JP5443838B2 - Tungsten oxide fine particles and method for producing the same - Google Patents

Description

  The present invention relates to monoclinic tungsten oxide fine particles having a controllable particle size, excellent agglomeration, excellent dispersibility, and uniform particle shape, and a method for producing the same. The tungsten oxide fine particles of the present invention can be suitably used for photocatalysts, electrochromic materials, alkali metal cation intercalation materials, positive or negative electrode active material coating materials, and various ceramic materials.

  Since tungsten oxide used as a photocatalyst or an electrochromic material is often used by being coated and supported with a binder or dispersed in a resin or the like, it is desired to be a powder having excellent dispersibility. In addition, in order to sufficiently exhibit the catalytic activity of tungsten oxide and to be suitably used as an electrochromic material, a particle size of submicron or less is desired, and particles having a small particle size and excellent dispersibility are required. ing.

Since tungsten oxide has a narrow band gap of 2.5 eV, it is promising as a visible light responsive photocatalytic material that is photoexcited in the visible light region that occupies about 50% of sunlight. Compared with TiO ₂ having a band gap of 3.0 to 3.2 eV, which is a photocatalyst, the redox power tends to be weak.

  Therefore, when using tungsten oxide as a photocatalyst, it is necessary to control the light absorption property, the oxidation-reduction reaction rate, and the electron-hole recombination rate in order to increase the photocatalytic activity.

  Specifically, the redox reaction rate can be increased by increasing the contact area between the electron-hole pair generated by photoexcitation and the reactive group. That is, tungsten oxide may be manufactured as fine particles having a specific surface area as large as possible. In addition, when the electron-hole pair generated by photoexcitation is recombined before coming into contact with the reactive group, it is clear that the oxidation-reduction reaction efficiency with respect to the light irradiation amount is lowered. In general, recombination of electron-hole pairs is said to occur due to lattice defects in the photocatalyst, and by producing highly crystalline particles, lattice defects can be reduced and an efficient photocatalytic material can be obtained. . In other words, high catalytic activity can be realized by producing tungsten oxide as fine particles with as small a particle size as possible and high crystallinity.

  Known methods for obtaining tungsten oxide particles as described above include a method of firing tungstic acid, a method of treating metallic tungsten or tungsten oxide with high-temperature plasma (see Patent Document 1), and the like. When tungsten oxide is used for a photocatalyst, its crystal structure is desirably monoclinic. However, when tungsten oxide is obtained from these existing methods, it is generated by tungstate or high-temperature plasma treatment. It is necessary to heat-treat the tungsten oxide-metal tungsten composite at 300 ° C. or higher. However, when the heat treatment is performed in the solid phase in this way, the particle size after the heat treatment becomes large, and thus a pulverization step is necessary to fully exhibit the functionality. It changes to oblique crystals.

  In addition, a method of synthesizing tungsten oxide fine particles in a liquid phase and preparing it as a dispersion is also known. According to Patent Document 2, tungstic acid or ammonium tungstate is dissolved in hydrogen peroxide and heated at 90 ° C. or higher. After that, it is described that ascorbic acid is added and further heated to obtain particles having an average particle diameter of 300 nm or less. By using this method, there is an advantage that a stable tungsten trioxide fine particle sol can be prepared. However, the produced fine particles are amorphous, and fine particles with high crystallinity cannot be obtained.

  In Patent Document 3, tungsten hexachloride particles of 200 nm or less are obtained by dissolving tungsten hexachloride in alcohol, evaporating the solvent or heating and refluxing, evaporating the solvent, and then firing at 100 to 500 ° C. Have been described. By using this method, there is an advantage that particles having good dispersibility can be produced at low cost. However, it is tungsten oxide hydrate that is produced in the stage where tungsten hexachloride is dissolved in alcohol and the solvent is evaporated or heated and refluxed, and then the solvent is evaporated, and heat treatment is required for dehydration. For this reason, when it is going to heat-process and obtain tungsten oxide with sufficient crystallinity, a particle diameter will become large.

In Patent Document 4, layered tungstic acid is prepared, and this and a basic compound are mixed in a solvent to simultaneously remove hydrogen contained between layers and insert a basic compound between layers. It is described that flaky tungsten oxide microparticles can be obtained by peeling off. In this method, a quaternary ammonium salt is used as a basic compound in an amount of 0.9 to 1.5 equivalents with respect to tungsten (W), and a large amount of ammonia is processed when producing flaky tungsten oxide fine particles. There is a need. In addition, although the particles obtained by this method are in the form of flakes, since tungsten oxide is generated by dehydrogenation from tungstic acid such as H ₂ W ₂ O ₇ and H ₂ WO ₄ , the basic compound is charged. In a range where the amount is small, tungstic acid is mixed as an impurity phase, and in order to obtain particles having a monoclinic crystal structure and high crystallinity even at an appropriate amount of the basic compound, it is 200 ° C. to 800 ° C. The baking process is required. However, when an operation for increasing crystallinity is performed by heat treatment, the particle diameter grows large.

