JP2002265223A - Titania ultrathin film and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that an ultrathin film made by laminating titania nanosheets and a polymer which is developed by this inventor previously is short of thermal stability, chemical stability and mechanical strength and has a fear of hindering a practical use depending on an application because the polymer is laminated within the thin film due to its role of a binder for the nanosheets, although the ultrathin film has an excellent characteristic such as highly efficient absorption for ultraviolet light. SOLUTION: The titania ultrathin film comprises a multi-layered structure of titania nanosheets consisting of ultrathin particles which are obtained by exfoliating laminar titanium oxide microcrystals and has no polymer interposing layer. By alternately laminating titania nanosheets consisting of ultrathin particles which are obtained by exfoliating laminar titanium oxide microcrystals and a cationic polymer on a substrate from liquid phases, respectively, the multi-layered structure film which is made by the titania ultrathin film and the polymer of interposed layer is once manufactured, but the polymer is removed from the film of the multi-layered structure film by heat treatment or electromagnetic irradiation such as ultraviolet rays and, thereby, the titania ultrathin film is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a titania nanosheet comprising flake particles obtained by exfoliating layered titanium oxide microcrystals (hereinafter referred to as "titania nanosheet" as appropriate).
And a method of manufacturing the same.

[0002]

2. Description of the Related Art In general, a titania thin film is produced by a sol-gel method by hydrolysis of a titanium salt, a CVD method by a gas phase reaction, a sputtering method, a laser ablation method, or the like. In this case, the film thickness is mostly sub-μm or more, and the obtained thin film is a dense aggregate of spherical fine particles.

[0003]

SUMMARY OF THE INVENTION The present inventors have previously described
We invented a titania multilayer ultrathin film obtained by alternately laminating a titania nanosheet and a polymer from a liquid phase on a substrate to a thickness of sub-nm to nm and a method for producing the same, and filed a patent application (Japanese Patent Application No. 2000-083654). . This titania multilayer ultra-thin film and its manufacturing method are significantly different in physical properties such as fine structure, controllability of film thickness and the like, and light absorption, as compared with the conventional thin film composed of titanium oxide fine particles, and have the following excellent features. Have.

(1) The thickness can be controlled with an accuracy of sub-nm to nm. (2) The film can be formed layer-by-layer, and the composition and structure of the thin film can be controlled with a high degree of freedom. (3) Low cost, energy saving, and low environmental impact due to adsorption and accumulation from the liquid phase. (4) Due to the use of nanosheets having extremely high two-dimensional anisotropy with a molecular level of thinness and a width of μm, they exhibit properties significantly different from conventional thin films in which spherical ultrafine particles are densely assembled. For example, the light absorption intensity is several tens of times that of the conventional one.

As described above, although a thin film has a number of advantages not found in the prior art, the polymer is laminated in the thin film as a binder of a nanosheet, so that the thermal stability and the chemical stability are improved. There is an aspect of lack of stability and mechanical strength, and there is a problem that it may hinder practical use depending on the application.

[0006]

Means for Solving the Problems Accordingly, the present inventors have:
With the idea of removing organic polymers,
We succeeded in producing an inorganic film with excellent stability without impairing the above advantages by irradiation with electromagnetic waves such as ultraviolet rays or heat treatment. That is, the present invention is an ultrathin titania thin film having a multilayer structure of titania nanosheets composed of flake particles obtained by exfoliating layered titanium oxide microcrystals and having no polymer intervening layer. Further, the present invention is the ultrathin titania thin film, wherein the nanosheet is converted into anatase or rutile by a heat treatment. Further, the present invention provides a flake particle having a composition formula
The ultra thin film described above, which is represented by Ti _1-d O ₂ (0 ≦ d ≦ 0.5). Further, the present invention is the above-mentioned ultrathin film, which absorbs ultraviolet light having a wavelength of 300 nm or less with high efficiency having a molar extinction coefficient of 10,000 mol ^-1 dm ³ cm ^-1 or more. Also,
The present invention is the above ultrathin film, which exhibits superhydrophilicity under ultraviolet light.

