SI23547A - COATING OF TiO2 RUTILE NANOPARTICLES IN A SUSPENSION WITH HYDRATED SiO2 AND Al2O3 - Google Patents

Abstract

The invention refers to a rapid, adaptable, manageable and reproducible approach to a synthesis of coated TiO2 nanoparticles with hydrated silicon and aluminium oxides with the use of harmless chemicals. The proposed method is based on a preparation of a well dispersed system of TiO2 nanoparticles and a controlled hydrolysis of Si02 from an alkaline precursor Na2Si03 in the presence of the mineral acid H2S04 or a hydrolysis of Al203 from an alkaline precursor of NaAlO2 in the presence of the mineral acid H2S04. The analyses have proved that the longer the coating times, the higher the SiO2 quantity and consequently the thickness of the coating. Controlled synthesis conditions were used to coat individual TiO2 nanoparticles. The mechanism of inorganic coating of TiO2 nanoparticles can be categorised as precipitation of hydrated oxides in the form of a thin layer onto already present particles (heterogeneous nucleation). The advantage of the method is its simplicity and the fact that a stable suspension of TiO2 nanoparticles is present in the whole method from the beginning to the end, which provides for healthy work conditions of industrial workers and users and also prevents negative effects on the environment (emission of nanoparticles).

Description

COATING OF ROUTE TIO2 nanoparticles in suspension with hydrated SiO ₂ and A1 ₂ O ₃

FIELD OF THE INVENTION

The object of the invention is the coating of rutile TiO ₂ nanoparticles in suspension with SiO ₂ and AI2O3.

The present invention relates to the field of colloidal chemistry and more specifically comprises suspensions of TiO ₂ nanoparticles in the crystalline structure of rutile, with emphasis on their surface treatment, using sodium silicate and sodium aluminate as coatings for nanoparticles with thin layers of hydrated silica and aluminum ₂ (SiO _2). and A1 ₂ O3).

TECHNICAL FIELD

The invention provides a controlled coating of TiO2 nanoparticles with thin layers of hydrated aluminum and silica (SiO ₂ and AI2O3). The decision to produce TiO2 nanoparticles, which exclusively produces suspension products, is based on the concern for healthy working conditions of employees in the production and use and the prevention of negative environmental impacts (nano-particle emission). The coating is based on the principle of hydrolysis of sodium silicate (Na ₂ SiO3) and sodium aluminate (NaA10 ₂ ). The purpose of coating TiO ₂ nanoparticles with inorganic oxides is to limit the formation of free radicals on the TiO ₂ surface and to change the electrokinetic properties. Surface treatment results in a change in surface charge and thus an increased dispersibility in polar media. Surface treatment with inactive inorganic materials results in the preparation of suspensions without major agglomerates. This increases the degree of transparency (coatings containing TiO ₂ coated nanoparticles) in the visible part of the spectrum and increases the coverage of paints / varnishes / coatings and the protection of the base substrate. Coated TiO ₂ nanoparticles defend their technological and usable properties while demonstrating even greater applicability, since the incorporation of UV absorbers such as TiO ₂ is an effective way to improve the UV resistance of materials. Coated TiO ₂ nanoparticles in the crystalline structure of rutile serve as UV absorbers, and there are various fields of application for them: cosmetics, plastics, paints, varnishes, coatings, textiles, etc. The coating according to the invention is affordable.