Japanese Patent No. 2629986 JP 2004-131346 A JP 2008-134658 A JP 2009-051687 A

  The present invention has been made in view of the circumstances as described above, and provides a monoclinic tungsten oxide fine particle having a controllable particle diameter, excellent agglomeration, excellent dispersibility, and uniform particle shape, and a method for producing the same. The purpose is to provide.

  As a result of diligent research, the present inventors have found that tungsten oxide fine particles having a uniform plate-like structure obtained by hydrothermal reaction of a tungsten source at a high temperature achieve the above object.

The present invention has been made on the basis of the above-mentioned findings. The average particle size is 1 to 500 nm, the relative standard deviation of the particle size distribution is 50.0% or less, the crystal structure is a monoclinic single phase , and the crystallite The present invention provides a tungsten oxide fine particle having a plate-like particle shape, wherein a value obtained by dividing a size by an average particle diameter is 0.40 or more.

Further, in the present invention, as a preferable production method for obtaining tungsten oxide fine particles having the plate-like particle shape, a solution or slurry containing a tungsten compound having a pH of less than 7 is subjected to a hydrothermal reaction under conditions of 200 ° C. or more and 300 ° C. or less. The manufacturing method of the tungsten oxide microparticles | fine-particles characterized by these is provided.

  The tungsten oxide fine particles of the present invention simultaneously achieve high homogenization, high dispersion, high crystallization, and fine particle formation, which were the problems of conventional tungsten oxide fine particles. For example, photocatalysts, electrochromic materials, alkali metals It is suitable as a cation intercalation material, a positive or negative electrode active material coating material, and various ceramic materials.

FIG. 1 is a TEM image (magnification: 60,000 times) showing the particle morphology of the plate-like tungsten oxide fine particles obtained in Example 1. FIG. 2 is a TEM image (magnification: 60,000 times) showing the particle morphology of the tungsten oxide fine particles obtained in Comparative Example 5. FIG. 3 is a TEM image (magnification: 60,000 times) showing the particle morphology in the tungsten oxide fine particle sol obtained in Comparative Example 8. FIG. 4 is a particle size distribution obtained by a laser diffraction / scattering method showing the dispersion state of the tungsten oxide fine particles obtained in Example 1, Comparative Example 5 and Comparative Example 8. FIG. 5 is an X-ray diffraction pattern obtained by comparing the crystallinity of the tungsten oxide fine particles obtained in Example 1 and Comparative Example 5.

  Hereinafter, although the tungsten oxide microparticles | fine-particles of this invention and its manufacturing method are described based on preferable embodiment, this invention is not restrict | limited to these description.

  The inventors of the present invention can control the particle size by hydrothermal reaction of a tungsten source at a temperature of 200 ° C. or higher, have no aggregation, have excellent dispersibility, and have a uniform monomorphic plate shape. It has been found that tungsten oxide fine particles can be obtained. The reason is not clear, but it is thought as follows. In the hydrothermal reaction of tungsten oxide microparticles, after primary particles are formed, particle growth occurs due to precipitation of solute on the surface of primary particles and reaggregation mechanism of primary particles. For this reason, by increasing the temperature of the hydrothermal reaction, precipitation of the solute is promoted and the particles have excellent crystallinity. In addition, in the case of a hydrothermal reaction, such a growth mechanism proceeds substantially uniformly throughout the reaction system, so that the particle shape after the reaction is uniform and the particle size distribution becomes sharp. As a result, reaggregation of the obtained tungsten oxide fine particles is relaxed, and dispersibility is improved.

  The tungsten oxide fine particles of the present invention have an average particle diameter of 1 to 500 nm, preferably 30 to 300 nm, and more preferably 50 to 200 nm. The average particle diameter of the tungsten oxide fine particles was determined by measuring the particle diameter of 200 or more arbitrary particles from a TEM image with a magnification of 60,000 based on observation with a transmission electron microscope (TEM). Ask. When the average particle diameter is smaller than 1 nm, the cohesive force of the particles becomes strong and the particles do not have excellent dispersibility. Further, when used as a photocatalyst, the band gap is narrowed due to the quantum size effect, and the redox power of the catalyst is weakened. Further, when the average particle diameter exceeds 500 nm, the dispersibility is excellent, but the particle packing property and the coating efficiency at the time of coating are deteriorated, and the specific surface area is also reduced, so that the photocatalytic efficiency is poor.

