KR20010075762A - the method coating titanium dioxide photo-catalyst on metal surface - Google Patents

Abstract

PURPOSE: A method for coating titanium oxide photocatalyst on metal surface is provided to increase photocatalyst activity and make coating effective by preventing cloudiness and delaying gelation of fixing solution for film formation. CONSTITUTION: Metal surface to be coated is treated with acid and alkali and washed with water in order to remove impurities. Titanium dioxide powder is added to phosphoric acid solution, and the metal surface is polished with this solution in order to make electron distribution of the metal surface uniform. The metal plate is immersed in a fixing solution in order to coat titanium oxide film. This metal plate is calcined longer than about 30 min at about 500-550 deg.C. The coating process is repeated at least 15 times until thicker than 0.4 micrometers of titanium oxide film is obtained.

Description

The method coating titanium dioxide photo-catalyst on metal surface}

The present invention relates to a method of coating a titanium oxide photocatalyst on a metal surface, and more particularly, a coating operation having a firm adhesive force and a white clouding phenomenon does not occur even when the coating is applied over an appropriate thickness. The present invention relates to a method of coating a titanium oxide photocatalyst on a metal surface.

In general, as the industry develops, organic substances harmful to various human bodies exist in the water and the air, such as rivers and lakes, and the amount thereof is gradually increasing. In addition, these harmful organic compounds and various bacteria are artificially removed from the self-cleaning ability of nature, and moreover, the hardly decomposable organic substances are not easily decomposed despite various treatments, and these harmful substances dissolved in water are It has a devastating effect.

Therefore, extensive researches on the treatment of such organic compounds have been conducted in various countries around the world, and one of the methods has recently been published in photocatalysts using photolysis reactions that react with the appropriate wavelength. The photolysis reaction is performed by irradiating an organic compound with light such as ultraviolet light having a constant light intensity, and is greatly accelerated in the presence of a photocatalyst such as titanium oxide. According to the present research, various kinds of photocatalysts are used. It is confirmed that not only organic compounds but also various bacteria that are not beneficial to the human body can be destroyed and killed.

For example, when the photolysis reaction is used for water treatment, the facility is simpler than the existing water treatment technology, and there are few chemicals, so the equipment cost and operation cost are low, and it is considerably effective because it uses light irradiation energy that does not cause secondary pollution. to be. In addition, the water treatment technology using the photolysis reaction is not only sludge generated but also has a high photo-degradation effect on the organic chlorine compounds, most of which are biologically difficult to decompose recently has been in the spotlight.

As the photocatalyst used for the photochemical treatment, TiO ₂ , WO ₃ , ZnO, SiC, Cds, GaAs, etc. are used, but most commonly titanium oxide (TiO ₂ ) is widely used. In addition, as a method of treating pollutants using the photocatalyst, a method of coating a thin film on a fixed carrier by using a powder form of titanium oxide or by using a sol-gel method has been widely adopted.

In the case of using a powdered titanium oxide, the surface area contributing to the photocatalytic activity is advantageous, which is advantageous over the latter case using a thin film, but the latter case is widely used in practical terms. That is, it is used as a method of using powdered titanium oxide, suspended in water to treat contaminants in water, and then reused through a recovery device. Suspensions of titanium oxide are fine particles of powder, which have a large specific surface area, and thus have high treatment efficiency due to many contact circuits with contaminants. However, since a separate recovery device is required, not only a large installation cost is required, but also a large facility such as water and sewage treatment is not applicable due to the limitation of the recovery device, and there is a problem that only a small device can be used.

In addition, if it cannot be reused due to coloring or contamination after a certain period of time, sludge is generated due to recovery and disposal. If too much titanium oxide is injected into the water, the yield of electron hole generation showing activity as a photocatalyst is lowered due to a decrease in light transmittance. There is a problem.

Due to the above problems, studies have recently been actively conducted to coat titanium oxide photocatalysts on fixed carriers and to use them for purification of water or air pollution. Titanium oxide has an anatase type or a rutile type depending on its crystal structure, and this crystal structure depends on the firing temperature of titanium oxide, and an anatase type is preferred to induce photoactivity. It is known to show much superior performance as a photocatalyst.

