WO2004062795A1

WO2004062795A1 - Optically transparent tio2 thin films on glass having anti-bacterial and anti-virus activities and method for preparing the same

Info

Publication number: WO2004062795A1
Application number: PCT/CN2004/000022
Authority: WO
Inventors: Jimmy Chai-Mei Yu; Jun Lin
Original assignee: Chinese University of Hong Kong CUHK
Current assignee: Chinese University of Hong Kong CUHK
Priority date: 2003-01-08
Filing date: 2004-01-07
Publication date: 2004-07-29
Anticipated expiration: 2005-07-08
Also published as: CN1245348C; HK1063620A1; CN1515514A

Abstract

The preferred embodiments provide a method for preparing an optically transparent TiO2 thin film on glass. The preferred embodiments also provide TiO2 thin films prepared by the method herein. The TiO2 thin films according to the invention have higher photocatalytic activity, and can be particularly used to photocatalytically degrade organic pollutants in air and to kill bacteria, germs and viruses therein.

Description

OPTICALLY TRANSPARENT TiO₂ THIN FILMS ON GLASS HAVING ANTI-BACTERIAL AND ANTI-VIRUS ACTIVITIES AND METHOD FOR PREPARING THE SAME

FIELD OF THE INVENTION The invention relates to a method for preparing nano-crystalline TiO thin films, and to applications of the thin films in cleaning itself, air and water where the films are located. Particularly, the invention is directed to use of the optically transparent nano-crystalline TiO thin films under ultraviolet irradiation in killing bacteria, germs and viruses in the environment.

BACKGROUND OF THE INVENTION

Glass has been widely used in modern construction industry. It is used as material not only for constituting common windows and doors as well as French windows, but also for constituting walls of buildings such as housings, offices, shopping malls and hospitals. Glass is also an important media with which people can view beauty of the Nature, and enjoy the sunlight under a desired and comfortable environment.

It has been known that TiO₂ thin films on substrates prepared by suitable processes have anti-bacterial and anti-virus activities. Therefore, it is desired that an optically transparent TiO₂ thin film having anti-bacterial and anti- virus activities is formed on the surface of glass used in the buildings mentioned above. It is more preferable if the thin film on glass can not only freshen air of the environment by decomposing organic pollutants existing in the environment but also kill germina and bacteria on contact, in view of more and more serious pollution of the environment. Meanwhile, the thin film does not reduce the transparency of the glass and visible light irradiation in the sunlight any more. This would greatly improve the environment of human livings.

The present inventors have disclosed in the Chinese patent application No. 02147089.8 a method f or p reparing n ano-crystalline TiO₂1 hin films o n a s ubstrate and u se o f t he t hin films . The method in this application includes providing a reverse micelle solution containing highly-dispersed water nano-droplets, which is made from an organic continuous phase, a non-ionic surfactant and water; adding a titanium alkoxide to the reverse micelle solution and subjecting the titanium alkoxide to hydrolysis in the water nano-droplets to form a TiO₂-containing solution; forming a wet film onto a substrate dipped into the TiO₂-containing solution by a dip coating technique; and drying the wet film and calcining the dried film. The thin films formed on the substrate disclosed in this application can kill the bacteria and viruses in the environment under ultraviolet irradiation.

Based on the Chinese patent application, the inventors have developed a new method for preparing TiO₂ thin film on common glass, and hereby provide the present invention. The thin l film prepared by the method in the present invention exhibits an excellent photocatalytic performance like that in the Chinese patent application. Moreover, the method also has several advantages that are not achieved in the prior art. For example, the thin film prepared by the method is optically transparent in visible range and very uniform after formed on a glass substrate; the reverse micelle solution used for preparing TiO₂ thin films in the present invention is more thermodynamic stable than that in the prior art. The present invention is provided to address these problems in the art.

SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide a method for preparing an optically transparent TiO₂ thin film on the surface of glass having fungicidal and antivirus activities. The method of the invention comprises the steps of: a) providing a reverse micelle solution containing highly-dispersed water nano-droplets, which is made from an organic continuous phase, a non-ionic surfactant, a co-surfactant and water; b) adding a titanium alkoxide to the reverse micelle solution and subjecting the titanium alkoxide to hydrolysis in the nano-droplets of the reverse micelle solution to form a

TiO₂-containing solution; c) forming a wet film onto a substrate of glass dipped into the TiO₂-containing solution by a dip coating technique; and d) drying the wet film and calcining the dried film.

