WO2012089840A2

WO2012089840A2 - THE METHOD OF PREPARATION OF NANOCOLLOIDAL SnO2 WATERSOL AND ITS USES

Info

Publication number: WO2012089840A2
Application number: PCT/EP2011/074334
Authority: WO
Inventors: Glen KELP; Tanel TÄTTE; Uno MÄEORG; Siim PIKKER; Aile TAMM; Madis PAALO
Original assignee: University Of Tartu
Priority date: 2010-12-31
Filing date: 2011-12-31
Publication date: 2012-07-05
Also published as: EE201000096A; WO2012089840A3

Abstract

The present invention concerns nano colloidal water-sol of SnO₂, and its use in the preparation of microspheres, micro and nano containers or solid and liquid filled microcapsules. According to the method firstly the stannic oxide is mixed with alcohol and the obtained mixture is poured quickly into the water. The resulting precipitate is transformed into homogeneous liquid whereas the alcohol is removed during transforming process. The aqueous solution is purified with alcohol and concentrated in vacuum at a temperature of 70-85 °C until the water solution becomes viscous syrop-like emulsion of the precursor.

Description

A METHOD OF PREPARATION OF NANOCOLLOIDAL SnO₂ WATERSOL AND

ITS USES

Technical Field

[0001] The object of current invention is a method for preparing a stable

nanocolloidal SnO2 watersol and its gelation and uses. The wet-chemical process is controlled by the nature of solvents, alkoxide, temperature, acidity and time. The sol is stable at room conditions. Depending on the configuration of the gelation system, the precursor can be gelled in order to get SnO2 materials of various geometrical shapes, including micro- and nanospheres, -torroidals and containers, thin films, fibres etc.

Background Art

[0002] SnO2 is a widely used metal oxide with great industrial importance. Using different synthesis methods, SnO2 has been prepared as nanofilms [13], powders [14], micro- and nanofibres [15-17] and microspheres [18-20]. For the enhancement of properties (electrical conductivity, optical

transmission) of these materials, SnO2 has been synthesized in

non-stoichiometric form or the material has been doped with F, Sb, CI, Bi [12].

[0003] A known method for preparing SnO2 structures is based on the sol-gel principle. As a neat compound, tin alkoxide (metoxide, etoxide, propoxide, i-propoxide, butoxide, t-butoxide, pentoxide) is left to react with water in solvents (alcohol, alkane) to grow the particle size of the precursor in order to set the viscous-elastic properties of the precursor into a range of 1 - 30000 P. To do that, 0 - 4 moles of water is added for each mole of alkoxide. When more water is added, the material gels by forming a 3-D network of particles. The growth of the particles occurs as a response to known chemical reactions: hydrolysis and polycondensation. The method is controlled by the nature and concentration of solvents (corresponding alcohols, alkanes), the nature of alkoxides, temperature, acidity-basicity of the system and water-alkoxide molar ratio, taken to grow the particles. However, there are several problems related to the use of tin alkoxides. Similarly to some other metal alkoxides: the oligomeric structure (particle size 0.5 - 2 nm) of alkoxides causes the decrease of their solubility [2]. Hydrolysis and condensation reactions of dissolved tin alkoxides are so rapid that the process is difficult to control [3]. To better control the reactions and material structure, inhibiting ligands and stabilizers are added to the solution [3, 4].

[0004] Another known wet-chemistry method to prepare tin oxide materials is based on using a substance expressed by formula RSn(C≡CR')3 [5]. This material acts as an alternative sol-gel precursor instead of alkoxides. It is possible to get tin oxide sheets separated by organic groups by using that kind of substance [6]. The same method enables also to prepare nanocrystalline nanoporous SnO2 [7]. Analogous is the method of functionalization of SnO2 nanoparticle surface by hydrolyzing XSn(C.CR) 3 in the presence of SnO2 nanoparticles. For different applications fluorinated chains [8] and dyes [9] have been used as the functionalizing group. The drawback with the usage of those substances is the presence of carbon in the final products, which changes the properties of the material.

