METAL OXIDE NANOPARTICLES

Abstract

The present invention relates to metal oxide nanoparticles, a method for their production, a coating, or printing composition, comprising the metal oxide nanoparticles and the use of the composition for coating of surface relief micro- and nanostructures (e.g. holograms), manufacturing of optical waveguides, solar panels, light outcoupling layers for display and lighting devices and anti-reflection coatings. Holograms are bright and visible from any angle, when coated, or printed with the composition, comprising the metal oxide nanoparticles.

Claims

1.-15. (canceled)

16. Process for the preparation of single, or mixed metal oxide nanoparticles comprising the following steps: a) preparing a mixture, comprising a metal oxide precursor compound(s), a solvent, a tertiary alcohol, or a secondary alcohol, wherein the tertiary alcohol and secondary alcohol eliminate water upon heating the mixture to a temperature of above 60° C., or mixtures, containing the tertiary alcohol(s) and/or the secondary alcohol(s) and optionally water, b) heating the mixture to a temperature of above 60° C., c) treating the obtained nanoparticles with a base, especially a base which is selected from the group consisting of alkali metal alkoxides, alkali metal hydroxides, alkali metal salts of carboxylic acids, tetraalkylammonium hydroxides, trialkylbenzylammonium hydroxides and combinations thereof, wherein the metal oxide precursor compound(s) is selected from the group consisting of metal alkoxides of formula Me(OR.sup.12).sub.x (I), metal halides of formula Me′(Hal).sub.x′ (II) and metal alkoxyhalides of formula Me″(Hal′).sub.m(OR.sup.12′).sub.n(III) and mixtures thereof, wherein Me, Me′ and Me″ are independently of each other titanium, tin, tantalum, niobium, hafnium, or zirconium; x represents the valence of the metal and is either 4 or 5, x′ represents the valence of the metal and is either 4 or 5; R.sup.12 and R.sup.12′ are independently of each other a C.sub.1-C.sub.8alkyl group; Hal and Hal′ are independently of each other Cl, Br or I; m is an integer of 1 to 4; n is an integer of 1 to 4; m+n represents the valence of the metal and is either 4 or 5; the solvent comprises at least one ether group and is different from the tertiary alcohol and the secondary alcohol; the ratio of the sum of moles of hydroxy groups of tertiary alcohol(s) and secondary alcohol(s) to total moles of Me, Me′ and Me″ is in the range 1:2 to 6:1.

17. The process according to claim 16, wherein the tertiary alcohol is selected from the group consisting of tert-butanol, 2-methyl-2-butanol, 3-methyl-3-pentanol, 3-ethyl-3-pentanol, 2-methyl-2-pentanol, 2,3-dimethyl-2-butanol, 1-methylcyclopentanol, 1-ethylcyclopentanol, 1-methylcyclohexanol, 1-ethylcyclohexanol, 1-vinylcyclohexanol, 2-methyl-2,4-pentanediol, 2,4-dimethyl-2,4-pentanediol, 2,3-dimethyl-2,3-butanediol, 2,5-dimethyl-2,5-hexanediol, 2,6-dimethyl-2-heptanol, 3,5-dimethyl-3-heptanol, 3,6-dimethyl-3-heptanol, 1-adamantanol, 2-methyl-3-buten-2-ol and 1-methoxy-2-methyl-2-propanol, 2-phenyl-2-propanol, 2-phenyl-2-butanol, 3-phenyl-3-pentanol, 2-methyl-1-phenyl-2-propanol, α-, β-, γ- or δ-terpineol, 4-(2-hydroxyisopropyl)-1-methylcyclohexanol (p-menthane-1,8-diol), 3,7-dimethylocta-1,5-dien-3,7-diol (terpenediol I), terpinen-4-ol (4-carvomenthenol), (±)-3,7-dimethyl-1,6-octadien-3-ol (linalool) and mixtures thereof.