Further, according to the present invention, the flaky particles obtained by peeling the layered titanium oxide microcrystals and the cationic polymer are alternately laminated on the substrate from the liquid phase, and the titania nanosheet and the polymer of the intervening layer are laminated. The method for producing an ultra-thin titania thin film according to the above, wherein a film having a multilayer structure is produced, and a polymer is removed from the film having the multilayer structure. Further, the present invention is the above-described method for producing a titania ultrathin film, wherein the polymer is removed by subjecting the film having a multilayer structure to heat treatment or electromagnetic wave irradiation.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the production method of the present invention, first, an operation of alternately immersing a substrate in a sol in which a titania nanosheet is suspended and a cationic polymer solution is repeated, whereby the nanosheet and the polymer are deposited on the substrate. Each component is adsorbed to a thickness of the sub-nm to nm level, and a multilayer film in which the component is alternately repeated is accumulated.

As a practical operation, a series of operations of (1) immersing a substrate in a titania sol solution → (2) washing with pure water → (3) immersing in an organic polycation solution → (4) washing with pure water. This is repeated as many times as necessary as one cycle.
Organic polycations include polydimethyldiallylammonium chloride (PDDA), polyethyleneimine (P
EI), polyallylamine hydrochloride (PAH) and the like are suitable.

Since titania nanosheets have a negative charge,
By combining with positively charged polymers (polydimethyldiallylammonium chloride (PDDA), polyethyleneimine (PEI), polyallylamine hydrochloride (PAH), etc.), self-assembled alternately on the appropriately treated substrate surface Each can be absorbed as a monolayer. By repeating this operation, a titania ultrathin film can be constructed layer by layer.

The titania nanosheet, which is a raw material alternately laminated on the substrate, is a lepidocrocite type titanate (Cs _x
Ti _{2-x / 4} O ₄ (where 0.5 ≦ x ≦ 1), A _x Ti _{2-x / 3} Li _{x / 3} O ₄ (where A = K, Rb, Cs; 0.5 ≦ x ≦ 1) First, layered titanium oxides such as trititanate (Na ₂ Ti ₃ O ₇ ), tetratitanate (K ₂ Ti ₄ O ₉ ), and pentatitanate (Cs ₂ Ti ₅ O ₁₁ ) are converted to hydrogen type (H _x Ti _{2-x / 4} O ₄・ nH ₂ O, H _{4x / 3} Ti _{2-x / 3} O ₄・ nH ₂ O, H ₂ Ti ₃ O ₇
・ NH ₂ O, H ₂ Ti ₄ O ₉・ nH ₂ O, H ₂ Ti ₅ O ₁₁・ nH ₂ O) .

The chemical treatment for converting to the hydrogen form is a combination of an acid treatment and a colloidal treatment. That is, when an acid aqueous solution such as hydrochloric acid is brought into contact with titanium oxide powder having a layered structure, and the product is filtered, washed, and dried, all the alkali metal ions present between the layers before the treatment are replaced with hydrogen ions, and the hydrogen type is formed. The substance is obtained.

Next, when the obtained hydrogen-type substance is placed in an aqueous solution of an amine or the like and stirred, it is converted into a colloid to form a sol solution.
At this time, the layers constituting the layered structure are peeled off one by one. In this sol solution, layers constituting the mother crystal, that is, nanosheets are dispersed one by one in water. The thickness of the nanosheet depends on the crystal structure of the starting mother crystal, but is extremely thin, around 1 nm. On the other hand, the lateral size is on the order of μm and has a very high two-dimensional anisotropy.

The substrate is basically a problem as long as it is a solid substance stable in an aqueous solution. The size is not limited in principle. Quartz glass plate, Si wafer, mica plate, graphite plate,
An alumina plate or the like can be given as an example. Before the lamination operation, the substrate surface is cleaned. Washing with detergent,
Degreasing with an organic solvent, washing with concentrated sulfuric acid and the like are performed. Subsequently, the substrate is immersed in an organic polycation solution to adsorb the polycation, thereby introducing a positive charge to the substrate surface. This is necessary for stably performing the subsequent lamination.

Among the process parameters of the adsorption cycle, the concentration of the solution, the pH and the immersion time are important for synthesizing a high quality ultrathin film. The titania sol concentration is 5
It is desirable that the content be not more than 0.1% by weight, especially 0.1% by weight. Further, since the nanosheet tends to aggregate on the acidic side, the pH needs to be 5 or more, and 7 or more is desirable for stable film formation. Organic polycation concentration is 10wt
% Or less, and the pH is desirably adjusted to be the same as that of the titania sol. The immersion time needs to be 10 minutes or more. If it is shorter than this, the substrate surface may not be sufficiently adsorbed and covered with the nanosheet or polymer. When the above conditions are satisfied, the film can be formed very stably.