On the other hand, in the method of coating the titanium oxide thin film on a widely used fixed carrier, there is a method using a glass, quartz material as a carrier and a metal. First, when titanium oxide is fixed by using quartz or glass as a fixed carrier, the photocatalyst thin film is easily broken, so it can be used experimentally, but it is not suitable for use as a real life device for removing air or water pollution. There is this.

And after coating to form a titanium oxide thin film on the quartz or glass, it is baked. As described above, in order to induce photoactivity, titanium oxide should be made of an anatase type, and for this, a baking temperature of about 500 ° C. is required. However, glass or quartz carriers have an unstable crystal structure when the firing temperature rises and are easily broken, so there is a problem that the use range of the carrier is limited.

Due to the above problems, recently, metals which are not affected by the firing temperature are frequently used as fixed carriers, and in particular, titanium oxide photocatalysts are coated on iron plates or stainless surfaces using the sol-gel method. However, currently used titanium oxide film coated on the metal surface has the following problems.

If the metal surface is not evenly coated and used in water, corrosion occurs after about one day, and the corrosion is severe, and there is a problem that it cannot be used for a long time.

In addition, the titanium oxide film coated on the current metal surface is weak in its adhesion (fixation), so it is easily eliminated even by a small impact, and many problems are still exposed for practical application. In order to reinforce this adhesion, some papers say that silica and titanium powder are properly mixed and immobilized on the metal surface. However, when the coating work is performed using titanium powder, the metal surface becomes rough, and thus the surface roughness As the number increases, not only the hardly decomposable substances or foreign substances that are not oxidized on the surface of the catalyst are caught, and the catalyst activity is not only lowered.

Therefore, the present invention is to solve the above problems, the titanium oxide photocatalyst on the metal surface to increase the adhesion of the titanium oxide on the metal surface is not easy to fall off and to form a dense thin film structure so that corrosion does not occur in water It is an object to provide a coating method.

In addition, the present invention is to distribute the thin film evenly on the metal surface, and the titanium oxide photocatalyst coating on the metal surface to maintain the sufficient thickness by the re-growth of the titanium oxide during repeated coating to maintain the optimum thickness and to prevent the clouding phenomenon It is another object to provide a method.

1 is a view showing a schematic process of a method of coating a titanium oxide photocatalyst on a metal plate surface according to the present invention.

Fig. 2 shows an electron micrograph showing the crystal structure of the titanium oxide photocatalyst according to the firing temperature.

Figure 3-Electron micrograph showing the change of titanium oxide photocatalyst on the surface of the metal plate by repeated coating and firing.

Figure 4-XRD graph showing the change in the titanium oxide photocatalyst of FIG.

According to the present invention for achieving the above object, the first step of washing with water after acid-alkali treatment to remove foreign substances on the surface of the metal plate to be coated; A second step of polishing and cleaning the surface of the metal plate with a material prepared by adding titanium dioxide powder to a phosphate solution for uniformity and densification of electron arrangement of the metal surface; A third step of coating the first titanium oxide thin film on the surface of the metal plate by immersing the metal plate, which has been abrasively washed in the immobilization solution, and pulling up at a rate of about 0.1 μm / s; A fourth step of baking the metal plate coated with the first titanium oxide thin film at a temperature of about 500 to 550 ° C. for about 30 minutes or more; A fifth process in which the titanium oxide thin film is formed to a thickness of 0.4 μm or more by repeating at least 15 times the coating and firing of the metal plate on which the first titanium oxide thin film is fired again; It is a technical point of the invention to provide a method for coating a titanium oxide photocatalyst on a metal surface. And, preferably, the metal plate is made of a stainless material.

Meanwhile, in the first process, the metal plate is immersed in a dichromic acid solution for about 30 minutes, neutralized in a caustic soda solution for about 30 minutes, and then washed with water. In addition, the immobilization solution of the third step is to prepare an immobilization base solution consisting of a weight ratio of alcoholic solvent: titanium isopropoxide: hydrochloric acid or nitric acid of 100: 10 to 20: 1 to 5 consisting of ethanol or isopropanol. Process; Adding a nonionic surfactant which induces an even dispersion of titanium propoxide in a weight ratio of 1 to 5% with respect to the prepared immobilized base solution; Stabilizing the sol-gel solution in the immobilized base solution to which the nonionic surfactant is added at a stirring speed of 3000 rpm or more for 50 minutes or more; and gelation of isopropanol to delay gelation of the stabilized sol-gel solution. The delay solvent is added to 10 to 20% by weight based on the weight ratio of the basic solution.