18. The process according to claim 16, wherein the solvent is selected from the group consisting of tetrahydrofuran, 2-methyltetrahydrofurane, tetrahydropyrane, 1,4-dioxane, cyclopen-tylmethyl ether, diisopropyl ether, di-n-propyl ether, di-isobutyl ether, di-tert-butyl ether, di-n-butyl ether, di(3-methylbutyl) ether (diisoamyl ether), di-n-pentyl ether, di-n-hexyl ether, di-n-octyl ether, ethylene glycol dimethyl ether, ethylene glycol di-ethyl ether, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, di(ethylene glycol) dimethyl ether, di(ethylene glycol) diethyl ether, di(ethylene glycol) di-n-propyl ether, di(ethylene glycol) di-n-butyl ether, 1,2-dimethoxypropane, 1,2-diethoxypropane, 1,3-dimethoxypropane, 1,3-diethoxypropane, 1,4-dimethoxybutane, 1,4-diethoxybutane, di(propylene glycol) dimethyl ether, di(propylene glycol) diethyl ether, tri(propylene glycol) dimethyl ether, tri(propylene glycol) diethyl ether, tri(ethylene glycol) dimethyl ether, tri(ethylene glycol) diethyl ether, tetra(ethylene glycol) dimethyl ether and tetra(ethylene glycol) diethyl ether and mixtures thereof.

19. The process according to claim 16, wherein the mixture in step a) comprises a metal alkoxide of formula (I) and a metal halide of formula (II).

20. The process according to claim 16, wherein Me, Me′ and/or Me″ are titanium.

21. The process according to claim 16, wherein the temperature in step b) is in the range 80 to 180° C.

22. The process according to claim 16, comprising the following steps: a) preparing a mixture, comprising a metal alkoxide of formula Ti(OR.sup.12).sub.4 (Ia), metal halide of formula Ti(Hal).sub.4 (IIa), wherein R.sup.12 and R.sup.12′ are independently of each other C.sub.1-C.sub.4alkyl; Hal is Cl; a solvent, a tertiary alcohol and optionally water, b) heating the mixture to a temperature of from 80° C. to 180° C., c) treating the obtained nanoparticles with a base, wherein the ratio of moles of hydroxy groups of tertiary alcohol to total moles of Ti is in the range 1:2 to 6:1; the base is selected from the group consisting of alkali metal alkoxides, especially potassium ethylate; alkali metal hydroxides, especially potassium hydroxide; alkali metal salts of carboxylic acids, especially potassium acrylate and methacrylate and combinations thereof, the solvent is selected from 2-methyltetrahydrofurane, tetrahydropyrane, 1,4-dioxane, cyclopentylmethyl ether, di-n-propyl ether, di-isobutyl ether, di-tert-butyl ether, di-n-butyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, di(ethylene glycol) dimethyl ether, di(ethylene glycol) diethyl ether, di(ethylene glycol) di-n-propyl ether, di(ethylene gly-col) di-n-butyl ether, di(propylene glycol) dimethyl ether, di(propylene glycol) diethyl ether, tri(propylene glycol) dimethyl ether, tri(propylene glycol) diethyl ether, tri(ethylene glycol) dimethyl ether, tri(ethylene glycol) diethyl ether, tetra(ethylene glycol) dimethyl ether and tetra(ethylene glycol) diethyl ether and mixtures thereof; the tertiary alcohol is selected from tert-butanol, 2-methyl-2-butanol, 3-methyl-3-pentanol, 3-ethyl-3-pentanol, 2-methyl-2-pentanol, 2,3-dimethyl-2-butanol, 1-methylcyclopentanol, 1-ethylcyclopentanol, 1-methylcyclohexanol, 1-ethylcyclohexanol, 2,3-dimethyl-2,3-butanediol, 2,5-dimethyl-2,5-hexanediol, 2,6-dimethyl-2-heptanol, 3,5-dimethyl-3-heptanol, 3,6-dimethyl-3-heptanol, 2-methyl-3-buten-2-ol, 2-phenyl-2-propanol, 2-phenyl-2-butanol, 3-phenyl-3-pentanol, 2-methyl-1-phenyl-2-propanol, α-, β-, γ- or δ-terpineol, 4-(2-hydroxyisopropyl)-1-methylcyclohexanol (p-menthane-1,8-diol), terpinen-4-ol (4-carvomenthenol), and wherein in step b) the alcohol R.sup.12OH is removed by distillation.

23. Metal oxide nanoparticles, obtainable according to the process of claim 16, especially titanium dioxide nanoparticles having a volume average particle size from 1 nm to 40 nm, and a film of the metal oxide nanoparticles, especially titanium dioxide nanoparticles which is dried at 100° C. for 1 minute shows a refractive index of greater than 1.70 (589 nm), especially of greater than 1.80, very especially of greater than 1.90 and dispersions of the metal oxide nanoparticles, especially the titanium dioxide nanoparticles in ethanol mixed with water (1:1 v/v) under vigorous stirring show a pH of higher than 3.5 and lower than 10.

