SI23501A - Synthesis method for obtaining anatase nanoparticles of high specificsurface area and spherical morphology - Google Patents

Abstract

The subject of the invention is anatase nanoparticles and ways of the synthesis thereof by a gel-sol method from sodium titanate, which is produced from metatitanic acid in a reaction with NaOH. The molar ratio between NaOH and TiO2 must be low for a synthesis of sodium titanate, if the goal is to synthetize anatase, i. e. is between 1:3 to 1:1. If the goal is to synthetize rutile, the molar ratio must be higher, at least between 3:1 to 4:1. In the gel-sol reaction between sodium titanate and acid very small anatase particles are formed that are of spherical morphology, polycrystalline and present in the acidic suspension. The size of crystallites that constitute the anatase particles is approximately 5 nm, wherein the particles themselves are bigger, somewhere between 40 to 60 nm. The subject of the invention is also a manner of stabilising the acidic suspension of nanoparticles, which allows an increase in pH value without causing excessive agglomeration, which is achieved by the use of an suitable dispersant, citric acid. A further subject of the invention is a hydrothermal method, which allows re-crystallisation of polycrystalline anatase nanoparticles into a monocrystalline form under hydrothermal conditions in an autoclave at a temperature about 130 stopinj C or more and at adequate pressure of the water vapour.

Description

Synthesis process for the production of high specific surface anatase nanoparticles and spherical morphology

The subject of the invention is a synthesis process for the production of high specific surface anatase nanoparticles and spherical morphology based on a gel-sol reaction, where metatitanic acid is used as the starting material, which is a nanocrystalline anatase gel and is a by-product of the production of TiO ₂ pigment and anatase nanoparticles. this procedure. The invention also provides sodium titanate and a synthesis process for the production of sodium titanate, the form of which is suitable for use in the synthesis process of the production of anatase nanoparticles. It is also an object of the invention to purify the resulting acid suspension of anatase nanoparticles from which excess acid is removed by centrifugation and further neutralize the purified suspension while raising the pH without agglomerating the nanoparticles. It is also an object of the invention to obtain monocrystalline nanoparticles from nanoparticles which are polycrystalline from sodium titanate synthesis by recrystallization in a hydrothermal reaction.

The invention provides a process whereby a suitable form of sodium titanate, which is suitable for the preparation of rutile nanoparticles or anatase nanoparticles in reaction with acid, is prepared from metatitanic acid, which is an intermediate in the process of producing pigment TiO ₂ . Sodium titanate is prepared by the reaction of metatitanoic acid with a concentrated NaOH base, and the ratio of the added NaOH to TiO ₂ in metatitanic acid directly determines the form of the titanium formed. The form of sodium titanate depends on whether rutile or anatase will form in the form of well-dispersed nanoparticles in reaction with the acid. The resulting particles are polycrystalline, which means that they consist of a number of smaller crystallites clustered together in a particle.

The invention also relates to the synthesis of anatase nanoparticles obtained by the reaction between a specific form of sodium titanate and hydrochloric acid in so-called. gel-salt reaction leading to the formation of well-dispersed nanoparticles in an acidic aqueous medium. After the synthesis, the nanoparticles are well dispersed and not agglomerated because they have a high zeta potential due to their low pH, which allows them to repel each other due to electrostatic forces. As the pH changes, the potential of the particles decreases, leading to agglomeration, which is undesirable since the nanoparticles no longer exhibit their original properties due to their large specific surface area. It is therefore necessary to raise the suspension pH in the presence of • ·

-2 of suitable dispersant, which is a surfactant component and prevents the agglomeration of nanoparticles based on the establishment of steric, electrostatic and / or electrostatic forces.

It is also an object of the invention to perform an appropriate stabilization of the nanoparticles in suspension, whereby a dispersant is added which prevents the nanoparticle agglomeration. This preserves the original properties of the nanoparticles, which is very important for a number of high-tech applications of TiO ₂ nanoparticles, which require the use of chemically neutral aqueous suspensions, including thin film coatings for self-cleaning coatings, electrochemical solar cells, photocatalysts, UV protective coatings, ... the syntheses are essentially polycrystalline, meaning that a single particle is composed of several smaller crystallites. This may affect some of the physical properties of the material, so in some cases it is desirable to prepare the particles in the so-called monocrystalline form. This means that the individual particle is also a crystal in itself. Transformation into a single crystalline form can be achieved by thermal treatment at a higher temperature, which is undesirable since there is always a large agglomeration of the product and potentially harmful dusting and high energy consumption. The present invention contemplates an alternative process based on the so-called hydrothermal method, in which the material in the form of an aqueous suspension is subjected to high temperature and pressure in an autoclave, whereby the polycrystalline particles are converted to a monocrystalline form. The advantage of the process is that in no step does the potentially harmful dusting occur, as well as the energy consumption is lower than in the case of heat treatment.

All of the processes contemplated by the invention are based on wet chemistry processes and carried out in aqueous suspension. Suspension processing is very important, as no intermediate step is created in any step, potentially harmful to the dusty phase, and well crystalline nanoparticles are formed as a final product, which eliminates the need for an energy-consuming calcination process and burdensome greenhouse gas environment.

Titanium dioxide in the form of pigment is a material with a wide range of applications and is useful for coatings, paints, as an adjunct to plastics, paper, cosmetics, the pharmaceutical industry and many other applications. Today, two processes for the production of pigment titanium dioxide, i.e. sulfate and chloride process. Both the sulfate and chloride process are based on high-temperature conversion of the corresponding components into pigment titanium dioxide. In the sulphate process, the pigment is formed in the process of calcination in the hydrolysis reactions of the obtained anatase gel, while in the chloride process, the pigment is formed.