[0005] Tin carboxylates and tin halogenides have also been used as precursors in the preparation of SnO2 materials by sol-gel method [12]. Most common halogenides have been SnCl2 and SnCLi [3]. The drawback with using those substances is the presence of chlorine anions in the final products, which changes the properties of the material [3]. This limits the usage of tin chlorides as a precursor for SnO2 preparation. The problem also is that toxic and corrosive HCI is released during the process.

[0006] The main deficiency of using the sol-gel method for tin oxide materials preparation is the presence of carbon residues in the final product [10, 11]. A method is known for preparing (fluorine doped) tin oxide materials with lesser carbon residues [12], but the residues are still present in trace amounts.

[0007] From the prior art a method is known for preparing an aqueous T1O2 sol and gelling it with dry n-butyl alcohol through the extraction of water to yield transparent T1O2 hollow microspheres [US pat 4349456]. The method of preparing this T1O2 watersol comprises of the following steps: - 5 parts of tetraisopropyl titanate was added to 1 part of hydrochloric acid (37%).

- The resulting sol was dried at room temperature and in ambient air.

12.5 g of the resultant dry gel was dispersed with ultrasonic agitation in 50 g of water, creating an aqueous T1O2 sol.

[0008] The resultant T1O2 watersol was gelled by the following method:

- The watersol obtained in previous section was poured into 100 g of dry n-butyl alcohol with constant stirring. Further 300 g of n-butyl alcohol was added, and the stirring was carried out for about 5 minutes. During the stirring T1O2 microspheres formed into liquid.

- T1O2 microspheres were separated from the liquid phase by filtering. The resultant microspheres obtained were liquid-filled, transparent shell-like structures.

[0009] The lack of this method is that it can be used to prepare T1O2 but not SnO2 materials. When applying it for preparation of SnO2 sols, solid saturation of SnO2 forms instead of a stable nanocolloidal solution. Nevertheless, the described method [US pat 4349456] is a prototype for the current invention.

Disclosure of Invention

[0010] The current invention is an improved method for preparing SnO2 water sols. The SnO2 precursor preparation method is similar to the T1O2 watersol preparation described above, differing in the following points:

- A tin alkoxide (Sn(OMe)₄, Sn(OEt)₄, Sn(OPr)₄, Sn(OBu)₄, Sn(OPe)₄, Sn(OHe)₄, Sn(Oi-Bu)₄, Sn(Ot-Bu)₄, Sn(Oi-Pr)₄ or other) is taken as a neat compound or as a solution in alcohol (MeOH, EtOH, PrOH, BuOH, PeOH or PrOH etc.) or in another solvent and mixed with water. The water-to-alkoxide molar ratio R is chosen to be at least 20 moles of water per a mole of alkoxide. When the water-to-alkoxide molar ratio R is sufficient stable nanocolloidal SnO2 dispersion in water is formed. When not enough water (R under 20) is taken, the reaction results in the formation of a solid precipitation of SnO2. When too much water is taken (e.g. R more than 1 000 000) then the process is too inefficient as the reaction is carried out in a huge amount of water. - As a result of a reaction with water, alkoxy-groups are released from the alkoxides and they dissolve in water as MeOH, EtOH, PrOH or, if they are non-dissoluble in water, form a separate layer on the surface as in the case of BuOH, PeOH etc. Release of alkoxy-groups occurs in 10 minutes to 2 days, from the mixing of solutions. Organics are removed from the system. In case of dissolving alcohols like MeOH, EtOH, PrOH, the separation is done by using evaporation. In case of non-dissolving alcohols, the layer which is formed is mechanically separated by decantation or a separation funnel for example.

- The obtained colloidal SnO2 dispersion in water is concentrated (when needed) to get a stable SnO2 watersol, which can be used as a precursor in synthesis of SnO2 materials as is or diluted in water.

[0011] The gelation of SnO2 watersol is achieved by:

- By removing water from the colloidal dispersion. It can be done by using extraction with dry alcohols (MeOH, EtOH, PrOH, BuOH, PeOH or PrOH etc.) or by using some other dry solvents and their mixtures, by drying in gaseous atmosphere, heating, vacuuming, osmosis etc.

