents that could modify the properties of the TiC>2 microparticles obtained.

[0064] A second aspect of the invention relates to the spherical, monodisperse titanium oxide (TiO2) microparticles obtained by the method of the present invention, with a particle size between 0.3 and 3 pm, preferably between 0.5 pm and 3 pm, more preferably between 0.60 pm and 3 pm, and still more preferably between 1 pm and 3 pm. Furthermore, these microparticles have a coefficient of variation of the degree of monodispersity less than or equal to 5%. These specific particles within these size ranges and with a coefficient of variation less than or equal to 5% offer the advantage of being similar in size to the wavelength of visible light, and their low size variability provides them with specific characteristics useful, for example, for forming tunable photonic crystals or exhibiting Mie resonances in an oxide with a high refractive index.

[0065] These specific solid TiO2 microparticles, of these sizes and with a low coefficient of variation, are highly advantageous for use in diverse fields such as renewable energy, the environment, optical applications, and even biotechnology. Examples of such applications include UV-active photocatalysts, solar cells, anode material for electrochemical cells, biotechnological applications, construction of high-performance, uniform, and dense particle networks for environmental and energy applications, radiative cooling, etc. Throughout the description and claims, the word "comprises" and its variations are not intended to exclude other technical features, additives, components, or steps. For those skilled in the art, other objects, advantages, and features of the invention will become apparent partly from the description and partly from the practice of the invention.The following examples and figures are provided for illustrative purposes and are not intended to be limiting to the present invention.

[0066] BRIEF DESCRIPTION OF THE FIGURES

[0067] FIG. 1.- TEM image of the particles obtained following the protocol described in Example number 1. The bar with the scale represents in this case 1 pm.

[0068] FIG. 2.- TEM image of the particles obtained following the protocol described in Example number 2. The bar with the scale represents in this case 1 pm.

[0069] FIG. 3.- TEM image of the particles obtained following the protocol described in Example number 3. The bar with the scale represents in this case 1 pm.

[0070] FIG. 4.- TEM image of the particles obtained following the protocol described in Example number 4. The bar with the scale represents in this case 5 pm.

[0071] FIG. 5.- TEM image of the particles obtained following the protocol described in Example number 5. The scale bar represents 5 pm in this case.

[0072] EXAMPLES

[0073] Prior to carrying out the following examples, all glass material was stored in an oven at 70 °C, to avoid contact with ambient humidity and thus eliminate possible traces of adsorbed water.

[0074] In all the examples (1-5) detailed below, the acetone, ethylene glycol (EG), and diethylene glycol (DEG) contained residual water, so they were pre-treated with zeolites to absorb this residual water. The zeolite treatment consisted of adding approximately 40 g of zeolites per 100 g of acetone or glycol, stirring manually, and keeping the zeolites submerged in the acetone and / or glycol for at least 2 days to dry, after which the zeolite was removed from the acetone and / or glycol.

[0075] Example 1 (particles of 0.48 m and coefficient of variation of 5%)

[0076] To form 0.48 pm monodisperse particles, a solution of 300 µL of titanium butoxide dissolved in 36 mL of acetone was first prepared. This solution was then stirred at 500 rpm for 20 h to form the acetone-titrogen alkoxide complex, which slows the subsequent hydrolysis of the titanium alkoxide by the hydrolysis reagent. Subsequently, 2 mL of DEG were added at the same stirring speed (500 rpm), followed by the addition of 50 µL of ammonium hydroxide (28% NH3 in H2O) as the hydrolysis reagent, also at the same stirring speed. Immediately afterward, 2 mL of EG were added, followed by stirring for a reaction time of 24 h. Once the reaction was complete, the resulting solid (T¡Ü2 microparticles) was separated from the reaction medium by centrifugation and subsequently washed with acetone and EtOH, and finally dried in a 50 °C oven.

[0077] Figure 1 shows a representative TEM image of the particles obtained from T¡Ü2 in Example 1. The samples were deposited on a TEM grid prior to drying the solid to improve image quality.

[0078] This method yields monodisperse microparticles of 0.48 pm, in a similar size range (0.2-0.5 pm) to that obtained in the article by Jiang et al. [X. Jiang et al, Adv. Mater. 15 (2003), 1205-1209]. Clearly, the use of diethylene glycol in this example 1, and especially the prior formation of the acetone-Ti alkoxide complex, results in an increase in efficiency related to the fact that the initial concentration of titanium alkoxide is significantly higher and therefore the amount of Ti₂ resulting after the synthesis method of the invention is greater.