Next, the present invention will be described in more detail through preferred embodiments of the present invention configured as described above. The embodiments described below are intended to explain the present invention in more detail, and it will be apparent to those skilled in the art that the present invention is not limited thereto.

Material Preparation

First, for the present invention, an immobilization solution prepared separately from a metal material serving as an immobilization carrier on which a titanium oxide photocatalyst coating is made is prepared.

As the metal material to be used, a metal such as iron or stainless may be used, and in this embodiment, stainless metal is used. The stainless metal is formed in a plate shape, and the titanium oxide photocatalyst is coated on both surfaces of the metal plate. The stainless metal is treated to prevent corrosion due to oxidation of iron itself, and is also a material suitable for use in water since its strength is hard and does not rust.

On the other hand, in the present invention, as a material for coating the titanium oxide photocatalyst on the metal plate, an immobilization solution prepared separately from the conventional one is used. The immobilization solution used in the present invention is an alcoholic solvent consisting of ethanol or isopropanol and titanium propoxide as a substrate for producing a thin film of titanium oxide, and hydrochloric acid or nitric acid as a reaction regulator in a ratio of 100: 10 to 20: 1 to 5 by weight. It is a separate treatment to the conventional immobilized solution for producing a conventional titanium oxide thin film made. That is, a dispersant is added to the conventional immobilization solution for even dispersion of titanium propoxide. As the dispersant, a nonionic surfactant is used, and the amount used is 1 to 5% by weight based on the immobilized base solution. The reason why the dispersant is required as described above is that the titanium propoxide is evenly dispersed in the alcoholic solvent. That is, conventionally, simply adding titanium propoxide to an alcoholic solvent and then inducing even dispersion by stirring, but gelation was easily performed because the even dispersion of the titanium propoxide was not achieved. Therefore, in order to delay this gelation, titanium propoxide should be evenly dispersed in the alcohol solvent. This requires a dispersant that does not cause a chemical reaction with the solvent and titanium propoxide, and can evenly disperse at both interfaces.

The present inventors have experimented with various dispersants and found that nonionic surfactants are suitable. Types of nonionic surfactants include fatty acid esters of polyoxy compounds, polyethylene oxide condensates, polyethylene imine condensates, and the like. Preferably, fatty acid esters of polyoxy compounds have the most effect. Oxyethylene nonyl (or octyl) phenyl ether (polyoxyethylene nonyl (or octyl) phenyl ether) was found to be the most effective. In addition, it was found that 1 to 5% / weight is appropriate as the amount of use, which is difficult to perform as a sufficient dispersant when used in a small amount, and the viscosity, surface tension and shear stress of the coating solution when used over 5% / weight ratio. Rather, it was found that the coating was prevented from being uniformly coated during thin film manufacturing.

And, by using the dispersing agent consisting of the nonionic surfactant, the coating was better made during the coating operation for manufacturing the thin film using the immobilized solution, and also the thickness of the coating is thicker, it can be seen that the coating can be made fewer times. there was. This is because the dispersant induces even dispersion of titanium propoxide, so that the titanium propoxide is evenly distributed over the entire surface during coating, and the dispersant is immobilized on the surface of the fixed carrier stainless metal plate for application. It is thought to act as an intermediate at both interfaces so that the solution can be more firmly attached.

Next, as described above, the immobilized base solution to which the nonionic surfactant is added is physically stirred for a predetermined time so that the titanium propoxide is evenly dispersed. This is to disperse the dispersant evenly at both interfaces of the alcoholic solvent and titanium propoxide. As a result of mixing at various stirring speeds and times, the stirring speed is to be stirred at 3000 rpm or more, and the stirring time is at least 40 minutes. It was found that it was most effective to make it ideal. Of course, as the stirring speed is increased, the stirring time can be reduced, and due to the agitation, the nonionic surfactant was sufficiently functioned, indicating that the titanium propoxide was uniformly stirred in the alcohol solvent, and thus gelling was delayed. .