-2Nano titanium dioxide (TiO ₂ ) is one of the new modern materials. Compared to pigment TiO ₂ has changed properties. TiO ₂ nanoparticles differ in size and physicochemical properties from pigment TiO ₂ , so their use is different from pigment. Particle size, specific surface area, tube part shape and crystal structure are key parameters that affect the properties and usability of the finished product. In terms of crystalline structure, TiO _{2 is a} substance with several known polymorphs. Of all the TiO ₂ crystal structures, however, only rutile and anatas are more important. The main properties of nano TiO ₂ are UV light absorption and photocatalytic activity. Already pigmented TiO ₂ is photocatalytically active, and this property is even more pronounced in TiO ₂ nanoparticles. TiO ₂ nanoparticles are most commonly used as UV absorbers and photocatalysts. TiO ₂ nanoparticles in the crystal structure of anatas have high photocatalytic activity, so they are used as self-cleaning agents, bactericides in wastewater treatment and also as semiconductors in photocell production. TiO ₂ nanoparticles in the crystalline structure of rutile are excellent UV absorbers and are added to different colors and coatings to achieve better UV and weather resistance. The properties of TiO2 nanoparticles can be improved by coating them with other materials. Surface treatment of TiO ₂ or. the coating is achieved by hydrolysis of the oxides to the surface of the TiO ₂ nanoparticle.

Water glass is chemically composed of sodium silicate (Na ₂ SiO ₃ ). It is formed by dissolving quartz sand with soda. Na ₂ SiO ₃ is water soluble. It is stable in alkaline medium at pH values greater than pH 10. Below this value, hydrolysis begins, Na ₂ SiO ₃ hydrolyzes and SiO ₂ is eliminated. Depending on the conditions of the hydrolysis of the oxide, nanoparticles with different characteristics can be formed during surface treatment with SiO ₂ . Hydrolysis at acidic or acidic. neutral pH leads to the formation of an amorphous SiO ₂ coating consisting of submicron particles bound in the gel structure. The resulting coating allows for better dispersibility (TiO ₂ ) of the particles in the medium and consequently an improvement in the optical properties. For neutral or weakly acidic conditions result in deposits with a lower degree of gloss or. gloss - matt. Slow hydrolysis at alkaline pH, however, leads to a denser glassy SiO ₂ coating, which results in improved durability. durability of the particle.

Sodium aluminate (NaA10 ₂ ) is an important inorganic chemical. It acts as an efficient source of aluminum oxide for many industrial and technical applications. Anhydrous sodium aluminate is a white crystalline substance of the formula NaA10 ₂ , Na ₂ O · A1 ₂ O ₃ , resp. At ₂ Al ₂ O4. Sodium aluminate is available in both solution and solid form. They produce it by dissolving it

Of -3 aluminum hydroxide in NaOH solution. Like Na2SiC> 3, NaAlCE is soluble in water. The aqueous NaAlCE solution is stable in an alkaline medium, at pH values greater than pH 10. Below this value, hydrolysis begins, NaAlCE hydrolyzes and AI2O3 is secreted. Surface treatment with hydrated alumina is probably the most common surface treatment of TiO2- Hydrated alumina can be hydrolyzed from acid-reacting sodium aluminate. The application morphology depends on the coating conditions. Hydrated aluminum oxides which hydrolyze from sodium aluminate under acidic conditions are amorphous forms. Hydrated AI2O3 particles on the surface of the T1O2 nanoparticle cause a decrease in the attractive forces between the particles and improve the dispersibility. In the case of multilayer surface treatment, AI2O3 is most often the last application.

Known state of the art

The properties of T1O2 nanoparticles can be improved by coating them with other materials. The most commonly used surface treatments are: treatment with S1O2, AI2O3 and / or ZrO2. Different approaches at the molecular level are known to produce oxide layers in controlled syntheses. The synthesis of T1O2 can proceed by different methods, but the more important of these is the solgel technique, which allows the formation of a microstructure in different choice of precursor type and process conditions. Despite all the advantages offered by the sol-gel process, many existing industrial production of TiCE-SiCE composites in this synthesis mode are most often hindered by factors such as: the use of expensive and hazardous chemicals in the sol-gel process; long synthesis times.

Important publications in which the authors have used sodium silicate (Na2SiCE) and sodium aluminate (NaAlCE) as the SiCE and AI2O3 source as the coatings for T1O2 nanoparticles are mentioned below.