Brief Description of Drawings

[0012] The present invention is prescribed in detail by following experimental details and examples with references to added drawings where in

Figure 1 is shown SAXS determined pair length distribution function for two different shapes of particle models: (a) for spheres and (b) for cross-sections of rod-shape particles of colloidal SnO2 in H2O;

Figure 2 is shown XRD 2θ-ω scans of a) liquid nanoparticle suspension, b) xerogel obtained by drying the suspension at room temperature in air for 7 days. Vertical bars represent relative intensities of the powder diffraction reference pattern ICDD 41-1445 of tetragonal Cassiterite SnO2 phase; Figure 3 is shown a schematic representation of SnO2 nanoparticle in H2 O;

Figures 4a-c are shown typical images of SnO2 microspheres prepared by ultrasonic mixing (figure 4a), injection mixing (figure 4b) and torroidal shape particles (figure 4c);

Figure 5 is shown a breaking of the spheres shows that opaque-looking spheres contain microbubbles inside their structure;

Figures 6a-6d are shown a results of the Raman measurements of the crystallization of the samples during the heat treatment;

Figure 7 is shown the Raman spectra of microsphere samples with different heat treatments (spectra are vertically shifted for clarity);

Figure 8 is shown image of broken microcapsule;

Figure 9 is shown tape casted SnO2 lines on glass substrate obtained by using structured , .doctoral blades,, (thickness of the lines is ~1 micron).

Best Mode for Carrying Out the Invention

[0013] Example 1

Precursor preparation

50 g of Sn(OBu)4 was mixed with 50 g of BuOH. The mixture was then poured rapidly into a beaker filled with 150 ml of water. Immediately after pouring a slurry-like voluminous saturation is formed. In the next 2 hours the mixture was transformed into a homogeneous slightly yellowish liquid. During the process butanol was released and it formed a separate layer on the water surface. To purify the water solution from additives, it was extracted 3 to 4 times with 10 ml of butanol until the layer on the solution remained uncolored. The resultant slightly yellow solution was used as a neat material in further experiments.

[0014] Precursor for materials preparation was obtained by concentrating the water solution in a 10 - 20 torr vacuum on a 70 °C water bath until it showed syrup-like viscous behavior. The concentration process was stopped as soon as the content started to solidify on the walls of the rotation bulb. Obtained matter was used as-prepared or diluted in water in further experiments.

[0015] The obtained precursor liquid is a watersol consisting of nanoparticles with rod-like shape with cross-sections of approximately 0.7 nm and lengths up to 7 nm, as determined by SAXS analysis. XRD measurements showed the cassiterite crystalline SnO2 phase in the nanoparticles.

[0016] Example 2

Precursor preparation

Precursor was prepared in a similar manner as in example 1 , except 50 g of Sn(OPe)4 and 50 g of PeOH were used for preparation, PeOH or BuOH were used for purification and precursor was obtained by concentrating the initial water solution in 5 torr vacuum.

[0017] Example 3

Precursor preparation

Precursor was prepared in a similar manner as in example 1 , except 50 g of Sn(OPr)4 and 50 g of PrOH were used for preparation, released propanol was removed by distillation or vacuum-rotation, PeOH or BuOH were used for purification and precursor was obtained To grow the size of precursor particles into rage 5 - 20 nm, the precursor was heat-treated at 300 °C in autoclave. Desired precursor concentration was achieved by concentrating the initial water solution in 5 torr vacuum or by diluting with water.

[0018] Example 4

Preparation of microspheres and toroid-shaped microparticles

The spheres were obtained by transforming the emulsion of precursor into solid as a result of extracting the water (solvent) from the precursor by using dry alcohol (for example butanol). Emulsions were generated by using 3 different mixing modes:

- Ultrasound mixing

0.1 ml of precursor and 2 ml of dry butanol were put into plastic ependorf so that they formed two separate liquid layers, the denser precursor in the bottom layer. For emulsion generation the liquids were mixed with Diagenode Nanoruptor ultrasonicator. A sample of structures was prepared at each available sonication mode (S, L1 , L2, H) as a result of 5 s sonication.