  In addition, the tungsten oxide fine particles of the present invention have a relative standard deviation of the particle size distribution determined by TEM of 50.0% or less, and preferably 45.0% or less. By taking such a narrow particle size distribution, it is possible to improve the filling properties when dispersed in the resin, the coating efficiency during coating, and the photocatalytic efficiency. If the relative standard deviation exceeds 50.0%, the particle filling property, coating efficiency, and photocatalytic efficiency are lowered, which is not preferable.

  Further, the crystal structure of the tungsten oxide fine particles of the present invention is monoclinic, and the presence of a heterogeneous phase is not desirable. In addition, the tungsten oxide fine particles of the present invention have a value obtained by dividing the crystallite size by the average particle diameter of 0.40 or more, and preferably 0.50 to 1.00. This is because when there is a heterogeneous phase and when the value obtained by dividing the crystallite size by the average particle size is less than 0.40, there are many interfaces and lattice defects in the fine particles, and the electron − generated by photoexcitation − This is because the positive electrode pair recombines in the interface and lattice defects, resulting in poor photocatalytic efficiency.

  The tungsten oxide fine particles of the present invention have a value obtained by dividing the median diameter obtained by the particle size distribution measurement by the average particle diameter of 1.5 or less, preferably 0.9 to 1.4, and the coefficient of variation by the particle size distribution measurement is 35. 0.0% or less, preferably 10.0 to 30.0%. When the value obtained by dividing the median diameter by the average particle diameter exceeds 1.5, or when the coefficient of variation by particle size distribution measurement exceeds 35.0%, the particles are aggregated when dispersed in the resin. There is a possibility that characteristics such as catalyst efficiency, filling properties and optical characteristics that can be expected from the size cannot be satisfactorily exhibited.

The tungsten oxide fine particles of the present invention have a good crystallinity as monoclinic fine particles by hydrothermal reaction of a solution or slurry containing a tungsten compound at 200 ° C. or higher and 300 ° C. or lower , preferably 250 to 300 ° C. The relative standard deviation of the diameter distribution can be manufactured within 50.0%. When the hydrothermal reaction is carried out at less than 200 ° C., the reactivity is lowered, so that different phases such as raw material particles tend to be mixed in the fine particles after the reaction.

  The hydrothermal reaction is desirably performed under a pressure of 0.1 MPa or more, more preferably 0.3 MPa to 30 MPa. By carrying out a hydrothermal reaction under such a pressure range, it is possible to produce tungsten oxide fine particles having a monoclinic crystal structure with high purity and uniformity. When the hydrothermal reaction is carried out at a pressure of less than 0.1 MPa, the reactivity decreases, so that not only the different phases such as the raw material particles are mixed in the fine particles after the reaction, but also the uniformity of the shape and particle diameter of the obtained particles. It tends to decline.

The solution or slurry containing the tungsten compound is prepared by dissolving or dispersing the tungsten compound in a solvent such as water. The concentration of the tungsten compound in the solvent is not particularly limited, but is preferably about 0.1 to 0.5 mol / L in terms of tungsten element. When the concentration of the tungsten compound is less than 0.1 mol / L in terms of tungsten element, it is not preferable from the viewpoint of productivity, and when it exceeds 0.5 mol / L, the reaction tends to proceed non-uniformly. The tungsten compound is not particularly limited, and examples thereof include tungstic acid and its salts, oxides, halides, oxyhalides, silicides, and phosphides.
Examples of the solvent include water, monohydric alcohols such as methanol, ethanol, propanol, and butanol, and polyols such as glycol.

  The pH of the solution or slurry during the hydrothermal reaction is less than 7, preferably pH 3.0 to 6.5. When the pH exceeds 7, the tungsten oxide fine particles after the hydrothermal reaction may contain crystal phases other than monoclinic crystals such as triclinic crystals.

  In the hydrothermal reaction, for the purpose of improving the catalyst efficiency, salts such as Ti, V, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Ta, and Pb are converted into each element in the above solution or slurry. In addition, 0.01 to 5 mol% may be added per tungsten. Although the salt to add is not specifically limited, For example, a chloride, nitrate, a sulfate, a hydroxide etc. are mentioned.