S. R. Frerichs and W. H. Morrison investigated the surface treatment of TiO2 nanoparticles with S1O2 and AI2O3 in the presence of citric acid. In their work, the authors used T1O2 nanoparticles in powder form to prepare the slurry. Surface treatment comprises: preparation of a slurry of T1O2 nanoparticles; citric acid supplement; addition of a metal oxide source selected from the group consisting of AI2O3 source and SiCE source; surface treatment of T1O2 surface treated nanoparticles. S1O2 hydrolysis was carried out at elevated T (T-30-100 ° C) at pH - 7. After surface treatment with Na2SiO3 and NaAlCE, the particles were dried, crushed and sieved. The coated T1O2 nanoparticles had a content of −44% S1O2 and 66% AI2O3. The particles coated according to the described procedure exhibited high light stability and had a less tendency to agglomerate.

C. R. Bettler and M. P. Deibold, however, investigated a process for the production of a highly persistent T1O2 pigment that is more easily dispersible. The process comprises the following steps: heating the slurry of T1O2 particles to T = 85-100 ° C; addition of citric acid; adjusting pH to pH> 10; addition of aqueous Na2SiO3 solution; neutralization of the slurry with mineral acid (nitric, hydrochloric or sulfuric acid); regulation T (T = 55-90 ° C); addition of aqueous NaAlCh solution; adjusting the pH to 5-9 z with a mineral acid (nitric, hydrochloric or sulfuric acid).

There is no information available in the literature on the described modification of TiO2 powderless nanoparticles. There is also no explanation for the stability of Na2SiC> 3 and NaAlCk precursors under different pH conditions using a titration approach and an explanation of the electrokinetic properties of T1O2 nanoparticles, which influence the determination of process conditions and ultimately the coating result. There is also no evidence of the washing effect of surface-treated T1O2 nanoparticles. The modification described above differs from other coating processes in that the process does not include the drying phase and, consequently, powder particles, which have a negative impact on production employees, users and the environment. Coated T1O2 nanoparticles will be available in suspension rather than powder form for various applications. This avoids the heating costs of calcining and agglomeration of the coated nanoparticles that occurs during calcination.

DESCRIPTION OF THE INVENTION

The invention is further explained by the figures:

Figure 1: TEM image of untreated TiO2 nanoparticles with crystalline rutile structure.

Figure 2: TEM image of submicron S1O2 particles formed after embodiment A bound to the gel structure.

Figure 3: TEM image of coated T1O2 nanoparticles having a crystalline rutile structure, with an amorphous layer of hydrated S1O2 formed by hydrolysis of Na2SiO3 according to embodiment C, which uniformly covers the surface of T1O2 nanoparticles.

-5Figure 4: TEM image of coated TiO2 nanoparticles with a crystalline rutile structure, with an amorphous layer of hydrated S1O2 and AI2O3 formed after embodiment D, which uniformly cover the surface of TiO2 nanoparticles.

Figure 5: SEM image of unwashed TiO2 nanoparticles with crystalline rutile structure, with an amorphous layer of hydrated S1O2 formed by hydrolysis of Na2SiO3 according to embodiment B, which uniformly covers the surface of TiO2 nanoparticles and salt crystals formed during surface treatment reactions.

Figure 6: SEM image of washed coated TiO2 nanoparticles with crystalline rutile structure, with an amorphous layer of hydrated S1O2 formed by hydrolysis of Na2SiC> 3 according to embodiment C, which uniformly covers the surface of T1O2 nanoparticles.

Figure 7: SEM image of washed lined TiO2 nanoparticles with crystalline rutile structure, with an amorphous layer of hydrated S1O2 and AI2O3 formed after embodiment D, which uniformly cover the surface of TiO2 nanoparticles.

t

The present invention describes the coating of T1O2 nanoparticles with thin layers of hydrated amorphous aluminum and silica (S1O2 and AI2O3). The essence of the coating of T1O2 nanoparticles according to the invention is controlled conditions that lead to the formation of homogeneous layers of hydrated oxides on the surface of T1O2 nanoparticles, which is reflected in the change in the electrokinetic properties of T1O2 nanoparticles. Physico-chemical conditions were determined and further applied that lead to the desired morphology and properties of the deposits. In the case of controlled coating of T1O2 particles, homogeneous deposits have formed on the surface of them, completely covering the surface of the particles. The advantage of the process described is that it does not include the final calcination phase.