-3High-temperature combustion of titanium tetrachloride with oxygen. Precisely the high-temperature calcination process, which is part of the sulfate and chloride process, prevents the possibility of producing nano-sized anatase particles, and it is also typical that high calcination temperature always leads to the conversion of anatase to the thermodynamically more stable TiO ₂ phase, rutile. The calcination process is also heavily burdened with high energy consumption and a high amount of greenhouse gas emissions is received. In addition, the resulting product is present in the form of dust which can be potentially harmful to health and the environment.

This is why many different processes for the production of anatase nanoparticles have been developed in recent years.

US 20060254461 discloses a process for the preparation of anatase nanoparticles by a sol-gel process, from starting organic alkoxides, which under specific reaction conditions hydrolyze and form the corresponding salt. The salt then undergoes an aging process at a temperature of 120-140 ° C, which triggers the nucleation and growth of well crystalline anatase nanoparticles.

US 4954476 discloses a process for the production of anatase nanoparticles by the hydrothermal method. The starting material is meta- or orthotitanoic acid, which is then subjected to a hydrothermal reaction under specific conditions. Typically, a hydrothermal reaction takes place for several hours at a temperature of about 180 ° C and an elevated pressure of the aqueous suspension. At lower temperature, hydrothermal synthesis takes a long time, even for several days.

US 7510694 discloses a process for the preparation of anatase nanoparticles from a solution of titanium tetrachloride to which an appropriate amount of hydrazine monohydrate is added. The reaction is carried out at room temperature at a pH of about 8, yielding a product of high specific surface area with very small basic anatase crystallites.

The above examples of synthesis of anatase nanoparticles lead to the desired product, but are unsuitable or less suitable for industrial production, since the processes described are either based on the use of expensive and / or toxic organic substances, or it takes a long and energy-consuming process to obtain the desired shape of the nanoparticles. A major disadvantage of the processes described is that the end product is a highly agglomerated powder of anatase nanoparticles, which reduces the possibility of application of the end product.

Anatase nanoparticles and the synthesis process for the production of anatase nanoparticles according to the invention eliminate the disadvantages given that they allow the production of uniformly large, good * ·

-4 crystalline and well-dispersed anatase nanoparticles in stable acid suspension, which greatly facilitates their use in high-tech applications.

The preparation of the nanoparticles according to the invention is based on gel-salt synthesis, where the raw material sodium titanate is obtained, which is obtained from metatitanic acid in reaction with an appropriate amount of NaOH base. The synthesis of sodium titanate is carried out at a temperature below 90 ° C, but the temperature may also be higher ah lower during the reaction.

In the next reaction with hydrochloric acid, sodium titanate is converted into a suspension of well-dispersed anatase nanoparticles, which are polycrystalline and spherical in shape and approximately 40 nm in size. The process allows to control their size by selecting the appropriate reaction parameters so that larger or smaller particles of 40 nm can also be prepared. The synthesis of anatase nanoparticles is carried out when heated to about 80 ° C, but the temperature may also be higher or lower during the reaction.

Since the suspension of anatase nanoparticles was formed after acid synthesis, this limits its use in certain applications. A particular application may require the use of a suspension of nanoparticles with a specific pH value, so it is very important that the suspension can be adjusted to the desired pH value, but at the same time it does not cause particle agglomeration. Several sessions have been developed to address this problem.

US7344591 describes a process for forming a suspension of TiO ₂ nanoparticles in the presence of a suitable dispersant (glycine, glycolic acid). The process is based on the synthesis of TiO ₂ nanoparticles in a medium in which the dispersant is already present and leads to the formation of a suspension in which the nanoparticles would be well dispersed, since the added dispersant would prevent agglomeration as early as the particle formation stage. However, the process was performed in a manner that prevented the formation of uniformly large particles, and the DLS (Dynamic Light Scattering) measurements performed were inappropriate, since the polydispersity factor of the suspensions was very low, meaning that the DLS measurement results provided do not show the actual state.

US20080317959 describes a process by which TiO ₂ nanoparticles are synthesized and stabilized using suitable surfactants such as Triton Χ-100. The downside of the process described is that it is based on the use of surfactants, which are relatively expensive and thus less suitable for use on a larger, industrial scale. Also, the resulting suspensions had a very low TiO ₂ content, which is very poor from an economic point of view.

The present invention solves these problems by describing a process by which the acid suspension of anatase nanoparticles is converted to a neutral suspension without the nanoparticles being

-5 over agglomerated. This is achieved by the use of a suitable dispersant which, while adding a base, prevents the agglomeration of the particles by binding to their surface. The dispersant used for this purpose is relatively inexpensive and allows the preparation of stable suspensions with a different TiC content of> 2, up to 50%.

The dispersant is added to the acidic suspension while stirring and dissolved in an aqueous medium to distribute evenly within the reaction medium. A concentrated base is then added, which may be NaOH or any other (NH3, KOH, Na2CO ₃ , ...). The base is added while stirring to the desired pH value, with the most appropriate pH value being about 7. The pH value can be appropriately adjusted by addition of the base and can be between 4 and 12, without suspending the stability of the suspension significantly.