24. Surface functionalized metal oxide nanoparticles, comprising the metal oxide nanoparticles of claim 23 treated with a) a phosphonate of formula ##STR00028## or a mixture of phosphonates of formula (V), wherein R.sup.1 and R.sup.2 are independently of each other hydrogen, or a C.sub.1-C.sub.4alkyl group, R.sup.3 is a group CH.sub.2═CH—, or a group of formula —[CH.sub.2].sub.n2—R.sup.4, wherein N2 is an integer of 1 to 12, when n>3 one —CH.sub.2— may be replaced by —S— with the proviso that S is not directly linked to P, or R.sup.4, R.sup.4 is hydrogen, or a group of formula ##STR00029## R.sup.5 is hydrogen, or a C.sub.1-C.sub.4alkyl group, R.sup.6 is hydrogen, or a C.sub.1-C.sub.4alkyl group, X.sup.1 is O, or NH, and b) bonded with an alkoxide of formula R.sup.7O.sup.− (VI) and/or ##STR00030## wherein R.sup.7 is a C.sub.1-C.sub.8alkyl group, which may be interrupted one or more times by —O— and/or substituted one or more times by —OH, R.sup.8 is hydrogen, or a C.sub.1-C.sub.4alkyl group, R.sup.9 is hydrogen, —CH.sub.2OH, —CH.sub.2SPh, —CH.sub.2OPh, or a group of formula R.sup.10—[CH.sub.2OH—O—CH.sub.2].sub.n1—, n1 is an integer of 1 to 5, X.sup.2 is O, or NH, R.sup.10 is a group of formula —CH.sub.2—X.sup.3—CH.sub.2—C(═O)—CR.sup.11═CH.sub.2, X.sup.3 is O, or NH, and R.sup.11 hydrogen, or a C.sub.1-C.sub.4alkyl group.

25. A coating, or printing composition, comprising the metal oxide nanoparticles according to claim 23, or the metal oxide nanoparticles obtained according to the process of claim 16, or the surface functionalized metal oxide nanoparticles according to claim 24 and optionally a solvent.

26. A security, or decorative element, comprising a substrate, which may contain indicia or other visible features in or on its surface, and on at least part of the said substrate surface, a coating, comprising the metal oxide nanoparticles according to claim 23, or the metal oxide nanoparticles obtained according to the process of claim 16, or the surface functionalized metal oxide nanoparticles according to claim 24.

27. A method for forming a surface relief micro- and nanostructure on a substrate comprising the steps of: a) forming a surface relief micro- and nanostructure on a discrete portion of the substrate; and b) depositing the coating, or printing composition according to claim 25, on at least a portion of the surface relief micro- and nanostructure; or a method for forming a surface relief micro- and/or nanostructure on a substrate comprising the steps of a′) providing a sheet of base material, said sheet having an upper and lower surface; b′) depositing the coating composition according to claim 25 on at least a portion of the upper surface; and c′) forming a surface relief micro- and/or nanostructure on at least a portion of the coating composition, and d′) curing the coating composition by exposing it to actinic radiation, especially, UV-light; or a method for forming a surface relief micro- and/or nanostructure on a substrate, comprising the steps of a″) providing a sheet of base material, said sheet having an upper and lower surface; b″) depositing the coating composition according to claim 25 on at least a portion of the upper surface; and c″) optionally removing a solvent; d″) curing the dry coating by exposing it to actinic radiation, especially UV-light; and e″) forming a surface relief micro- and/or nanostructure on at least a portion of the coating composition.

28. The method according to claim 27, wherein step a) comprises a1) applying a curable compound to at least a portion of the substrate; a2) contacting at least a portion of the curable compound with surface relief micro- and nanostructure forming means; and a3) curing the curable compound.

29. Use of the coating, or printing composition according to claim 25 for coating holograms, manufacturing of optical waveguides and solar panels.

30. Use of the metal oxide nanoparticles according to claim 23, or the metal oxide nanoparticles obtained according to the process of claim 16, or the surface functionalized metal oxide nanoparticles according to claim 24 in light outcoupling layers for display and lighting devices, high dielectric constant (high-k) gate oxides and interlayer high-k dielectrics, anti-reflection coatings, etch and CMP stop layers, protection and sealing (OLED), organic solar cells, optical thin film filters, optical diffractive gratings and hybrid thin film diffractive grating structures, or high refractive index abrasion-resistant coatings.

Description

EXAMPLES

Measurement of pH of Dispersions in Ethanol

[0261] The aliquots of nanoparticles dispersions in ethanol were mixed with water (1:1 v/v) under vigorous stirring and pH was measured in the resulting mixture by means of pH meter.

