JP6061872B2 - Titanium oxide mesocrystal - Google Patents

Description

  The present invention relates to a titanium oxide mesocrystal.

Titanium oxide, particularly chemically stable and highly active anatase titanium oxide (TiO ₂ ) nanoparticles, has been widely used in various applications such as photo-water decomposition, environmental purification photocatalysts, and dye-sensitized solar cells. . However, titanium oxide nanoparticles tend to aggregate randomly, and this contributes to a decrease in photoactivity (photocatalytic activity, etc.), photoenergy conversion efficiency, and the like due to a reduction in surface area and interface mismatch.

In order to solve this problem, development of titanium oxide mesocrystals in which titanium oxide nanoparticles are regularly and densely integrated is underway (see, for example, Non-Patent Documents 1 and 2). However, in a titanium oxide mesocrystal whose size is 1 μm or more, the specific surface area is much larger than that of a general titanium oxide nanoparticle (the typical photocatalyst titanium oxide nanoparticle P25 is about 50 m ² / g). Lower than 10 m ² / g. Therefore, at present, sufficient photocatalytic activity and the like are not obtained.

  On the other hand, a titanium oxide crystal having a large specific surface area is also known (Non-patent Document 3), but its size (length) is as small as 450 nm or less. When such a small crystal is used, it tends to aggregate randomly and the photocatalytic activity or the like is lowered as in the case of the titanium oxide nanoparticles, and thus a fundamental solution has not been reached.

L. Zhou et al., J. Am. Chem. Soc. 2008, 130, 1309-1320 J. Feng et al., CrystEngComm, 2010, 12, 3425-3429 J. Ye et al., J. Am. Chem. Soc. 2011, 133, 933-940 L. Zhou et al., Small 2008, 4, 1566-1574 G. Liu et al., Chem. Commun. 2011, 47, 6763-6783 J. Zhu et al., CrystEngComm, 2010, 12, 2219-2224 T. Tachikawa et al., J. Am. Chem. Soc. 2011, 133, 7197-7204

  An object of the present invention is to provide a titanium oxide mesocrystal having a large size and specific surface area. Another object of the present invention is to provide a titanium oxide mesocrystal having excellent photocatalytic activity, photoluminescence characteristics, photoinduced charge separation characteristics, and the like.

  As a result of intensive studies in view of the above-mentioned purpose, TiF₄, NH₄NO₃, NH₄It has been found that a precursor aqueous solution containing a predetermined amount of F and water is fired in an air atmosphere and then fired at a higher temperature in an oxygen atmosphere to provide a titanium oxide mesocrystal having a large size and specific surface area. The present invention has been further researched and completed. That is, the present invention includes the following configurations.
Item 1. The ratio of the average width to the average thickness (average width / average thickness) is 10 to 100, and the specific surface area is 10 m.²/ G or more of titanium oxide mesocrystal.
Item 2. Item 2. The titanium oxide mesocrystal according to Item 1, having an average width of 3 to 5 µm.
Item 3. Item 3. The titanium oxide mesocrystal according to Item 1 or 2, wherein the average thickness is 50 to 300 nm.
Item 4. Item 4. The titanium oxide mesocrystal according to any one of Items 1 to 3, which is an aggregate of anatase-type titanium oxide nanocrystals.
Item 5. Item 5. The titanium oxide mesocrystal according to any one of Items 1 to 4, which is a single crystal.
Item 6. Item 6. The titanium oxide mesocrystal according to any one of Items 1 to 5, which is plate-shaped.
Item 7. TiF₄, NH₄NO₃, NH₄Contains F and water, and TiF₄And NH₄NO₃And the content ratio is 1: 4 to 15 (molar ratio), and TiF₄And NH₄Item 6. The oxidation according to any one of Items 1 to 6, comprising a step of firing an aqueous precursor solution having a content ratio with F of 1: 1 to 9 (molar ratio) in an oxygen atmosphere at 400 to 700 ° C. Method for producing titanium mesocrystal.
Item 8. TiF₄, NH₄NO₃, NH₄Contains F and water, and TiF₄And NH₄NO₃And the content ratio is 1: 4 to 15 (molar ratio), and TiF₄And NH₄An aqueous precursor solution having a content ratio with F of 1: 1 to 9 (molar ratio) is calcined under a condition of 250 to 700 ° C. in an air atmosphere or an oxygen atmosphere, and is then subjected to a condition of 400 to 700 ° C. in an oxygen atmosphere. Item 7. A method for producing a titanium oxide mesocrystal according to any one of Items 1 to 6, further comprising a step of firing.
Item 9. Item 7. A composite material in which a layer made of the titanium oxide mesocrystal according to any one of Items 1 to 6 is formed on the surface of the functional material.
Item 10. The ratio of average width to average thickness (average width / average thickness) is 10 to 100, NH ₄TiOF₃crystal.
Item 11. TiF₄, NH₄NO₃, NH₄Contains F and water, and TiF₄And NH₄NO ₃And the content ratio is 1: 4 to 15 (molar ratio), and TiF₄And NH₄Item 11. The NH according to Item 10, comprising a step of calcining a precursor aqueous solution having a content ratio with F of 1: 1 to 9 (molar ratio) in an air atmosphere at 250 to 300 ° C.₄TiOF₃Crystal production method.

According to the present invention, it is possible to provide a titanium oxide mesocrystal having a large size and specific surface area. In particular, the specific surface area can be larger than 50 m ² / g of titanium oxide nanoparticles P25, which is a typical photocatalyst. The titanium oxide mesocrystal of the present invention is excellent in photocatalytic activity, photoluminescence characteristics, photoinduced charge separation characteristics, and the like.