In the multilayer ultrathin film obtained by the above operation, the nanosheet and the polymer are repeated with a sub-nm thickness, and the polymer is an intervening layer. This polymer layer is removed in the next step. The removal of the polymer component can be performed by ultraviolet irradiation or heat treatment. As the ultraviolet light source, various high-pressure mercury lamps, xenon lamps, and the like can be used. There is no change in appearance due to irradiation, and the sample shows high transparency after irradiation for several tens of hours, as before.

The removal rate of the polymer depends on the composition and structure of the thin film,
Although it depends on parameters such as UV intensity, typical thin films, namely, multilayer thin films (total 10 layers) of nano-sheets Ti _1-d O ₂ obtained by exfoliating lepidocrocite-type titanic acid and PDDA are alternately For example, a film thickness of 14 nm
When irradiated with light having an ultraviolet intensity of 1 mW cm ^-2 of 0 nm or less, the polymer component can be decomposed and removed in about 24 hours. The change in the X-ray diffraction data at that time is as shown in FIG. The halo at 2θ = 15 to 30 ° in FIG. 1 is due to the quartz glass substrate.

The pre-irradiation sample gives a diffraction peak with a spacing of 1.4 nm corresponding to the repetition period of the nanosheet and the polymer. Irradiation with ultraviolet light reduces the spacing with time, resulting in a diffraction peak of 0.94 nm in about one day. It reaches a certain value (FIG. 2). This reduction in the interplanar spacing can be considered to be because the polymer was decomposed by the photocatalytic action of the titania nanosheet. Also, while the diffraction line becomes sharp,
The fact that the third-order reflection can be detected also means that the stacking order of the nanosheets has increased, and that the polymer that caused broadening of diffraction lines due to structural disorder has been removed. Indirectly suggests.

More direct evidence can be obtained from infrared absorption spectra (FIG. 3) and X-ray photoelectron spectroscopy data (FIG. 4). Infrared spectrum before irradiation whereas clearly indicates the presence of water molecules and the polymer, after the irradiation changes greatly, the absorption band of from 3,000 to 2,800 cm ^-1 and from 1,500 to 1,350 cm ^-1 derived from a polymer Disappears. Instead, a spectrum is obtained indicating the presence of NH ₄ ⁺ . This can be explained by the photodegradation of PDDA and finally the generation of NH ₄ ⁺ as a cation that compensates for the negative charge of the nanosheet.

In addition, X-ray photoelectron spectroscopy data shows a significant decrease in the carbon component due to irradiation, suggesting that the polymer has been removed. On the other hand, there was no significant change in the nitrogen component, and the chemical shift value almost coincided with that of NH ₄ ⁺ , which coincided with the infrared absorption data. From the above, by irradiating the ultra-thin titania nanosheet / polymer multilayer thin film with ultraviolet light,
It was supported that the polymer could be photolyzed and converted to an ultrathin film consisting of only inorganic components.

On the other hand, the polymer can be decomposed by heat treatment. When the nanosheet / polymer multilayer ultrathin film is heated, the spacing decreases from around 200 ° C, and reaches 0.95 nm at 400 ° C. This value is the mother crystal of the present nanosheet _{H x Ti 2-x / 4} O 4 · nH 2 O spacing of, namely, since it matches the host layer interval before peeling, the polymer at this temperature almost pyrolyzed It is considered removed. At 500 ° C, the diffraction peak at 0.94 nm also disappears, and the film changes to an amorphous thin film. When heating is further continued, a thin film composed of anatase at 600 ° C. or more and rutile at 900 ° C. or more can be obtained.

The titania nanosheet shows a sharp and strong absorption peak in the ultraviolet region. This is the region where the nanosheets are said to exhibit a quantum size effect in the titanium oxide system.
This is considered to be mainly due to the miniaturization to a molecular level of less than m. The thin film from which the polymer has been removed by ultraviolet irradiation or heat treatment (<400 ° C.) gives the titania nanosheet a characteristic absorption spectrum (see FIGS. 5 and 7).

