JP6328407B2 - Titania nanostructure and manufacturing method thereof - Google Patents

Description

  The present invention relates to a titania nanostructure having a novel structure and a method for producing the same. The present invention further relates to an ultraviolet absorber containing the titania nanostructure, an ultraviolet absorbing coating material, and an article subjected to the ultraviolet absorbing coating.

Since titania and titanate are abundant, non-toxic and highly stable, they are mainly involved in photoreactions such as solar power generation, degradation of contamination by photocatalysis, and production of H ₂ by photocatalysis. It is an important material for many applications (Non-Patent Documents 1 to 3). Titania nanostructure control is a proven method for optimizing performance and searching for new applications (eg, solid acid catalyst of Non-Patent Document 4). Therefore, many titania nanostructures such as nanotubes and nanosheets (and respective integrated structures or aggregates) have been actively synthesized (Non-Patent Documents 5 to 11).

  It is an object of the present invention to provide a titania nanostructure having a novel nanostructure, and thereby further expand the applications in which titania and titanate can be used.

According to one aspect of the present invention, the formula _{H x M y Ti 2- (x} + ny) / 4 O 4 (M is one element selected from the group consisting of alkali metals and alkaline earth metals, n represents The valence, x and y are represented by positive values satisfying 0 <x + ny <8), and a nanofiber- shaped titania nanostructure having a multi-core structure is given.

  Here, the outer diameter may be 2 to 10 nm.

  Moreover, length may be 100-1000 nm.

  The inner diameter of the air core may be 0.5 to 1 nm.

  Moreover, a refractive index may be 1.5-1.7.

  According to another aspect of the present invention, any one of the above titania and titania by hydrothermally treating the layered titanate in the presence of at least one selected from the group consisting of quaternary ammonium, hydroxide and fluoride. A method of manufacturing a nanostructure is provided.

Here, the layered titanate may be K ₂ Ti ₂ O ₅ .

  The group consisting of quaternary ammonium, hydroxide and fluoride may also include tetrapropylammonium hydroxide and ammonium fluoride.

  The hydrothermal treatment temperature may be 250 ° C. or lower.

  Moreover, you may wash | clean the gel-like product by the said hydrothermal treatment with water.

  According to still another aspect of the present invention, there is provided an ultraviolet absorber containing any one of the above titania nanostructures.

  According to still another aspect of the present invention, there is provided an ultraviolet absorbing coating material comprising any of the above titania nanostructures and an organic polymer.

  According to still another aspect of the present invention, there is provided an article having an ultraviolet absorbing coating containing any of the above titania nanostructures and an organic polymer.

  The novel titania nanostructure of the present application can provide characteristics such as low refractive index and low photocatalytic activity that could not be obtained with conventional titanates and titania.

(A), (b) SEM images of _K 2 _Ti 2 _{O 5} used in Examples of the present invention. (C) SEM image of titania MCNF of an example of the present invention. (D) HRTEM image of titania MCNF of an example of the present invention. (E) The figure which shows the EDS spectrum of the titania MCNF of the Example of this invention. _{(A) K 2 Ti 2 O} 5 and shows the X-ray diffraction pattern of the titania MCNF. The inset shows the crystal structure of K ₂ Ti ₂ O ₅ (the sphere shows the K ions between the layers). (B) N ₂ adsorption / desorption isotherm of K ₂ Ti ₂ O ₅ and titania MCNF. (A) P25, shows diffuse reflective UV-vis spectra of titania MCNF and _K 2 _Ti 2 _{O 5,} as well as UV-bis absorption spectra of the eluate with water titania MCNF. (B) from cyclohexane by photocatalysis (CH), cyclohexanone (CHone), shows the time accumulation of cyclohexanol (CHnol) and the amount of CO _2. Data when P25 is used as a photocatalyst and data when titania MCNF is used are shown. The reaction conditions are as follows: CH (2 mL), O ₂ saturated acetonitrile (18 mL), catalyst (60 mg), light irradiation (using a solar simulator with an illuminance of 1000 W · m ⁻² and a wavelength> 320 nm). (C) A photograph showing the state of the immersion method test for determining the refractive index of titania MCNF. 10 mg powder sample was dispersed in 10 mL organic solvent. The photograph which shows the UV deterioration test result of the rhodamine 101 film | membrane without a coating | cover, and a PCL coating containing P25 and a PCL coating containing titania MCNF.

