JP2004238226A - Anatase nano-crystal, thin film of the same and method of manufacturing them - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel anatase nano-crystal having orientation in the c-axial direction and a nano level thickness, a crystal thin film of the same and a simple and low cost manufacturing method of them. <P>SOLUTION: The anatase crystal having orientation in the c-axial direction or its thin film is obtained and recovered by using a titania nano-sheet obtained by peeling a layered titanium oxide in nano-level thickness as a starting material, laminating the titania nano-sheet on an optional substrate treated with a cationic polymer solution with controlled thickness and heating. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to c-axis oriented anatase nanocrystals expected to be used for ultraviolet blocking coating materials, photoelectric conversion thin films, superhydrophilic thin films, photocatalysts, and the like, and thin films thereof, and methods for producing them.
[0002]
[Prior art]
Anatase is used as a semiconductor material having a band gap of 3.2 eV or slightly higher than that of rutile 3.0 eV in a wide variety of applications such as ultraviolet cut coating, solar cells, photocatalysts, and photoinduced superhydrophilic thin films. It is known that an anatase having an orientation is used in order to exhibit its excellent performance as much as possible and to further improve the performance. In particular, the c-axis orientation surface shows better optical characteristics than other orientation surfaces. (For example, see Non-Patent Document 1).
[0003]
On the other hand, a means generally known as a means for obtaining oriented anatase includes a gas phase method such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD), and forms a film-shaped gas on a substrate. It is obtained by phase growth (see Non-Patent Documents 2 and 3). However, in order to obtain anatase oriented by these gas phase methods, a crystal having a lattice spacing close to the lattice spacing of anatase must be selected and used as a substrate to be used. The material had to be extremely limited, which was one of the causes of the cost increase of the oriented anatase to be obtained. In addition, there was a problem that obtaining a large area was technically difficult.
[0004]
Also, it is known that an anatase crystal thin film can be obtained by applying a titania sol obtained by hydrolyzing a titanium compound (for example, titanium tetrachloride, titanium alkoxide, titanium isopropoxide, etc.) on a substrate and heating the substrate. However, in the hydrolysis method using these specific titanium compounds, it has been difficult to control the orientation of the anatase nanocrystals.
[0005]
[Non-Patent Document 1] Jpn. J. Appl. Phys. 2000, 39, 169-171.
[Non-Patent Document 2] Chen et al. J. Vac. Sci. Technol. A 1993, 11, 2419-2429.
[Non-Patent Document 3] Sugimura et al. Jpa. J. Appl. Phys. 1997, 36, 7358-7359.
[Non-Patent Document 4] Wang et al. Chem. Mater. 1999, 11, 3113-3120.
[0006]
[Problems to be solved by the invention]
The present invention provides a novel anatase nanocrystal having a nano-level thickness and a crystal thin film having a nano-level thickness, which is different from a conventional anatase crystal or a thin film thereof, and is oriented in a specific direction, that is, a c-axis direction. And to provide a low-cost manufacturing method.
[0007]
[Means for Solving the Problems]
Therefore, the present inventors have conducted intensive studies and have found that a titania nanosheet Ti _1- δO ₂ (0 ≦ δ ≦ 0.15) obtained by exfoliating a layered titanium oxide with a nano-level thickness is used as a starting material. By using this titania nanosheet adsorbed on an arbitrary substrate treated with a cationic polymer solution, titanium oxide with a two-dimensional lattice can be laminated on the substrate by controlling the thickness to several atomic layers. By heating the sample laminated to this controlled thickness, it can be easily converted to an anatase nanocrystal oriented in the c-axis direction. It has been found that a thin film body can be obtained. The present invention has been made based on this series of findings, and has a configuration as described in the following (1) to (6).
[0008]
(1) Anatase nanocrystals having a shape anisotropy of a particle size of 20 nm in height × 500 nm in horizontal size or less and having c-axis orientation.
(2) Titanium nanosheets in which titanium and oxygen atoms are two-dimensionally constrained, obtained by exfoliating a layered titanium oxide crystal to a nano-level thickness, are used as an initial substance, heated, and converted to an anatase structure. The anatase nanocrystal having the c-axis orientation according to the above (1), which is obtained by:
(3) A titania nanosheet in which titanium atoms and oxygen atoms are two-dimensionally constrained, obtained by exfoliating a layered titanium oxide crystal to a nano-level thickness, is used as an initial material, and the substrate is formed into a one-layer or three-layer film. An anatase nanocrystalline thin film having a c-axis orientation, wherein the nanocrystalline thin film has a c-axis orientation by being adsorbed thereon, heated, and converted into an anatase structure to obtain an anatase crystalline thin film having a c-axis orientation.
