WO2013062491A1

WO2013062491A1 - Synthesis method for obtaining anatase nanoparticles of high specific surface area and spherical morphology

Info

Publication number: WO2013062491A1
Application number: PCT/SI2011/000061
Authority: WO
Inventors: Dejan VERHOVŠEK; Nika Veronovski; Aljaž SELIŠNIK; Miran Ceh; Zoran SAMARDŽIJA
Original assignee: Cinkarna Metalurško Kemična Industrija Celje, D.D.
Priority date: 2010-10-25
Filing date: 2011-10-25
Publication date: 2013-05-02
Also published as: SI23501A

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 °C or more and at adequate pressure of the water vapour.

Description

Synthesis method for obtaining anatase nanoparticles of high specific surface area and spherical morphology

The subject of the invention is a synthesis method for obtaining anatase nanoparticles of high specific surface area and spherical morphology based on a gel-sol reaction, wherein metatitanic acid is used as a starting material, which is a nanocrystalline anatase gel and a semi-product in the production of the Ti0₂ pigment, as well as anatase nanoparticles obtained by said method. The subject of the invention is also sodium titanate and a synthesis method for obtaining sodium titanate, the form of which is suitable for use in the synthesis method for obtaining anatase nanoparticles. A further subject of the invention is purifying the obtained acid suspension of anatase nanoparticles, from which the excess acid is removed by centrifugation and further neutralisation of the purified suspension by increasing the pH value without the nanoparticles getting agglomerated. The subject of the invention is also a manner, in which monocrystalline nanoparticles are obtained from the nanoparticles that are polycrystalline after the synthesis from sodium titanate, namely with re-crystallisation in a hydrothermal reaction.

The invention relates to a method, based on which a suitable form of sodium titanate, which is suitable either for the preparation of rutile nanoparticles or anatase nanoparticles in a reaction with an acid, is prepared from metatitanic acid, which is an intermediate in the process of manufacturing the Ti0₂ pigment. Sodium titanate is prepared in a reaction of metatitanic acid with a concentrated NaOH base, wherein the ratio between the added NaOH and Ti0₂ in the metatitanic acid directly defines the form of the obtained titanate. The form of the sodium titanate defines whether rutile or anatase in the form of well dispersed nanoparticles will be formed in a reaction with an acid. The obtained particles are polycrystalline, which means they are composed of a multitude of small crystallites agglomerated into a particle.

The invention further relates to a synthesis of anatase nanoparticles obtained in a reaction between a specific form of sodium titanate and hydrochloric acid in the so-called gel-sol reaction, which leads to a formation of well dispersed nanoparticles in an acidic aqueous medium. After the synthesis is completed, the nanoparticles are well dispersed and are not agglomerated, as they have a high value of zeta potential due to their low pH value, which enables them to mutually reflect due to electrostatic forces. A change in the pH value decreases the potential of the particles, which results in agglomeration. This is undesirable, since the nanoparticles no longer exhibit their original properties that are a consequence of their huge specific surface area. Thus, the pH of a suspension needs first be increased in the presence of a suitable dispersant, which is a surface active component and prevents the agglomeration of nanoparticles by establishing steric, electrostatic and/or electrosteric forces.

The subject of the invention is also achievement of suitable stabilisation of nanoparticles in a suspension, wherein a dispersant is added that prevents the nanoparticles from agglomerating. The original properties of nanoparticles are herewith preserved, which is very important for a series of high technology applications of Ti0₂ nanoparticles that require a use of a chemically neutral aqueous suspension, like thin-layer coatings for self-cleaning coatings, electrochemical solar cells, photocatalysts, UV protective coatings, ... The particles after the synthesis are polycrystalline, i. e. each particle is composed of a multitude of small crystallites. This may have influence on some physical properties of a material, so it is desirable in some cases that the particles are prepared in the so-called monocrystalline form. It means that each individual particle is a crystal in itself as well. Transformation into a monocrystalline form can be achieved by thermal treatment at an increased temperature, which is, however, undesirable, as considerable agglomeration of the product, potentially harmful powdering and huge energy consumption are observed. The present invention relates to an alternative method based on the so-called hydrothermal method, in which a material in the form of an aqueous suspension is subjected to a high temperature and pressure in autoclave, wherein the polycrystalline particles convert into a monocrystalline form. An advantage of said method lies in that no potentially harmful powdering is present in any step, and what's more, less energy products are used than in the case of thermal treatment.

All methods treated by the invention are based on the methods of »wet« chemistry and are performed in an aqueous suspension. Processing in the form of a suspension is very important, since no step is accompanied by formation of an intermediate potentially health harmful powder phase and well crystalline nanoparticles are formed as the final product, which excludes a need for a calcination method, which is energetically consuming and

environmentally unfriendly due to the release of greenhouse gases. Titanium oxide in the form of a pigment is a material with a wide range of applications. It can be used for coatings, paints, as additive to plastics, paper, in cosmetics, pharmaceutical industry and in many other applications. Nowadays mostly two methods of obtaining titanium oxide pigment are in use, i. e. a sulphate and a chloride method. Both, the sulphate and the chloride method are based on a high-temperature conversion of adequate components into a titanium dioxide pigment. In the sulphate method the pigment is formed in the process of calcination in reactions of hydrolysis of the obtained anatase gel, wherein in the chloride method the pigment is formed at high-temperature combustion of titanium tetrachloride with oxygen. It is exactly the method of high-temperature calcination, which is part of both the sulphate and the chloride method, that prevents a possibility of obtaining anatase particles of nano sizes, moreover, it is typical that high temperature of calcination always leads to conversion of anatase into a thermodynamically more stable phase of Ti0₂, rutile. The calcination method is heavily burdened with high energy consumption and consequently with high greenhouse gas emissions. Moreover, the obtained product is present in the form of powder, which can be potentially harmful to health and environment.