The present application provides novel titania nanostructures that are synthesized by methods inspired by zeolite synthesis and have optical properties that none of the available titanias will ever exhibit. That is, the nanostructure is a nanofiber as a whole, but a plurality of channels (holes) along the longitudinal direction of the nanofiber are further opened in one nanofiber. In the present application, this structure is referred to as a multi-air core structure in the sense that there are a plurality of hollow cores (that is, two or more) empty cores. The outer diameter of the nanostructure is about 2 to 10 nm, the length is about 100 to 1000 nm, the number of internal channels is about 2 to 10, and the inner diameter of each channel is about 0.5 to 1 nm. The chemical formula _{H x M y Ti 2- (x} + ny) / 4 O 4 (M is one element selected from the group consisting of alkali metals and alkaline earth metals of the material, n represents the valence, x and y is a positive value satisfying 0 <x + ny <8)
It is represented by The titania nanostructure of the present application has a specific gravity and refractive index lower than those of conventional titania-based materials due to its multi-core structure, which is about 2.5 to 3.5 and about 1.5 to 1.7, respectively. It is feasible.

  Although the synthesis of zeolites, or microporous aluminosilicates that are widely used in the industry, has traditionally been trial and error, more rational approaches have recently become available. As one of these methods, hydrothermal conversion of one zeolite to another zeolite, that is, zeolite conversion (interzeolite conversion) has attracted attention (Non-Patent Documents 12 to 14). In zeolite conversion, the zeolite under hydrothermal conditions is decomposed to form an aluminosilicate species having a local ordered structure (hereinafter referred to as “nanopart”). This structure is similar to the local structure of the original zeolite. Such nanoparts are assembled with the help of structure-directing agents and mineralizers to become another zeolite. This zeolite conversion method has been successfully used to synthesize new, valuable or high-quality zerolite from cheap and abundant zeolites (Non-Patent Document 14).

In this application, a novel nanofibrous structure having a one-dimensional multiple channel by converting a layered titanate, such as but not limited to K ₂ Ti ₂ O ₅ , by a method similar to zeolite conversion. It shows that titania nanostructures can be obtained. This novel titania nanostructure exhibits little photocatalytic activity as a result of the unique nanostructure and has a very low refractive index. Due to this feature, the nanofibrous titania nanostructures of the present invention can be used as UV (UV) absorbers embedded in commonly used organic polymers and are very effective on UV sensitive substrates. It can be a transparent UV protective coating.

Titania has been sought to be used as such a UV absorber. However, the photocatalytic activity and high refractive index of titania cause problems of polymer photodegradation and white turbidity in the polymer in which it is embedded, respectively, which places great restrictions on the use of titania in this field. Since various titanias with different local structures are available, the synthesis method of the present invention becomes a rational synthesis method of titania nanostructures, and the application of titania is realized in most existing titania nanostructures. Fields that require highly controlled optical properties that cannot be performed (for example, UV absorbers (Non-Patent Document 15), fillers for polymers (Non-Patent Document 16), photocatalysts for fine chemical synthesis (Non-Patent Documents 17 and 18) Etc.). Also, other metal oxide nanostructured materials with high performance and novel applications can be designed by the method of the present invention inspired by this zeolite conversion.
If the refractive index of the organic polymer at the embedding destination is almost the same as the refractive index of the titania nanostructure, the composite material after embedding becomes extremely transparent. However, even if both refractive indexes differ to some extent, the refractive index of this nanostructure is much closer to the refractive index of the organic polymer than other titania-based materials, as will be described later. A new application is expected in a field that allows a slight degree of cloudiness.

The inventor of the present application designed a novel titania nanostructure by applying zeolite conversion to the layered titanate compound K ₂ Ti ₂ O ₅ (Non-patent Document 19) for the first time. K ₂ Ti ₂ O ₅ has a plate-like morphology. The lateral size is several μm and the thickness is several hundreds of nanometers (FIGS. 1A and 1B), and a corrugated layer composed of a pair of triangular TiO ₅ pyramids and K between the layers. It is made of ions, and the basic surface interval (repetition interval between layers) is 0.64 nm (inset of FIG. 2A). Hydrothermal treatment under alkaline conditions in the presence of tetrapropylammonium hydroxide (TPAOH) and ammonium fluoride (NH ₄ F) converts K ₂ Ti ₂ O ₅ into a completely new product. . The TPAOH and NH ₄ F used here are often used as mineralizers and / or structure indicators and mineralizers in the zeolite conversion, respectively. Also, this hydrothermal treatment is not limited to the specific reagents and processing conditions described above. For example, any quaternary ammonium, hydroxide, or fluoride can be used in place of TPAOH or NH ₄ F, and the temperature range can be increased to a general hydrothermal treatment temperature range (˜250 ° C.). The reaction time varies depending on various conditions.