(4) using a titania nanosheet in which titanium atoms and oxygen atoms are two-dimensionally constrained, obtained by exfoliating a layered titanium oxide crystal to a nanometer-level thickness, and converting it to anatase by heating; The particle size has a shape anisotropy of height 20 nm × width 500 nm or less, and an anatase nanocrystal having c-axis orientation is obtained and collected, and the particle size is height 20 nm × width 500 nm or less. A method for producing anatase nanocrystals having shape anisotropy and c-axis orientation.
(5) A titania nanosheet in which titanium atoms and oxygen atoms are two-dimensionally constrained, obtained by exfoliating a layered titanium oxide crystal to a nano-level thickness, is used as an initial material, and one to three layers are formed on an arbitrary substrate. A method for producing an anatase nanocrystalline thin film having c-axis orientation, comprising adsorbing in a layered form, heating to convert to anatase, obtaining and recovering an anatase crystal thin film having c-axis orientation.
[0009]
The titania nanosheet used as a starting material in the present invention can be obtained by the following process. First, a layered titanium oxide is prepared. Examples of the layered titanium oxide include lepidocrocite type titanate Cs _x Ti _{2-x / 4} O ₄ (here, 0.5 ≦ x ≦ 1), K _0.8 Ti _1.73 Li _0.27 O ₄ , Trititanate (Na ₂ Ti ₃ O ₇ ), tetratitanate (K ₂ Ti ₄ O ₉ ), pentatitanate (Cs ₂ Ti ₅ O ₁₁ ), and the like. These titanates acid treatment to hydrogen form _{(H x Ti 2-x /} 4 O 4 · nH 2 O, H 2 Ti 3 O 7 · nH 2 O, H 2 Ti 4 O 9 · nH 2 O, After conversion into H ₂ Ti ₅ O ₁₁ .nH ₂ O), this is shaken in an aqueous solution of a suitable amine or the like using “titania sol and its production method” (Patent Document 1) invented by the present inventors. In this way, the layered titanium oxide is peeled off to obtain a sol.
[0010]
Next, the obtained release sol, that is, a plate-like titania nanosheet, is treated with a titania nanosheet and a cationic titania nanosheet by the method disclosed in “Ultra-thin titania thin film and method for producing the same” invented by the present inventors (Patent Document 2). A polymer is self-assembled into a film on a substrate, and the titania nanosheet is converted to anatase by heating. By using this method, it is possible to manufacture a c-axis oriented anatase thin film more simply and more economically in a large area than the conventional gas phase method.
[0011]
[Patent Document 1] Japanese Patent No. 2,671,949 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-270022
The actual procedure is that the substrate is immersed in a polycation solution, then washed with pure water, and immersed in a nanosheet colloid solution. At this time, the nanosheet is basically adsorbed on the substrate surface as a monolayer (thickness of 1 nm or less). Furthermore, when laminating a plurality of nanosheets, the sheet is immersed again in a polycation solution, washed with pure water, and then immersed in a nanosheet colloid solution. By repeating this process, the number of stacked nanosheets is controlled.
[0013]
Titania nanosheets have high crystallinity and are composed of two layers of titanium / oxygen and one layer of oxygen above and below it, and are extremely thin nanomaterials with a thickness of 0.45 nm. . This substance is charged and can be controlled in units of one layer, that is, in units of two titanium atomic layers, by utilizing the technology of the self-assembly reaction.
There is basically no problem if the substrate is stable in an aqueous solution, and the size is not limited in principle. Further, the present invention has a great advantage that it is not necessary to select the crystal plane of the substrate. For example, quartz glass, a Si wafer, a mica plate, a graphite plate, an alumina plate, a transparent electrode substrate, and the like can be given as specific examples.
[0014]
Since anatase is an oxide crystal, the heat treatment is basically performed in the atmosphere. Heating is performed at an appropriate heating rate, desirably at a temperature gradient of 3 ° C./min to 8 ° C./min, and maintained for an arbitrary time to produce c-axis oriented anatase nanocrystals. At this time, by adjusting the holding time, the crystal growth of the c-axis oriented anatase nanoparticles can be controlled, and the crystal can be grown to a height of 20 nm and a horizontal size of 500 nm.
[0015]
In the present invention, a structural feature in which the titania nanosheet is composed of a limited number of titanium atomic layers is used. First, using ultra-thin titania nanosheets in the titanium atomic layer state, which is about the number of atomic layers of titanium necessary to construct an anatase unit cell, or less, changes the heating characteristics significantly from bulk titanium oxide. This produces an anatase c-axis oriented nanocrystal.
[0016]
That is, while the bulk undergoes a phase transition to anatase at around 400 ° C., as shown in FIG. 1, in an ultrathin film with a limited number of titanium atoms, the nanosheet structure does not change up to a high temperature range of 600 ° C. This is because the titanium in the nanosheet is bound by its crystal structure, and in the low temperature region (600 ° C. or lower), the diffusion of the substance is small, and titanium atoms for forming the growth nucleus of anatase are insufficient. It is a phenomenon that occurs. By further reaching the high temperature region (600 ° C. or higher), the diffusion of atoms is promoted to generate and grow anatase crystal nuclei.