Lepidocrocite type titanium oxide H _x Ti
_{2-x / 4 O 4 ·} nH 2 when O super thin film obtained by accumulating the titania nanosheet obtained by peeling the absorbance per nanosheets 1 Reya spans 0.14. Since the thickness of the nanosheet is 0.75 nm, the absorbance per 1 nm of titania exceeds 0.18. This is 24000 mol ^-1 dm ³ cm in terms of molar extinction coefficient
Equivalent to ^-1 .

By the way, the equivalent absorbance of a thin film in which ultrafine titania particles having an average diameter of 3 nm are laminated is about 0.00.
5 (Liu et al. J. Phys. Chem. B, 10
1, 1385, 1997), indicating that this thin film has an extremely high efficiency of absorbing ultraviolet light. Such high-efficiency absorption of ultraviolet light suggests that it is very effective for use as an ultraviolet cut coating, a solar cell member, a photoluminescence material, a sensor, and the like.

On the other hand, titanium oxide is expected to be used as an environmental purification material utilizing its excellent photocatalytic ability, and also as a coating material utilizing photo-induced superhydrophilization. Has been reached. Because of its unique structure and characteristics, titania nanosheets are expected to be more functional than conventional materials, such as ultrafine titanium oxide particles, even in this field.

In fact, as shown in FIG. 8, the ultrathin titania thin film of the present invention was found to have a water droplet contact angle close to 0 ° in a short time under irradiation of weak ultraviolet rays, and was effective as a superhydrophilic thin film. Is shown. In applications related to the photocatalyst and superhydrophilicity, there is a concern that the presence of a polymer in a thin film may adversely affect these functions from the viewpoints of denaturation, decomposition, and the like. It can be seen that the time required for superhydrophilization is about three times as long as the thin film containing the polymer.

Thus, the fact that the chemically unstable polymer was removed by the present invention to form an inorganic film containing nanosheets as a main component opened a way to apply the excellent properties of titania nanosheets to this field. It can also be said.

[0028]

Next, examples of the present invention will be described. [Example 1] formula _{Cs x Ti 2-x / 4} O 4 (x~0.7) lepidocrocite type layered cesium titanate to an acid treatment is obtained by layered titanate powder represented by (H _x Ti _{2- (} 0.5 g of _{x /} 4O ₄ .nH ₂ O) was added to 100 cm ³ of the tetrabutylammonium hydroxide solution, and the mixture was shaken (150 rpm) at room temperature for about one week to obtain a milky white titania sol.

A 50-fold diluted solution thereof and a 2 wt% aqueous solution of polydimethyldiallylammonium (PDDA) chloride were prepared, and the pH was adjusted to 9. An appropriately sized Si wafer plate or quartz glass plate was treated with a neutral detergent, methanol / hydrochloric acid (1: 1) and concentrated sulfuric acid, and then sufficiently washed with Milli-Q pure water. Finally, the substrate was immersed in a 0.25 wt% aqueous solution of polyethyleneimine for 20 minutes, washed with water and dried.

The substrate thus cleaned and pre-treated is immersed in (1) the titania sol solution described above. (2) 20
After a lapse of minutes, the plate is sufficiently washed with Milli-Q pure water and dried by blowing an argon stream. (3) Next, the substrate is immersed in a PDDA solution for 20 minutes. (4) Subsequently, by repeating the operation of sufficiently washing with Milli-Q pure water, a multilayer ultrathin film in which titania nanosheets and PDDA were alternately accumulated was synthesized.

An Xe lamp manufactured by Sanei Electric Co., Ltd.
(XEF-501S) was used to irradiate ultraviolet rays. The ultraviolet light intensity below 300 nm, which is the main light absorption region of the nanosheet, was set to about 1 mW cm ^{-2 on the} substrate surface by placing the sample at a position 15 cm from the ultraviolet light source. The diffraction line reflecting the repetition period of the nanosheet and the polymer of the multilayer film gradually moved toward the high angle side with the ultraviolet irradiation, and a decrease in the interplanar spacing as shown in FIG. 2 was observed.