  Hereinafter, the present invention will be described in more detail on the basis of examples. However, it should be understood that the examples are not intended to limit the present invention in any way, but are presented for the purpose of assisting understanding of the present invention. It should be noted that there are.

The novel titania nanostructure of the present invention was specifically produced as follows. First, as shown in Non-Patent Document 19, K ₂ Ti ₂ O ₅ was prepared by a solid-phase reaction between K ₂ CO ₃ and TiO ₂ (rutile type). In a stainless steel autoclave lined with Teflon (registered trademark) (“Teflon (registered trademark)” is a registered trademark of EI DuPont de Nemours and Company), TPAOH, water, and NH ₄ F were mixed in a molar ratio of K ₂ Ti _2. O ₅ : TPAOH: H ₂ O: NH ₄ F = 1: 0.8: 5: 0.2 The mixture was heated to 170 ° C. and held for 1 week. After this hydrothermal treatment, the dried gel product was washed with water and dried at 70 ° C.

This product has a fibrous morphology (nanofibers) up to several nanometers in diameter and up to several hundred nanometers in length as seen in the scanning electron microscope (SEM) image shown in FIG. . This product (hereinafter referred to as titania MCNF (titanate multi-channel nanofiber)) showed an X-ray diffraction (XRD) pattern almost completely different from the starting material K ₂ Ti ₂ O ₅ (FIG. 2 (a)). ). Titania MCNF showed steep N ₂ absorption at low partial pressure, as seen in the N ₂ adsorption / desorption isotherm in FIG. 2 (b), indicating the presence of micro (micro) pores. . From the high-resolution transmission electron microscope image (HRTEM) shown in FIG. 1 (d), it was found that each nanofiber had several one-dimensional channels having a diameter of about 0.9 nm. The fact that this product cannot have a layered structure and actually has a one-dimensional multichannel structure will be described later. Although it examined in detail by SEM and HRTEM, particles other than MCNF, such as a nanotube and a nanofiber (nanorod) without a hole, were not found.

It was found from energy-dispersive X-ray spectroscopy (EDS) shown in FIG. 1 (e) that O, K and Ti are present in titania MCNF but F is not present. The contents of Ti and K were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) of dissolved titania MCNF (Ti: 42.1 wt%, K: 12.1 wt%). The differential thermo-thermogravimetric analysis (TG-DTA) curve of titania MCNF recorded in air showed no weight loss due to oxidative degradation of TPA, indicating that TPA was not incorporated into titania MCNF. confirmed. Therefore, it can be seen that TPAOH functions as a mineralizer rather than as a structure indicator. This TG-DTA curve also showed a weight loss (about 2.3 wt%) attributed to dehydration and condensation of surface OH groups. When the chemical formula of titania MCNF was calculated based on the amounts of Ti, K and OH, it was H _0.52 K _0.60 Ti _1.72 O ₄ (H is present as OH).

When H _0.52 K _0.60 Ti _1.72 O ₄ is treated with an aqueous solution containing dodecylammonium ions, 0.8 mmol · g ⁻¹ ammonium ions are adsorbed and simultaneously incorporated K Ions were desorbed by a semi-quantitative amount (0.4 mmol · g ⁻¹ ). That is, dodecyl ammonium was not quantitatively exchanged with K ions in a one-to-one manner and adsorbed, but a considerable amount was adsorbed.

More specifically, this ion exchange treatment was performed as follows. First, titania MCNF powder (200 mg) was dissolved in an aqueous solution of dodecylamine hydrochloride (100 mL, 19.4 mmol·L ⁻¹ ), and the mixture was stirred at room temperature for 6 hours. The resulting product was separated by centrifugation, washed with water and dried at room temperature. The amounts of adsorbed dodecylammonium and desorbed K ions were determined from the TG-DTA curve of the dried product and ICP-AES of the dissolved product, respectively.

This fact suggests that K ions are located on the surface of the one-dimensional channel as exchangeable ions. Here, even after replacing K with bulky dodecylammonium, the XRD pattern of H _0.52 K _0.60 Ti _1.72 O ₄ does not change significantly (for example, the lowest diffraction peak has a low 2θ It should be noted that it did not shift to the side. _{Thus, H 0.52 K 0.60 Ti 1.72 O} 4 has that do not have a layer structure is confirmed (Non-Patent Document 20).