Due to the above, many various methods of obtaining anatase nanoparticles have been developed in past years.

The patent US 20060254461 discloses a method for forming anatase nanoparticles by a sol-gel method from starting organic alkoxydes that hydrolyse under specific reaction conditions and form an adequate sol. The sol is then subjected to aging process at a temperature 120 - 140 °C, which triggers a process of nucleation and growth of good crystalline nanoparticles of anatase. The patent US 4954476 describes a method of producing anatase nanoparticles with a hydrothermal process. Meta- or ortho-titanic acid is used as a starting material, which is then exposed to a hydrothermal reaction under specific conditions. A hydrothermal reaction is typically performed for several hours at a temperature of about 180 °C and increased pressure of the aqueous suspension. The hydrothermal synthesis at a lower temperature takes more time, even several days.

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

The examples of synthesis of anatase nanoparticles given above lead to the desired product, yet are unsuitable and less adequate for industrial production, since the processes described are either based on the use of expensive and/or toxic organic substances or a lengthy and energy consuming process needs to be used in order to obtain the desired form of nanoparticles. A considerable disadvantage of the disclosed methods lies in that the final product is a heavily agglomerated powder of anatase nanoparticles, which reduces the application scope of the final product.

The anatase nanoparticles and the synthesis method for obtaining anatase nanoparticles of the invention do away with said disadvantages, since they provide for obtaining equally sized, well crystalline and well dispersed anatase nanoparticles in a stable acid suspension, which makes their use in high technology applications considerably easier.

The obtaining of nanoparticles of the invention is based on a gel-sol synthesis, in which sodium titanate is used as raw material. Sodium titanate is obtained from metatitanic acid in a reaction with a suitable quantity of NaOH base. The synthesis of sodium titanate is carried out at a temperature below boiling point, approximately at 90 °C, but the temperature during the reaction can also be higher or lower.

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

The suspension of anatase nanoparticles obtained after the synthesis is acidic, which limits its use in certain applications. A certain application may require the use of the nanoparticle suspension with a specific pH value, it is therefore very important that the suspension can be adjusted to the desired pH value without causing agglomeration of particles. Several methods have been developed to solve this problem.

The patent US7344591 discloses a method of formation of Ti0₂ nanoparticle suspension in the presence of a suitable dispersing agent (glycine, glycolic acid). The method is based on the synthesis of Ti0₂ nanoparticles in a medium, in which a dispersing agent is already present or is supposed to lead to the formation of a suspension, in which nanoparticles would be well dispersed, as the added dispersing agent would prevent agglomeration as early as the phase of particle formation. Yet this method is carried out in a way that prevents formation of equally sized particles. Moreover, the DLS measurements (Dynamic Light Scattering) were inadequate, as the factor of polydispersion of suspensions is very low, which means that the presented results of the DLS measurements fail to show the actual situation.

The patent US20080317959 discloses a method, with which Ti0₂ nanoparticles are synthetized and stabilised by using suitable surfactants, like Triton X-100. A disadvantage of the disclosed method is that it is based on the use of surfactants, which are relatively expensive and consequently less suitable for use on a larger, industrial scale. The obtained suspensions moreover had a very low Ti0₂ content, which is very poor from the economic aspect.

The present invention solves the mentioned problems, as it describes a method, with which an acid suspension of anatase nanoparticles is converted to a neutral suspension without the nanoparticles getting too agglomerated. This is achieved by the use of a suitable dispersing agent, which prevents agglomeration of particles during an addition of base by binding to their surface. The dispersant used to this purpose is relatively cheap and allows a preparation of stable suspensions with various Ti0₂ contents, even up to 50 %.

The dispersant is added to the acidic suspension during stirring and is dissolved in an aqueous medium thus equally distributing inside the reaction medium. A concentrated base like NaOH or any other (NH₃, KOH, Na₂C0₃, ...) is then added. The base is added during stirring until the desired pH is obtained, wherein the most adequate pH value is about 7. The pH value can be adequately adjusted by adding base and can be between 4 and 12, wherein the stability of the suspension does not considerably change.

After the synthesis and/or stabilisation a suspension is obtained, in which the nanoparticles are well dispersed at the desired pH value. As the nanoparticles are basically polycrystalline, it means they do not exhibit optimal physical and chemical properties, since each particle is composed of several small crystallites, the boundaries between them can present an obstacle for a certain process (e. g. electron flow). Optimal properties of nanoparticles are only achieved if the particles have a shape of a monocrystal, since their properties in this case are not influenced by possible defects in the internal structure, like boundaries between individual crystallites. The methods that allow formation of a monocrystalline form are usually based on thermal treatment, in which exposure to high temperature leads to consolidation of small crystallites into a large one. Anyway, with this method being energetically consuming and leading to the formation of a dry, potentially health harmful powder phase, it is less suitable. An alternative method is based on the so-called hydrothermal method, in which the monocrystalline form of a material can be formed in an aqueous medium at a temperature above boiling point and adequately high pressure. The hydrothermal method is a well-known synthesis method and is used in various inventions.