And, it was found that the gelation is delayed than before when the immobilization solution is prepared while the stirring is completed. That is, the immobilization solution prepared in the above state was found to inhibit the progress of gelation for at least 15 days even in the presence of external air and humidity. However, it was found that the gelation to some extent progressed more than 10 days. The present inventors have conducted various experiments to further delay the gelation, and it has been found that the gelation can be further delayed by adding isopropanol as the gelling delay solvent in the above-mentioned stirring state. When the isopropanol was added to the stirred solution at a ratio of 10 to 20% / weight, it was found that gelation was inhibited for at least 80 days. The isopropanol not only plays a role of inhibiting gelation, but also changes the immobilization solution, which has already undergone gelation, to some extent in the sol state.

As described above, in the present invention, by using the immobilization solution prepared separately, it was possible to prevent the loss of material due to the gelation of the immobilization solution during the coating operation takes a long time.

Next, the method of coating the titanium oxide photocatalyst on the surface of the stainless metal plate according to the present invention using the material prepared as described above will be described in detail with reference to FIG.

Pretreatment for removing foreign substances from metal plate surface

Foreign materials such as moisture and fat are attached to the surface of the metal used. If the coating is performed in the presence of such foreign substances, even coating is not performed. Therefore, pretreatment is performed to remove these foreign substances. As a pretreatment method, foreign substances are washed with acid and alkali. That is, the prepared metal plate is immersed in a dichromic acid solution for about 30 minutes to decompose and remove foreign substances. In general, pickling is performed by surface treatment of metals such as iron, and especially hydrochloric acid or sulfuric acid is used as a strong acid, but in the present invention, since the surface of the metal plate is corroded when the strong acid is used, bichromic acid is used to prevent this. It was used, and soaked for about 30 minutes. At this time, it was found experimentally that the foreign substances were effectively removed without any corrosion of the metal plate. After the pickling, an alkali, preferably caustic soda, is neutralized to neutralize the acid component remaining on the surface of the metal plate. This is also immersed for about 30 minutes, after the process, washed with water to wash the acid and alkali.

Through such a pretreatment process, since there are no foreign substances on the surface of the metal plate, the titanium oxide to be coated increases the adhesion with the surface of the metal plate, thereby making it possible to perform the immobilization work better. When the coating is performed without such a process, the adhesion of titanium oxide to the surface of the metal plate is weak, so that the thin film is easily peeled off, or rapidly oxidizes to cause partial corrosion, resulting in roughening of the surface, thereby degrading activity as a photocatalyst.

Uniformity of Electron Array on Metal Plate Surface

Titanium oxide is chemically bonded to iron atoms on the surface of the metal plate in order to adhere to the surface of the metal plate. Therefore, in order to achieve such a smooth connection, evenly arranged electrons should be made through the entire surface of the metal plate, and the surface roughness is densified and homogenized to form a stable titanium oxide coating film, and even after repeated coating for immobilization, the properties are uniform on the surface. Corrosion will not occur.

To this end, titanium dioxide powder is added to a predetermined amount of phosphoric acid solution to make the powder in a sticky state, and then the powder is applied to the surface of the metal plate in a certain amount and polished with a soft cloth or the like. During the polishing process, the titanium dioxide powder is evenly distributed on the surface of the metal plate, and the friction causes the surface of the metal plate to have an even electron arrangement. Then, the titanium dioxide powder is washed to remove the titanium dioxide powder on the surface of the metal plate. Since the method for uniformly arranging electrons on the surface of the metal plate is generally used in metal surface treatment, detailed description thereof will be omitted.

First coating

As described above, the first photocatalyst coating operation of titanium oxide is performed by using an immobilization solution on the surface of the metal plate on which the metal plate is subjected to pretreatment and electron array uniformity. The photocatalytic coating is generally repeated several times, and the most problematic in this repetition is cloudiness and surface roughness and weakening of adhesion. Therefore, the first coating is the most important process because it determines the removal of the white turbid phenomenon, the surface roughness and the adhesion of the subsequent repeated coating depending on how the initial coating is made.

In general, the main cause of the turbidity is that the immobilization solution is prepared incorrectly, or the influx of moisture generated by the hydrolysis caused by the reaction of the substances added during the preparation of the solution is the largest factor. Another factor is the method of coating a thin film of titanium oxide photocatalyst on the surface of a metal plate to form a thin film of uniform thickness on the entire surface, thus forming a laminated structure instead of re-growing of titanium oxide upon repeated coating. As a result, cloudiness may occur. That is, in the conventional thin film forming method, the spin coating method does not evenly apply the entire surface of the metal plate due to the difference in the centrifugal force of each part as the metal plate is immersed in the immobilization solution and then rotated. As a result, as the thickness of each part is changed, turbidity occurs when the coating is repeated. Therefore, it is important to prepare the immobilized solution, to prepare the immobilized solution in which the hydrolysis reaction does not occur, and to form a uniform thin film.