- Mechanical shaking

The liquids were transferred to the ependorf in the same way as for the ultrasound mixing experiment. Emulsification was done by a mechanical shaking mixer. Three different mixing rates (800, 1500 and 3000 rpm) were applied.

- Injection into alcohols

A beaker was filled with 10 ml of dry butanol. For colloid generation 0.1 ml of precursor was injected into the liquid with an insulin syringe in 0.1 s. During injection, the tip of the needle was kept in the liquid. The experiment was repeated using dry pentanol as the extracting liquid.

[0019] The resultant spheres are smooth-surfaced, and the diameters and optical transparencies of the spheres are given in table 1.

Table 1. Experimental results of SnO2 micro and nanospheres preparation process

Extracting Diameter of Transparency of

Method of mixing

medium spheres the spheres

Dry up to 20-30

ultrasound S opaque

butanol micron

Dry up to 20-30

ultrasound L1 opaque

butanol micron

Dry up to 20-30

ultrasound L2 opaque

butanol micron

Dry up to 20-30

ultrasound H opaque

butanol micron

mechanical shaking 800 Dry

2-100 micron opaque rpm butanol

mechanical shaking 1500 Dry

2-150 micron opaque rpm butanol

mechanical shaking 3000 Dry

2-200 micron opaque rpm butanol

Dry

injecting 500nm-10 micron opaque

butanol

Dry

injecting 500nm-10 micron transparent pentanol

[0020] Figures 4a-c shows typical images of SnO2 microspheres prepared by ultrasound mixing (figure 4a), injection mixing (figure 4b), and toroid-shape particles (figure 4c). Micro-Raman spectroscopy showed the amorphous SnO2 structure of the as-prepared spheres. The opaque-looking spheres contained sub-micron size bubbles in the structure, as seen on figure 5. [0021] In addition, different microchannel chips can be used to generate micro- and nanodroplets of precursor and transform them to micro spheres.

Specific drying conditions can lead to the formation of toroid-shaped particles due to volume shrinkage.

[0022] Heat-treatment of the spheres

[0023] In all cases, the spheres prepared in butanol or pentanol were left to

sediment for 24 h, after which they were decanted. The materials were left exposed to ambient air until alcohols evaporated. Heat treatment (1 h; 400, 600, 800 and 1000 °C) was applied on the materials prepared in butanol; the resultant spheres are depicted on figures 6a-d.

Heat-treatment causes the growth of SnO2 crystallites and the spheres heated at or over 600 °C are rough-surfaced. Estimated size of SnO2 crystallites in microspheres determined from micro-Raman spectroscopy measurements, using a method proposed by Dieguez et al [21], is given in table 2.

Table 2. Size estimations of SnO2 nanocrystallites in microsphere samples heat-treated at different temperatures

Temperature Estimated size

Room temperature Amorphous

400 °C 15-30 nm

600 °C 25-40 nm

800 °C 30-60 nm

1000 °C 30-60 nm

[0024] Example 5

Preparation of micro- and nanocontainers

The microparticles described in example 4 and made from the precursors obtained as described in examples 1 - 3 can be prepared as liquid-, gas- or solid-filled. For that the following should be done:

- To get gas-filled microcapsules, the precursor should be dispersed into extracting liquid as gas-filled bubbles.

- To get liquid-filled microcapsules, the precursor should be dispersed into extracting liquid as liquid-filled bubbles. - To get solid-filled microcapsules, the precursor should be dispersed into extracting liquid as bubbles containing solid content or, as an alternative, the precursor should be dispersed into extracting liquid as liquid-filled bubbles and the content should be solidified afterwards. Figure 8 shows an image of a broken microcapsule.

[0025] Example 6

The preparation of micro and nanofibres

The precursors obtained as described in examples 1 - 3 could be spun or drawn into fibres by using extrusion, direct drawing, etc. The continuous liquid precursor jets could be transformed to solid fibres by using extraction of the water by dry liquid mediums (like alcohols).