[0079] In summary, in Example 1, 0.48 pm monodisperse particles were obtained using diethylene glycol (DEG) in addition to ethylene glycol (EG). This is a representative example of how the slowing of the Ti alkoxide hydrolysis rate can be achieved through the prior formation of a complex with acetone. This complex only forms after the excess water is removed from the acetone (in this case, using zeolites). Visually, the progression of complex formation is observed by the formation of an initial transparent solution with a slight yellow tint, the intensity of which increases with stirring time due to the aforementioned complex formation between acetone and titanium alkoxide.In fact, if the acetone used is not pre-treated to remove residual water, the excess water in the solvent activates the alkoxide hydrolysis reaction, and titania (T1U2) precipitates uncontrollably.

[0080] Example 2 (particles of 0.72 m and coefficient of variation of 5%).

[0081] In another representative example of the method's versatility, monodisperse particles of 0.72 pm were obtained. Microparticles larger than 0.5 pm were achieved thanks to rigorous control of the water content of the solvents and co-solvents.

[0082] In this case, the protocol differs from that described in Example 1, which illustrates the versatility of the method proposed here, since the addition of reagents is not limited to a single procedure. Thus, two solutions, 1 and 2, were prepared separately.

[0083] Solution 1 was obtained by dissolving 75 pL of water in 7.5 mL of ethylene glycol, and then stirring at 500 rpm for at least 10 min. Solution 2 was obtained by dissolving 250 pL of titanium butoxide in 45 mL of acetone, and then stirring at 200 rpm for 10 min.

[0084] As in example 1, removing wastewater prevents the hydrolysis reaction of titanium alkoxide with water, which is a hydrolysis reagent per se, thus preventing titania (TiO2) from precipitating uncontrollably in solution 2.

[0085] Once the two solutions were prepared, solution 1 was quickly added to solution 2, and the resulting solution was kept stirring at 500 rpm for a reaction time of 3 hours.

[0086] Once the reaction was complete, the resulting solid (TiO2 microparticles) was separated from the reaction medium by centrifugation and subsequently washed with acetone and ethanol (EtOH), and finally dried in a 50°C oven.

[0087] Figure 2 shows a representative TEM image of the TiO2 microparticles obtained in Example 2. The samples were deposited on a TEM grid prior to drying the solid to improve image quality.

[0088] Example 3 (particles of 1.5 pm and coefficient of variation of 3%)

[0089] Example 3 is another representative example of the versatility of the method, where monodisperse particles of 1.5 pm were obtained by changing the reactant addition protocol in order to illustrate the importance of excess water in the co-solvents used (ethylene glycol in this specific case, although it also applies to other types of glycols).

[0090] In this example 3, solution 1 was obtained by dissolving 40 pL of ammonium hydroxide (28% NH3 in H2O) in 36 mL of acetone with stirring at 200 rpm for 10 min. Solution 2 was obtained by dissolving 300 pL of titanium butoxide in 5 mL of EG with stirring at 500 rpm for 10 min.

[0091] Once the two solutions were prepared, solution 1 was quickly added to solution 2, followed by stirring at 500 rpm for a reaction time of 3 hours. The resulting solid (TiO2 microparticles) was then separated from the reaction medium by centrifugation, subsequently washed with acetone and EtOH, and finally dried in an oven at 50 °C.

[0092] Figure 3 shows a representative TEM image of the particles obtained in Example 3. The samples were deposited on a TEM grid prior to drying the solid to improve image quality.

[0093] Note that in example 3, the glycol is mixed with the alkoxide in solution 2, and the fact that the residual water has been removed from the glycol in particular (although also from the ketone in solution 1) is important because, although glycols can dissolve alkoxides in the presence of residual water, the presence of this water adds more water to the system, favoring the rapid nucleation of microparticles, which prevents obtaining microparticles larger than 0.5 pm. This results in obtaining particles of 1.5 pm, well above the 0.5 pm limit mentioned in previous work [X. Jiang et al, Adv. Mater. 15 (2003), 1205-1209].

[0094] Example 4 (particles of 2.1 pm and a coefficient of variation of 2%) Example 4 is another example that shows how, by varying the concentration of reactants in order to slow down nucleation, monodisperse particles of a size above 2 pm can be obtained. In this case, the protocol is similar to that of Example 2, but replacing water with ammonium hydroxide and varying the concentrations of the other reactants, thus making it possible to go from a size of 0.72 pm (Example 2) to 2.1 pm (this Example 4).