To this end, in the present invention, since the gelation as described above is delayed and an immobilization solution having an even dispersion is used, the occurrence of turbidity due to the immobilization solution is prevented. In order to form an even thin film having a uniform thickness over the entire surface of the metal plate, an even coating using capillary phenomenon generated by the surface tension due to the viscosity of the immobilization solution was performed.

In detail the method, the present inventors were able to achieve an even thin film formation by using a method of immersing the metal plate in the immobilization solution at the end of repeated experiments, and slowly pulling up at a rate of about 0.1 ㎛ / s. In other words, if the speed of lifting the carrier is too high, the solutions have weak surface tension and the interface is broken, and thus, foreign matters (moisture, dust, etc.) in the air or other air penetrate between the broken interfaces so that an even thin film is not formed. When the spin coating method was used, invasion of foreign materials could be prevented, but also a nonuniform coating was made due to the difference in centrifugal force of each part. On the contrary, when using the method of pulling up at a rate of 0.1 μm / s in the immobilization solution used in the present invention, not only does the centrifugal force difference occur, but also the surface tension is generated between the surface of the metal plate and the fixed liquid due to the viscosity of the fixed liquid, thereby causing capillary phenomenon. This occurred, and thus it was found that all parts of the surface of the metal plate were uniformly coated. Therefore, it was found that even when the coating was applied again, the layered structure did not occur on the thin film, and thus the titanium oxide was regrown to prevent cloudiness, surface roughness, and adhesion.

Plastic working

After the coating is evenly formed on the surface of the metal plate as described above, the crystal structure of the titanium oxide is changed into an anatase form through plastic working at a temperature of about 500 to 550 ° C. for at least about 30 minutes, thereby allowing complete adhesion.

That is, as shown in various research data and papers, titanium oxide has a crystal structure of anatas type or rutile type depending on plastic processing, and it can be seen that the optimum activity as a photocatalyst is indicated by the anatas type. Therefore, as a result of the experiment for the expansion of the crystal structure and adhesion of the anatase type, it can be seen that the above temperature and time zone is suitable. Although according to other papers in Japan, it is known that the anatase type is formed by plastic working at different temperatures and time zones than the present invention, but this may be due to a difference in immobilization solution. In the present invention, the temperature and time zone suitable for this by using the immobilization solution different from the conventional one appeared as described above.

As shown in FIG. 2, it can be seen that during plastic working at different temperatures, the growth of titanium oxide crystals and the sufficient anatase type are not achieved. That is, when the plastic processing at 300 ℃ as shown in the figure, the crystals did not grow enough during the repeated coating again, and also the crystal structure was not made of sufficient anatas type. Although not shown, the peeling phenomenon and the growth of titanium oxide did not occur properly during firing for less than 30 minutes even under different time periods, for example, 500 to 550 ° C., resulting in rough surfaces and reduced efficiency. It was found that the titanium regrowth and anatase type were made under the above conditions.

Repeat coating and repeat firing

As described above, the thickness of the titanium oxide coating film formed by the primary coating and the firing was about 0.03 μm. Generally, the titanium oxide coating film should have a thickness of about 0.4 μm or more in order to exhibit optimal photoactivity. Therefore, it is necessary to repeat coating and repeated firing to form the thickness.

The most problematic problem in repetitive coating is the occurrence of cloudiness due to uneven coating film. In general, turbidity occurs when the coating is repeated 7 times or more. In the conventional method, the coating is generally not performed more than 10 times, and a method of maintaining a thickness of 0.4 μm by only a few coatings by applying a thick coating at one time is applied. When using the method, the thickness can be maintained, but due to the difficulty of plastic processing, titanium oxide is not fully grown, which leads to a weakening of the adhesion force, making it difficult to use for a long time. In this case, the regrowth of titanium oxide should be induced, but in this case, the clouding phenomenon occurred and only one of the two requirements could be satisfied.