At 24 hours after irradiation, the surface spacing is almost constant 0.94 n
m, the diffraction line became sharper, and it was possible to detect even higher-order reflections (Fig. 1). The above data is interpreted as a result of the decomposition of the polymer by the photocatalytic action of the titania nanosheet. FIG. 3 shows FT-IR spectra before and after irradiation. In the spectrum before irradiation, the broad absorption centered at 3500 cm ^-1 and the band around 1630 cm ^-1 were assigned to stretching vibration and bending mode of water molecule, suggesting the presence of hydration water.

Further, the signals at 3024, 2959, 2926, and 2853 cm ^-1 show the stretching vibration of -CH ₃ and -CH _2- at 1468 and 1356 cm ^-1.
Can be assigned to those bending vibrations, and the presence of the polymer could be clearly confirmed. In contrast, after irradiation, the peak derived from the polymer component disappears,
A broad band at 3190, 3035, 2847, 1420 cm ^-1 was suggested, suggesting the formation of NH ₄ ⁺ , while supporting the photolysis.

This is derived from N in PDDA, and is considered to play a role of charge compensation of the negatively charged nanosheet. FIG. 4 shows the results of measuring the X-ray photoelectron spectrum before and after the irradiation. Before the irradiation, all the peaks of the elements C, N, Ti, O constituting the multilayer ultra-thin film were detected.

On the other hand, after irradiation, the peak intensities of N, Ti, and O remained almost unchanged, while only the C component was significantly reduced. Calculating the molar ratio based on the sensitivity of each element shows that Ti: C: N = 1: 3: 0.22 before irradiation and Ti: C: N after irradiation.
= 1: 0.48: 0.21 Given that XPS has a very sensitive sensitivity to C and usually senses background carbon, this data can be interpreted as an effective removal of the polymer.

Based on the above data, the nanosheet / polymer multilayer ultra-thin film was irradiated with ultraviolet light,
₄ ⁺ was concluded that can be converted into inorganic thin film formed by laminating in a manner sandwiching. The ultraviolet / visible absorption spectrum of the obtained thin film did not change from that before irradiation, and gave a sharp and high-intensity profile derived from titania nanosheet having a peak at 265 nm (FIG. 5).

Example 2 After synthesizing a multilayer ultrathin film of titania nanosheets and PDDA in the same procedure as in Example 1, heating was performed in an electric furnace to remove the polymer. Heat treatment is 100
The temperature was held at each temperature up to 1000 ° C. in increments of 1 ° C. for 1 hour.

FIG. 6 shows an X-ray diffraction pattern obtained by tracing a change accompanying the heat treatment of the multilayer ultra-thin film by an X-ray diffraction method. In FIG. 6, ● and ▲ indicate peaks of anatase and rutile. 2θ = 15
The halo of 3030 ° is due to the quartz glass substrate. FIG.
As shown in the figure, the nanosheet / polymer repetition period of 1.4 nm began to shrink from 200 ° C,
It became 0.95 nm at 00 ° C. This value is H _x Ti _{2-x / 4} O ₄ before peeling
・ Approximately the host layer spacing of nH ₂ O is 0.94 nm.
This suggests that PDDA has been degraded and removed.

When the temperature reached 500 ° C., the Bragg peak indicating the spacing between the multilayers disappeared, and the film became amorphous. When heating was further continued, peaks appeared at around 2θ = 25 ° and 48 ° at 600 ° C, and crystallization of anatase was confirmed. At 800 ° C, the phase transition from anatase to rutile started, and at 900 ° C, it became a rutile single phase.

FIG. 7 shows the change in the ultraviolet / visible absorption spectrum during this heating process. No significant change was observed in the sharp absorption profile derived from the nanosheets up to 400 ° C, but broadening was observed above 500 ° C, and a shift to the longer wavelength side of the absorption edge was observed.At 900 ° C, bulk rutile was observed. Approximately matched the spectrum. This is
It was in good agreement with the results of the phase transition obtained by the XRD data.

Water droplets were dropped on the thin film samples (substrate: silicon) prepared in Examples 1 and 2, and the time change of the contact angle when ultraviolet light was irradiated was measured. The contact angle was measured using a Kyowa Interface Science CA-XP, and the ultraviolet light source was a Sanei Electric Xe lamp (XEF-501S) with an intensity of 0.6 mW cm ^-2 .