Another object of the invention is to provide an optically transparent TiO₂ thin film prepared by the method of the invention.

With the use of a specific co-surfactant, the invention has many advantages that the TiO₂ thin film formed onto glass is optically transparent; anatase TiO₂ in the film is highly crystalline and nano-crystalline TiO₂ has an obvious quantum size effect; the thin film can be formed on glass in various shapes in no need of any specific manufacturing apparatus; the phase constitution and composition of the TiO₂ thin film can be readily controlled upon the need, for example, pure anatase TiO₂ in the thin film can be obtained; and the thin film prepared in the presence of different specific surfactants and c o-surfactants has a specific p orous structure and distribution, resulting in a higher content of hydroxyl group at the surface of the thin film.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is the polycrystalline X-ray diffraction patterns of nano-TiO₂ thin films of the present invention on glass; Fig. 2 is an ultraviolet absorption spectrum of nano-TiO₂ thin films of the present invention on glass;

Fig. 3 shows the photo-induced antibacterial activities of thin films of the present invention on glass;

Fig. 4 shows the hydrophilicity of thin films of the present invention on glass;

Fig. 5 shows the photocatalytic activities of thin films of the present invention on glass for the degradation of acetone in air; and

Fig. 6 shows the formation process of amorphous TiO₂ by controlled hydrolysis and condensation of titanium alkoxide in reverse micelles in a continuous phase in the present invention.

DETAILED DESCRIPTION OF THE INVENTION In a full consideration of the most advanced technologies of preparing nano-materials and thin film in the art, the invention provides an effective method for preparing optically transparent TiO₂ thin films on glass. The TiO₂ thin films prepared in the invention are highly optically transparent in the visible range. Upon UV irradiation, the thin films can kill all kinds of bacteria on contact, such as colon bacillus, comma bacillus, and pathogenic protoplast, and photocatalytically degrade organic pollutants in the environment, so as to clean the environment where the films are located.

In the method for preparing TiO₂ thin films of the invention, water phase is highly dispersed as nano-droplets in an organic continuous phase in the presence of a surfactant and a co-surfactant to form a reverse micelle solution. Then, titanium alkoxide is added to the reverse micelle solution. The titanium alkoxide in the reverse micelle solution penetrates the mono-layer of the surfactants into water nano-droplets to form amorphous TiO₂ by controlled hydrolysis and condensation in the reverse micelles. In the reverse micelle solution, the formation process of amorphous TiO₂ is shown in Fig.

6.

In the method of preparing TiO₂ thin film on glass in accordance with the invention, the organic continuous phase may be non-polar or lower-polar organic solvents at the ambient temperature. Examples of the organic continuous phases may include: unsubstituted alkanes or alkanes substituted with one or more substituents (substituting groups), unsubstituted alkenes or alkenes substituted with one or more substituents, unsubstituted alkynes or alkynes substituted with one or more substituents, and unsubstituted aromatic hydrocarbons or aromatic hydrocarbons substituted w ith o ne o r more s ubstituents. The s ubstituents o r s ubstituting groups u sed h erein includes, but not limited to, lower alkyl, halide, hydroxyl, lower alkoxyl, cyanide, nitro and the like. Preferably, the continuous phase used in the invention is C₃.₈ alkane or cycloalkane, more ^• preferably, it is C₅.₆ clycloalkane, and most preferably, it is cyclohexane. The number of the substituents maybe from 1 to 3, and preferably it is 1.

Unless specifically indicated otherwise, alkane, alkene and alkyne used in the invention include both straight chain and branch chain and cyclic alkane, alkene, and alkyne. In the invention, non-ionic surfactants used include polyols partial fatty acid esters,- poly oxyethylene aliphatic alcohol ether, polyoxyethylene alkyl phenol ether, Triton series. In the invention, Triton series such as Triton X-100 and Triton X-405 are preferably surfactants, of which Triton X-100 is more preferable.