The patent US20090223412 discloses a hydrothermal method, with which monocrystalline Ti0₂ particles are formed, wherein a water soluble component of titanium is used as a starting material. This is exposed to hydrothermal conditions, i. e. temperature of 160 °C for a period of 16 hours. A similar method is disclosed in the patent US5776239, wherein the hydrothermal method was performed at a higher temperature and within a shorter time period, yet again only on the basis of a water soluble titanium component.

A disadvantage of both methods is that the shape of the final product is defined only during reaction course, since the particles are formed from a water soluble component. Both methods described are characterized by a long-lasting reaction, which could be a great problem in case of industrial applicability. This is even rendered harder by very high temperatures described in both methods.

The present invention refers to a method that is shorter and is carried out at a relatively low temperature and based on a hydrothermal method that allows a conversion of polycrystalline anatase particles to monocrystalline particles. The material used is a stable suspension of anatase nanoparticles with a pH value from 4 to 12. The suspension is subjected to

hydrothermal conditions, to a temperature ranging between 130 and 250 °C and the pressure that coincides with this temperature. The hydrothermal reaction takes 2 to 4 hours, wherein monocrystalline anatase nanoparticles are formed. The obtained particles have a size and shape similar to those of the starting, polycrystalline particles.

The main goal of the present invention is disclosure of methods that are suitable for industrial obtaining 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 possibly not agglomerated. The main advantages of the present invention are as follows:

- the method is based on the use of a material, sodium titanate, which can be produced from the so-called metatitanic acid, which is an important semi-product in the production of a Ti0₂ pigment by a sulphate method,

sodium titanate is produced from metatitanic acid at a relatively low temperature (below 100 °C) in a reaction with NaOH, - the quantity of NaOH or the molar ratio between Ti0₂ and NaOH is used to define the form of the sodium titanate, which is a raw material for the synthesis of Ti0₂ nanoparticles. If the molar ratio of NaOH : Ti0₂ is between 3 and 4, sodium titanate is obtained that is suitable only for the obtaining of Ti0₂ nanoparticles of the crystal rutile structure. If the molar ratio is smaller, between 1 and 2, sodium titanate is formed that is suitable for the obtaining of Ti0₂ nanoparticles of the crystal anatase structure without a simultaneous formation of rutile.

- for the purpose of obtaining anatase nanoparticles first sodium titanate is obtained, which can be adequately purified in order to remove the excess base with filtration, decanting and/or centrifugation,

- purified sodium titanate is re-suspended in water and hydrochloric or nitric (V) acid is added up to an adequate concentration and a synthesis of anatase nanoparticles is performed in a reaction at increased temperature, yet below boiling point (below 100 °C),

nanoparticles are formed in the form of an acidic suspension, in which they are well dispersed,

- nanoparticles are formed in the form of good polycrystalline particles, which means there is no need for thermal treatment, which saves energy products and prevents the release of greenhouse gases into the environment,

- nanoparticles are equally sized, wherein their size can be controlled by a selection of suitable reaction parameters,

the acidic suspension is easy to neutralise by using a base in the presence of a dispersant, citric acid, which prevents the undesired agglomeration of nanoparticles. The stability of the suspension mostly depends on the quantity of the added dispersant and should be at least 10 % based on the Ti0₂ mass in the suspension.

- the obtained suspension can be subjected to the so-called hydrothermal method. The suspension is exposed to high temperatures, above boiling point, and adequate pressure in an autoclave, wherein polycrystalline particles are transformed into monocrystalline particles.

the hydrothermal method is carried out in a suspension, which prevents the formation of a potentially harmful powder phase,

- all said methods have a very high yield exceeding 95 %. Description of the invention

The invention will be described in the continuation by a description of methods, figures and examples that adequately illustrate the method of synthesis of anatase nanoparticles, its stabilisation with a suitable dispersant and use of hydrothermal method for the obtaining of the monocrystalline form of nanoparticles. The invention will also contain a detailed description of the starting material, sodium titanate, which can - in dependence on the synthesis method performed - represent a starting material for the obtaining of rutile or anatase nanoparticles.

The figures show:

Figure 1 : Sodium titanate suitable for the synthesis of anatase nanoparticles in a gel-sol reaction obtained by the invention in Example 1 and recorded with a scanning electron microscope.

Figure 2: Anatase nanoparticles obtained by the invention in Example 1 , recorded with a scanning electron microscope.

Figure 3 : X-ray powder diffractogram showing deflections typical of the crystal structure of anatase obtained in the example 1.

Figure 4: Sodium titanate synthetized by a ratio NaOH:Ti0₂ 4 and suitable for the synthesis of rutile nanoparticles in a gel-sol reaction obtained by the invention in Example 2 and recorded with a scanning electron microscope.

Figure 5: Sodium titanate synthetized by a ratio NaOH:Ti0₂ 3 and suitable for the synthesis of rutile nanoparticles in a gel-sol reaction obtained by the invention in Example 2 and recorded with a scanning electron microscope.

Figure 6: X-ray powder diffractogram showing deflections typical of the crystal structure of rutile obtained in Example 2.

Figure 7: Rutile nanoparticles obtained in a gel-sol reaction, when sodium titanate obtained with a molar ratio NaOH:Ti0₂ between 3 and 4 is used, obtained by the invention in Example 2 and recorded with a scanning electron microscope.