The mechanism of forming titania MCNF from K ₂ Ti ₂ O ₅ was examined. When K ₂ Ti ₂ O ₅ , TPAOH, NH ₄ F and water were reacted at 170 ° C., only a dry gel product was found. This product was washed with water to obtain titania MCNF. The yield was 76% evaluated based on the amount of Ti. On the other hand, a colorless and transparent eluate with water (that is, washing water) was also obtained, but this should contain 24% of Ti. From this, it is considered that the titania species (probably “titania nanoparts”) incorporated in the titania MCNF remain in the eluate. In the ultraviolet-visible (UV-vis) spectrum of the eluate shown in FIG. 3 (a), a sharp absorption peak centered at 270 nm was observed. This absorption peak does not appear in the UV-vis spectrum of titania MCNF. Zeolite or mesoporous silica having an isolated tetrahedral Ti species (highly dispersed titania species) exhibits absorption at 220 nm in the UV-vis spectrum (Non-patent Document 21). In addition, in the UV-vis spectrum of the titania nanosheet obtained by peeling of the layered titania (delamination), an absorption peak that grows largely around 270 nm is often observed. This peak is explained by the presence of nanosheets of molecular thickness (Non-Patent Documents 7 and 22). In the SEM image of the dried eluate, no nanosheet-like particles or aggregates derived from K ₂ Ti ₂ O ₅ were observed (the presence of Ti, K and O in the corresponding SEM-EDS spectrum). Was confirmed, but only KOH particles up to several hundred nm in size were observed). Furthermore, in the XRD pattern of titania MCNF, the (020) peak due to diffraction in the titania layer of K ₂ Ti ₂ O ₅ was still observed (FIG. 2 (a)). Therefore, Ti species in the eluate is not a Ti atom by complete decomposition of _K 2 _Ti 2 _{O 5,} consists TiO ₅ triangular bipyramidal units K 2 _Ti 2 _{O 5} (inset of FIG. 2 (a)) It can be concluded that it is a “titania nanopart” with a significantly lower molecular weight than the delaminated K ₂ Ti ₂ O ₅ single layer nanosheet.

When TPAOH and / or NH ₄ F was not added to the starting mixture, a much lower crystalline and porous product was obtained. Thus, these two mineralizers played an important role in the formation of titania nanoparts and assembly into MCNF.

It is well known that TiO ₂ (anatase phase, rutile phase, brookite phase, or amorphous phase) can be converted to titania nanotubes under alkaline hydrothermal conditions without using any organic ammonium or fluoride. In addition, it is generally accepted that the formation of titania nanotubes is done by decomposing titania microparticle precursors and then recrystallizing them into single or multilayer nanosheets which are then rolled. (Non-patent document 5). Non-porous titania nanofibers (or nanorods) have been obtained under similar hot water conditions but at higher temperatures (Non-Patent Document 5). Furthermore, K ₂ Ti ₂ O ₅ is treated with acid (interlayer K ions are replaced with protons), and then heated to change to nonporous anatase (or TiO ₂ (B)) having a fibrous morphology. It has been reported (Non-Patent Documents 23 to 25). Here, it is noteworthy that the XRD pattern of the titania MCNF of the present application does not match that of the conventional titania nanostructure as described above. This fact also indicates that the titania MCNF of the present application is formed through a new route related to the formation of “titania nanoparts”.

Furthermore, it should also be noted that the layered niobate _{K 4 Nb 6 O 17 · 3H} 2 O is changed in nanotube niobate in the same hydrothermal conditions as in the synthesis of titania MCNF. This fact strongly suggests that the synthesis method of the present application can be widely used in producing various metal oxide nanostructures.

The light-related characteristics of the titania MCNF of the present application were examined. 3 (a) it is shown with the spectrum of the _K 2 _Ti 2 _{O 5} and a commercially available TiO ₂ particles P25 which is a starting material of the UV-vis spectra of titania MCNF recorded with diffuse reflective mode, for comparison. The absorption edge (380 nm) of titania MCNF was shifted to the longer wavelength side than the absorption edge (345 nm) of K ₂ Ti ₂ O ₅ . This result shows that MCNF of the present application is advantageous as a UV absorbing material. This is because most of the damaging natural UV radiation is between 290 nm and 350 nm (26).