Measurement of Refractive Indices of the Coatings by Ellipsometry

[0262] The nanoparticles-containing dispersions were coated onto silicon wafers to obtain coatings with thicknesses of at least 200 nm (thickness was measured with KLA Tencor Alpha-Step D-100 Stylus Profiler). The data was acquired in Reflectance mode at 65°, 70° and 75° angles, using Woollam M-2000-R19 ellipsometer, and the obtained data was fitted using the Cauchy model with WVase32 software.

Measurement of Particle Size Distribution by DLS

[0263] The measurements were performed using Malvern Zetasizer Nano ZS device with ca. 3% w/w dispersions of nanoparticles in a suitable solvent. Measurements in ethanol were performed in presence of acrylic acid (15% w/w of acrylic acid relative to particles weight was added). Measurements in water were performed in presence of 1 mM HCl. D10, D50 and D90 values are given for volume distributions.

Measurement of Solids Content

[0264] The solids content of powders and dispersions was determined using Mettler-Toledo HR-73 halogen moisture analyzer at 100° C.

XRD Measurements

[0265] Powder samples were loaded on to a special flat plate Silicon sample holder, taking special care on producing a flat and smooth surface with the correct alignment to the sample holder zero-reference to avoid large systematic errors. The silicon sample holder was manufactured such that the it does not produce sharp diffraction features but only a weak and smooth background.

[0266] The sample on the sample holder was loaded in to a Panalytical 'XPert3 Powder equipped with a sealed Cu tube producing a characteristic X-ray lines Cu K.sub.α and Cu K.sub.β with wavelengths λ.sub.1=1.54056 Å (Cu K.sub.α1), λ.sub.2=1.54439 Å (Cu K.sub.α2), I.sub.2/I.sub.1=0.5 and λ.sub.2=1.3922 Å (Cu K.sub.β). The contribution of the latter (Cu K.sub.β) was removed introducing a Ni-filter on the incident beam of the diffractometer right after the Cu-tube.

[0267] Diffraction data was collected from 10 to 80°2θ, using a step of 0.026°2θ for a total time of 2 h and spinning the sample around its axis at a rate of 0.13 rate/s in order to increase the sampling statistic.

[0268] The analysis of the diffraction patterns in terms of crystallographic phase analysis and average domain size was performed using the Panalytical HighScore software (v 4.8+) and the Bruker Topas6 program, obtaining consistent results.

[0269] The volume weighted domain size of diffraction (Dv) was evaluated using the Schemer equation (B. E. Warren, X-Ray Diffraction, Addison-Wesley Publishing Co., 1969) Dv=K λ/[β cos(θ)], where K(˜1) is the shape factor, dependent on the shape and reciprocal space direction, A the wavelength, β the integral breadth of the diffraction peak and θ the scattering half-angle. To ensure a correct determination of the Dv, the integral breadth β was amended of the instrumental contribution. To achieve this, the line-broadening of the powder reference material LaB.sub.6 was measured and evaluated according to the same procedure, as described above.

Example 1

Step 1. Synthesis of TiO.SUB.2 .Nanoparticles

[0270] Di(propyleneglycol) dimethyl ether (400 g) was placed in a 1 L double-wall reactor, equipped with a mechanical stirrer and a distillation head with a Liebig condenser. 2-Methyl-2-butanol (282.1 g) was added, followed by addition of tetraethyl orthotitanate (273.8 g), and the mixture was stirred for 5 min. Titanium tetrachloride (75.9 g) was added dropwise with stirring and the reaction mixture was heated to 120° C., during which time distillation has begun. The reaction mixture was stirred at 120° C. internal temperature (with jacket temperature control) for 24 h, upon which time distillate (440 g) was collected and the beige precipitate has formed. After that, the reaction temperature was increased to 150° C. and the stirring was continued for 5 h at this temperature.

[0271] The reaction mixture was cooled to 25° C., iso-propanol (400 g) was added and stirring was continued for 1 h. The mixture was filtered under vacuum through a paper filter (20 μm pore size), the product was washed on the filter with iso-propanol (500 g) and dried on the filter for 10 min after washing was complete. The beige powder (285.7 g) was obtained, which was resuspended in iso-propanol (550 g) in a 1 L 3-neck round-bottom flask, equipped with a magnetic stirring bar. This suspension was stirred for 2 h at 50° C. and then filtered under vacuum through a paper filter (20 μm pore size). The beige wet powder of TiO.sub.2 nanoparticles agglomerates was obtained (294.4 g). Solids content at 100° C. 66.5% w/w. XRD analysis showed anatase to be the predominant phase with crystalline domain size of 2.7±1 nm. D10(v)=2.3 nm, D50(v)=3.3 nm, D90(v)=5.2 nm (in 1 mM HCl in water).