Figure 8: Diagram of distribution of particle/agglomerate sizes formed after neutralisation with a base according to Example 3 and defined by DLS measurement (dynamic light scattering). Figure 9: Diagram of distribution of particle/agglomerate sizes formed after neutralisation with a base according to Example 4 and defined by DLS measurement (dynamic light scattering). Figure 10: Diagram of distribution of particle/agglomerate sizes formed after neutralisation with a base according to Example 5 and defined by DLS measurement (dynamic light scattering).

Figure 11 : Diagram of distribution of particle/agglomerate sizes formed after neutralisation with a base according to Example 6 and defined by DLS measurement (dynamic light scattering).

Figure 12: High-resolution image of anatase nanoparticles obtained in Example 1 recorded with a transmission electron microscope.

Figure 13: Anatase nanoparticles obtained by the invention in Example 7 recorded with scanning electron microscope.

Figure 14: High-resolution image of anatase nanoparticles obtained in example 7 recorded with a transmission electron microscope.

Figure 15: X-ray powder diffractogram showing the most intense deflection typical of the anatase crystal structure obtained in Example 7.

Figure 16: Anatase nanoparticles obtained by the invention in Example 8 recorded with a scanning electron microscope.

Figure 17: High-resolution image of anatase nanoparticles obtained in Example 8 recorded with a transmission electron microscope.

Figure 18: Anatase nanoparticles obtained by the invention in Example 9 recorded with a scanning electron microscope.

Figure 19: High-resolution image of anatase nanoparticles obtained in Example 9 recorded with a transmission electron microscope.

Figure 20: Anatase nanoparticles obtained by the invention in Example 10 recorded with a scanning electron microscope.

Figure 21 : High-resolution image of anatase nanoparticles obtained in Example 10 recorded with a transmission electron microscope.

The synthesis method of anatase nanoparticles is based on the use of metatitanic acid, which is a semi-product in the production of the Ti0₂ pigment. The metatitanic acid is a nanocrystalline anatase agglomerate with a size ranging between 0.5 and several micrometres. The metatitanic acid is a raw material for the synthesis of the so-called sodium titanate, which is a starting material for a further gel-sol synthesis of Ti0₂ nanoparticles. Sodium titanate is formed in a reaction between metatitanic acid and concentrated NaOH base. Mass concentration of NaOH is approximately 750 g/L and is added up to the total molar ratio NaOH : Ti0₂ 4:1 to 1 :2.

It needs to be stressed that the molar ratio between NaOH and Ti0₂ in a reaction defines the specific form of sodium titanate. The form of sodium titanate primarily defines the form of Ti0₂ which is formed in the following gel-sol reaction with an acid. It is therefore typical that sodium titanate, which is formed at a higher molar ratio of NaOH : Ti0₂ promotes the formation of Ti0₂ nanoparticles with a crystal structure of rutile, whereas sodium titanate obtained with a lower molar ratio of NaOH : Ti0₂ promotes the formation of Ti0₂

nanoparticles with anatase crystal structure. If rutile nanoparticles are to be obtained in the gel- sol reaction of synthesis of Ti0₂ nanoparticles, sodium titanate must be formed that is formed in a reaction with a molar ratio of NaOH : Ti0₂ between 4:1 to 3:1. If anatase nanoparticles are to be obtained in the gel-sol reaction of the synthesis of Ti0₂ nanoparticles, sodium titanate must be formed that is formed in a reaction with a molar ratio of NaOH : Ti0₂ between 1 : 1 to 1 :2. The shape of the obtained Ti0₂ product is mostly defined by the form of the synthetized sodium titanate, which considerably contributes to an easier carrying out of the reaction.

If anatase nanoparticles are to be formed in the gel-sol reaction, a synthesis of sodium titanate is to be performed in a reaction between the cone. NaOH and metatitanic acid, in which the molar ratio between NaOH and Ti0₂ is max. 1 :1, but can even be lower, namely 1 :3. The synthesis reaction of sodium titanate is performed at a temperature between 80 and 120 °C, whereas the concentration of Ti0₂ in a suspension is between 50 and 400 g/L, preferably between at least 200 and 400 g/L. Prior to the gel-sol reaction the obtained sodium titanate must be purified with a mineral acid. The excess base and a major part of sulphate ions must be removed with filtration. After purification, sodium titanate is re-suspended to an adequate quantity of water, with which the adequate mass concentration of Ti0₂ is adjusted. Sodium titanate is normally re-suspended up to a mass concentration of Ti0₂ between 90 and 180 g/L. The obtained suspension is stirred and hydrochloric acid is added while stirring up to mass concentration between 20 and 100 g/L, preferably 50 g/L. A changing mass concentration of the acid leads to the formation of variously sized anatase nanoparticles, which are

polycrystalline.

After the mass concentration of hydrochloric acid is adjusted to the desired value, the suspension is slowly heated up to a temperature about 80 °C, at which the reaction is most intense. The reaction is carried out for two hours and in this time sodium titanate completely converts into well dispersed and equally sized anatase nanoparticles. The reaction temperature can be changed from 50 °C to boiling point, which does not have a significant effect on the final shape of particles.

The particles formed in the gel-sol reaction have the following characteristics:

• anatase nanoparticles are polycrystalline, which means they are composed of individual small anatase crystallites. Crystallites have an average size of about 5 nanometres. The nanoparticles themselves are composed of several crystallites and have a size between 25 and 60 nanometres, depending on the acid concentration used in the gel-sol reaction.