For surface treatment, a suspension of T1O2 nanoparticles with a mass concentration of γ = 100 - 300 g / L was used. The average size of each T1O2 nanoparticle in suspension is 80x20 nm. The specific surface area of T1O2 nanoparticles is 130 m ² / g. The analysis of electrokinetic properties determined the isoelectric point (IET) of T1O2 nanoparticles, which is located at pH ~ 6.5. Increasing the pH of the suspension of T1O2 nanoparticles from strongly acidic to pH 10.5 is achieved by the addition of a base. 10-60 wt. Is used as a base for pH adjustment. % sodium hydroxide. Stabilization of the suspension of T1O2 nanoparticles with a basic pH is carried out in the temperature range between 40 and 100 ° C and for two hours or less. 0.5 - 2% by weight of citric acid is added as a stabilizing agent to prevent the agglomeration of T1O2 nanoparticles in the pH range near the isoelectric point. The reaction with citric acid takes place in the temperature range between 40 and 100 ° C and over time

-6hours or less with agitation at 150 to 400 rpm. Na2SiO ₃ were used as precursors _; 10-30 wt% (ysi02 = 220 g / L) and NaAlCh; 10-30 wt% (yai2O3 = 280.3 g / L). Synthesis of S1O2 nanoparticles from sodium silicate with mineral acid addition. As a mineral acid, it uses 10-60 wt% to recover S1O2 nanoparticles. % sulfuric acid. The elimination of S1O2 nanoparticles is achieved at pH 7 at which the suspension is stirred for three hours or less. Diluted H2SO4 is used as the initiator of hydrolysis and condensation, since concentrated acid could cause local or homogeneous nucleation of S1O2, which in our case is a side effect. Synthesis of the coating of AI2O3 nanoparticles from sodium aluminate is carried out with the simultaneous addition of mineral acid at a pH between 6.5 and 7.5. The resulting suspension is stirred over a temperature range of 40 to 100 ° C and for an hour or less. and as a means of regulating pH conditions, 10 -60 wt. % mineral sulfuric acid and 10 - 60 wt. % NaOH. Prior to the surface treatment process, both precursors were analyzed using titrations, while the T1O2 nanoparticles were determined to have electrokinetic properties or. isoelectric point (IET). The pH of aqueous Na2SiO ₃ (ysiO2 = 220 g / L) was determined to be 1 wt. % Na2SiO ₃ solution was 10.3, while the pH of undiluted aqueous Na2SiO _{3 was} 11.7. The alkaline solution was titrated with Chci = 0.1 mol / L. The Na2SiO ₃ solution was still stable at pH 10.5, while at pH <10 the solution was no longer stable, as evidenced by the course of the curve and the turning point. The pH value of the dilute aqueous NaAlCE is ~ 11.5, with the addition of acid or. lowering the pH results in the hydrolysis of hydrated AI2O3. The IET of TiC> 2 nanoparticles was determined using a Particle Charge Detector (PCD). The IET is at pH 6.5. At a given pH value, the stability of the T1O2 aqueous suspension is lowest, and the T1O2 nanoparticles have a high tendency to agglomerate. The surface charge of the particles determines the stability of the dispersions, so the charge on the surface of the particles is created by adjusting the pH to pH 10.5. At this pH, the stability of both precursors was also determined, providing optimum conditions under which homogeneous thin layers of hydrated aluminum and silicon oxide (S1O2 and A ^ CE) will be formed, which will completely cover the surface of individual T1O2 nanoparticles rather than agglomerates. .

After the surface treatment is completed, the particles are further washed on a centrifuge, removing the impurities contained, in particular the various salts formed by the surface treatment of the T1O2 nanoparticles.