In the invention, the term "co-surfactant" used in the invention means organic agents with a polarity between those of water and the organic continuous phase used in the reverse micelle solution in the invention, and may help the non-ionic surfactant of the invention stabilize water nano-droplets in the reverse micelle solution so as to keep the formed TiO₂ particle as small as possible, hereby to produce the thin film having higher optical transparency. In the invention, suitable co-surfactants are selected from C₄.₈ alkanol, and n-hexanol is preferable. In the invention, the term "alkyl titanate" has the same meaning as "titanium alkoxide".

Alkyl titanate according to the invention is selected from those that can be easily hydrolyzed to

TiO in the reverse micelles. Alkyl portion in alkyl titanate may be selected from C alkyl, more preferably from C . alkyl, and most preferably ethyl and iso-propyl.

The concentration of the non-ionic surfactant in the reverse micelle solution is generally from 0.20 to 0.40M, and preferably 0.3M. The concentration of titanium alkoxide in the reverse micelle solution of the invention is generally from 0.1 to 0.25M, and preferably from 0.15 to

0.20M. The molar ratio of water to the surfactant used in the invention may be from 1.0 to 2.5, preferably from 1.5 to 2.0. In the method of the invention, the molar ratio of the surfactant to the co-surfactant may be from 0.1 to 0.2, preferably 0.15. In addition, a small amount of a stabilizer can be added to the reverse micelle solution to control the rate of hydrolysis of the titanium alkoxide. In the invention, organic compounds of

2,4-diketone may be used as the stabilizer of the invention. It is well-known for those skilled in the art to select a proper stabilizer in the invention and the amount thereof used in the invention.

In general, the amount of the stabilizer used in the invention is ranged from 1 to 10% by volume of the reverse micelle solution. Preferably, acetyl acetone is used in the invention and accounts for

2 to 5% by volume in the reverse micelle solution.

The withdrawal speed in step c) of the method of the invention can be adjusted based on desired properties of thin films prepared. The speed is normally set at 3-6mm/s, preferably 4mm/s. The drying temperature of the wet film in step d) is in general at 80°-120°C, and preferably at 100°C for 10 minutes to 1.5 hours. In this step, the dried film may be calcined at a temperature ranging from 400° to 580°C, preferably from 470° to 550°C, and more preferably at 550°C. The dried film may be calcined for 1 to 4 hours, preferably 1.5 to 2.5 hours, and more preferably 2 hours. After c onducting c areful i nvestigation, t he i nventors h ave found t hat t he p resence o f a co-surfactant together with a surfactant in a reverse micelle solution can help stabilize the water nano-droplets and inhibit the aggregation and growth of the nano-droplets. Thus, the TiO₂ reverse micelle solution formed by controlled hydrolysis and condensation of titanium alkoxide in the reverse micelles exhibits the following advantages: (1) the formed amorphous TiO₂ particles in the reverse micelles are very fine; (2) the formed TiO₂ reverse micelle solution is very thermodynamically stable, and no TiO₂ particles aggregation and precipitation occur; and (3) the formed TiO₂ reverse micelle solution can produce a uniform and optically transparent nano-crystalline TiO thin film on a glass substrate by dip-coating technology.

In the invention, after glass is dipped into the reverse micelle solution, a homogenous amorphous TiO₂ gel layer is formed onto the surface of glass with the dip coating technique. An optically transparent crystalline TiO₂ thin film will be formed after calcined.

The substrate of glass used in the invention is preferably cleaned before dipped in the reverse micelle solution so as to achieve a good affinity between the TiO thin film and the substrate. The dip coating technique used in the method of the invention is the same as that used in the prior art. Detailed information on the dip coating technique can be referred to R. Reisfeld and C. K. Jorgensen, 77 Structure and Bonding; Chemistry: Spectroscopy and Applications of Sol-gel Glass, Springer- Verlag, 1992, Berlin, pp91-95.

The invention will be further described by the following examples.