• anatase nanoparticles have rough spherical morphology and have a relatively narrow size distribution.

At the end of the synthesis method an acidic suspension of anatase nanoparticles is obtained, which can be the final synthesis product or the acidic suspension can adequately be purified of undesired ions. Two processes have been developed to this purpose:

• Increase in pH: pH of the acidic suspension can be increased by an addition of a base, with which the surface charge of nanoparticles near the isoelectric point is adjusted, which causes the nanoparticles to agglomerate, to deposit and to get filtered. The agglomerated anatase nanoparticles can be purified by an addition of an adequate quantity of water in the filtration phase, with which undesired ions are continuously washed. In this way the excess acid and water soluble salts can be removed.

• Centrifugation of the acidic suspension of anatase nanoparticles: centrifugation of the suspension makes it possible to separate the liquid and solid phase and to remove the acidic liquid phase by decanting. If centrifugation is not sufficient for the separation of both phases, a small quantity of aluminium sulphate can be added to the solution to promote deposition. After the acidic liquid fraction is poured away, the solid Ti0₂ phase is re-suspended in water and stirred, then the centrifugation cycle is repeated. This is repeated until the major part of the acid or the present water soluble salts are removed.

By using both purifying processes well purified anatase nanoparticles are obtained at the end, which can be used in certain applications. Purifying by an addition of a base is principally less preferred, as it leads to irreversible agglomeration of nanoparticles, which thus lose their characteristic physical and chemical properties.

Purifying of an acid with centrifugation does not cause agglomeration or causes less agglomeration, but does not quantitatively remove the acid. Centrifugation alone is thus not sufficient to obtain entirely neutral suspensions, in which the particles would be well dispersed. This can be avoided by adding a certain quantity of a dispersant to the suspension of Ti0₂ nanoparticles. The dispersant sticks to the surface of a particle and maintains the dispersion of the system based on the mechanism of stabilisation. There are several types of dispersants and several mechanisms of stabilisation accordingly, namely electrostatic stabilisation, steric stabilisation and electrosteric stabilisation.

The dispersant used was citric acid and it functions on the basis of electrostatic stabilisation. This means that the citric acid molecules, when binding to the surface of a Ti0₂ nanoparticle, effectively increase the electric charge and thus cause Coulomb reflection force, which makes it possible that the nanoparticles do not agglomerate not even in the case when the pH of the suspension is changed.

Stabilisation with citric acid was performed on a suspension of anatase nanoparticles that were formed in the gel-sol reaction between sodium titanate, formed during a synthesis between NaOH and metatitanic acid in a molar ratio NaOH:Ti0₂ 1 : 1, and hydrochloric acid with a final concentration in the suspension 50 g/L. The suspension was first centrifugated in order to remove the major part of the acid. The nanoparticles deposited at the bottom of the centrifuge tube and formed a thick Ti0₂ cake. The cake was then re-suspended in water up to mass concentration between 50 and 400 g/L and then citric acid was added up to the content 1 - 20 % based on the mass of the Ti0₂ present in the suspension. The level of agglomeration of particles in the suspension after neutralisation with a base depended on the quantity of added dispersant.

Citric acid can be added both in a dry or dissolved form. Citric acid was well stirred, pH of the suspension was then increased by adding a base. The selected base was NaOH with a mass concentration ~ 750 g/L, but any other water soluble base could be used and even at a lower mass concentration. The pH value was adjusted between 5 and 10. The obtained suspensions exhibited a greater or smaller extent of agglomeration of particles, which was mostly dependent on the quantity of added citric acid. A bigger addition of citric acid always led to the formation of less viscous suspensions that exhibited good dispersity of basic particles. Dispersity and agglomeration of particles after the addition of a base was measured by the method of dynamic light scattering (DLS). The DLS measurements allowed us to determine the quantity of citric acid sufficient to reach good dispersity and low agglomeration level.

Since neutralisation takes place during the addition of a base between the remaining part of the acid and the added base, water soluble salts are present consequently. These are undesirable, as they could be disturbing in the use of neutral nanoparticle suspensions in some applications, so they are preferably removed. The salts formed during neutralisation can be removed due to high ionic power of the suspension, which reduces the electrostatic reflection of nanoparticles. The nanoparticles can thus be separated from neutralised suspensions by centrifugation, wherein the clear, liquid portion above the cake contains plenty of water soluble salts and can be discarded. The cake can then be re-suspended in water up to a certain mass concentration of Ti0₂, which can be even up to 30 % of solid substance. In this way an entirely stable suspension of nanoparticles is formed that does not deposit, since the ionic power of the obtained suspension is too low due to part of the salts being removed by centrifugation. The shading effect of the electrostatic charge of the colloidal double layer of nanoparticles Ti0₂ is no longer present and these particles mutually reflect and form a stable suspension.

Although stabilisation method can be used to form suspensions having an optional pH value, the starting material does not change and preserves its properties, which is very important for many applications, especially photocatalytic functioning.