After synthesis and / or stabilization, we have a suspension in which the nanoparticles are well dispersed at the desired pH. However, since the nanoparticles are essentially polycrystalline, this means that they do not exhibit optimal physical and chemical properties, since a single particle is composed of several smaller crystallites, and the boundaries between them can be a barrier to a particular process (eg electron flow). The optimum properties of the nanoparticles are achieved only if the particles are in the form of a single crystal, since in this case their properties are not affected by any defects in the internal structure, such as e.g. the boundaries between the individual crystallites. The processes that allow the formation of a single crystalline form are usually based on thermal treatment, in which exposure to high temperature leads to the consolidation of smaller crystallites into larger ones. Because this process is energy consuming and leads to the formation of a dry, potentially harmful dust phase, it is less appropriate. The alternative procedure is based on i.i. a hydrothermal process in which the monocrystalline form of the material can be formed in an aqueous medium at a temperature above boiling point and at this suitable high pressure. The hydrothermal method is a well-known synthesis method and is contemplated by various inventions.

US20090223412 discloses a hydrothermal method for the formation of single crystalline T1O2 nanoparticles using a water-soluble titanium component as the feedstock. It is exposed to hydrothermal conditions at a temperature of 160 ° C for a period of 16 hours. A similar process is described in US5776239, where the hydrothermal method was performed at a higher temperature and for a shorter time, but again only on the basis of a water-soluble titanium component.

The disadvantage of both processes is that it is only during the course of the reaction that the form of the final product is defined, since the particles are formed from the water-soluble component. It is also described for both

-6 The process is characterized by the fact that the reaction is long-lasting, which is a big problem for a possible transfer to the industrial level. This is all the more aggravated by the very high temperatures described in both procedures.

The present invention contemplates a shorter process carried out at a relatively low temperature based on a hydrothermal method that enables the conversion of polycrystalline anatase particles to monocrystalline ones. The raw material to be used is a stable suspension of anatase nanoparticles with a pH between 4 and 12. The suspension is subjected to hydrothermal conditions, at a temperature between 130 and 250 ° C and a pressure that coincides with that temperature. The hydrothermal reaction takes 2 to 4 hours to form monocrystalline anatase nanoparticles. The resulting particles are very similar in size to the starting, polycrystalline particles and are also similar in shape.

The main object of the present invention is to provide processes suitable for the industrial production of anatase nanoparticles of various sizes and crystallinity in the form of acidic, basic and / or neutral suspensions in which the nanoparticles are well dispersed and preferably non-agglomerated. The main advantages of the present invention are:

that the process is based on the use of a raw material, sodium titanate, which can be produced from so-called metatitanic acid, which is an important intermediate in the production of TiO ₂ pigment by the sulphate process, that sodium titanate is produced from metatitanic acid at a relatively low temperature (below 100 ° C) in reaction with NaOH to define the form of sodium titanate, which is the raw material for the synthesis of TiO ₂ nanoparticles, by the amount of NaOH or the molar ratio between TiO ₂ and NaOH. If the molar ratio of NaOH: TiO _{2 is} between 3 and 4, sodium titanate is obtained, which is only suitable for obtaining TiO ₂ nanoparticles of the crystalline structure of rutile. If the molar ratio is smaller between 1 and 2, sodium titanate is formed, which is suitable for the production of TiO ₂ nanoparticles of the crystalline structure of anatase without simultaneously forming rutile.

to obtain anatase nanoparticles to obtain sodium titanate, which can be properly purified to remove excess base by filtration, decantation and / or centrifugation, to resuspend the purified sodium titanate in water and to add hydrochloric acid or nitric acid to the appropriate concentration, and synthesize anatase nanoparticles in a reaction at elevated temperature but below boiling point (below 100 ° C) to form the nanoparticles as an acidic suspension in which they are well dispersed,

-7 that the nanoparticles are formed in the form of well polycrystalline particles, which means that there is no need for thermal treatment, which saves energy and prevents the consequent release of greenhouse gases into the environment, that the nanoparticles are evenly sized, and their size can be controlled by choice appropriate reaction parameters to easily neutralize the acidic suspension using a base in the presence of a dispersant, a citric acid that prevents unwanted agglomeration of the nanoparticles. Suspension stability is conditional on the amount of dispersant added and should be at least 10% by weight of TiO ₂ in suspension.

that the resulting suspension may be subjected to, e.g. a hydrothermal process where the suspension is subjected to high temperatures above the boiling point and to this appropriate pressure in the autoclave, resulting in the transformation of the polycrystalline particles into monocrystalline ones.

- that the hydrothermal method is carried out in suspension, which prevents the formation of a potentially harmful dust phase, that all of these processes have a very high efficiency, above 95%

Description of the invention

The invention will hereinafter be described by process descriptions, figures and embodiments that adequately illustrate the process of synthesis of anatase nanoparticles, its stabilization with a suitable dispersant, and the use of a hydrothermal method to obtain the monocrystalline form of nanoparticles. The invention will also contain a detailed description of the feedstock, sodium titanate, which, depending on the synthesis process performed, may be the feedstock for the production of rutile nanoparticles or anatase nanoparticles.