As a result, as shown in FIG. 8, it was revealed that the contact angle of the thin film from which the polymer was removed by ultraviolet light irradiation or heat treatment became 5 ° or less in about 5 minutes, and showed a so-called superhydrophilic property. In contrast, the thin film before processing,
In other words, it was found that it took 15 minutes or more for the superhydrophilicity to be exhibited in the multilayer film of the polymer and the titania nanosheet, indicating a large difference.

[0043]

According to the present invention, the titania nanosheet /
The characteristics of nanosheet / polymer composite thin film (fine-precision control of nano-thickness, derived from nanosheet) by removing polymer by secondary treatment such as ultraviolet irradiation or heating to modify polymer into inorganic film , Etc.), and a thin film having more excellent chemical stability can be synthesized.

The ultrathin titania thin film of the present invention absorbs ultraviolet rays with high efficiency, and can be used for window materials and other ultraviolet shielding coatings. It is also expected as a thin film having a superhydrophilic surface, a thin film for photoelectric conversion, a photochromic material, a photocatalytic thin film, and a sensor. Further, by having a high refractive index and being able to finely control the film thickness, it is possible to form a color by an interference effect, and it can be various coloring materials and paints.

[Brief description of the drawings]

FIG. 1 shows titania nanosheet / PDDA of Example 1.
5 is an X-ray diffraction pattern showing an ultraviolet irradiation effect on a multilayer ultrathin film.

FIG. 2 shows titania nanosheet / PDDA of Example 1.
4 is a graph showing a relationship between a multilayer plane interval of a multilayer ultrathin film and an ultraviolet light irradiation time.

FIG. 3 shows titania nanosheet / PDDA of Example 1.
4 is an FT-IR absorption spectrum graph of a multilayer ultrathin film before and after irradiation with ultraviolet light.

FIG. 4 shows titania nanosheet / PDDA of Example 1.
It is an X-ray photoelectron spectrum graph before and after ultraviolet light irradiation of a multilayer ultra-thin film.

FIG. 5 shows titania nanosheet / PDDA of Example 1.
It is an ultraviolet-visible absorption spectrum graph before and after ultraviolet light irradiation of a multilayer ultra-thin film.

FIG. 6 shows titania nanosheet / PDDA of Example 2.
4 is a graph showing a change in an X-ray diffraction pattern accompanying a heat treatment of a multilayer ultrathin film.

FIG. 7 shows the titania nanosheet / PDDA of Example 2.
It is an ultraviolet-visible absorption spectrum graph accompanying the heat treatment of a multilayer ultra-thin film.

FIG. 8 is a graph showing a relationship between a contact angle of a water drop and an ultraviolet irradiation time in Examples 1 and 2.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Jun Suzuki 1-1-1, Namiki, Tsukuba, Ibaraki Pref., Japan Inorganic Materials Research Laboratories, Ministry of Education, Culture, Sports, Science and Technology 4G047 CA02 CB05 CD02

Claims (7)

[Claims] 1. An ultrathin titania thin film comprising a multilayer structure of titania nanosheets comprising flake particles obtained by exfoliating layered titanium oxide microcrystals and having no polymer intervening layer. 2. The ultrathin titania thin film according to claim 1, wherein the nanosheet is converted into anatase or rutile by a heat treatment. 3. The ultra-thin film according to claim 1, wherein the flake particles are represented by a composition formula Ti _1-d O ₂ (0 ≦ d ≦ 0.5). 4. The ultra-thin film according to claim 1, wherein the ultra-thin film absorbs ultraviolet light having a wavelength of 300 nm or less with high efficiency having a molar extinction coefficient of 10,000 mol ⁻¹ dm ³ cm ⁻¹ or more. 5. The ultrathin film according to claim 1, which exhibits superhydrophilicity under ultraviolet light. 6. A titania nanosheet composed of flake particles obtained by exfoliating layered titanium oxide microcrystals and a cationic polymer are alternately laminated on a substrate from a liquid phase, respectively. The method for producing an ultrathin titania thin film according to any one of claims 1 to 5, wherein a film having a multilayer structure is prepared, and a polymer is removed from the film having a multilayer structure. 7. The method for producing an ultra-thin titania thin film according to claim 6, wherein the polymer having a multilayer structure is subjected to heat treatment or electromagnetic wave irradiation to remove the polymer.