Nevertheless, it is desirable for some applications that Ti0₂ is present in the form of monocrystalline particles, because in principle they possess better physical properties. As it is impossible to form monocrystalline particles with the gel-sol synthesis, another method, which allows it, must be used. Such method is a hydrothermal synthesis, which is based on the use of autoclave, in which a solute is dissolved and crystallised into a monocrystalline form. The hydrothermal method is usually based on relatively high temperatures and consequently adequate water pressure and can be performed for a longer period of time, since the solute must first dissolve and only then crystallise into a final product. A long reaction time could be avoided if the solute were present in the form of particles having high specific surface area. So the starting material used in our experiments was a previously prepared and stabilised suspension of anatase nanoparticles with pH in the range from 5 to 10 and mass concentration of Ti0₂ between 40 and 350 g/L. When such a suspension is subjected to hydrothermal conditions, the polycrystalline anatase nanoparticles first dissolve, which is performed relatively rapidly due to a very large surface, which originates from the small size of Ti0₂ particles. The initial step of dissolving is followed by a step of formation of particles that are formed in their monocrystalline form due to specific reaction conditions. The reaction time needed for the production of the final product was between 2 and 5 hours, which mostly depended on the temperature of the medium, which ranged between 180 and 210 °C.

Hydrothermal conditions are relatively mild and the reaction is quite rapid due to the form of the starting material (stabilised, well dispersed suspension of anatase nanoparticles).

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

The synthesis of anatase nanoparticles is carried out in two steps:

• synthesis of an adequate precursor for the synthesis of nanoparticles, the precursor being sodium titanate,

• conversion of sodium titanate to anatase nanoparticles.

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

In a typical experiment metatitanic acid of Ti0₂ mass concentration 290 - 300 g/L and of NaOH mass concentration 750 g/L was used for the synthesis of sodium titanate. A typical reaction mixture for the synthesis of sodium titanate contains NaOH and Ti0₂ in a molar ratio NaOH : Ti0₂ between 1 : 1 and 1 :2.

Conversion of metatitanic acid to sodium titanate lasted for 2 hours at a temperature of 90 °C. At the end of reaction a basic suspension of sodium titanate was obtained, which was then subjected to intense washing or rinsing of the excess NaOH and the salts that formed during the process. It is very important that sulphate ions present in the starting material, metatitanic acid, and the excess base quantity, which could cause excessive neutralisation of the mineral acid in the subsequent gel-sol reaction, were removed. Washing was performed until the excess of NaOH and sulphate (S0₄ ^") ions were present. Sodium titanate obtained by the above method is shown in Figure 1.

The washed sodium titanate was then re-suspended in water up to mass concentration of 120- 125 g/L (calculated to Ti0₂). This was a starting material for a further synthesis of anatase nanoparticles. The synthesis of anatase nanoparticles from sodium titanate was carried out by a simple method, in which titanate suspension was heated to an adequate temperature and hydrochloric acid was added in an adequate concentration sufficient for a quantitative conversion to anatase. In the experiment, to 500 mL of suspension of sodium titanate of a mass concentration of 120 - 125 g/L 37 % hydrochloric acid was added up to final mass concentration of 50 g/L. The obtained suspension was heated at 80 °C for 1 hour and the reaction mixture was constantly stirred at 200 rpm. Acidic suspension of anatase nanoparticles of mass concentration - 100 g/L was obtained. The suspension can be purified of the excess acid with an addition of an adequate quantity of NaOH, which would neutralise the acid, but simultaneously also irreversibly agglomerate the nanoparticles, which would considerably worsen their basic physical and chemical properties. This is why we preferred purification by centrifugation, with which the excess acid was removed without agglomerating the nanoparticles. The excess acid cannot be quantitatively removed by centrifugation, so some of it always remains. The rest of the acid can be important, as it keeps the low pH level, when the nanoparticles are re- suspended in water, which consequently adjusts the nanoparticles to a high pH value and thus provides for a good stability of the obtained suspension.

So, after centrifugation a suspension was obtained from the cake with the content of dry substance up to 30 %.

Anatase nanoparticles that formed in the gel-sol reaction are shown in Figure 2 and were recorded with a scanning electron microscope. Anatase nanoparticles have an average size of 50 nanometres and have a spherical morphology. The crystal structure of the obtained nanoparticles was checked by X-ray powder diffraction (XRD). The diffractogram showing deflections typical of the anatase crystal structure is shown in Figure 3.

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

The synthesis reaction of rutile nanoparticles was carried out following a method very similar to that of the synthesis of anatase nanoparticles, except that sodium titanate, which was used, was synthetized in a different molar ratio between NaOH and Ti0₂ in metatitanic acid. The molar ratio between NaOH and Ti0₂ in the reaction of preparation of sodium titanate was 4, in principle it could be even lower, up to 3.

The sodium titanate thus obtained was washed in a similar way as described in Example 1. The specific form of sodium titanate that formed in a molar ratio between NaOH and Ti0₂ in metatitanic acid equalling 4 and was suitable for a gel-sol rutile synthesis is shown in Figure 4. The specific form of sodium titanate that formed in a molar ratio between NaOH and Ti0₂ in metatitanic acid equalling 3 and also suitable for a gel-sol rutile synthesis is shown in Figure 5. For the synthesis of rutile nanoparticles sodium titanate, shown in Figure 4, was re-suspended in water up to mass concentration 120-125 g/L and 37 % hydrochloric acid was added up to the final mass concentration 70 g/L. The gel-sol reaction was performed at a temperature of 80 °C for 2 h.

An acidic suspension of rutile nanoparticles of mass concentration ~ 100 g/L was prepared. The crystal structure of obtained nanoparticles was checked with X-ray powder diffraction (XRD). The diffractogram showing deflections typical of the rutile crystal structure is shown in Figure 6. Figure 7 shows rutile nanoparticles obtained by the above method and was taken with a scanning electron microscope.