Pictures show:

Figure 1: Sodium titanate suitable for the synthesis of anatase nanoparticles in the gel-sol reaction obtained according to the invention in embodiment 1 and taken with a scanning electron microscope,

Figure 2: Anatase nanoparticles obtained according to the invention in embodiment 1, taken with a scanning electron microscope,

Figure 3: X-ray powder diffraction pattern showing the defects characteristic of the crystal structure of anatase obtained in Example 1, «*

-8Figure 4: Sodium titanate synthesized with a NaOH ratio: TiC> 2 4 and suitable for the synthesis of rutile nanoparticles in the gel-salt reaction obtained according to the invention in embodiment 2 and taken with a scanning electron microscope,

Figure 5: Sodium titanate synthesized by NaOLLTiCE 3 ratio and suitable for synthesis of rutile nanoparticles in the gel-sol reaction obtained according to the invention in embodiment 2 and taken with a scanning electron microscope,

Figure 6: X-ray powder diffraction pattern showing the defects characteristic of the crystalline structure of rutile obtained in Example 2,

Figure 7: Rutile nanoparticles obtained in a gel-sol reaction when using sodium titanate obtained with a NaOHiTiCh molar ratio of 3 to 4 obtained according to the invention in embodiment 2 and taken with a scanning electron microscope,

Figure 8: Particle size / agglomerate size distribution diagram after base neutralization according to Example 3 and determined by DLS (dynamic light scattering) measurement, Figure 9: Particle size / agglomerate size distribution diagram after base neutralization Embodiment 4 and was determined by DLS (dynamic light scattering) measurement, Figure 10: Particle size / agglomerate size distribution chart after neutralization with base according to Embodiment 5 and determined by DLS (dynamic light scattering) measurement, Figure Figure 11: Particle size / agglomerate particle size distribution diagram after neutralization with base according to embodiment 6 as determined by DLS (dynamic light scattering) measurement, Figure 12: High resolution image of anatase nanoparticles taken with a transmission electron microscope obtained according to Example 1,

Figure 13: Anatase nanoparticles obtained according to the invention in Embodiment 7, taken with a scanning electron microscope,

Figure 14: High resolution image of anatase nanoparticles taken with a transmission electron microscope obtained from embodiment 7,

Figure 15: X-ray powder diffraction pattern showing the most intense deflection characteristic of the crystal structure of anatase obtained in Example 7,

Figure 16: Anatase nanoparticles obtained according to the invention in embodiment 8, taken with a scanning electron microscope,

Figure 17: High resolution image of anatase nanoparticles taken with a transmission electron microscope obtained from embodiment 8,

Figure 18: Anatase nanoparticles obtained according to the invention in Embodiment 9, taken with a scanning electron microscope, * ·

-9Figure 19: High resolution image of anatase nanoparticles taken with a transmission electron microscope obtained from embodiment 9,

Figure 20: Anatase nanoparticles obtained according to the invention in Embodiment 10, taken with a scanning electron microscope,

Figure 21: High resolution image of anatase nanoparticles taken with a transmission electron microscope obtained from embodiment 10.

Process for the synthesis of nanoparticles of anatase is based on the use of metatitanic acid, which is a semi-product in the production of the pigment TiO _second Metatitanic acid is an anatase nanocrystalline agglomerate with a size between 0.5 and several micrometers. Metatitanoic acid is the raw material for the synthesis of so-called sodium titanate, which is the starting material for further gel-salt synthesis of TiO ₂ nanoparticles. Sodium titanate is formed in the reaction between metatitanoic acid and concentrated NaOH base. The mass concentration of a NaOH is about 750 g / L and added to a final molar ratio of NaOH: TiO ₂ 4: 1 to 1: 2.

It should be noted that the specific molar ratio of NaOH to TiO ₂ determines the specific form of sodium titanate in the reaction. The form of sodium titanate, however, primarily determines the form of TiO ₂ which is formed in the subsequent gel-salt reaction with acid. Thus, it is characteristic that sodium titanate formed at a higher molar ratio of NaOH: TiO ₂ promotes the formation of TiO ₂ nanoparticles with the crystalline structure of rutile, whereas sodium titanate obtained with a lower molar ratio of NaOH: TiO ₂ promotes the formation of TiO ₂ nanoparticles with the crystalline structure of anatase . In order to obtain rutile nanoparticles in the gel-sol reaction of synthesis of TiO ₂ nanoparticles, sodium titanate is formed which is formed by reaction with a molar ratio of NaOH: TiO ₂ between 4: 1 to 3: 1. However, if an anatase nanoparticle is to be obtained in the gel-sol reaction of the synthesis of TiO ₂ nanoparticles, then sodium titanate is formed which is formed by reaction with a molar ratio of NaOH: TiO ₂ between 1: 1 to 1: 2. The form of the synthesized sodium titanate is largely determined by the form of the resulting TiO ₂ product, which greatly facilitates the reaction itself.

In order to form anatase nanoparticles in the gel-sol reaction, the synthesis of sodium titanate in the reaction between conc. NaOH and metatitanoic acid, in which the molar ratio of NaOH to TiO _{2 is} maximum 1: 1, but may be lower, up to 1: 3. The sodium titanate synthesis reaction is carried out at a temperature between 80 and 120 ° C, while the concentration of TiO ₂ in the suspension is between 50 and 400 g / L, and at least between 200 and 400 g / L is desirable. The resulting sodium titanate must be properly purified before the gel-salt reaction with the mineral acid, and the excess base and most of the sulfate ions must be removed by filtration. After cleaning the titanium sodium • ·

- 10 is resuspended in an appropriate amount of water, thereby adjusting the appropriate mass concentration of T1O2. Typically, sodium titanate is resuspended to a T1O2 mass concentration between 90 and 180 g / L. The resulting suspension is stirred and hydrochloric acid is added to the concentration between 20 and 100 g / L while stirring, and a concentration of 50 g / L is desired. Changing the acid mass concentration leads to the formation of differently sized anatase nanoparticles, which are polycrystalline.

After adjusting the hydrochloric acid mass concentration to the desired value, the suspension is slowly warmed up to a temperature of about 80 ° C, at which the reaction is most intense. The reaction was allowed to run for two hours, during which time the sodium titanate was completely transformed into well dispersed and uniformly sized anatase nanoparticles. The reaction temperature can be varied from 50 ° C to boiling point, which does not significantly affect the final shape of the particles.