It is a conceptual diagram explaining a mesocrystal. N-I, N-II and N-III represent various titanium oxide nanocrystals in which titanium oxide nanoparticles are regularly arranged. M-I, M-II, and M-III represent titanium oxide mesocrystals in which these are regularly arranged. It is a conceptual diagram explaining the arrangement | sequence of the titanium oxide nanocrystal in the conventional nanoparticle type | system | group and the titanium oxide mesocrystal of this invention. It is a conceptual diagram which shows an example of the manufacturing method of this invention. It is a graph which shows the result of the powder X-ray diffraction (XRD) of the crystal | crystallization of Examples 1-3 and Comparative Examples 4, 5 and 11. For reference, peaks of anatase-type titanium oxide, NH ₄ NO ₃ , and NH ₄ TiOF ₃ are also shown. It is a graph which shows the result of the powder X-ray diffraction (XRD) of the crystal | crystallization of Comparative Examples 1-2. It is a figure which shows the result of the powder X-ray diffraction (XRD) and electron microscope (SEM) observation of the crystal | crystallization of the comparative example 3. It is a figure which shows the result of an electron microscope (SEM and TEM) observation of the titanium oxide mesocrystal of Example 2. 3 is a graph showing the distribution of thickness of the titanium oxide mesocrystal of Example 2. It is a figure which shows the result of the electron microscope (SEM) observation of the crystal | crystallization of Examples 1, 3 and Comparative Examples 4, 5. It is a figure which shows the result of the electron microscope (SEM) observation of Example 5 and Comparative Examples 6-8. It is a figure which shows the result of the electron microscope (SEM) observation of Examples 6-7 and Comparative Examples 9-10. It is a figure which shows the result of the electron microscope (TEM) observation of the comparative example 1. It is a figure which shows the result of the electron microscope (a: TEM, b: SEM) observation of the comparative example 2. It is a graph which shows the photocatalytic oxidation activity of the p-chlorophenol of the crystal | crystallization of Examples 1-4 and Comparative Examples 1-3. It is a graph showing the photocatalytic reduction activity of ^{Cr 6+} crystals of Examples 1 to 4 and Comparative Examples 1 to 3 and 5. It is a graph which shows the photocatalytic oxidation activity of RhB of the crystal | crystallization of Examples 1-4 and Comparative Examples 1-3. It is a conceptual diagram explaining an example of the arrangement | sequence of a nanocrystal and the movement of an electron in an Example and Comparative Examples 1-2. It is a graph which shows the measurement result of the time-resolved diffuse reflectance in Example 2 and Comparative Example 1. It is a graph which shows the measurement result of the adsorption | suction behavior of MTPM on the titanium oxide crystal surface in Example 2 and Comparative Example 1. It is a graph which shows the measurement result of the time-resolved diffuse reflectance in Examples 3-4. 10 is a graph showing measurement results of time-resolved diffuse reflectance in Comparative Example 2. It is a graph which shows the measurement result of the single particle fluorescence spectroscopy in Example 2 and Comparative Example 1. In Example 2, it is a graph which shows the measurement result about the photocurrent response by UV irradiation intensity. It is a graph which shows the comparison result of the photocurrent response of the crystal of Example 2 and Comparative Example 1. It is a graph which shows the contrast of a photocurrent response by the difference in the width | variety and thickness of a titanium oxide mesocrystal. It is a figure which shows the result of the electron microscope (SEM) observation of the composite material of the titanium oxide mesocrystal-functional material of Example 8.

1. Titanium oxide mesocrystal The titanium oxide mesocrystal of the present invention has a ratio of average width to average thickness (average width / average thickness) of 10 to 100, and a specific surface area of 10 m ² / g or more.

  In the present invention, the “titanium oxide mesocrystal” means a mesosize (specifically, about 1 to 10 μm) crystalline superstructure in which titanium oxide nanocrystals are regularly arranged. Note that the superstructure means a structure in which nanoparticles or nanocrystals are not randomly aggregated but regularly arranged (see FIG. 1 and FIG. 1 of Non-Patent Document 4). In FIG. 1, N-I, N-II, and N-III represent various titanium oxide nanocrystals in which titanium oxide nanoparticles are regularly arranged. M-I, M-II, and M-III represent titanium oxide mesocrystals in which these are regularly arranged. At this time, each titanium oxide nanoparticle is regularly arranged in the titanium oxide mesocrystal. As described above, the titanium oxide mesocrystal of the present invention is a large-sized crystal in which the titanium oxide nanocrystals are not randomly aggregated but regularly arranged, and therefore, disordered aggregation can be suppressed.

  Further, in the titanium oxide mesocrystal of the present invention, since the titanium oxide nanocrystals are regularly arranged as described above, it is possible to form a single crystal as a whole.

  In FIG. 1, Amorphous Solid and Polycrystal shown in the lower left represent a state in which titanium oxide nanoparticles or titanium oxide nanocrystals are randomly aggregated.

  In addition, as shown in FIG. 2, in the conventional disordered aggregate, there are few contacts between the titanium oxide nanocrystals (in FIG. 2, they are in contact only with dots), and electrons generated by sunlight are difficult to conduct. However, in the titanium oxide mesocrystal of the present invention, the titanium oxide nanocrystals are regularly arranged, and there are many contact points between the titanium oxide nanocrystals (in FIG. 2, they are in contact with each other), which facilitates conduction of electrons and is high. Electrical conductivity is obtained. Further, since the titanium oxide mesocrystal of the present invention can suppress aggregation, it can maintain a high specific surface area compared to the case of using titanium oxide nanoparticles having the same specific surface area, and has a photocatalytic activity. Can be dramatically improved.

  One feature of the titanium oxide mesocrystal of the present invention is that the width is sufficiently larger than the thickness. Specifically, the ratio of the average width to the average thickness (average width / average thickness) is about 10 to 100, preferably about 40 to 50. By increasing the ratio of the average width to the average thickness, the size of the titanium oxide mesocrystal can be increased, and disordered aggregation can be suppressed, but the above range is preferable from the viewpoint of being easily broken when it is too large. .

  Moreover, since the ratio of the average width to the average thickness is large as described above, the titanium oxide mesocrystal of the present invention can have a {001} plane having a high surface energy as a main crystal plane. Since the {001} plane is attracting attention as a highly active crystal plane (Non-Patent Document 5), the titanium oxide mesocrystal of the present invention has a {001} plane as a main crystal plane, thereby providing photocatalytic activity and the like. Can be improved.

  Thus, since it is preferable to increase the ratio of the average width to the average thickness, it is preferable that the average width is large. Moreover, the one where average thickness is smaller is preferable. Specifically, the average width is preferably about 2 to 8 μm, and more preferably about 4 to 5 μm. The average thickness is preferably about 50 to 300 nm, more preferably about 70 to 110 nm.