Example 3: Stabilisation of a suspension of anatase nanoparticles with citric acid in a quantity of 5 % based on the mass of Ti0₂.

Stabilisation of the suspension was carried out on an acidic suspension of anatase nanoparticles obtained by centrifugation and re-suspending of the formed cake as described in Example 1. The starting suspension of nanoparticles had a mass concentration of Ti0₂ about 400 g/L and pH value about 1. To the acidic suspension citric acid monohydrate was added (dry form, 100 %, C₆H₈0₇ H₂0) in a quantity of 5 % based on the mass of the Ti0₂ present in the starting acidic suspension. After citric acid was added, the suspension was stirred for about 30 minutes until complete dissolution of citric acid. The obtained suspension was then neutralised by using concentrated NaOH with a mass concentration of ~ 750 g/L. Neutralisation was performed up to pH 7.5. While NaOH was added the viscosity of the suspension decreased, which is indicative of increased dispersity of particles in the suspension. Dispersity was determined by DLS measurement, which is based on light scattering on the particles in the suspension. DLS measurement and distribution of particle sizes after neutralisation is shown in Figure 8. As the obtained suspension contained quite much salts that formed upon neutralisation of acid with NaOH, it was desired that the salts were removed as much as possible. This could be achieved by centrifugation, which caused depositing of nanoparticles and Ti0₂ could thus be separated from the liquid medium, in which salts were present. Centrifugation and consequently deposition of nanoparticles is possible exactly due to the presence of salts that increase the ionic power of the suspension medium and thus decrease reflection forces among nanoparticles. When centrifugation is completed, the pure fraction can be decanted and the cake re-suspended. As the obtained suspension has a considerably lower ionic power, the reflective forces among the particles are consequently more prominent and the obtained suspension is completely stable, thus neither centrifugation can separate the Ti0₂

nanoparticles.

Example 4: Stabilisation of a suspension of anatase with citric acid in the quantity of 10 % based on the mass of Ti0₂.

Stabilisation experiment was carried out in a similar way as described in Example 3, except that the quantity of added citric acid was 10 % based on the mass of Ti0₂. Since more citric acid was added, the dispersity of the system after the addition of NaOH was better. Dispersity was determined with DLS measurement and is shown in Figure 9.

Example 5: Stabilisation of a suspension of anatase nanoparticles with citric acid in the quantity of 15 % based on the mass of Ti0₂.

Stabilisation experiment was carried out in a similar way as described in Example 3, except that the quantity of added citric acid was 15 % based on the mass of Ti0₂. Since more citric acid was added, the dispersity of the system after the addition of NaOH was better. Dispersity was determined with DLS measurement and is shown in Figure 10.

Example 6: Stabilisation of a suspension of anatase nanoparticles with citric acid in the quantity of 20 % based on the mass of Ti0₂.

Stabilisation experiment was carried out in a similar way as described in Example 3, except that the quantity of added citric acid was 20 % based on the mass of Ti0₂. Since more citric acid was added, the dispersity of the system after the addition of NaOH was better. Dispersity was determined with DLS measurement and is shown in Figure 11.

Example 7: Hydrothermal synthesis of monocrystalline anatase particles from a stabilised suspension of polycrystalline anatase nanoparticles of mass concentration 40 g/L and pH 9. The starting material used for the hydrothermal synthesis was a suspension of anatase nanoparticles, which was stabilised by an addition of 10 % of citric acid and had pH 9 after neutralisation. After neutralisation with NaOH no purification with centrifugation was performed, as the fundamental suspension was used.

The suspension was subjected to hydrothermal conditions. It was heated up to a temperature of 210 °C for 3 hours. An entirely monocrystalline product was obtained in the hydrothermal synthesis from the polycrystalline nanoparticles shown in Figure 12. The obtained

nanoparticles were monocrystals, which was proved by an analysis performed on a

transmission electron microscope. Figure 13 is a SEM figure and shows the formed

nanoparticles at small magnification, whereas 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 obtained material was checked with X-ray powder diffraction, with which it was identified that the obtained product was exclusively anatase. The X-ray powder diffractogram of the obtained product is shown in Figure 15 and indicates the most intense deflection.

Example 8: Hydrothermal synthesis of monocrystalline anatase particles from a stabilised suspension of polycrystalline anatase nanoparticles of mass concentration 200 g/L and pH 9. The starting material used for the hydrothermal synthesis was a suspension of anatase nanoparticles, which was stabilised by an addition of 10 % of citric acid and had pH 9 after neutralisation. The mass concentration of Ti0₂ in the suspension was ~ 200 g/L.

The suspension was subjected to hydrothermal conditions. It was heated up to a temperature of 210 °C for 3 hours. Figure 16 is a SEM figure and shows the obtained nanoparticles at small magnification, whereas Figure 17 is a high resolution image obtained with a transmission electron microscope (TEM) and shows individual particles with the characteristic crystalline nature typical of a monocrystal.

Example 9: Hydrothermal synthesis of monocrystalline anatase particles from a stabilised suspension of polycrystalline anatase nanoparticles of mass concentration 300 g/L and pH 8.5. The starting material used for the hydrothermal synthesis was a suspension of anatase nanoparticles, which was stabilised by an addition of 10 % of citric acid and had pH 8.5 after neutralisation. The mass concentration of Ti0₂ in the suspension was - 300 g/L. The suspension was subjected to hydrothermal conditions. It was heated up to a temperature of 210 °C for 3 hours. Figure 18 is a SEM figure and shows the obtained nanoparticles at small magnification, whereas Figure 19 is a high resolution image obtained with a transmission electron microscope (TEM) and shows individual particles with the characteristic crystalline nature typical of a monocrystal.