Particles formed in the gel-sol reaction are characterized by:

• Anatase nanoparticles are polycrystalline, which means they are made up of individual smaller anatase crystallites. The crystals average about 5 nanometers in size. The nanoparticles themselves are composed of several crystallites and, depending on the concentration of acid used in the gel-sol reaction, are between 25 and 60 nanometers in size.

• Anatase nanoparticles have roughly spherical morphologies and have a relatively narrow size distribution.

At the end of the synthesis process, an acidic suspension of the anatase nanoparticles is obtained, which can be the final product of the synthesis, and the acidic suspension can also be appropriately purified from unwanted ions. To this end, we have developed two procedures, namely:

• Increase in pH: Acid suspensions can be adequately raised by the addition of a base to adjust the surface charge of nanoparticles near the isoelectric point, resulting in their agglomeration and deposition and filtration capacity. The agglomerated anatase nanoparticles can be purified by adding an appropriate amount of water during the filtration phase, thus continuously washing away the unwanted ions. In this way, excess acid and water-soluble salts can be quantitatively removed.

• Centrifugation of acid suspension of anatase nanoparticles: Centrifugation of the suspension allows the separation of the liquid and solid phases and the removal of the acidic liquid phase by decantation. In case centrifugation is not sufficient to separate the two phases, a smaller amount of aluminum sulphate solution can be added to the suspension to accelerate deposition. Po

After casting the acidic liquid fraction, the TiO ₂ solid phase is resuspended in water and stirred and the centrifugation cycle is repeated. This is repeated until most of the acid or water soluble salts are removed.

With both cleaning processes, we end up with well-purified anatase nanoparticles that can be used in specific applications.

Cleaning with the addition of a base is, in principle, less desirable as it leads to irreversible agglomeration of the nanoparticles, which can lead to the loss of their characteristic physical and chemical properties. Purification of the acid by centrifugation does not lead to agglomeration or is less agglomerated, but does not quantitatively remove the acid. Thus, only neutral suspensions in which the particles would be well dispersed cannot be obtained by centrifugation.

This is avoided by adding a certain amount of dispersant to the suspension of TiO ₂ nanoparticles, which adheres to the surface of the particle and maintains the dispersion of the system based on the stabilization mechanism. There are many forms of dispersants and several stabilization mechanisms, such as electrostatic stabilization, steric stabilization and electrosteric stabilization.

The dispersant we used is citric acid and it works by electrostatic stabilization. This means that citric acid molecules, when bonded to the TiO ₂ nanoparticle surface, effectively increase the electrical charge, thereby causing a Coulomb repulsive force, which allows the nanoparticles not to agglomerate even when the pH of the suspension is changed. Stabilization with citric acid was performed on a suspension of anatase nanoparticles formed in a gel-sol reaction between sodium titanate formed during the synthesis between NaOH and metatitanic acid in a molar ratio of NaOH: TiO ₂ 1: 1, and hydrochloric acid having a final concentration in a suspension of 50 g / L. The suspension was first centrifuged to remove most of the acid. The nanoparticles settled on the bottom of the centrifuge and formed a thick cake of TiO ₂ . The cake was then resuspended in water to a mass concentration between 50 and 400 g / L and then citric acid was added to a content of 1 - 20% by weight in suspension of TiO ₂ present. The amount of dispersant added depended on the degree of particle agglomeration in suspension after neutralization with the base.

Citric acid can be added in both dry and dissolved form. The citric acid was mixed well and then the pH of the suspension was raised by adding a base. The base selected was NaOH of a mass concentration of ~ 750 g / L, but any other base that is water-soluble in nature could be used and could also be used at a lower mass concentration. The pH value

-12 We adjusted between 5 and 10. The resulting suspensions showed a greater or lesser degree of particle agglomeration, which was mainly dependent on the amount of citric acid added. Higher addition of citric acid always led to the formation of less viscous suspensions, which showed good dispersion of the basic particles.

The dispersion and agglomeration of the particles after addition of the base was measured by the DLS (dynamic light scattering) method. Based on DLS measurements, we were able to determine the amount of citric acid that was sufficient to achieve good dispersion. low agglomeration rates.

However, since neutralization is carried out between the rest of the acid and the added base during addition of the base, water-soluble salts are consequently present. These are undesirable since they may interfere with the use of neutral nanoparticle suspensions in some applications and are therefore desirable to remove. Removal of salts formed during neutralization is possible due to the resulting high ionic strength of the suspension, which reduces the electrostatic repulsion of the nanoparticles. As a result, nanoparticles can be separated from neutralized suspensions by centrifugation, with the clear, liquid portion above the cake containing many water-soluble salts and discarded as such. The cake can then be resuspended in water to a certain mass concentration of TiO ₂ , which can be as high as 30% dry matter. This creates a completely stable suspension of the nanoparticles, which is not possessed, since the ionic strength of the resulting suspension is too low as some of the salts are removed by centrifugation. Thus, there is no longer an effect of shading the electrostatic charge of the colloidal bilayer of TiO ₂ nanoparticles, which subsequently repel each other and form a stable suspension.

Although suspensions of any pH can be formed by the stabilization process, the starting material does not change and thus retains its properties, which is very important for many applications, especially photocatalytic activity.