[0017]
At this time, since the anatase crystal nucleus is generated based on the titania nanosheet, the c-axis oriented anatase nanocrystal having only 200 anatase diffraction lines in the plane as shown in FIG. Is generated. Therefore, it is possible to form a film even on a substrate on which c-axis oriented anatase could not be formed conventionally.
[0018]
In the present invention, a dip coating, a Langmuir-Blodgett film forming method, or the like can be used as a film forming method other than the self-assembly reaction between the titania nanosheet and the cationic polymer.
[0019]
Anatase is a photocatalytic substance with a wide range of applications, including UV-cut coatings, solar cells, photocatalysts, and photo-induced superhydrophilic thin films. In particular, the c-axis oriented surface shows better optical properties than other oriented surfaces. This is as described in (0002). The c-axis oriented anatase nanocrystals and their thin films according to the present invention and their production methods are very effective methods for such applications, and are expected to have great industrial effects such as simple, low cost, and no choice of substrate. You.
[0020]
Hereinafter, the present invention will be specifically described based on examples. However, these examples are provided for the purpose of easily understanding the present invention, and are not intended to limit the present invention.
[0021]
Example 1;
Cesium carbonate (Cs ₂ CO ₃ ) and titanium dioxide (TiO ₂ ) are mixed at a molar ratio of 1: 2.65 and fired at 800 ° C. for 2 days to obtain orthorhombic cesium titanate (composition formula Cs _{0 0.7} Ti _1.825 O ₄ ) was synthesized. This powder was stirred in a 1N aqueous hydrochloric acid solution for 3 days, then filtered and air-dried to obtain a layered titanic acid powder (H _0.7 Ti _1.825 O ₄ .nH ₂ O).
0.4 g of the obtained various layered titanic acid powders was added to 100 cm ³ of a tetrabutylammonium hydroxide solution, and shaken at room temperature for about one week (150 rpm) to obtain a milky white titania sol Ti _0.91 O ₂ . A solution obtained by diluting them 50-fold and an aqueous solution of polyethyleneimine (concentration: 2.5 gdm ⁻³ ) were adjusted to pH 9.
[0022]
A Si wafer plate or quartz glass plate of about 4 cm × 1 cm was immersed in a hydrochloric acid / methanol 1: 1 mixed solution for 30 minutes, and then sufficiently washed with Milli-Q pure water. Next, it was immersed in concentrated sulfuric acid for 30 minutes, and washed well with Milli-Q pure water again.
The substrate thus washed and pre-treated was immersed in the above-mentioned polyethylene imine solution for 20 minutes, and after adsorbing polyethylene imine on the substrate surface, the substrate was washed with Milli-Q pure water. Next, the substrate was immersed in the titania sol solution for 20 minutes and washed with Milli-Q pure water to synthesize an ultrathin film in which the titania nanosheet was adsorbed on the substrate.
[0023]
The above ultra-thin film was heated at a rate of 5 ° C./min, kept at 600 ° C., 700 ° C., 800 ° C., and 900 ° C. for 1 hour, and allowed to cool, and then subjected to total reflection X-ray absorption fine structure (XAFS; X-ray Absorption Fine Structure (X-ray Absorption) analysis, X-ray in-plane diffraction measurement, and atomic force microscopy (AFM) observation were performed to examine the heating characteristics of the titania nanosheet.
[0024]
FIG. 1 shows an X-ray absorption edge structure pattern (XANES; X-ray Absorption Near Edge Structure) of an ultrathin film synthesized using Ti _0.91 O ₂ after heat treatment. Focusing on the vicinity of the energy indicated by the broken line in the figure, it was found that the nanosheet structure was maintained up to 600 ° C., and that anatase generation started at 700 ° C. and changed to a single anatase phase at 900 ° C. .
The X-ray plane diffraction pattern shown in FIG. 2 shows that the titania nanosheet does not lose its nanosheet structure even when subjected to a heat treatment at 600 ° C. When a heat treatment at 700 ° C. or 800 ° C. is performed, a mixed phase of titania nanosheet and anatase is formed, but the generated anatase shows only 200 reflections, and almost no diffraction line containing a component in the c-axis direction is detected. This is because the generated anatase is c-axis oriented. Here, the 200 reflection of anatase is observed at a position very close to the peak around 48.3 ° of the nanosheet, but as shown in the enlarged view of the profile in FIG. , It is possible to identify.