However, in the present invention, evenly formed thin films are formed due to the uniformity of the initial coating, and thus, white turbidity is prevented even after more than 20 coatings and firings, and the adhesion to the surface of the metal plate is increased due to the regrowth of titanium oxide. Figure 3 is an electron micrograph of the surface of the titanium oxide coated metal plate produced by the present invention, Figure 4 is an x-aldiphotograph. As shown in the photo above, the coating was maintained at a thickness of 0.4 μm or more due to the 20th coating, but the surface was kept in a clear golden color without the occurrence of cloudiness, and the ideal coating that clearly reflected the shape of the object like the surface of a mirror It could be seen.

On the other hand, during repeated coating it is possible to increase the speed of pulling up the metal plate gradually and the firing time can also be shortened. That is, at the time of initial and repeated coating up to 2 to 4 times, it maintains a speed of 0.1 μm / s for uniform thin film formation and adhesion strength, and also maintains the firing time for about 30 minutes. The lifting speed can be about 4 to 5 times faster. This subsequent coating is made on the titanium oxide thin film, not the surface of the metal plate, so that the adhesion by the same material can be increased, and the firing time is also sufficiently possible in about 10 to 20 minutes because the regrowth is performed on the same material.

As described above, according to the present invention, when the coating on the metal plate surface of the titanium oxide photocatalyst can increase the adhesive force to make a solid structure, the titanium oxide photocatalyst coating film is made to increase the photocatalytic activity by preventing the occurrence of cloudiness with an appropriate thickness There is an effect that can produce a metal plate made.

As even thin film can be formed and densified on the surface of metal plate, it can be used for a long time because it is prevented from being used in water, and it is prevented from being broken due to the increase of adhesion force, so it can be used in poor environment as well as cost and water quality. There are other effects that can be used in or for general purposes.

In addition, since the gelation of the immobilization solution for thin film production is delayed, it is possible to use the immobilization solution prepared for a long time, thereby suppressing the generation of unnecessary gelation solution, and there is another effect of efficiently performing a long time coating operation.

On the other hand, as the gelation of the immobilization solution is delayed, by overcoming the rapid curing phenomenon due to the presence of air contact or humidity, which is a disadvantage of the conventional sol-gel solution, a separate vacuum clean bench is not necessary, so only a small work can be performed under a conventional vacuum facility. There are other effects that can work up to large plants.

Claims (5)

A first step of washing with an acid-alkali treatment to remove foreign substances on the surface of the metal sheet to be coated; A second step of polishing and cleaning the surface of the metal plate with a material prepared by adding titanium dioxide powder to a phosphate solution for uniformity and densification of electron arrangement of the metal surface; A third step of coating the first titanium oxide thin film on the surface of the metal plate by immersing the metal plate, which has been abrasively washed in the immobilization solution, and pulling up at a rate of about 0.1 μm / s; A fourth step of baking the metal plate coated with the first titanium oxide thin film at a temperature of about 500 to 550 ° C. for about 30 minutes or more; A fifth process in which the titanium oxide thin film is formed to a thickness of 0.4 μm or more by repeating at least 15 times the coating and firing of the metal plate on which the first titanium oxide thin film is fired again; Method for coating a titanium oxide photocatalyst on a metal surface characterized in that made. The method of claim 1, wherein the metal plate is made of a stainless material. [Claim 3] The titanium oxide photocatalyst on the metal surface of claim 1 or 2, wherein the first step is to immerse the metal plate in a dichromic acid solution for about 30 minutes, neutralize the caustic soda solution for about 30 minutes, and then wash with water. How to coat. The method of claim 1 or 2, wherein the immobilization solution of the third process, Preparing an immobilized base solution comprising an alcoholic solvent consisting of ethanol or isopropanol: titanium isopropoxide: hydrochloric acid or nitric acid in a weight ratio of 100: 10 to 20: 1 to 5; Adding a nonionic surfactant which induces an even dispersion of titanium propoxide in a weight ratio of 1 to 5% with respect to the prepared immobilized base solution; Stabilizing the sol-gel solution in the immobilized base solution to which the nonionic surfactant is added at a stirring speed of 3000 rpm or more for 50 minutes or more; And A method of coating a titanium oxide photocatalyst on a metal surface, comprising adding a gelling delay solvent composed of isopropanol to the stabilized sol-gel solution by 10 to 20% with respect to the weight ratio of the base solution. . 5. The method of claim 4, wherein the nonionic surfactant is polyoxyethylene ether and the gelling delay solvent is isopropanol.