[0095] Thus, two solutions, 1 and 2, were prepared separately. Solution 1 was obtained by dissolving 30 pL of ammonium hydroxide (28% NH3 in H2O) in 3.5 mL of ethylene glycol previously treated with zeolites, stirring at 500 rpm for at least 10 min. Solution 2 was obtained by dissolving 250 pL of titanium butoxide in 18 mL of acetone previously treated with zeolites, followed by stirring at 200 rpm for 10 min.

[0096] Solution 1 was rapidly added to solution 2, followed by stirring at 500 rpm for a reaction time of 4.5 hours. Once the reaction was complete, the resulting solid (TiO2 microparticles) was separated from the reaction medium by centrifugation, then washed with acetone and EtOH, and finally dried in an oven at 50 °C.

[0097] Figure 4 shows a representative TEM image of the particles obtained in Example 4. The samples were deposited on a TEM grid prior to drying the solid to improve image quality.

[0098] Example 5 (particles of 2.9 pm and coefficient of variation of 4%)

[0099] Example 5 is also a representative example of how, by increasing the concentration of reactants to slow nucleation, even monodisperse particles as small as around 3 pm can be obtained. Furthermore, compared to Example 4, it shows how a greater quantity of TiO2 microparticles can be obtained by increasing the reaction volume from around 20 mL (Example 4) to around 108 mL (Example 5).

[0100] The preparation of solutions 1 and 2, as well as their mixing and reaction conditions, were the same as in Example 4, but the amount of reactants and the volume were different. In this Example 5, solution 1 was prepared from 80 pL of ammonium hydroxide (28% NH3 in H2O) and 18 mL of ethylene glycol, and solution 2 from 1250 pL of titanium butoxide and 90 mL of acetone (the total reaction volume being therefore approximately 108 mL).

[0101] Figure 5 shows a representative TEM image of the particles obtained in Example 5. The samples were deposited on a TEM grid prior to drying the solid to improve image quality.

Claims

CLAIMS 1. A method for obtaining spherical, monodisperse titanium oxide (T₂) microparticles comprising the steps of: a) preparing at least one solution comprising a ketone, a glycol, a hydrolysis reagent selected from water, ammonium hydroxide, tetraalkyl (C₁-C₅) ammonium hydroxide, and any combinations thereof, and a titanium alkoxide, b) optionally stirring the mixture of step (a), preferably at 200-1000 rpm for at least 5 min, and c) separating the solid obtained in step (a) or (b), wherein: the ketone and the glycol are pre-treated to step (a) to remove the residual water they contain.

2. Method according to claim 1, wherein step (a) comprises: i. preparing a solution 1 comprising the hydrolysis reagent and a co-solvent selected from glycol and ketone, and / or i. prepare a solution 2 comprising titanium alkoxide and a solvent selected from glycol and ketone, wherein: if the solvent is glycol or ketone, the co-solvent is ketone or glycol, respectively, and solutions 1 or 2 are mixed with the remaining components of step a).

3. Method according to claim 1 or 2, wherein the ketone and / or glycol are treated with zeolites to remove the wastewater they contain.

4. Method according to claim 3, wherein at least 40% weight of zeolites is immersed in the ketone and / or glycol with respect to the weight of the glycol and / or ketone, respectively.

5. Method according to claim 3 or 4, wherein the ketone and / or glycol in which the zeolites are immersed is stirred for at least 1 minute, before being left to stand for at least 2 days, and subsequently separating the zeolite from the ketone and / or glycol.

6. Method according to any one of claims 1 to 5, wherein the tetraalkyl (C1-C5) ammonium hydroxide is selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, and any combinations thereof.

7. Method according to any one of claims 1 to 6, wherein the titanium alkoxide is selected from titanium butoxide, titanium isopropoxide, and any combinations thereof.

8. Method according to any one of claims 1 to 7, wherein the ketone is selected from acetone, acetyl ketone, and any combination thereof.

9. Method according to any one of claims 1 to 8, wherein the glycol is selected from ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, and any combinations thereof.

10. Method according to any of claims 1 to 9, wherein step c) is performed by centrifugation.

11. Method according to any of claims 1 to 10, wherein after step c), a step d) is performed comprising washing the solid at least once with a polar solvent, preferably selected from acetone, ethanol, isopropanol, and any combination thereof.

12. Method according to any of claims 1 to 11, wherein after step c) or d), a drying step of the obtained solid is carried out, preferably by heating the solid to a temperature between 50 °C and 70 °C.

13. Spherical, monodisperse titanium oxide (TiO2) microparticles obtained by the method described in any of claims 1 to 12, having a size of particle size between 0.5 pm and 3 pm and a coefficient of variation of the degree of monodispersity less than or equal to 5%.

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