  In addition, in the titanium oxide mesocrystal of the present invention, the average width means an average value of the sides of the estimated square or rectangle when the surface is regarded as a plate-like crystal having a square or rectangle. In addition, the average thickness of the titanium oxide mesocrystal of the present invention is an average value of the thickness in the case of a plate-like crystal, and an average value of the thickness when it is assumed to be a plate-like crystal in the case of not being plate-like. These widths and thicknesses can be measured, for example, by observation with an electron microscope (SEM or the like).

Moreover, the specific surface area of the titanium oxide mesocrystal of the present invention is 10 m ² / g or more. When the specific surface area of the titanium oxide mesocrystal is too small, the photocatalytic activity and the photocurrent life are inferior. In the present invention, the specific surface area of the titanium oxide mesocrystal can be larger than 50 m ² / g of the titanium oxide nanoparticle P25 which is a typical photocatalyst. In addition, about 10-80 m < ² > / g is preferable and, as for the specific surface area of a titanium oxide mesocrystal, about 50-70 m < ² > / g is more preferable. The specific surface area of the titanium oxide mesocrystal of the present invention can be measured by, for example, the BET method.

  The titanium oxide nanocrystal constituting the titanium oxide mesocrystal of the present invention may be anatase type or rutile type. Among these, since the catalytic activity is high, the titanium oxide mesocrystal of the present invention is preferably an aggregate of anatase-type titanium oxide nanocrystals. The crystal structure of the titanium oxide nanocrystal can be measured by, for example, powder X-ray diffraction.

  In addition, the average particle size of the titanium oxide nanocrystals constituting the titanium oxide mesocrystal of the present invention is preferably about 30 to 70 nm, more preferably about 35 to 40 nm, from the viewpoint of superior photocatalytic activity. The average particle diameter of the titanium oxide nanoparticles can be measured by, for example, powder X-ray diffraction (using Scherrer's equation) or the like.

The pore diameter and pore volume in the titanium oxide mesocrystal of the present invention are preferably larger from the viewpoint of further improving the specific surface area. Specifically, the average pore diameter is preferably about 3 to 8 nm. The average pore volume is preferably about 0.1 to 0.2 cm ³ / g. These can be measured from an adsorption isotherm by the BJH model.

  In the titanium oxide mesocrystal of the present invention, the purity of titanium oxide can be improved and a crystal substantially free of impurities such as nitrogen and fluorine can be obtained. This can be confirmed from the fact that the band gap of the titanium oxide mesocrystal of the present invention is comparable to that of titanium oxide.

  The shape of the titanium oxide mesocrystal of the present invention may be a plate shape or another shape. Among these, in order to have a {001} plane having a high surface energy as a main crystal plane, a plate-like shape is preferable because the ratio of the average width to the average thickness is preferably large.

2. NH ₄ TiOF ₃ crystal
  As described later, the titanium oxide mesocrystal of the present invention described above is the NH of the present invention.₄TiOF ₃It can be obtained by a topotactic reaction using crystals. In this topotactic reaction, the crystal shape is substantially maintained (see Non-Patent Document 1, etc.), so the crystal shape is the same as that of the titanium oxide mesocrystal of the present invention. Specifically, it is as follows.

The NH ₄ TiOF ₃ crystal of the present invention is characterized by a sufficiently large width compared to the thickness. Specifically, the ratio of the average width to the average thickness (average width / average thickness) is about 10 to 100, preferably about 40 to 50. By increasing the ratio of the average width to the average thickness, the size of the titanium oxide mesocrystal finally obtained can be increased, and disordered aggregation of the titanium oxide mesocrystal can be suppressed. From the viewpoint of the above, the above range is preferable.

In addition, since the NH ₄ TiOF ₃ crystal of the present invention has a large ratio of the average width to the average thickness as described above, the finally obtained titanium oxide mesocrystal is a main crystal with a {001} plane having a high surface energy. Can have as a face. Since the {001} plane is attracting attention as a highly active crystal plane (Non-Patent Document 5), the titanium oxide mesocrystal of the present invention has a {001} plane as a main crystal plane, thereby providing photocatalytic activity and the like. Can be improved.

  Thus, since it is preferable to increase the ratio of the average width to the average thickness, it is preferable that the average width is large. Moreover, the one where average thickness is smaller is preferable. Specifically, the average width is preferably about 2 to 8 μm, and more preferably about 4 to 5 μm. The average thickness is preferably about 50 to 300 nm, more preferably about 70 to 110 nm.

  The average width and average thickness are those described above.

The shape of the NH ₄ TiOF ₃ crystal of the present invention may be a plate shape or another shape. Among these, in the finally obtained titanium oxide mesocrystal, in order to have a {001} plane having a high surface energy as a main crystal plane, the ratio of the average width to the average thickness is preferably large, and thus a plate shape is preferable. .

3. Production method of titanium oxide mesocrystal and NH ₄ TiOF ₃ crystal The titanium oxide mesocrystal of the present invention contains TiF ₄ , NH ₄ NO ₃ , NH ₄ F and water, and the content ratio of TiF ₄ and NH ₄ NO ₃ is A precursor aqueous solution having a content ratio of 1: 4 to 15 (molar ratio) and TiF ₄ and NH ₄ F of 1: 1 to 9 (molar ratio) is 250 to 700 ° C. in an air atmosphere or an oxygen atmosphere. It can manufacture by the method provided with the process of baking on 400-700 degreeC conditions in oxygen atmosphere after baking on conditions.

  In the method for producing a titanium oxide mesocrystal of the present invention, first, the precursor aqueous solution is fired under conditions of 250 to 700 ° C. in an air atmosphere or an oxygen atmosphere.

  Specifically, a liquid layer composed of an aqueous precursor solution may be formed on a substrate and fired under conditions of 250 to 700 ° C. in an air atmosphere or an oxygen atmosphere.

  There is no restriction | limiting in particular as a board | substrate, It has a smooth surface at normal temperature, The surface may be a plane or a curved surface, and may deform | transform by stress. Specific examples of substrates that can be used include silicon, various glasses, and transparent resins. However, since it is necessary to fire at 400 ° C. or higher as described later, it is preferable to use silicon, glass, or the like.

  The thickness of the liquid layer is not particularly limited, but is usually preferably about 2 mm or less.

  The formation method of a liquid layer is not specifically limited, A well-known method is employable suitably according to the kind of board | substrate to be used. For example, deep coating, spin coating, or the like may be performed on the substrate, or a mixed solution of the substrate material and the precursor aqueous solution may be dropped onto silicon, glass, or the like.