Nevertheless, for some applications it is desirable that TiO _{2 be} in the form of monocrystalline particles, since these generally have better physical properties. Since it is impossible to form monocrystalline particles with gel-salt synthesis, a method permitting this must be used. Such a method is a hydrothermal synthesis based on the use of an autoclave in which the solute dissolves and crystallizes into a monocrystalline form. The hydrothermal method is usually based on relatively high temperatures and the associated water pressure, and can be performed for a longer period since the solute must first dissolve and then crystallize into the final product. Long reaction times could thus be avoided if the solute were present in the form of particles with a high specific surface area. Therefore, in our experiments, a previously prepared and stabilized suspension of anatase nanoparticles having a pH range between 5 and 10 and a mass concentration of TiO ₂ between 40 and 350 g / L was used as starting material. When such a suspension is exposed to hydrothermal • ·

- 13 conditions, the dissolution of polycrystalline anatase nanoparticles is first carried out, but expires relatively quickly, as a very large surface area is available, resulting from the small size of T1O2 particles. The initial stage of dissolution is followed by the stage of particle formation, which, due to specific reaction conditions, is formed in monocrystalline form. The reaction time required for the formation of the final product was between 2 - 5 hours, which was mainly dependent on the temperature of the medium, which was between 180 and 210 ° C.

Hydro thermal conditions are therefore relatively mild, and due to the shape of the starting material (stabilized, well-dispersed suspension of anatase nanoparticles), the reaction is also quite rapid.

Embodiment 1: Synthesis of high specific surface anatase nanoparticles and spherical morphology

Synthesis of anatase nanoparticles is carried out in two stages, namely:

• synthesis of a suitable precursor for the synthesis of sodium titanate nanoparticles; • conversion of sodium titanate into anatase nanoparticles.

The starting material for the synthesis of anatase nanoparticles is so-called metatitanic acid, which is a nanocrystalline anatase agglomerate with a very high specific surface area (over 200 m ² / g). The constituent crystallites of anatase in metatitanic acid are approximately 5 nm in size. Metatitanic acid is converted to sodium titanate by reaction with NaOH.

In a typical experiment, metatitanic acid of a mass concentration of T1O2 290 - 300 g / L and a NaOH mass concentration of 750 g / L were used for the synthesis of sodium titanate. A typical reaction mixture for the synthesis of sodium titanate contains NaOH and T1O2 in a molar ratio of NaOH: TiO ₂ between 1: 1 and 1: 2.

The conversion of metatitanoic acid to sodium titanate takes 2 hours at 90 ° C.

At the end of the reaction, a basic suspension of sodium titanate is obtained, which is then subjected to intensive washing or washing of excess NaOH and during the formation of salts. It is of particular importance to remove the sulfate ions present in the starting material, metatitanic acid, and the excess amount of base that could over-neutralize the mineral acid in the subsequent gel-salt reaction. The washing is carried out as long as excess NaOH and sulfate (SO4 ') ions are present. The titanium sodium obtained by the procedure described above is shown in Figure 1.

The washed sodium titanate is then resuspended in water to a mass concentration of 120-125 g / L (calculated on T1O2). It is the starting material for further synthesis of anatase nanoparticles.

• · ·

- 14The synthesis of anatase nanoparticles from sodium titanate is carried out by a simple process where the titanate suspension is heated to a suitable temperature and hydrochloric acid of a suitable concentration sufficient for quantitative conversion to anatase is added.

In the experiment, 37% hydrochloric acid was added to a 500 mL suspension of sodium titanate with a mass concentration of 120-125 g / L to a final mass concentration of 50 g / L. The resulting suspension was heated at 80 ° C for 1 hour with constant stirring of the reaction mixture at 200 rpm. An acidic suspension of ~ 100 g / L anatase nanoparticles is obtained. The suspension can be purified from excess acid by adding an adequate amount of NaOH to neutralize the acid but at the same time irreversibly agglomerate the nanoparticles, which would significantly impair their basic physical and chemical properties. For this reason, we preferred to use a centrifugation cleaning method to remove excess acid without agglomerating the nanoparticles. Centrifugation does not remove excess acid quantitatively so that something always remains. The residual acidity can be important as it provides a low pH value when resuspended nanoparticles in water, which in turn gives the nanoparticles a high pH value and thus provides good stability of the resulting suspension.

Thus, a suspension with a dry matter content of up to 30% could be prepared from the cake after centrifugation.

Anatase nanoparticles formed in the gel-sol reaction are shown in Figure 2 and were taken with a scanning electron microscope. Anatase nanoparticles average 50 nanometers in size and have spherical morphologies. The crystal structure of the resulting nanoparticles was verified by X-ray powder diffraction (XRD). A diffractogram showing the defects characteristic of the crystal structure of the anatase is shown in Figure 3.

Example 2: Synthesis of high specific surface rutile nanoparticles and anisotropic morphology

The synthesis of rutile nanoparticles is carried out in a very similar procedure to the synthesis of anatase nanoparticles, except that the sodium titanate used was synthesized in a different molar ratio between NaOH and TiO ₂ in metatitanoic acid. The molar ratio of NaOH to TiO ₂ in the sodium titanate preparation reaction was 4, but in principle may be lower, up to 3.

The sodium titanate thus obtained was washed in a similar manner to that described in embodiment 1. The specific form of sodium titanate formed at a molar ratio of NaOH to TiO ₂ in metatitanic acid equal to 4 and suitable for gel-salt synthesis of rutile is shown on • *

- 15 Fig. 4. The specific form of sodium titanate, which is formed by the molar ratio of NaOH to T1O2 in metatitanic acid equal to 3 and is also suitable for the gel-salt synthesis of rutile, is shown in Fig. 5.

For the synthesis of rutile nanoparticles, the sodium titanate shown in Figure 4 was resuspended in water to a mass concentration of 120-125 g / L and 37% hydrochloric acid was added to a final mass concentration of 70 g / L. The gel-sol reaction was carried out at 80 ° C for 2 h.