Step 2. Neutralization/Re-Dispersion of TiO.SUB.2 .Nanoparticles

[0272] The powder, obtained in Step 1 (290 g), was resuspended in absolute ethanol (400 g), the temperature of the mixture was raised to 50° C. and the pH of the mixture was brought to 4 via dropwise addition of 24% w/w potassium ethylate solution in absolute ethanol with stirring. Upon addition of potassium ethylate solution the turbidity of the mixture was strongly reduced due to the re-dispersion of TiO.sub.2 nanoparticles agglomerates. The mixture was centrifuged at 3000 G for 30 min to remove the formed potassium chloride along with the traces of non-re-dispersed TiO.sub.2 nanoparticles and the brown supernatant, containing redispersed TiO.sub.2 nanoparticles, was collected (755 g). Solids content at 100° C. 22% w/w. D10(v)=2.0 nm, D50(v)=3.0 nm, D90(v)=5.3 nm (in presence of acrylic acid in ethanol).

Step 3. Formulation of TiO.SUB.2 .Nanoparticles as a UV-Curable Ink.

[0273] To the dispersion of TiO.sub.2 nanoparticles, obtained in Step 2 (25 g), dipropyleneglycol diacrylate (0.825 g) was added and the mixture was concentrated on rotary evaporator to the total solids content (including acrylate) of 50% w/w. Photoinitiator Irgacure 819 (25 mg) was added. The obtained dispersion was diluted with 1-methoxy-2-propanol to the total solids content of 25% to obtain a UV-curable ink.

Example 2

Step 1. Synthesis of TiO.SUB.2 .Nanoparticles

[0274] All operations were carried out under dry nitrogen atmosphere. Di(propylene glycol) dimethyl ether (400 g) was placed in a 1 L double-wall reactor, equipped with a mechanical stirrer and a distillation head with a Liebig condenser. 2,5-Dimethyl-2,5-hexanediol (234 g) was added, followed by addition of tetraethyl orthotitanate (273.8 g). The mixture was heated to 65° C. over 30 min with stirring and was kept for 15 min at this temperature. Titanium tetrachloride (75.9 g) was added dropwise with stirring and the reaction mixture was heated to 130° C. over 2 h, during which time distillation has begun. The reaction mixture was stirred at 125-130° C. internal temperature (with constant jacket temperature) for 3 h, upon which time distillate was collected and the beige precipitate has formed. After that, the internal reaction temperature was increased to 150° C. over 2 h and stirring was continued for 5 h at this temperature. In total, 315 g distillate was collected.

[0275] The reaction mixture was cooled to 77° C., absolute ethanol (200 g) was added and stirring was continued for 5 h at 77° C. The mixture was cooled to 25° C., isopropanol (300 g) was added, the mixture was stirred for 30 min at 25° C. and filtered under vacuum through a paper filter (20 μm pore size). The product was washed on the filter with iso-propanol (1000 g) and absolute ethanol (300 g) and dried on the filter for 1 min. The beige powder of TiO.sub.2 nanoparticles agglomerates was obtained (247 g). Solids content at 100° C. 61.7% w/w. XRD analysis showed anatase to be the predominant phase with crystalline domain size of 3.1±0.3 nm. D.sub.10(v)=2.1 nm, D.sub.50(v)=3.0 nm, D.sub.90(v)=4.8 nm (in 1 mM HCl in water).

Step 2. Neutralization/Re-Dispersion of TiO.SUB.2 .Nanoparticles

[0276] The powder, obtained in Step 1 (227 g), was resuspended in absolute ethanol (450 g). The temperature of the mixture was raised to 75° C., acetylacetone (5.6 g) was added and the pH of the mixture was brought to 4.5 via dropwise addition of 24% w/w potassium ethylate solution in absolute ethanol (98.6 g) with stirring at 75° C. Upon addition of potassium ethylate solution the turbidity of the mixture was strongly reduced due to the re-dispersion of TiO.sub.2 nanoparticles agglomerates. The mixture was cooled to 25° C. and filtered through the depth filter sheet (Seitz® KS 50) under 2.5 Bar pressure to remove the formed potassium chloride along with the traces of non-re-dispersed TiO.sub.2 nanoparticles. The brownish filtrate, containing re-dispersed TiO.sub.2 nanoparticles, was collected (730 g). Solids content at 100° C. 18.1% w/w. D.sub.10(v)=2.0 nm, D.sub.50(v)=2.8 nm, D.sub.90(v)=4.2 nm (in presence of acrylic acid in ethanol).