  As the substrate, in addition to the above, a functional material can be used to easily manufacture a composite material with the titanium oxide mesocrystal of the present invention. Examples of the functional material include tin-doped indium oxide (ITO) and fluorine-doped tin oxide (FTO).

  As the titanium precursor contained in the precursor aqueous solution, TiF₄Is used. TiF₄F inside ⁻Ions are strongly adsorbed on the {001} plane of the titanium oxide nanocrystal, and a titanium oxide mesocrystal having a {001} plane as a main crystal plane, that is, a large size is obtained.

As a titanium precursor, it is also known to use TiCl ₄ which is much cheaper than TiF ₄ (Non-patent Document 2, etc.), but actually, titanium oxide nanocrystals are not regularly arranged. That is, titanium oxide mesocrystals are not obtained, and only aggregates are obtained. In addition, when Ti (OC ₄ H ₉ ) ₄ , Ti (SO ₄ ) ₂ or the like is used as the titanium precursor, only aggregates can be obtained.

In the production method of the present invention, NH ₄ NO ₃ is used to control the crystal structure of the titanium oxide mesocrystal. At this time, the amount of _NH 4 NO ₃ is, TiF ₄ and _NH 4 content ratio of NO ₃ is 1: 4 to 15 (molar ratio), preferably 1: 6-9 (molar ratio). If the amount of NH ₄ NO ₃ used is too small, only aggregates of titanium oxide nanoparticles having no crystal structure can be obtained, and titanium oxide mesocrystals cannot be obtained. On the other hand, if the amount of NH ₄ NO ₃ used is too large, it becomes a mixture of titanium oxide mesocrystals and titanium oxide nanoparticles, and aggregation of titanium oxide nanoparticles cannot be prevented. By setting the amount of NH ₄ NO ₃ used within the above range, a titanium oxide mesocrystal with higher crystallinity can be obtained.

In the production method of the present invention, NH ₄ F is used to control the size (width) and thickness of the titanium oxide mesocrystal. At this time, the amount of NH ₄ F used is such that the content ratio of TiF ₄ and NH ₄ F is 1: 1 to 9 (molar ratio), preferably 1: 3 to 5 (molar ratio). If the amount of NH ₄ F used is too small, the average width becomes small and the thickness becomes large, and aggregation cannot be prevented. On the other hand, when the amount of NH ₄ F used is too large, the width of the titanium oxide mesocrystal becomes too large, and the thickness becomes too thin and the shape collapses so that aggregation cannot be prevented.

In the production method of the present invention, water is used as the solvent. When the raw material is an organic material, an organic solvent can be used. However, in the present invention, an inorganic solvent is used, and therefore an aqueous solvent, particularly water, is preferable. The amount of water used may be excessive with respect to the other components, but it should not be too much for film formation. Specifically, the content ratio of TiF ₄ and water may be about 1: 100 to 1000 (molar ratio).

  In the production method of the present invention, a titanium oxide mesocrystal is produced by regularly arranging titanium oxide nanocrystals without using a surfactant used in a conventional method (Non-patent Document 1 or the like). Is possible.

In the production method of the present invention, the above-described precursor aqueous solution is first fired under an air atmosphere at 250 to 700 ° C., preferably 250 to 300 ° C., more preferably 250 to 290 ° C. (first firing). . Thereby, the above-described NH ₄ TiOF ₃ crystal of the present invention can be manufactured. At this time, the NH ₄ TiOF ₃ crystal of the present invention can be produced even in an oxygen atmosphere. Further, when the firing temperature is too low, the mixture of _NH 4 NO _3. Furthermore, when the firing temperature is too high, a titanium oxide crystal having a specific surface area of 10 m ² / g or less is obtained. In the first baking, if baking is performed at 400 to 700 ° C. in an oxygen atmosphere, the titanium oxide mesocrystal of the present invention can be obtained in one pot without passing through the NH ₄ TiOF ₃ crystal.

Thereafter, the produced NH ₄ TiOF ₃ crystal is fired under an oxygen atmosphere at 400 to 700 ° C. (second firing). By this firing, a topotactic reaction (see Non-Patent Document 1, etc.) is caused to obtain the titanium oxide mesocrystal of the present invention. At this time, the firing may be performed in the same furnace as in the above-described firing in an air atmosphere or in a different furnace. As described above, in the first firing, when the firing is performed at 400 to 700 ° C. in an oxygen atmosphere, the titanium oxide mesocrystal of the present invention can be obtained without performing the second firing. .

  By setting the atmosphere of the second baking (the first baking when only the first baking is performed) to be an oxygen atmosphere, it is possible to suppress the entry of impurities such as nitrogen and fluorine into the titanium oxide mesocrystal. In the present invention, the oxygen atmosphere is preferably a 100% oxygen gas or a mixed gas atmosphere of oxygen and air having an oxygen concentration of 90% or more.

Moreover, the calcination temperature of 2nd baking is 400-700 degreeC, Preferably it is 400-600 degreeC, More preferably, it is 450-550 degreeC. If the firing temperature is too low, NH ₄ TiOF ₃ cannot be sufficiently converted to TiO _2, and a titanium oxide mesocrystal cannot be obtained. Further, the specific surface area cannot be increased. On the other hand, if the calcination temperature is too high, titanium oxide mesocrystals can be obtained, but the specific surface area decreases, and the photocatalytic activity and the like deteriorate. Normally, titanium oxide cannot maintain a highly active anatase type when baked at a high temperature, and the structure changes to a rutile type. However, in the present invention, anatase can be baked at a high temperature (near the upper limit of the above temperature range). The mold can be maintained.

  An example of the concept of the manufacturing method of the present invention described above is shown in FIG.

4). Applications The titanium oxide mesocrystal of the present invention has a large specific surface area as described above, and is a regular arrangement of titanium oxide nanocrystals, and has a large size and can suppress aggregation, so that photocatalytic activity and photoluminescence characteristics are achieved. The photo-induced charge separation characteristics are high and the conductivity is high. In addition, according to the present invention, since titanium oxide mesocrystals can be produced by a very simple method, the mass productivity is excellent. Therefore, it can be applied to various uses such as an environmental purification photocatalyst, a hydrogen generation photocatalyst, a dye-sensitized solar cell, and a lithium ion battery.

  The present invention will be specifically described based on examples, but the present invention is not limited to these examples.