Example 1

Preparation of Optical Transparent nano-TiO₂ Thin Films onto the Surface of Glass

To 100ml of cyclohexane were added Triton X-100 (18.35g), n-hexanol and water, and resultant mixture was stirred for 1 hour to obtain a reverse micelle solution, i the solution, the concentration of Triton X-100 is 0.3M, the molar ratio of Triton X-100 to n-hexanol was 0.15 and that of water to Triton X-100 was 2. In this Example, isopropyl titanate together with 5ml of acetyl acetone was added to the reverse micelle solution. The concentration of the titanate was kept at 0.2M in the reverse micelle solution. After resultant solution was continuously stirred for about 1 hour to have isopropyl titanate hydrolyzed and condensed in nano-droplets of the reverse micelle solution, the solution became homogenous and optically transparent. Glass as a substrate was then dipped into the reverse micelle solution and a wet TiO₂ film was formed onto the substrate by the withdrawing technique, of which the withdrawal speed was controlled at 4mm/s. The wet film was dried at 100°C for 60 minutes and then calcined at 550°C in a muffle roaster for 4 hours in the presence of oxygen. Afterwards, the substrate was cooled to the room temperature to thereby obtain optically transparent crystalline TiO₂ thin films.

A typical polycrystalline X-ray diffraction pattern (Diffraction peak of antanase TiO₂ (101), Bruker Advanced D8) of TiO₂ thin films on glass prepared in this Example was shown in Fig. 1. The size of the TiO₂ crystalline granule was estimated to be less than lOnm calculated by the Scherrer Formula based on the (101) diffraction peak. Ultraviolet absorption spectrum (Variant 100 Scan) of TiO₂ thin film on glass prepared in

Example 1 was shown in Fig. 2. The thin film is highly transparent in the visible range. The absorption edge starts at a wavelength shorter than 387.5nm (3.2eV) indicating the quantum-sized effect.

Experimental Example 1

Photocatalytic Antibacterial Activities of TiO₂ Thin Films

Photocatalytic antibacterial activities of TiO₂ thin film prepared in Example 1 was evaluated by killing Bacillus pumilusi in an aqueous solution under ultraviolet irradiation. The procedures were shown as follows. 2ml of an aqueous solution of Bacillus pumilusi having a concentration of 1 x 10⁷ CFU/ml was pipetted onto the TiO₂ thin film on glass as prepared in

Example 1. The glass was illuminated by a 15W 365nm UV lamp (Cole-Parmer Instrument Co.) at a light intensity of 0.63mW/cm². 20 or 40μl of diluted solution containing Bacillus pumillusi was dispensed into 1 ml of phosphate buffer. Resultant solution was plated on Luria-Bertani (LB) agar plates. The plate was then incubated at 37°C for 24h, and the number of colonies on the plate was counted at an interval o f 20 minutes. The change in the number of b acteria on the surface TiO₂ thin films on glass was calculated.

Results were shown in Fig. 3, in which the curve at the upper portion was obtained by blank control. Fig. 3 showed that the thin films of the invention had excellent photocatalytic antibacterial activities.

Experimental Example 2

Hydrophilicity of TiO₂ Thin Films

Hydrophilicity of a TiO₂ thin film prepared in Example 1 was evaluated with a change of the contact angle between water and the thin film under ultraviolet irradiation (15W 365nm, Cole-Parmer Instrument Co.). The contact angle was measured by an instrument, Model CA-XP, Kyowa Interface Science Co. Ltd., Japan. A thin film on glass as prepared in Example 1 was exposed to air for 2 months to have the contact angle of water on the thin film increased to 43°. Then, the thin film was irradiated by ultraviolet light to measure the contact angle at different time. The results were shown in Fig. 4, which indicated that the contact angle was decreased from 43° to around 5° after the thin film was irradiated for 2 hours. It was understood that thin film of the invention had an excellent hydrophilicity.

Experimental Example 3

Photocatalytic Activities of TiO₂ thin films The photocatalytic activity of the TiO₂ thin film as prepared in Example 1 was evaluated by the degradation of acetone in air under ultraviolet irradiation. The area of the TiO₂ thin film used for experiment was 150cm² in a 7,000ml reactor. Before switching on the ultraviolet source, the equilibrium concentration of acetone was controlled at 407+lppm, and the initial concentration of water vapor was adjusted to 1.2+0.01 vol%, and the temperature was regulated at 25±1°C. The ultraviolet w as generated by a 1 5W 365nm UV lamp (Cole-Parmer Instrument C o.). The UV lamp was positioned 2.5 cm above the glass coated with the TiO₂ thin film. The concentrations of carbon dioxide, water vapor and acetone were measured on line with a Photoacoustic IR Multigas Monitor (INNOVA Air Tech Instruments Model 1312, Denmark). The total analysis time for the thin film sample was 60 minutes, 10-minute interval each. Results were shown in Fig. 5, which indicated that, the TiO₂ thin film on glass also have good photocatalytic activity for the degradation of acetone in air.