An acidic suspension of rutile nanoparticles of a mass concentration of ~ 100 g / L is obtained.

The crystal structure of the resulting nanoparticles was verified by X-ray powder diffraction (XRD). A diffractogram showing the defects characteristic of the crystalline structure of rutile is shown in Figure 6. Figure 7 shows the rutile nanoparticles formed by the procedure described above and made on a line electron microscope.

Embodiment 3: Stabilization of the suspension of anatase nanoparticles with citric acid in an amount of 5% by weight of T1O2.

Suspension stabilization was performed on the acid suspension of anatase nanoparticles obtained by centrifugation and resuspension of the resulting cake, as described in the embodiment

1. The starting suspension of the nanoparticles had a T1O2 mass concentration of about 400 g / L and a pH value of about 1. Citric acid monohydrate (dry form,

100%, C ₆ H ₈ O7-H ₂ O), in an amount of 5% by weight of the T1O2 present in the initial acidic suspension. After the addition of citric acid, the suspension was stirred for approximately 30 minutes, until complete dissolution of the citric acid. Subsequently, the resulting suspension was neutralized with a concentrated NaOH mass concentration of ~ 750 g / L. Neutralization of the session was performed to a pH of 7.5. During the addition of NaOH, the viscosity of the suspension decreases, indicating an increase in the dispersion of the particles in the suspension. The dispersion was determined by measuring DLS (dynamic light scattering) based on the scattering of light on particles in suspension. The DLS measurement and particle size distribution after neutralization is shown in Figure 8.

Since the resulting suspension contains quite a few salts which have been formed by the neutralization of the acid with NaOH, it is desirable to remove the latter as much as possible. This can be achieved by centrifugation to obtain the deposition of the nanoparticles, thus separating T1O2 from the liquid medium in which the salts are present. Centrifugation and subsequent deposition of nanoparticles is possible precisely because of the presence of salts which increase the ionic strength of the suspension medium and thus reduce the repulsive forces between the nanoparticles. After centrifugation, the pure fraction can be decanted and the cake resuspended. Because the resulting suspension has much lower ionic strength, they are consequently repulsive

-16 forces between the particles are more pronounced and the resulting suspension is completely stable and therefore even TiO ₂ nanoparticles cannot be separated by centrifugation.

Embodiment 4: Stabilization of the suspension of anatase nanoparticles with citric acid in an amount of 10% by weight of TiO ₂ .

The stabilization experiment was performed in a similar manner to that described in Embodiment 3, except that the amount of citric acid added was 10% by weight of TiO ₂ .

As more citric acid was added, the dispersion of the system after NaOH addition was also better. The dispersion was determined by measuring DLS and is shown in Figure 9.

Embodiment 5: Stabilization of the suspension of anatase nanoparticles with citric acid in an amount of 15% by weight of TiO ₂ .

The stabilization experiment was performed in a similar manner to that described in Embodiment 3, except that the amount of citric acid added was 15% by weight of TiO ₂ .

As more citric acid was added, the dispersion of the system after NaOH addition was also better. The dispersion was determined by DLS measurement and is shown in Figure 10.

Embodiment 6: Stabilization of the suspension of anatase nanoparticles with citric acid in an amount of 20% by weight of TiO ₂ .

The session stabilization experiment was performed in a similar manner to that described in embodiment 3, except that the amount of citric acid added was 20% by weight of TiO ₂ .

As more citric acid was added, the dispersion of the system after NaOH addition was also better. The dispersion was determined by DLS measurement and is shown in Figure 11.

Embodiment 7: Hydrothermal synthesis of monocrystalline anatase nanoparticles from a stabilized suspension of polycrystalline anatase nanoparticles with a mass concentration of 40 g / L and a pH value of 9.

As the starting material for hydrothermal synthesis, a suspension of anatase nanoparticles was used, which was stabilized with 10% citric acid added and had a pH value after neutralization. After neutralization with NaOH, the purification of the resulting salts by centrifugation was not carried out, but the basic suspension was used.

The slurry was exposed to hydrothermal conditions and heated to 210 ° C for 3 hours. From the polycrystalline nanoparticles shown in Fig. 12, we are in hydrothermal

-17 syntheses were able to obtain a completely monocrystalline product. The nanoparticles formed are single crystals, as evidenced by analysis on a screening microscope. Figure 13 is a SEM image and shows the nanoparticles formed at a smaller magnification, and Figure 14 is a high resolution image obtained with a transmission electron microscope (TEM). The figure clearly shows the monocrystalline nature of the particle.

The crystal structure of the resulting material was verified by X-ray powder diffraction, which determined that the resulting product was exclusively anatas. The powder diffraction pattern of the resultant product is shown in Figure 15 and shows the most intense scattering.

Embodiment 8: Hydrothermal synthesis of monocrystalline anatase nanoparticles from a stabilized suspension of polycrystalline anatase nanoparticles with a mass concentration of 200 g / L and a pH value of 9.

Anatase nanoparticle suspension stabilized with 10% citric acid added and pH pH 9 neutralized after neutralization was used as the starting material for hydrothermal synthesis. The concentration of T1O2 in the suspension was ~ 200 g / L.

The slurry was exposed to hydrothermal conditions and heated to 210 ° C for 3 hours. Figure 16 is a SEM image and shows the nanoparticles formed at a smaller magnification, and Figure 17 is a high resolution image obtained by a transmission electron microscope (TEM) showing single particles with characteristic crystallinity characteristic of a single crystal.