  <Examples and Comparative Examples>
  Example 1: Firing at 400 ° C. (Meso-TiO ₂ -400)
  First, TiF₄, NH₄NO₃, NH₄Precursor aqueous solution (composition is molar ratio, TiF ₄: NH₄NO₃: NH₄: H₂A liquid layer composed of O = 1: 6.6: 4: 117) was formed on a silicon substrate. Specifically, the precursor aqueous solution was dropped on the substrate. The thickness of the liquid layer was set to 1 mm or less. The liquid layer formed on the silicon substrate is sintered at 250 ° C. for 2 hours in an air atmosphere, so that NH is formed on the silicon substrate.₄TiOF₃Crystals formed. Then, the titanium oxide mesocrystal of Example 1 was formed on the silicon substrate by baking at 400 degreeC under oxygen atmosphere (oxygen 100%) for 8 hours. In addition, as for the raw material used for precursor aqueous solution, all used the commercial item.

Example 2: Firing at 500 ° C. (Meso-TiO ₂ -500)
An NH ₄ TiOF ₃ crystal and a titanium oxide mesocrystal of Example 2 were produced in the same manner as in Example 1 except that the firing temperature in an oxygen atmosphere was 500 ° C.

Example 3: Firing at 600 ° C. (Meso-TiO ₂ -600)
An NH ₄ TiOF ₃ crystal and a titanium oxide mesocrystal of Example 3 were produced in the same manner as in Example 1 except that the firing temperature in an oxygen atmosphere was 600 ° C.

Example 4: Firing at 700 ° C. (Meso-TiO ₂ -700)
An NH ₄ TiOF ₃ crystal and a titanium oxide mesocrystal of Example 4 were produced in the same manner as in Example 1 except that the baking temperature in an oxygen atmosphere was 700 ° C.

Example 5: Firing at 500 ° C., TiF ₄ : NH ₄ NO ₃ = 1: 9
A titanium oxide mesocrystal of Example 5 was produced in the same manner as in Example 2 except that TiF ₄ : NH ₄ NO ₃ = 1: 9 (molar ratio) in the precursor aqueous solution.

Example 6: Firing at 500 ° C., TiF ₄ : NH ₄ F = 1: 2
A titanium oxide mesocrystal of Example 6 was prepared in the same manner as in Example 2 except that TiF ₄ : NH ₄ F = 1: 2 (molar ratio) in the precursor aqueous solution.

Example 7: Firing at 500 ° C., TiF ₄ : NH ₄ F = 1: 8
A titanium oxide mesocrystal of Example 7 was produced in the same manner as in Example 2 except that TiF ₄ : NH ₄ F = 1: 8 (molar ratio) in the precursor aqueous solution.

Comparative Example 1: Titanium oxide nanocrystals (Nano-TiO ₂ )
According to Non-Patent Document 6, a titanium oxide nanocrystal of Comparative Example 1 was produced. This sample was baked at 600 ° C. for 8 hours in an oxygen atmosphere.

Comparative Example 2: Titanium oxide polycrystal (Poly-TiO ₂ )
First, a liquid layer composed of an aqueous precursor solution containing TiF ₄ (composition is molar ratio, TiF ₄ : H ₂ O = 1: 117) was formed on a silicon substrate. Thereafter, the liquid layer formed on the silicon substrate was sintered at 500 ° C. for 2 hours in an air atmosphere. Furthermore, it was baked at 500 ° C. for 8 hours in an oxygen atmosphere to obtain a titanium oxide polycrystal of Comparative Example 2.

Comparative Example 3: Titanium oxide microcrystal (Micro-TiO ₂ )
In accordance with Non-Patent Document 7, a titanium oxide microcrystal of Comparative Example 3 was produced. This sample was baked at 600 ° C. for 8 hours in an oxygen atmosphere.

Comparative Example 4: Firing at 300 ° C. (TiO ₂ -300)
A titanium oxide crystal of Comparative Example 4 was produced in the same manner as in Example 1 except that the firing temperature in an oxygen atmosphere was 300 ° C.

Comparative Example 5: 800 ° C. firing (Meso-TiO ₂ -800)
A titanium oxide crystal of Comparative Example 5 was produced in the same manner as in Example 1 except that the firing temperature in an oxygen atmosphere was 800 ° C.

Comparative Example 6: 500 ° C. _{calcination, TiF 4: NH 4 NO 3} = 1: 0
A crystal of Comparative Example 6 was produced in the same manner as in Example 2 except that NH ₄ NO ₃ was not used (TiF ₄ : NH ₄ NO ₃ = 1: 0 (molar ratio)) in the precursor aqueous solution.

Comparative Example 7: Firing at 500 ° C., TiF ₄ : NH ₄ NO ₃ = 1: 3
A crystal of Comparative Example 7 was produced in the same manner as in Example 2 except that TiF ₄ : NH ₄ NO ₃ = 1: 3 (molar ratio) in the precursor aqueous solution.

Comparative Example 8: Firing at 500 ° C., TiF ₄ : NH ₄ NO ₃ = 1: 20
A crystal of Comparative Example 8 was produced in the same manner as Example 2 except that TiF ₄ : NH ₄ NO ₃ = 1: 20 (molar ratio) in the precursor aqueous solution.

Comparative Example 9: Firing at 500 ° C., TiF ₄ : NH ₄ F = 1: 0
A crystal of Comparative Example 9 was produced in the same manner as in Example 2 except that NH ₄ F was not used in the precursor aqueous solution (TiF ₄ : NH ₄ F = 1: 0 (molar ratio)).

Comparative Example 10: Firing at 500 ° C., TiF ₄ : NH ₄ F = 1: 10
A crystal of Comparative Example 10 was produced in the same manner as in Example 2 except that TiF ₄ : NH ₄ F = 1: 10 (molar ratio) in the precursor aqueous solution.

Comparative Example 11: 250 ° C. firing (TiO ₂ -250)
A titanium oxide crystal of Comparative Example 11 was produced in the same manner as in Example 1 except that the firing temperature in an oxygen atmosphere was 250 ° C.

<Evaluation>
Test Example 1: Specific surface area The specific surface areas of the crystals of Examples 1 to 4 and Comparative Examples 1 to 5 were measured by the BET method. The results are shown in Table 1.

Test Example 2: Pore diameter and pore volume The pore diameter and pore volume of the crystals of Examples 1 to 4 and Comparative Examples 1 to 5 were measured by the BJH method. The results are shown in Table 1.