Application Example 1

[0277] a) Preparation of Thin Films with High Refractive Index

[0278] The TiO.sub.2 nanoparticles dispersion, obtained in Step 2 of Example 1, was diluted with absolute ethanol to the concentration of 5% w/w of solids. This dispersion was spin-coated onto a silicon wafer and dried at 100° C. for 1 min to obtain a 200 nm thick layer with a refractive index of 1.96 at 589 nm wavelength.

b) Preparation of UV-Cured Films with High Refractive Index.

[0279] The ink, obtained in Step 3 of Example 1, was spin-coated onto a silicon wafer, dried at 100° C. for 1 minute and the dry coating was cured using a medium pressure gallium-doped mercury UV lamp to obtain a 290 nm thick cured coating with a refractive index of 1.87 at 589 nm wavelength.

Comparative Example 1 (p-Xylene as Non-Ethereal Solvent)

Step 1. Synthesis of TiO.SUB.2 .Nanoparticles

[0280] p-Xylene (150 g) was placed in a 0.5 L double-wall reactor, equipped with a mechanical stirrer and a distillation head with a Liebig condenser. 2-Methyl-2-butanol (70.5 g) was added, followed by addition of tetraethyl orthotitanate (68.4 g), and the mixture was stirred for 5 minutes. Titanium tetrachloride (19.0 g) was added dropwise with stirring and the reaction mixture was heated to 120° C., during which time distillation has begun. The reaction mixture was stirred at 120° C. internal temperature (with jacket temperature control) for 24 h, upon which time distillate (105 g) was collected and the white precipitate has formed. After that, the reaction temperature was increased to 135° C. and the stirring was continued for 5 h at this temperature.

[0281] The reaction mixture was cooled to 25° C., iso-propanol (100 g) was added and stirring was continued for 1 h. The mixture was filtered under vacuum through a paper filter (20 μm pore size), the product was washed on the filter with iso-propanol (150 g) and dried on the filter for 10 min after washing was complete. The beige powder (116 g) was obtained, which was resuspended in iso-propanol (150 g) in a 0.5 L 3-neck round-bottom flask, equipped with a magnetic stirring bar. This suspension was stirred for 2 h at 50° C. and then filtered under vacuum through a paper filter (20 μm pore size). The beige wet powder of TiO.sub.2 nanoparticles agglomerates was obtained (119 g). Solids content at 100° C. 41.4% w/w.

Step 2. Neutralization/Re-Dispersion of TiO.SUB.2 .Nanoparticles

[0282] The wet filter cake, obtained in Step 1 (112 g), was resuspended in absolute ethanol (105 g), the temperature of the mixture raised to 50° C. and the pH of the mixture was brought to 4 via dropwise addition of 24% w/w potassium ethylate solution in absolute ethanol (26.9 g) with stirring. Upon addition of potassium ethylate solution no significant re-dispersion of TiO.sub.2 nanoparticles agglomerates occurred.

Comparative Example 2 (a Secondary Alcohol which does not Eliminate Water Upon Heating the Mixture to a Temperature of Above 60° C.)

[0283] Dipropylenglycol dimethylether (100 g) was placed in a 0.5 L double-wall reactor, equipped with a mechanical stirrer and a distillation head with a Liebig condenser. 2-Methylcyclohexanol (91.3 g) was added, followed by addition of tetraethyl orthotitanate (68.4 g), and the mixture was stirred for 5 min. Titanium tetrachloride (19.0 g) was added dropwise with stirring and the reaction mixture was heated to 120° C., during which time distillation has begun. The reaction mixture was stirred at 120° C. internal temperature (with jacket temperature control) for 72 h, upon which time distillate (35 g) was collected but no precipitate has formed. After that, the reaction temperature was increased to 130° C. and the stirring was continued for 24 h at this temperature. No precipitate of TiO.sub.2 nanoparticles was formed.

Comparative Example 3 (pH of Nanoparticles Dispersion According to Example 2 of WO19016136A1)

[0284] The transparent foamy material, obtained in Example 2 of WO19016136A1, was dissolved in water at 5% w/w concentration and pH was measured with ph meter. pH<1 was found.