Embodiment 9: Hydrothermal synthesis of monocrystalline anatase nanoparticles from a stabilized suspension of polycrystalline anatase nanoparticles with a mass concentration of 300 g / L and a pH value of 8.5.

Anatase nanoparticle suspension, stabilized with 10% citric acid added and having a pH of 8.5 after neutralization, was used as the starting material for hydrothermal synthesis. The mass concentration of T1O2 in the suspension was ~ 300 g / L.

The slurry was exposed to hydrothermal conditions and heated to 210 ° C for 3 hours. Figure 18 is a SEM image and shows the nanoparticles formed at a small magnification, and Figure 19 is a high resolution image obtained by a transmission electron microscope (TEM) showing single particles with crystallinity characteristic of a single crystal.

- 18Example 10: Hydrothermal synthesis of monocrystalline anatase nanoparticles from a stabilized suspension of polycrystalline anatase nanoparticles with a mass concentration of 300 g / L and a pH value of 8.5.

Anatase nanoparticle suspension stabilized with 10% citric acid added and having a pH value of 8.5 was neutralized as the starting material for hydro thermal synthesis. The mass concentration of TiO ₂ in suspension was ~ 300 g / L.

The suspension was exposed to hydrothermal conditions and heated to 180 ° C for 5 hours. Figure 20 is a SEM image and shows the nanoparticles formed at a small magnification, and Figure 21 is a high resolution image obtained by a transmission electron microscope (TEM) showing single particles with crystallinity characteristic of a single crystal.

Claims (11)

1. A synthesis process for the production of anatase nanoparticles of high specific surface area and spherical morphology based on a gel-sol reaction, where metatitanoic acid, which is a nanocrystalline anatase gel, is used as a starting material and is a by-product of the production of TiO ₂ pigment, - that the metatitanic acid is first converted to sodium titanate by reaction with NaOH, where the molar ratio of NaOH: TiO _{2 is} between 1: 1 or 1: 3, - that the titanium suspension of sodium titanate resuspended in water is purified and a TiO ₂ mass concentration suspension between 90 and 180 g / L is obtained, having a basic pH value, that the sodium titanate suspension is used as a raw material for the direct synthesis of anatase nanoparticles in the following reaction, - the synthesis of anatase nanoparticles from a suspension of sodium titanate with the addition of hydrochloric acid of a mass concentration between 20 and 100 g / L is carried out to react at a temperature of 80 ° C or in a temperature range between 50 ° C and boiling point for two hours or less, - that the anatase nanoparticles obtained in the form of an acidic suspension have an approximate mass concentration of 100 g / L, so that the nanoparticles can be purified from the acidic suspension by raising the pH value, thereby achieving the nanoparticle agglomeration or by centrifuging the acidic suspension while adding a small amount of aluminum sulfate, the spin cycle is repeated several times. 2. A process according to claim 1, characterized in that the variation of the reaction mass of the hydrochloric acid hydrochloric acid makes it possible to control the size of the nanoparticles. Method according to claim 1, characterized in that the size of the anatase nanoparticles can be adjusted to about 50 nm or less, with the anatase nanoparticles in acid suspension having a mass concentration of ~ 100 g / L and that the nanoparticles can be isolated and cleaned while the nanoparticles are inherently polycrystalline, and therefore there is no need for energy-consuming calcination of the product, and they are the basic crystallites that make up a single nanoparticle of about 5 nm in size. A method according to claim 1, by which the acid suspension of the anatase nanoparticles can be purified by excess acid by centrifugation and resuspended the resulting cake in water to a mass • -20concentrations between 50 and 400 g / L, forming a completely stable suspension with a low pH. 5. The method of claim 4, wherein the acid suspension of anatase nanoparticles can be converted into a stable suspension with a neutral or basic pH by adding an appropriate amount of dispersant and using as a dispersant citric acid in dry or dissolved form in an amount of from 5 to 20% relative to mass of TiO ₂ . The process according to claims 4 to 5, wherein the pH value of the suspension of anatase nanoparticles in which the dissolved citric acid is raised with a base which may be NaOH or any other water-soluble base with various mass concentrations ranging from saturated to diluted. The method of claims 4 to 6, wherein the pH is raised between 5 and 10, whereby the nanoparticles do not agglomerate excessively due to the dispersant action, thus forming a relatively well-dispersed system. The method of claims 4 to 7, wherein by centrifuging the neutralized suspension of the anatase nanoparticles, the salts formed during neutralization with the base are removed, since the nanoparticles settle to the bottom of the centrifuge, while the salts are mostly present in the pure aqueous fraction above the cake; the cake of TiO ₂ nanoparticles can be resuspended in water after centrifugation, forming a completely stable suspension with a dry matter content of up to 30%. The method of claims 4 to 8, wherein a neutralized or slightly basic suspension of polycrystalline anatase nanoparticles can be used to perform hydrothermal synthesis by which it can obtain monocrystalline anatase nanoparticles. A method according to claim 9, wherein an autoclave is used for hydrothermal synthesis in which a neutral or slightly basic suspension of the anatase nanoparticles is introduced to a suitable charge and subjected to hydrothermal conditions; hydrothermal synthesis is carried out with suspensions with a pH range between 7.5 and 10 and a mass concentration of between 40 and 350 g / L at a temperature of between 130 and 210 ° C, performed over a period of 2.5 to 5 hours, and form monocrystalline anatase nanoparticles that are between 30 and 50 nm in size. Process according to Claims 1 to 10, characterized in that all the processes are continuously carried out in the wet, ie in the form of a suspension, which prevents the occurrence of potentially harmful dusting.