Example A:

500 mL of distilled water is poured into the beaker and the pH is adjusted to pH 9.5 - 10.5 by adding 10 - 60 wt. % NaOH. At the same time, it is heated to a temperature of 40 -100 ° C At this pH, Na2SiO ₃ is still present

-7stable. Slowly (dropwise) 10-30 wt% Na2SiO3 is added. At ₂ SiC> 3 in this case represented the source of SiO ₂ . The suspension was stirred for 5-30 minutes, then added 10-60 wt. % H2SO4 until a pH of 6.5-7.5 is reached. The suspension was stirred at this pH for 0.5 to 4 hours to recover all SiO ₂ . Maturation follows.

Example B:

Pour 500 mL of a prepared suspension of rutile TiO ₂ nanoparticles with a mass concentration of γ = 100 - 300 g / L previously dispersed using an Ultra Turrax T25 dispersant (IKA, Germany) at 8000 - 13500 rpm. The suspension of rutile TiO ₂ nanoparticles after synthesis is strongly acidic (pH ~ 0). 0.5 - 2 wt. Is added to the suspension when mixed with the propeller mixer at 150 400 rpm. % citric acid in solution form. The particles can be highly agglomerated during surface treatment during the isoelectric point transition and as such are unsuitable for SiO ₂ coating. When coating TiO ₂ nanoparticles with SiO _2, it is necessary to prevent particle agglomeration, ie to prepare a stable suspension in water. Agglomeration can be prevented by binding citric acid to the surface. The acid bound on the surface sterically prevents the nanoparticles from agglomerating, while providing a high charge on their surface, which contributes to the electrostatic stabilization of the suspension (electrostatic stabilization). The mixture was heated to 40 - 100 ° C, with stirring at 150 - 400 rpm for 0.5 - 2 hours. Meanwhile, citric acid chemically bound to the surface of the nanoparticles. The pH of the suspension was then adjusted to pH 10.5 by adding 10-60 wt. % NaOH. At this pH, the particles on the surface have a high negative charge, which is manifested as a high potencial-potential that prevents their agglomeration. The suspension was stirred for 0.5 - 2 hours in order to obtain a uniform charge distribution on the TiO ₂ nanoparticle surface. With further heating and stirring, 10-30 wt% on ₂ SiO3 is added. Na ₂ SiO3 in this case represents the source of SiO ₂ . The suspension should be stirred for 5-30 minutes, then added 10-60 wt. % H ₂ SO4 until a pH of 7 is reached. At this pH, the suspension is stirred for 0.5 - 3 hours to eliminate all SiO ₂ . This is followed by maturation and centrifugation.

Example C:

The start of Execution Procedure B is the same as Execution Procedure B. After maturation, centrifugation and washing follow. The washing process of the coated TiO ₂ nanoparticles is an important stage as the washing removes the resulting salts in the SiO ₂ hydrolysis reactions and other impurities that could have a negative effect on the applicability properties of the coated TiO ₂ nanoparticles.

-8Example D:

The start of Execution Procedure B is the same as Execution Procedure A. After 0.5 - 3 hours of stirring the suspension at a neutral pH value, once all SiO ₂ is depleted, the coating is continued by adding 10-30 wt. % NaA10 ₂ , with 10-60 wt. % H ₂ SC> 4, maintaining the pH at 6.5-7.5. The suspension is then stirred and heated at a constant temperature for another 10-60 minutes. This is followed by the maturation and washing of the surface-treated TiO ₂ nanoparticles, where the resulting salts are removed by the SiO ₂ and A1 ₂ C> 3 and> impurity reactions.