[0026] Example 7

Preparation of micro- and nanofilms

The precursors, prepared as described in examples 1 - 3 were used to coat substrates by using the following coating methods:

- Spin coating

For that the precursor was dropped on spinning substrates rotating at different speeds. If carried out correctly, homogeneous films with thicknesses in the micro- or nanoscale were obtained.

- Dip coating

For that the precursor was dissolved in suitable amount of water or used as prepared. The substrate was dipped momentarily in the solution and then removed. If carried out correctly, homogeneous films were produced.

- Spray coating

For that the precursor was dissolved in suitable amount of water or used as prepared. The material was carried onto the substrate by spraying it trough the nozzle.

- Flow coating

For that the precursor was dissolved in suitable amount of water or used as prepared. The material was let to flow over substrate and form nano- or microfilm on it.

- Tape casting For that the precursor was dissolved in suitable amount of water or used as prepared. To obtain the films, thin strips of metal were used to smear the precursor on the substrate surface.

[0027] The exact parameters of the preparation processes depend on desired properties of the films (thickness, structure) and properties (viscosity, concentration, temperature) of the precursor, and processing. Figure 9 shows tape casted SnO2 lines on glass substrate obtained by using structured thin blades. Thickness of the lines is approximately 1 micron.

[0028] Example 8

Micromolding

The precursors, prepared as described in examples 1 - 3 were processed by micromolding following these steps:

- An amount of precursor was carried onto the solid substrate.

- A structured PDM mold was pressed on the substrate so that the

precursor filled the structures of the mold.

- During the next few hours or more the precursor solidified between the stamp and substrate.

- The stamp was carefully cleaved from the surface so that the solid structures remained on the substrate.

[0029] Example 9

Formation of monolayer of SnO2 nanocrystals

The precursors, prepared as described in examples 1 - 3 were taken into a beaker as prepared or dissolved in water. Si-substrate was immersed in the solution for 1 minute. Then the substrate was washed for 3 times with pure water. As a result of washing only the particles bound covalently directly to the substrate remained on the surface as a monolayer of randomly oriented individual particles.

[0030] Example 10

The preparation of Micro and nanoporous thick films

[0031] Saturation

The spheres, toroids, nano- and microcontainers obtained as prepared (examples 4 - 5) or heated as described in example 1 1 were suspended in water or some other liquid medium and let to saturate a layer on the surface of the vessel. The liquid medium was then decanted and the layer left to dry in ambient air or at a slightly elevated temperature depending on the boiling point of the liquid medium. The neat layer was used as a filter as prepared or after heat treatment. To make the layer even harder, special additives (alkoxides, glues, polymers, oils etc.) were added to allow the structures to stick stronger together.

[0032] Filtering

The spheres, balls, toroids, nano- and microcontainers obtained as prepared as in examples 4 - 5 or heated as described in example 11 were suspended in water or some other liquid medium and filtered out, resulting in a filter "cake" on the filter surface. To remove the layer, the cake was cleaved from the filter, left to dry in ambient air or at a slightly elevated temperature, depending on the boiling point of the liquid medium. To make the layer even harder, special additives (alkoxides, glues, polymers, oils etc.) were added to stick the structures stronger together.

[0033] Example 11

Post-treatment of the structures

All groups of materials described above were heated up to 1400 °C with no fragmentation after 2 - 3 days of aging at normal ambient air conditions.

[0034] Example 12

Final application: transparent electrodes

The materials prepared in the forms described in examples 4 - 10 can be used as transparent electrodes if doped with antimony, bismuth, chlorine or fluorine, or used unstoichiometrically. The electrodes could be used in solar-cells, heating elements on (transparent) materials like window glasses, etc.

[0035] Example 13

Final application: gas sensors

The materials prepared in the forms described in examples 4 - 10 can be used in gas sensing elements. The sensing is possible by using electrical conductance or receptivity as feedback, since organic vapors largely influence the electrical parameters of these materials.

[0036] Example 14 Final application: organic vapors sensors

The materials prepared in the forms described in examples 4 - 10 can be used as sensing elements in organic (alcohol) vapors sensors. The sensing is possible by using electrical conductance or receptivity as feedback, since organic vapors largely influence electrical parameters of these materials.