It is understood that the above Examples and description are only used to illustrate the invention, and that any varieties or modifications to the present invention without departing from the spirit of the invention will be fallen into the scope of the invention which is defined by appended claims.

Claims

What we claimed is:

1. A method for preparing an optically transparent TiO₂ thin film on glass having photo-induced antibacterial and antivirus activities comprising the steps of: a) providing a reverse micelle solution containing highly-dispersed nano-droplets, which is made from an organic continuous phase, a non-ionic surfactant, a co-surfactant and water; b) adding a titanium alkoxide to the reverse micelle solution and subjecting the titanium alkoxide to hydrolysis in said nano-droplets of the reverse micelle solution to form a TiO₂-containing solution; c) forming a wet film onto a glass substrate by dipping said the substrate into the

TiO₂-containing solution with a dip coating technique; and d) drying the wet film and calcining the dried film.

2. The method of claim 1, wherein said reverse micelle solution further comprises a stabilizer comprising a 2, 4-diketone.

3. The method of claim 1 or 2, wherein said calcining the dried film in step d) is preformed in the presence of oxygen.

4. The method of claim 1 or 2 or 3, wherein said organic continuous phase is selected from C₃.₈ alkane and cycloalkane; said non-ionic surfactant is Triton; said titanium alkoxide is selected from C_..₆ alkyl titanate; and said co-surfactant is selected from C .₈ alkanol.

5. The method of claim 4, wherein said non-ionic surfactant is Triton X-100; said organic continuous phase is cyclohexane; and the titanium alkoxide is selected from ethyl titanate, propyl titanate, iso-propyl titanate, n-butyl titanatem and iso-butyl titanate; and said auxiliary surfactant is n-hexanol.

6. The method of claim 5, wherein said non-ionic surfactant has a molar concentration of 0.2 to 0.4M; the titanium alkoxide has a molar concentration of 0.1 to 0.4M in the reverse micelle solution.

7. The method of claim 6, wherein said non-ionic surfactant has a molar concentration of 0.3M; and the titanium alkoxide has a molar concentration of 0.2M in the reverse micelle solution.

8. The method of claim 5 or 6 or 1, wherein the molar ratio between water and the surfactant is from 1.0 to 2.5.

9. The method of claim 5 or 6 or 1, wherein the molar ratio between the surfactant and the co-surfactant is from 0.1 to 0.2.

10. The method of claim 8, wherein the molar ratio of the surfactant to the co-surfactant is

11. The method of claim 9, wherein the molar ratio of water to the surfactant is from 1.0 to 2.5; and the molar ratio of the surfactant of the co-surfactant is 0.15.

12. The method of claim 10, wherein the molar ratio of the surfactant to the co-surfactant is 0.15.

13. The method of claims 1 or 2 or 3, wherein said step c) is performed with a withdrawal speed of 3-6mm/s, and said step d) is performed with said wet film drying at a temperature ranging from 80° to 100°C for 10 minutes to 1.5 hours and said dried film calcining at 400° to 580°C for 1 to 4 hours.

14. The method of claim 13, wherein said step c) is performed with a withdrawal speed of 4mm/s, and said step d) is performed with said dried film calcining at 470° to 550°C for 1.5 to 2.5 hours.

15. The method of claim 14, wherein said step d) is performed with said dried film calcining at 550°C for 2 hours.

16. The method of claims 5 or 6 or 7, wherein said step c) is performed with a withdrawal speed of 3-6mm/s, and said step d) is performed with said wet film drying at a temperature ranging from 80° to 100°C for 10 minutes to 1.5 hours and said dried film calcining at 400° to 580°C for 1 to 4 hours.

17. A method of claim 16, wherein said step c) is performed with a withdrawal speed of 4mm/s, and said step is performed with said wet film drying at 100°C for 0.5 to 1.5 hours and said dried film calcining at a temperature ranging from 470° to 550°C for 1.5 to 2.5 hours.

18. The method of claim 17, wherein said step d) is performed with said dried film calcining at 550°C for 2 hours.

19. A TiO₂ thin film prepared by the method of any of claims 1 to 18.