Example 10: Hydrothermal synthesis of monocrystalline anatase particles from a stabilised suspension of polycrystalline anatase nanoparticles of mass concentration 300 g/L and pH 8.5. The starting material used for the hydrothermal synthesis was a suspension of anatase nanoparticles, which was stabilised by an addition of 10 % of citric acid and had pH 8.5 after neutralisation. The mass concentration of Ti0₂ in the suspension was - 300 g/L.

The suspension was subjected to hydrothermal conditions. It was heated up to a temperature of 180 °C for 5 hours. Figure 20 is a SEM figure and shows the obtained nanoparticles at small magnification, whereas Figure 21 is a high resolution image obtained with a transmission electron microscope (TEM) and shows individual particles with a crystalline nature typical of a monocrystal.

Claims

Claims:

1. A synthesis method for obtaining anatase nanoparticles of high specific surface area and spherical morphology based on a gel-sol reaction, wherein metatitanic acid is used as a starting material, which is a nanocrystalline anatase gel and a semi-product in the production of a Ti0₂ pigment, characterised in that

- metatitanic acid is first converted to sodium titanate in a reaction with NaOH, wherein the molar ratio of NaOH : Ti0₂ is between 1:1 or 1 :3,

purification of the sodium titanate suspension is performed, which is re-suspended in water and the obtained suspension has a mass concentration of Ti0₂ between 90 and 180 g/L and has a basic pH value,

the sodium titanate suspension is used as a raw material for a direct synthesis of anatase nanoparticles in a reaction that follows,

- the synthesis of anatase nanoparticles is performed from a suspension of sodium titanate with an addition of hydrochloric acid having a mass concentration of between 20 and 100 g/L, the reaction is carried out at a temperature of 80 °C or at a range of temperatures between 50 °C and boiling point and the time period of two hours or less,

the obtained anatase nanoparticles are in the form of an acidic suspension of an

approximate mass concentration of 100 g/L,

- the nanoparticles can be purified from the acidic suspension by increasing the pH value, with which agglomeration of nanoparticles is achieved, or by centrifugation of the acidic suspension by a simultaneous addition of a small quantity of aluminium sulphate, wherein the centrifugation cycle is repeated several times.

2. Method of claim 1, characterised in that the size of nanoparticles can be controlled by changing the mass concentration of the hydrochloric acid used in the reaction.

3. Method of claim 1, characterised in that the size of anatase nanoparticles can be adjusted to about 50 nm or less, wherein the anatase nanoparticles are present in the form of an acidic suspension with a mass concentration ~ 100 g/L and that the nanoparticles can be isolated and purified, wherein the nanoparticles are polycrystalline in their nature and there is no need for an energy consuming calcination of the product and the basic crystallites that compose each nanoparticle have an approximate size of 5 nm.

4. Method of claim 1, with which the acidic suspension of anatase nanoparticles can be purified of the excess acid by centrifugation and the obtained cake is then re-suspended in water up to a mass concentration between 50 and 400 g/L, wherein a stable suspension with a low pH value is formed.

5. Method of claim 4, with which the acidic suspension of anatase nanoparticles is converted into a stable suspension having a neutral or basic pH by adding a suitable quantity of a dispersant, wherein citric acid in a dry or dissolved form is used as dispersant in a quantity between 5 and 20 % based on the mass of Ti0₂.

6. Method of claim 4 to 5, wherein pH of the suspension of anatase nanoparticles, in which citric acid is dissolved, can be increased by a base, which can be NaOH or any other water soluble base with various mass concentrations ranging from saturated to diluted.

7. Method of claims 4 to 6, wherein pH value is increased to between 5 and 10, wherein the nanoparticles do not excessively agglomerate due to the activity of the dispersant and thus form a relatively well dispersed system.

8. Method of claims 4 to 7, wherein the neutralised suspension of anatase nanoparticles is centrifugated in order to remove the salts produced during neutralisation with a base, because the nanoparticles deposit on the bottom of the centrifuge tube, whereas the salts are mostly present in the pure water fraction above the cake; after centrifugation, the cake of Ti0₂ nanoparticles can be re-suspended in water, wherein an entirely stable suspension is formed having a dry substance content of up to 30 %.

9. Method of claims 4 to 8, with which the neutralised or slightly basic suspension of polycrystalline anatase nanoparticles can be used to carry out a hydrothermal synthesis, with which monocrystalline anatase nanoparticles are obtained.

10. Method of claim 9, in which an autoclave is used for the hydrothermal synthesis, into which autoclave a neutral or slightly basic suspension of anatase nanoparticles is inserted up to adequate filling and subjected to hydrothermal conditions; the hydrothermal synthesis is performed with suspensions having a pH range from between 7.5 and 10 and mass

concentration between 40 and 350 g/L at a temperature between 130 and 210 °C, which is carried out in a time period of between 2.5 and 5 hours, wherein monocrystalline anatase nanoparticles are formed with a size between 30 to 50 nm.

1 1. Method of claims 1 to 10, characterised in that all processes are carried out the whole time in wet, i. e. in the form of a suspension, which prevents a possible occurrence of potentially harmful powdering.