It should be noted that titania MCNF absorbs UV light as effectively as P25. From this point of view, titania MCNF, like P25, may be considered to exhibit photocatalytic activity when irradiated with UV light, but the actual result was the opposite. FIG. 3 (b) shows the change over time in the yield of the product during irradiation of a solar simulator (λ> 320 nm) with cyclohexane (CH) on P25 or titania MCNF in the presence of dissolved oxygen. When P25 was used, cyclohexanone (CHone) and cyclohexanol (CHnol) and a considerable amount of CO ₂ were generated. This is because the cyclohexyl radical generated by reduction with a hydroxyl radical derived from OH ⁻ reduced by a hole in the valence band of P25 or a hole in the valence band is converted to O by a hydroxyl radical or a conduction band electron. This is because it is oxidized by an oxidizing species such as a superoxide anion generated by reduction of ₂ to form CHone and CHnol, and they are converted to CO ₂ by the overoxidation that occurs easily (Non-patent Document 27). ). In contrast, titania MCNF produced little CO ₂ even.

This means that when titania MCNF was irradiated with UV light, it showed only negligible photocatalytic activity. This result can be an advantage of the present titania MCNF. This is because today's advanced technology titania nanomaterials have higher photoactivity than conventional titania (eg, P25 with a surface area of about 50 m ² · g ⁻¹ ) due to several factors such as a larger surface area. It is because it shows. For example, titania nanotubes after firing have a surface area of about 150 m ² · g ⁻¹ and are effective UV-induced photocatalysts of various organic compounds (even titania nanotubes before firing have some photocatalytic activity. (Non-patent Document 28)). The surface area of titania MCNF (approximately 240 m ² · g ⁻¹ as shown in FIG. 2B) is even better or equivalent to the state-of-the-art titania nanostructure. The possible reason that titania MCNF has such a very low photocatalytic activity is that its crystallinity is low, as shown in FIG. 2 (a). In general, titania having a higher degree of crystallinity (or fewer surface and bulk defects) is considered to exhibit higher photocatalytic activity for various reactions because of a lower electron-hole recombination rate (non-non-reactive). Patent Document 29). Since the photocatalytic activity to completely oxidize organic compounds to CO ₂ is sufficiently suppressed, the titania MCNF of the present application is not only used as a UV absorber described later, but also suitable modifications such as firing and metal loading (non-patent literature) 30), and may be used as a photocatalyst (Non-patent Documents 17 and 18) for partially oxidizing an organic substrate (that is, for synthesizing a basic chemical).

Another notable property of titania MCNF is its extremely low refractive index. The refractive index of titania MCNF was measured using the immersion method. Titania MCNF powder (or P25 as a comparison) was dispersed in various organic solvents having different refractive indexes, and the refractive index of titania MCNF was evaluated from the transparency of the dispersion observed visually (FIG. 3 (c)). P25 was opaque in all organic solvents. This is because P25 is mainly composed of rutile type and anatase type TiO ₂ , and their refractive indexes are 2.71 and 2.53 (Non-patent Document 31), which are much larger than those of organic solvents. It can be explained that it is high (as another possible reason, the primary particle size of P25 is about 20 nm, but this may have aggregated in a solvent to scatter visible light into larger particles) . On the other hand, titania MCNF is almost transparent in diiodomethane having a refractive index of 1.74 (refractive index matching), and the refractive index is estimated to be about 1.7. This refractive index value is extremely lower than the refractive index of TiO ₂ (2.4 to 2.7 (Non-patent Document 31)). It has been found that the refractive index (n) and density (d) of the crystalline compound approximately follows the relationship (n-1) / d = constant, and this relationship has also been found to be true for titania ( Non-patent document 31). Therefore, the most probable reason that the titania MCNF of the present application has such a very low refractive index is the low density of this material in view of its high porosity. Titania MCNF may also aggregate in organic solvents. However, as is apparent from FIGS. 1 (c) and 2 (b), since this substance has mesopores, these act to lower the refractive index of the aggregate.

  Due to the unique optical properties described above, the titania MCNF of the present invention can be suitably used as a UV absorber embedded in commonly used organic polymers. In order to evaluate the actual UV protection performance of the titania MCNF of the present application, a highly UV sensitive organic dye (rhodamine 101) film is coated with polycaprolactam (polycaprolactam, PCL, refractive index 1.53) embedded with titania MCNF. The film thus coated was irradiated with strong UV light (λ> 300 nm). For comparison, a film coated with PCL embedded with P25 instead of titania MCNF was also prepared, and the same UV irradiation was performed. The titania MCNF could be uniformly embedded in PCL (in the sense of being totally transparent when viewed with the naked eye), and the titania MCNF-containing PCL was able to form a highly transparent coating on the organic dye film.