Test Example 3: X-ray diffraction The characteristics of the crystals of Examples 1 to 3 and Comparative Examples 4, 5, and 11 were measured by powder X-ray diffraction (XRD). Moreover, about the crystal | crystallization of Examples 1-4 and Comparative Examples 1-5, the particle diameter of the titanium oxide nanocrystal which comprises each crystal | crystallization was evaluated from the X-ray diffraction peak using Scherrer's formula. The results are shown in Table 1 and FIG.

  For reference, the results of X-ray diffraction of the crystals of Comparative Examples 1 to 3 are also shown in FIGS. In both cases, anatase-type titanium oxide has been detected.

Test Example 4: Electron Microscope Observation The characteristics of the crystal of Example 2 were observed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results are shown in FIGS.

  As shown in FIG. 7a, the titanium oxide mesocrystal of the present invention (particularly Example 2) was a plate-like crystal having a substantially square surface.

Further, as shown in FIG. 7b, the titanium oxide nanocrystals were regularly arranged. Further, pores of about several nm were generated. The pore structure was also confirmed by TEM (FIG. 7d), and a single crystal anatase crystal along the {001} plane was confirmed from the limited-field electron diffraction (SAED) pattern on the crystal. From the high-resolution transmission electron microscope (HRTEM) image of the contact point of the titanium oxide particles, the single crystal lattice showed an atomic plane of anatase (200) or (020) crystal plane having a lattice spacing of about 0.189 nm (FIG. 7e). ). These results strongly suggest that the {001} plane is exposed on the surface of the titanium oxide mesocrystal of the present invention. Furthermore, uniform lattice fringes were clearly obtained, and perfectly oriented titanium oxide nanocrystals were confirmed. By nanocrystals regularly arranged, on the surface a large number of defects or holes were formed (Fig. 7b and 7 e). A TEM image of a portion having a hole at the end of the crystal showed that the nanocrystals were aligned in a regular structure (FIG. 7f).

  The average thickness of the titanium oxide mesocrystal was about 80 nm (FIG. 7c), and was distributed in the range of 50 to 300 nm (FIG. 8).

  In other examples, similar results were obtained in the shape and crystal structure of the titanium oxide mesocrystal of the present invention (see FIGS. 4 and 9).

Moreover, the scanning electron microscope (SEM) image of Example 5 and Comparative Examples 6-8 is shown in FIG. When NH ₄ NO ₃ was not used or when the amount was small, aggregates of primary particles due to recombination of titanium oxide particles were observed (FIGS. 10a and 10b). When the amount of NH ₄ NO ₃ was increased, plate-like titanium oxide mesocrystals were formed (FIG. 10c). When the amount of NH ₄ NO _{3 was} further increased, a mixture of plate crystals and particles was obtained (FIG. 10d). For this reason, it is suggested that only Example 5 can suppress aggregation of titanium oxide nanoparticles.

Furthermore, the scanning electron microscope (SEM) image of Examples 6-7 and Comparative Examples 9-10 is shown in FIG. When NH ₄ F was not used, the thickness of the titanium oxide crystal was about 300 to 500 nm and the width was about 1 μm (FIG. 11 a). When the amount of NH ₄ F was increased, the thickness was decreased and the width was increased (FIGS. 11b to 11c). Specifically, it was possible to set the thickness to about 70 to 110 nm and the width to about 4 to 5 μm. In addition, the width | variety and thickness of the titanium oxide mesocrystal of this invention do not depend on a calcination temperature, and the crystal | crystallization of the same grade is obtained. When the amount of NH ₄ F was further increased, the thickness was too small, the width was too large, and the shape of the titanium oxide crystal collapsed, resulting in a mixture with fragmented small masses (FIG. 11d). For this reason, it is suggested that Examples 6-7 can enlarge ratio of a width | variety and thickness, suppressing aggregation of a titanium oxide nanoparticle.

  For reference, the results of electron microscope (SEM and TEM) observation of the crystals of Comparative Examples 1 to 3 are also shown in FIGS. 6 (Comparative Example 3), 12 (Comparative Example 1) and 13 (Comparative Example 2). Comparative Examples 1 and 2 are agglomerated, and Comparative Example 3 has a small width to thickness ratio.

Test Example 5 Elemental Analysis Elemental analysis of the titanium oxide mesocrystal of Example 2 was performed by energy dispersive X-ray analysis (EDX). As a result, elements other than the constituent elements of titanium oxide such as nitrogen and fluorine were not detected.

  From the steady state diffuse reflectance spectrum, the band gap of the titanium oxide mesocrystal of Example 2 was calculated to be 3.2 eV. Since it is almost the same as the band gap of titanium oxide, this also suggests that impurities are not mixed in the titanium oxide mesocrystal of Example 2.

  In other examples, similar results were obtained for the elemental analysis of the titanium oxide mesocrystal of the present invention.

Test Example 6: Photocatalytic activity (p-chlorophenol)
The photocatalytic oxidation of p-chlorophenol was measured.

Using 4 mL (0.2 g / L) of a dispersion of the crystals of Examples 1 to 4 and Comparative Examples 1 to 3 and 5 containing 1.0 × 10 ⁻⁴ M of p-chlorophenol, ultrasonic decomposition was performed for 20 minutes. And transferred into a quartz cuvette. The photocatalytic reaction was initiated by irradiation with ultraviolet light from a mercury light source (REX-120) using a filter (365 nm) at room temperature. The intensity of ultraviolet light was 70 mW / cm ² and the reaction time was 4 hours. After the ultraviolet light irradiation was stopped, the sample was centrifuged at 10000 rpm (HITACHI himac CF16RX) to remove particles. The concentration of unreacted molecules was analyzed with an ultraviolet spectrophotometer (UV-3100, Shimadzu Corporation) at the intrinsic wavelength, and the decomposition rate was calculated.

  The results are shown in FIG. The titanium oxide mesocrystals of the examples showed a photocatalytic activity superior to that of any other comparative examples (about twice or more). Especially, Example 2 baked at 500 ° C. showed the most excellent photocatalytic activity.

Test Example 7: Photocatalytic activity (Cr ⁶⁺ )
The photocatalytic reduction of Cr ⁶⁺ was measured.