  The tungsten oxide fine particles of the present invention obtained as described above have an average particle size of 1 to 500 nm, a relative standard deviation of the particle size distribution is 50.0% or less, and the crystallite size is the average particle size. Single crystal fine particles having a divided value of 0.40 or more are obtained. The crystal structure is monoclinic and the shape of the fine particles is plate-like.

  Since the tungsten oxide fine particles of the present invention are plate-like and substantially single crystal particles, for example, when the tungsten oxide fine particles of the present invention are applied as a photocatalyst, they can be arranged with good filling properties on the coated surface. In addition, since the crystal structure is a monoclinic single phase and the composition of the fine particles is almost a single crystal, compared to the case where a crystal structure other than the monoclinic crystal is present in the particles and in the fine particle powder structure, Since the movement of electrons and holes is smooth, the probability of arrival of electrons and holes on the surface is improved, and efficient catalytic properties are exhibited.

  Further, the tungsten oxide fine particles of the present invention are produced by particle growth by nucleation and precipitation of solute on the particle surface in a solvent under high temperature and high pressure conditions, and thus have a relatively sharp particle size distribution. Therefore, in combination with the plate-like particle shape, particles having excellent dispersibility having a value obtained by dividing the median diameter obtained by particle size distribution measurement by the average particle diameter is 1.5 or less, and the coefficient of variation by particle size distribution measurement is 35.0% or less. It becomes.

  Therefore, when the tungsten oxide fine particles of the present invention are applied or dispersed in a resin, reaggregation of the fine particles can be suppressed to obtain a uniform material, and the transmittance required for photocatalysts and electrochromic materials The optical characteristics such as can be satisfied.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not restrict | limited to these Examples etc.

(Examples 1-7 and Comparative Examples 1-4)
[Production of tungsten oxide fine particles]
Using tungstic acid (H ₂ WO ₄ ) as a tungsten source, 700 ml of a tungstic acid slurry (solvent: water) having a tungstic acid concentration (converted to tungsten element) shown in Table 1 was prepared. Depending on the comparative example, sodium hydroxide (NaOH) or ammonium hydrogen carbonate (NH ₄ HCO ₃ ) was added to the tungstic acid slurry as shown in Table 1 for the purpose of adjusting the pH. The pH of the slurry is listed in Table 1. While the prepared tungstic acid slurry was stirred in an autoclave, a hydrothermal reaction was performed under the conditions shown in Table 1. After completion of the reaction, the reaction mixture was cooled to room temperature, and the product was filtered, washed with water, and dried to obtain the target tungsten oxide fine particles.

(Comparative Examples 5-7)
[Production of tungsten oxide fine particles by solid phase method]
Using tungstic acid as a tungsten source, firing was performed under the conditions shown in Table 2. Firing was performed in the air using an electric furnace. After the firing, the electric furnace was naturally cooled to room temperature, the target product was taken out, and the target product was crushed with an agate mortar.

(Comparative Example 8)
[Production of tungsten oxide fine particle sol by liquid phase method]
100 g of tungstic acid was added to 466.8 g of 35% aqueous hydrogen peroxide and stirred for 2 hours, then heated to 90 ° C. and stirred for 1 hour. Thereafter, 21.1 g of ascorbic acid was added and further stirred at 90 ° C. for 1 hour to obtain a yellow sol.

  About the tungsten oxide fine particles obtained in Examples 1 to 7 and Comparative Examples 1 to 7, X-ray diffraction (qualitative analysis, crystallite size analysis), specific surface area, average particle diameter, particle shape, particle size distribution (median diameter and variation) The coefficient was evaluated according to the following (1) to (4).

(1) X-ray diffraction (qualitative analysis, crystallite size analysis)
The crystallite size of the monoclinic crystal was determined by qualitative analysis and Pawley method using an X-ray diffractometer (D8 ADVANCE / V) manufactured by Bruker AXS. In Tables 1 and 2, monoclinic tungsten oxide is m-WO ₃ , triclinic tungsten oxide is tri-WO ₃ , hexagonal tungsten oxide and tungsten oxide hydrate are h-WO _3, and Indicated.

(2) Specific surface area The specific surface area was measured with a fully automatic BET specific surface area measuring device (Macsorb HM Model-1210) manufactured by Mountec.

(3) Measurement of average particle diameter and observation of particle shape Using a transmission electron microscope (TEM), the particle diameter of 200 or more arbitrary particles was measured from a TEM image with a magnification of 60,000 times, and the average The average particle diameter was determined from the value. The particle shape was also observed with a TEM image.