  The production of this organic dye (rhodamine 101) film, the coating with titania MCNF or PCL containing P25, and the evaluation of the UV protection properties by UV irradiation were specifically performed as follows.

Rhodamine 101 (10.2 mol·L ⁻¹ ) and clay (synthetic saponite distributed by the Japan Clay Society as standard data, 5.0 g · L ⁻¹ ) in ethanol aqueous solution (30 mL, v / v = 1/1) A part (2 mL) of the mixed solution mixed with was cast on a glass substrate (width 28 mm, length 48 mm), and the solvent was evaporated to produce a rhodamine 101 film. A solution of PCL (200 mg) in chloroform (100 mL) was mixed with MCNF or P25 (20 mg each), and a portion (1.0 mL) of the mixture was cast onto a Rhodamine 101 membrane. After evaporation of the solvent, the coated film was irradiated with UV light at a room temperature using a solar simulator with an illuminance of 1000 W · m ⁻² (wavelength> 300 nm). For comparison, UV irradiation was also applied to an uncoated Rhodamine 101 film.

  FIG. 4 shows that the above-mentioned UV irradiation was performed for 0 minutes, 240 minutes (4 hours), and 1440 minutes (24 hours) on the rhodamine 101 film without coating, P25-filled PCL coating, and titania MCNF-filled PCL coating from the left column. The photograph of the result is shown. As can be seen from FIG. 4, as a result of UV light irradiation, the coating containing titania MCNF was able to effectively suppress UV damage (fading) of the organic dye. On the other hand, in the case of PCL coating containing P25, the refractive index of P25 is much higher than that of PCL, and P25 has high photocatalytic activity, so that the coating matrix (PCL) becomes cloudy (irradiation 0 minutes) At that time, photodegradation of both the matrix and the organic dye occurred immediately after irradiation.

  To date, titania has been widely applied to sunscreen products (primarily as UV shielding materials, not as UV absorbers). However, for such applications, it is often required to suppress the high photocatalytic activity of titania by performing an appropriate surface treatment. In addition, since titania has a high refractive index and loses transparency in the visible range, its use in applications that require high transparency such as optical devices is greatly limited. From all the results described above, it can be seen that the titania MCNF of the present application has a high possibility of becoming a universal UV protection material without such problems.

  As described above, the novel titania nanostructure of the present application having a multi-core structure has novel properties not familiar to conventional titanates and titania, such as low refractive index and low photocatalytic activity. Although not limited to this, it can be widely used in new fields such as a transparent and long-life UV absorber for polymers.

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Claims (10)

It is represented by the following formula, and is a nanofiber in which a plurality of hollow channels along the longitudinal direction are provided,
The outer diameter is 2 to 10 nm,
The length is 100-1000 nm,
A titania nanostructure having an inner diameter of the channel of 0.5 to 1 nm.
_{H x M y Ti 2- (x} + ny) / 4 O 4 (M is one element selected from the group consisting of alkali metals and alkaline earth metals, n represents the valence, x and y are 0 <x + ny <Positive numerical value satisfying 8) The titania nanostructure according to claim 1, having a refractive index of 1.5 to 1.7. The method for producing a titania nanostructure according to claim 1 or 2 , wherein the layered titanate is hydrothermally treated in the presence of at least one selected from the group consisting of quaternary ammonium, hydroxide and fluoride. . The method for producing a titania nanostructure according to claim 3 , wherein the layered titanate is K ₂ Ti ₂ O ₅ . The method for producing a titania nanostructure according to claim 3 or 4 , wherein the group consisting of quaternary ammonium, hydroxide and fluoride contains tetrapropylammonium hydroxide and ammonium fluoride. The method for producing a titania nanostructure according to any one of claims 3 to 5 , wherein a temperature of the hydrothermal treatment is 250 ° C or lower. The method for producing a titania nanostructure according to any one of claims 3 to 6 , wherein the gel-like product obtained by the hydrothermal treatment is washed with water. UV absorber containing titania nanostructure according to claim 1 or 2. An ultraviolet absorbing coating material comprising the titania nanostructure according to claim 1 or 2 and an organic polymer. An article having an ultraviolet absorbing coating, comprising the titania nanostructure according to claim 1 or 2 and an organic polymer.