Not a dispersion containing 1.0 × 10 ⁻⁴ M of p-chlorophenol, but a dispersion containing 4.0 × 10 ⁻⁴ M of Cr ⁶⁺ and adjusted to pH 3 with H ₂ SO ₄ Was the same as in Test Example 6 to calculate the decomposition rate of Cr ⁶⁺ .

  The results are shown in FIG. The titanium oxide mesocrystals of the examples showed a photocatalytic activity superior to that of any other comparative examples (about twice or more). Especially, Example 2 baked at 500 ° C. showed the most excellent photocatalytic activity.

Test Example 8: Photocatalytic activity (RhB)
The photocatalytic oxidation of rhodamine B (RhB) was measured.

Test except that a dispersion containing 1.0 × 10 ⁻⁵ M of rhodamine B (RhB) was used instead of a dispersion containing 1.0 × 10 ⁻⁴ M of p-chlorophenol, and the reaction time was 10 minutes. In the same manner as in Example 6 , the decomposition rate of RhB was calculated.

  The results are shown in FIG. The titanium oxide mesocrystals of the examples showed a photocatalytic activity superior to that of any other comparative examples (about twice or more). Especially, Example 2 baked at 500 ° C. showed the most excellent photocatalytic activity.

  The crystals of Comparative Examples 1 and 2 (particularly Comparative Example 1) have a specific surface area comparable to that of the Examples, but the photocatalytic activity is significantly reduced. This suggests that in the crystal of Comparative Example 1, the titanium oxide nanocrystals are not regularly arranged as in the titanium oxide mesocrystal of the present invention but are randomly aggregated (see FIG. 17).

Test Example 9: Time-resolved diffuse reflectance time-resolved reflectance spectroscopy was performed in order to evaluate the lifetime of the charge separation state. This measurement is a criterion for evaluating the efficiency of the photocatalytic reaction.

  Specifically, it was performed as follows.

Time-controlled Q-switch Nd ³⁺ using a delay generator (Stanford Research Systems, DG535): Third high frequency (355 nm, 5 ns (full width at half maximum)) from a YAG laser (Continuum, Surelite II-10), 1.5 mJ / pulse) was used as the excitation light source for titanium oxide. Analytical light from a pulsed 450W xenon arc lamp (Ushio, UXL-451-0) is collected in a focusing lens and directed through a spectrometer (Nikon, G250) to a silicon avalanche photodiode detector (Hamamatsu Photonics, S5343) It was. Transient signals were recorded with an oscilloscope (Tektronix, TDS 580D). All experiments were performed at room temperature.

  As shown in FIG. 18, in the titanium oxide mesocrystal of Example 2, transient absorption was confirmed in a wide wavelength range from the visible region to the near infrared region. In this range, absorption bands of trapped holes (mainly 440 to 600 nm) and trapped electrons (660 to 900 nm) overlap.

  Here, 4- (methylthio) phenylmethanol (MTPM) was selected as a probe molecule, and the lifetime of the charge separation state was evaluated. This is because by using MTPM as a probe molecule, a transient signal of a one-electron oxidant generated by reaction with holes can be recognized more clearly (FIG. 18). This is also because MTPM has a structure similar to p-chlorophenol.

  Here, when the adsorption behavior of MTPM on the titanium oxide crystal surface was measured, the same behavior was shown in Example 2 and Comparative Example 1 as shown in FIG. For this reason, if Example 2 and Comparative Example 1 are compared, the contribution of adsorption to the reaction efficiency can be minimized.

  Here, MTPM generated by one-electron oxidation of MTPM adsorbed by photogenerated holes ^{● +}The rate of charge recombination with electrons in titanium oxide can be estimated from the absorption data of the 550 nm absorption band. In FIG. 18, these comparisons are made, and the half-life is 2 μs in Example 2, which is much longer than 0.5 μs in Comparative Example 1. Similar results were obtained for the titanium oxide mesocrystals of Examples 3 to 4 other than Example 2 (FIG. 20). On the other hand, the crystal of Comparative Example 2 had a very weak transient signal and a very short half-life (FIG. 21).

Test Example 10: Single-particle fluorescence spectroscopic light Clear photoluminescence is obtained by recombination of electrons and holes generated. Single particle fluorescence spectroscopy was performed to evaluate the photoluminescence properties of titanium oxide. Fluorescence spectroscopy is a powerful technique that can observe surface reactions with high spatial resolution.

  Single particle fluorescence images and emission decay curves were measured with a confocal scanning microscope system (PicoQuant, Micro Time 200) equipped with an Olympus IX71 inverted fluorescence microscope. Samples were passed through an oil immersion objective (Olympus, UPLSAPO 100XO; 1.40 NA, 100 ×), a 380 nm circularly polarized pulse laser (Spectra-Physics, MAI TAI HTS with automatic frequency doubler) controlled by a PDL-800B driver (PicoQuant) -W, Inspire Blue TAST-W; 0.8 MHz repetition rate, 10 μW excitation power). All experiments were performed at room temperature.

  As shown in FIG. 22, in the titanium oxide mesocrystal of Example 2, broad light emission due to the charge trapped on the surface at 450 to 600 nm was obtained at both the center and the end of the crystal.

  In addition, as a result of evaluating the emission decay curve at different locations, the intensity weighted average decay lifetime was 5.9 ns at the center and 7.1 ns at the end in the titanium oxide mesocrystal of Example 2. Moreover, about Example 4, the center part was 6.6 ns and the edge part was 15.2 ns. In contrast, Comparative Example 1 was about 2.0 ns. As described above, in the titanium oxide mesocrystal of the present invention, charge recombination occurs later, so that charge separation can be promoted.

Test Example 11: Current Measurement AFM Measurement In order to evaluate the photoinduced charge separation characteristics, current measurement AFM measurement with a UV light source was performed.

  AFM images and IV curves were obtained with an MFP-3D atomic force microscope (Asylum Research, Santa Barbara, CA) equipped with an ORCA module (Asylum Research, Model 59) attached to an Olympus IX71 inverted fluorescence microscope. . All current measurements were performed using Rocky Mountain Nanotechnology, LLC solid platinum cantilevers (25Pt300B; tip radius less than 20 nm). The electron transport properties were evaluated from an IV curve recorded at points marked on the sample surface with or without UV irradiation. 365 nm light from an LED (OPTO-LINE, MS-LED-365) that has passed through a two-color beam splitter (Olympus, DM410) is a neutral density filter (ND filter; Olympus) and an objective lens (Olympus, UPLSAPO 100XO; 1.40). The sample was irradiated after passing through NA, 100 ×).