(4) Particle size distribution (median diameter and coefficient of variation)
Tungsten oxide fine particles (20 mg) were added to a 0.2 wt% aqueous sodium hexametaphosphate solution (30 ml) and dispersed with a homogenizer (at 360 to 600 W for 30 to 90 seconds). The dispersion was measured with a Horiba laser diffraction / scattering particle size distribution measuring device (LA-950) and a dynamic light scattering particle size distribution measuring device (LB-550) manufactured by Horiba. The coefficient of variation of the diameter and particle diameter was determined.

  Tables 1, 2 and 3 show the results of the above evaluation, and the relative standard deviation, crystallite size / average particle size, and median size / average particle size obtained from the results.

  From the results of Examples 1 to 7, it was proved that monoclinic single-phase tungsten oxide fine particles can be obtained by performing a hydrothermal reaction at a high temperature of 200 ° C. or higher regardless of the concentration of the raw material solution. The tungsten oxide fine particles obtained in Examples 1 to 7 were plate-like. As a typical example of the shape, a TEM image of the tungsten oxide fine particles obtained in Example 1 is shown in FIG.

  The tungsten oxide fine particles obtained in Examples 1 to 7 had a value obtained by dividing the crystallite size by the average particle size from the average particle size obtained from the TEM image and the crystallite size obtained from the X-ray diffraction was 0.40 or more. It was confirmed that fine particles close to a single crystal were obtained. This means that there are few interfaces per particle, and it can be said that the electron-hole pairs generated by photoexcitation act efficiently on the reactive group.

On the other hand, from the results of Comparative Examples 1 to 4, in the case of a hydrothermal reaction at a temperature lower than 200 ° C. (Comparative Example 1), tungsten oxide and tungsten oxide hydrate are mixed and the reaction does not proceed sufficiently. Can be confirmed. In addition, as a result of pH adjustment by adding NaOH or NH ₄ HCO ₃ , when a raw material solution having a pH of 7 or higher is subjected to a hydrothermal reaction (Comparative Examples 2 to 4), not only a crystal phase such as triclinic crystal is mixed. It can be confirmed that oxide particles may not be obtained.

  In addition, from the results of Comparative Examples 5 to 7, a monoclinic tungsten oxide single phase was obtained in the firing reaction by the solid phase method using tungstic acid, but the crystallite size obtained from X-ray diffraction was from TEM. As can be seen from the TEM image of the tungsten oxide fine particles obtained in Comparative Example 5 shown in FIG. 2, it can be confirmed that the particle shape is poor in uniformity as compared with the obtained average particle diameter.

  In addition, the tungsten oxide fine particle sol obtained in Comparative Example 8 had a yellow color and was relatively stable, but as can be seen from the TEM image shown in FIG. 3, the particle shape in the sol was amorphous. The particle size was about 300 to 500 nm.

  As shown in Table 3, the median diameters obtained by the dynamic light scattering method of the tungsten oxide fine particles obtained in Examples 1 to 7 are 1.5 times or less of the respective average particle diameters and are very dispersed. It can be confirmed that the powder has excellent properties.

  The tungsten oxide fine particles obtained by the solid phase method obtained in Comparative Examples 5 to 7 generally have a tendency to aggregate, and as shown in Table 3, all the median diameters are 3 μm or more. Further, as a result of measuring the median diameter of the particles in the tungsten oxide fine particle sol obtained in Comparative Example 8 by the laser diffraction / scattering method, it has a very wide distribution as shown in FIG. The diameter is also strongly agglomerated at 12.5 μm.

  Further, from the X-ray diffraction pattern shown in FIG. 5, the tungsten oxide fine particles obtained in Example 1 are superior in crystallinity as compared with the tungsten oxide fine particles obtained by the solid phase method obtained in Comparative Example 5. Was confirmed.

Claims (4)

The average particle size is 1 to 500 nm, the relative standard deviation of the particle size distribution is 50.0% or less, the crystal structure is a monoclinic single phase , and the value obtained by dividing the crystallite size by the average particle size is 0.40 or more A tungsten oxide fine particle having a plate-like particle shape characterized by   The fine particles of tungsten oxide according to claim 1, wherein the average particle size is 50 to 200 nm.   The tungsten oxide fine particles according to claim 1 or 2, wherein a value obtained by dividing a median diameter obtained by particle size distribution measurement by an average particle diameter is 1.5 or less, and a coefficient of variation by particle size distribution measurement is 35.0% or less. 2. The method for producing tungsten oxide fine particles according to claim 1, wherein a solution or slurry containing a tungsten compound and having a pH of less than 7 is hydrothermally reacted under conditions of 200 ° C. or more and 300 ° C. or less. A method for producing fine particles.