[0037] Example 15

Final application: filtering materials

The materials prepared in the forms described in examples 6 and 10 can be used as micro and nano-filtering materials. The filters can be purified after use by heat or UV radiation treatment, and reused.

[0038] Example 16

Final application: optical ring-resonators

The materials prepared in the forms described in examples 4 - 7 can be used as optical resonators in sensors or data processing.

[0039] Example 17

Final application: light generation

The materials prepared in forms described in examples 4 - 10 and heat treated as described in example 11 can be used to generate light. Light generation is possible by using the materials in their pure form, or doped by rare earth ions or some other materials. Light generators can also be used as optical sensors, since the ability of these materials to generate light depends strongly on surrounding gases.

[0040] Example 18

Final application: catalyst carrier

The materials prepared in forms described in examples 4 - 10 and heat treated as described in example 11 can be used as catalyst carriers in chemical synthesis. The application is possible at least up to the temperature of 1400 °C.

[0041] Example 19

Final application: thermal isolators

The materials prepared in forms described in examples 4 - 10 and heat treated as described in example 11 can be used as thermal isolators. The application is possible at least up to the temperature of 1400 °C.

[0042] Example 20

Final application: textiles

The materials prepared in form described in example 6 and heat treated as described in example 1 1 can be used in textiles. The application is possible at least up to the temperature of 1400 °C.

References

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Claims

1. A method of preparation of nanocolloidal SnO2 watersol and its gelation

comprising the steps for

- mixing a tin alkoxide solution in an organic solvent or pure tin alkoxide with water, whereas the water-to-alkoxide molar ratio R is at least 20, to form a nanocolloidal SnO2 watersol, thereafter

- removing the released alcohol and solvents from the formed nanocolloidal SnO2 watersol to form a precursor material, thereafter

- gelling the precursor material by removing the water from said material.

2. The method according to the claim 1 wherein the solvent is selected from

MeOH, EtOH, PrOH, BuOH, PeOH or some other organic solvent or any mixture thereof.

3. The method according to the claim 1 wherein tin alkoxide is selected from

Sn(OMe)4, Sn(OEt)4, Sn(OPr)4, Sn(OBu)4, Sn(OPe)4, Sn(OHe)4,

Sn(Oi-Bu)4, Sn(Ot-Bu)4, Sn(Oi-Pr)4 or any other tin alkoxide.

4. The method according to claims 1 - 3 wherein acids or bases are added to the water prior to mixing with tin alkoxide.

5. The method according to claims 1 - 4, wherein during the forming the SnO2 watersol a temperature treatment in the range of 0 - 500 °C is applied to the mixture to modify the structure of the formed watersol.

6. The method according to claims 1-5 wherein nanocolloidal SnO2 watersol is gelled by removing water from it by evaporation in a gas atmosphere, supported by lowered pressure or increased temperature or by extraction with a dry alcohol or other liquid or by osmosis.

7. The method according to the claim 6 wherein the gelation of the precursor prepared according to claim 1 - 5 is used in emulsified form in a liquid system in order to prepare spherical particles in diameter range from 10 nm to 1 mm.

8. The method according to claims 1-6 wherein coating procedures like spray-, dip-, spin-, flow coating, tape-casting etc. are used before gelation in order to carry the precursor prepared according to claims 1 - 5 onto the substrates and subsequently gel the precursor layer to obtain thin nanocrystalline SnO2 films on substrate surfaces.

9. The method according to claim 6 wherein the gelation of the precursor is carried out in molds in order to prepare nanocrystalline oxide materials with desired geometry.

10. The method according to claim 6 wherein the gelation of the precursor is

carried out in the form of a jet in a dry liquid system in order to prepare spherical shape particles.

11. Use of a material obtained according to claims 7 - 10 as gas sensor materials.

12. Use of a material obtained according to claims 7 - 10 as transparent

electrodes.

13. Use of the materials obtained according to claims 7 - 10 as light emitters.

14. Use of the materials obtained according to claims 7 - 10 as catalyst carriers.

15. Use of the materials obtained according to claims 7 - 10 as construction

materials.