  In the titanium oxide mesocrystal of Example 2, when UV irradiation was not performed, a current response to the applied voltage was not observed, but a clear photocurrent response was observed by UV irradiation (FIG. 23).

In addition, as a result of measuring IV curves for the samples of Examples 2 and 4 and Comparative Example 1, Example 2 showed a more remarkable current response to UV irradiation of the same intensity. It was found that the UV irradiation intensity up to 5 nA may be smaller in Example 2 (FIG. 24). Specifically, when UV of 1.9 mW / cm ² is irradiated, the photocurrent response is about 1.0 nA in Example 2, about 2.0 nA in Example 4, and about 0.2 in Comparative Example 1. It was 5 nA.

Furthermore, as a result of measuring IV curves of various samples, it was suggested that the photocurrent response hardly changes depending on the width of the titanium oxide mesocrystal, but varies greatly depending on the thickness (FIG. 25). The photoconductivity obtained from the relationship between the IV value and the thickness of the titanium oxide mesocrystal was 1.9 × 10 ⁻² Ω ⁻¹ m ⁻¹ .

  Further, when the current decay of the crystals of Examples 2 and 4 and Comparative Example 1 after the UV irradiation was stopped was measured, Example 2 was larger than 20 minutes, Example 4 was about 1 minute, and Comparative Example 1 was about 1 minute. 5 minutes, and the titanium oxide mesocrystal of Example 2 was most maintained.

From the results of Test Examples 9 to 11 described above, the rate of charge recombination of electrons and holes can be reduced, and the electron conductivity can be improved. The utility to the electrode material of a battery is suggested.

Example 8: Titanium oxide mesocrystal-ITO film
  First, TiF₄, NH₄NO₃, NH₄Precursor aqueous solution (composition is molar ratio, TiF ₄: NH₄NO₃: NH₄: H₂A liquid layer composed of O = 1: 6.6: 4: 117) was formed on the ITO film substrate. Specifically, the precursor aqueous solution was dropped on the substrate. The thickness of the liquid layer was set to 1 mm or less. The liquid layer formed on the ITO substrate was sintered at 400 ° C. for 2 hours in an air atmosphere to form titanium oxide mesocrystals on the silicon substrate. Thereafter, the titanium oxide mesocrystal of Example 1 was formed on the ITO substrate by baking at 400 ° C. for 8 hours in an oxygen atmosphere.

  About the obtained titanium oxide mesocrystal, the result of having observed with the electron microscope (SEM) similarly to Test Example 4 is shown in FIG.

Claims (15)

A titanium oxide mesocrystal having a ratio of an average width to an average thickness (average width / average thickness) of 10 to 100 and a specific surface area of 10 m ² / g or more.
(Here, the average width refers to the average value of the sides of the square or rectangle when the surface of the crystal is considered to be a square or rectangle, and the average thickness is the case of a plate crystal. (The average value of the thickness is the average value of the thickness when it is regarded as a plate-like crystal if it is not a plate shape.) The surface has a [001] plane as the main crystal plane, the ratio of the average width of the surface to the average thickness (average width / average thickness) is 10 to 100, and the specific surface area is 10 m. ² / G or more of titanium oxide mesocrystal. The titanium oxide mesocrystal of Claim 1 or 2 whose average width | variety is 2-8 micrometers. The titanium oxide mesocrystal in any one of Claims 1-3 whose average thickness is 50-300 nm. The titanium oxide mesocrystal according to any one of claims 1 to 4 , which is an aggregate of anatase-type titanium oxide nanocrystals. The titanium oxide mesocrystal according to any one of claims 1 to 5 , which is a single crystal. The titanium oxide mesocrystal according to any one of claims 1 to 6 , which has a plate shape. TiF ₄ , NH ₄ NO ₃ , NH ₄ F and water are contained, the content ratio of TiF ₄ and NH ₄ NO ₃ is 1: 4 to 15 (molar ratio), and TiF ₄ and NH ₄ F The titanium oxide mesocrystal according to any one of claims 1 to 7 , comprising a step of firing a precursor aqueous solution having a content ratio of 1: 1 to 9 (molar ratio) under a condition of 400 to 700 ° C in an oxygen atmosphere. Manufacturing method. TiF ₄ , NH ₄ NO ₃ , NH ₄ F and water are contained, the content ratio of TiF ₄ and NH ₄ NO ₃ is 1: 4 to 15 (molar ratio), and TiF ₄ and NH ₄ F A precursor aqueous solution having a content ratio of 1: 1 to 9 (molar ratio) is fired under a condition of 250 to 700 ° C. in an air atmosphere or an oxygen atmosphere, and then fired under a condition of 400 to 700 ° C. in an oxygen atmosphere. It comprises a method of manufacturing an oxide Chitanmeso crystal according to any one of claims 1-7. The method for producing a titanium oxide mesocrystal according to claim 8 or 9, wherein a liquid layer composed of the precursor aqueous solution is formed on a substrate and then fired. A composite material in which a layer made of the titanium oxide mesocrystal according to any one of claims 1 to 7 is formed on a surface of a substrate . The composite material according to claim 11, wherein the substrate is tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO). NH ₄ TiOF ₃ crystal having a ratio of average width to average thickness (average width / average thickness) of 10 to 100.
(Here, the average width refers to the average value of the sides of the square or rectangle when the surface of the crystal is considered to be a square or rectangle, and the average thickness is the case of a plate crystal. (The average value of the thickness is the average value of the thickness when it is regarded as a plate-like crystal if it is not a plate shape.) TiF ₄ , NH ₄ NO ₃ , NH ₄ F and water are contained, the content ratio of TiF ₄ and NH ₄ NO ₃ is 1: 4 to 15 (molar ratio), and TiF ₄ and NH ₄ F The method for producing an NH ₄ TiOF ₃ crystal according to claim 13 , comprising a step of firing an aqueous precursor solution having a content ratio of 1: 1 to 9 (molar ratio) in an air atmosphere at 250 to 300 ° C. . The NH according to claim 14, wherein a liquid layer comprising the precursor aqueous solution is formed on a substrate and then fired. ₄ TiOF ₃ Crystal production method.

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