Coating of the rutile nanoparticles of the invention is therefore characterized in that the acidic suspension of TiO2 nanoparticles is converted into a stable suspension with a basic pH by adding an appropriate amount of citric acid in dissolved form to react with citric acid in the temperature range between 40 and 100 ° C and for two hours or less while stirring at 150 to 400 rpm to raise the pH from strongly acidic to pH 10.5 by adding a base to stabilize the suspension of TiO2 nanoparticles with a basic pH in the temperature range between 40 and at 100 ° C and for two hours or less that the synthesis of the deposition of SiO2 nanoparticles from sodium silicate with the addition of mineral acid is carried out, yielding a pH of 7 at which the suspension is stirred for three hours or less, that the synthesis is carried out application of A1 ₂ O3 sodium aluminate nanoparticles with the simultaneous addition of mineral acid at a pH between 6.5 and 7.5 to mix the resulting suspension over a temperature range of between 40 and 100 ° C, and for one hour or less, that the coated TiO2 nanoparticles can be purified by centrifugation, the cycle of spinning being repeated several times.

The suspension of TiO ₂ nanoparticles has a mass concentration of TiO ₂ between 100 and 300 g / L and has an acidic pH value. Citric acid between 0.5 and 2% by weight is used as a stabilizer. 10 - 60 wt. % sodium hydroxide. The SiO ₂ source represents 10 - 30% by weight of sodium silicate with a SiO ₂ concentration of 220 g / L. As the mineral acid to izobarjanje SiO ₂ nanoparticles used 10-60 wt. % sulfuric acid. Source A1 ₂ C> 3 represents 10-30% by weight of sodium aluminate with an ALO3 mass concentration of 280 g / L. As a mineral acid for

-9Displacement of AI2O3 nanoparticles uses 10-60 wt. % sulfuric acid. Centrifugation of the neutralized suspensions of the coated TiO ₂ nanoparticles removes the salts formed during neutralization with the base and the acid.

Claims (8)

Coating of rutile TiO ₂ nanoparticles in suspension with SiO ₂ and AfCfi, the process being developed from an acidic suspension of TiO ₂ nanoparticles in a crystalline structure, characterized in that the acid suspension of TiO2 nanoparticles is converted into a stable suspension with a basic pH by adding an appropriate amount citric acid in dissolved form to react with citric acid in the temperature range between 40 and 100 ° C and stirring at 150 to 400 rpm for two hours or less in order to raise the pH from strongly acidic to pH 10 , 5 with the addition of a base that stabilization of the suspension of TiO2 nanoparticles with a basic pH is carried out in the temperature range between 40 and 100 ° C and for two hours or less that a synthesis of the sodium silicate nanoparticles with the addition of mineral acid is carried out, recovery achieved at pH 7 at which the suspension is stirred for three hours or less gives a synthesis of the deposition of ΑΕΟ3 nanoparticles from sodium aluminate with simultaneous addition of mineral acid at a pH of between 6.5 and 7.5 so that the resulting suspension is stirred over a temperature range of 40 to 100 ° C and for one hour or less to allow the coated TiO2 nanoparticles to be purified by centrifugation, the spin cycle can be repeated several times. Coating according to claim 1, characterized in that the suspension of TiO2 nanoparticles has a mass concentration of TiO ₂ between 100 and 300 g / L and has an acidic pH value. Coating according to claim 1, characterized in that citric acid between 0.5 and 2% by weight is used as a stabilizer. Coating according to claim 1, characterized in that 10-60 wt. % sodium hydroxide. 5. The coating techniques according to claim 1, characterized in that the source of SiO ₂ representing 10 - 30 wt.% Sodium silicate the mass concentration of SiO _{2 of} 220 g / L. Coating according to claim 1, characterized in that 10-60% by weight is used as a mineral acid for the recovery of SiO ₂ nanoparticles. % sulfuric acid. -117. Coating according to claim 1, characterized in that the source of AI2O3 represents 10-30% by weight of sodium aluminate with a mass concentration of AI2O3 280 g / L. Coating according to claim 1, characterized in that 10-60% by weight is used as a mineral acid for the recovery of AI2O3 nanoparticles. % sulfuric acid. Coating according to claim 1, characterized in that by centrifuging the neutralized suspensions of the coated TiO ₂ nanoparticles, salts formed during neutralization with the base and acid are removed.