FIG. 3 shows the process of changing the ultrathin film by the heat treatment using AFM. Titania nanosheets do not change their nanosheet structure when heated up to 600 ° C when they are single-layer or two-layer, but heat treatment at 700 ° C partially generates nano-sized anatase with shape anisotropy. Can be seen. This is because the change to anatase starts preferentially from the portion where two or more nanosheets overlap. It can be seen that the heat treatment at 800 ° C. changes most of the overlapped portions to anatase, but at this stage, a single-layer nanosheet remains without changing the structure. This enlarged view shows an example of the outer shape of the generated anatase, and it can be seen that the nanoparticles have a lateral size of about 100 nm and an average thickness of about 4 nm. Furthermore, when heated at 900 ° C., almost all nanosheets, whether single-layer or multi-layer, were changed to anatase.
[0025]
Example 2;
Monotitanium titanium oxides H ₂ Ti ₄ O ₉ .nH ₂ O and H ₂ Ti ₅ O ₁₁ .nH ₂ O were exfoliated into a sol in the same manner as in Example 1 to form a tetratitanic acid-type nanosheet Ti _The solution in which _0.89 O ₂ and the pentatitanic acid type nanosheet Ti _0.90 O ₂ were dispersed was adjusted to a concentration of 0.08 gdm ⁻³ and a pH of 9. Then, using the same method as in Example 1, the above titania nanosheet was adsorbed on a Si wafer to synthesize an ultrathin film, which was heated at a rate of 5 ° C./min and held at 800 ° C. for 1 hour. did. These nanosheets also produced nano-sized particulate anatase as in Example 1, as shown in the AFM images of FIGS.
[0026]
【The invention's effect】
Anatase crystals are used in a wide variety of applications such as ultraviolet cut coatings, solar cells, photocatalysts, and photoinduced superhydrophilic thin films, and are used. .
The present invention provides a flaky titanium oxide (titania nanosheet) obtained by exfoliating a layered titanium oxide crystal of an anatase nanocrystal having the expected c-axis orientation or a thin film thereof on a substrate. It is obtained by heating at a temperature 300 ° C. or more higher than the normal phase transition temperature of anatase (approximately 400 ° C.) after adsorbing and controlling the titania nanosheets to have one or two or three layers.
A new c-axis oriented anatase nanocrystal with two-dimensional anisotropy that differs in shape, size, and orientation from crystals obtained by conventional vapor deposition methods such as physical vapor deposition and chemical vapor deposition, and sol-gel methods It is technically excellent in that a crystal can be obtained, and it is possible to increase the size and cost, and its significance is extremely large.
[Brief description of the drawings]
FIG. 1 is a graph showing an XANES spectrum when the titania nanosheet of Example 1 was heated.
FIG. 2 is a graph showing an X-ray surface diffraction pattern and an enlarged view of a peak around 48 ° when the titania nanosheet of Example 1 is heated.
FIG. 3 is a graph showing an AFM morphological observation image of a titania nanosheet ultrathin film of Example 1 after heating.
FIG. 4 is a graph showing an AFM morphological observation image of the ultrathin tetratitanate-type titania nanosheet of Example 2 after heating at 800 ° C.
FIG. 5 is a graph showing an AFM morphology observation image of the ultra-thin titanate-type titania nanosheet thin film of Example 2 after heating at 800 ° C.

Claims (5)

Anatase nanocrystals having a shape anisotropy of a particle size of 20 nm in height × 500 nm in horizontal size or less and having c-axis orientation. Using a titania nanosheet in which titanium atoms and oxygen atoms are two-dimensionally constrained and obtained by exfoliating the layered titanium oxide crystal to a nano-level thickness, heating and converting it to an anatase structure An anatase nanocrystal having a c-axis orientation according to claim 1. Titanium nanosheets in which titanium atoms and oxygen atoms are two-dimensionally constrained, obtained by exfoliating a layered titanium oxide crystal to a nano-level thickness, are used as an initial material, and this is formed on a substrate in a one- or three-layer film form. An anatase nanocrystalline thin film having a c-axis orientation, wherein the anatase nanocrystalline thin film has a c-axis orientation. Using a titania nanosheet in which titanium atoms and oxygen atoms are two-dimensionally constrained, obtained by exfoliating the layered titanium oxide crystal to a nano-level thickness, and converting it to anatase by heating, the particle size is reduced. An anatase nanocrystal having a shape anisotropy of a height of 20 nm × horizontal size of 500 nm or less and having c-axis orientation is obtained and collected. A method for producing an anatase nanocrystal having anisotropy and c-axis orientation. Titanium nanosheets in which titanium atoms and oxygen atoms obtained by exfoliating a layered titanium oxide crystal to a nanometer thickness are two-dimensionally bound are used as an initial substance, and this is used as a single-layer or three-layer film on any substrate. A method for producing an anatase nanocrystalline thin film having a c-axis orientation, characterized in that the anatase nanocrystalline thin film having a c-axis orientation is obtained by collecting and collecting an anatase crystal thin film having a c-axis orientation.

Images