JP2009506975A - Method for synthesizing rutile single-phase titanium oxide with nanostructures - Google Patents

Abstract

  A method for synthesizing nanostructured titanium oxide, at least mostly consisting of a rutile phase, comprising a step of providing a nanostructured template, coating the template with a colloidal phase containing titanium oxide, heat treating the titanium oxide coated template, A synthesis method comprising a step of forming a crystal phase at least mostly consisting of an anatase phase, and a step of further heat-treating the titanium oxide-coated template to convert the anatase phase into a rutile phase.

Description

  The present invention relates to a method for synthesizing nanostructured titanium oxide, nanostructured titanium oxide synthesized by the synthesis method, and a colloidal phase used for producing the nanostructure.

Titania (titanium oxide, TiO ₂ ) is one of the most important transition metal oxides in the industry, and is used as a color pigment or a photocatalyst (Non-Patent Documents 1 to 3). World production is 4Mt per year.

Titania has a band gap of 3.2 eV, and therefore can absorb light with a wavelength of 320 nm or less, activates water molecules, has a strong oxidizing power, and a strongly reducing oxygen ion. (O ₂ ⁻ ) is generated.

Therefore, titania can not only oxidatively decompose ethylene but also decompose harmful organic compounds (Non-patent Document 4), and reduce harmful gases (NO _x , SO _x ) in the environment. it can. Furthermore, in recent years, it has also been clarified that NASA can sterilize killing anthrax spores and bacteria (Non-patent Document 5).

The excellent photocatalytic activity of titania is also used in so-called dye-sensitized TiO ₂ solar cells (Non-patent Documents 6 and 7). Particularly in this application, titania particles have a high specific surface area in order to adsorb more dye molecules (mostly ruthenium complexes) on the nanostructure surface and increase the photolysis efficiency of the solar cell. It is better to do.

Over the last few years, titania has received more attention as a sensor material for H ₂ , O ₂ , CO _x , and other gases (Non-Patent Documents 8-10). The strongly reduced titania material (Magneli phase (Non-Patent Documents 11 and 12)) has a very high electrical conductivity, corrosion resistance, and a high overvoltage for gas generation reaction, and thus can also be used as an electrode material. Furthermore, titania has been suitably used as a catalyst and a catalyst support for many years (Non-patent Documents 13 and 14). In most of these applications, it is important that titania has a high ability to form a substoichiometric phase and oxygen vacancies (Non-Patent Document 15). This type of configuration and the impact of the configuration on catalyst performance and electrical performance have been intensively studied over the past years (Non-Patent Documents 16-22).

In view of the immense importance of TiO _{2 in} the field of photocatalysts, environmental purification catalysts, and electrochemistry, attention has been focused on producing the materials in nanoscale anisotropic form.

According to Grimes, by using titania (TiO ₂ -NT) as a nanotube coated with palladium particles (hereinafter, the nanotube may be referred to as “NT”), it detects minute hydrogen concentration in the atmosphere. (Non-Patent Document 23), which is suitably applied to fuel cell technology. In addition, a sensor having a skeleton of TiO ₂ -NT can be operated at room temperature and exhibits a long-term service life (Non-Patent Document 24). In addition, there is a group that studies the applicability of TiO ₂ -NT to dye-sensitized solar cells, and extremely high photolysis efficiency has been reported (Non-patent Document 25). All of these studies was to evaluate the differences in physical and chemical properties between the TiO 2 _-NT and TiO ₂ particles.

It has been proposed that TiO ₂ -NT can also be used as a precursor for superconducting titanate NT (Non-patent Document 23). Superconducting titanate NT has two main crystal phases, anatase and rutile. Today, anatase is not only inexpensive, but is also available in a more finely divided form. In the field of photocatalyst and environmental purification catalyst, it is in the stage of material selection regardless of the fact that rutile can be superior in performance. In particular, rutile absorbs light over a wider wavelength than anatase and is whiter than anatase.

In the first reported method for synthesizing titania, a polymer mold was used, and titanium oxide was electrochemically disposed on the mold (Non-patent Document 26). The most common and simple synthesis method is hydrothermal treatment of TiO ₂ , which was first reported by Kasuga et al. (Non-patent Document 27) and later improved by various research groups (Non-patent Documents 28-31). ). However, the resulting TiO ₂ nanotubes had a trititanate structure rather than anatase or rutile.

There is also a group producing TiO ₂ -NT using porous alumina (Non-Patent Documents 32-34) or polymer yarns or fibers (Non-Patent Document 35) as a template. If the template is coated with titanium oxide by the sol-gel method (Non-patent Document 36) and then subjected to heat treatment to remove the template, titania nanotubes mostly composed of anatase type can be obtained.

There is no example of successfully converting a nanotube made of anatase or anatase / rutile mixture to rutile while retaining the structure as a nanotube. According to the work of Sun et al. (Non-patent Document 37) and Wang et al. (Non-patent Document 38), it has been reported that a TiO ₂ coating is successfully formed on a carbon nanotube template. Sun et al. (Non-Patent Document 37) were coated with TiO ₂ (having an anatase phase or an anatase / rutile mixed phase) using multi-walled carbon nanotubes (MWCNTs) and coating by an improved sol-gel method. Nanotubes having a diameter of 300-800 nm were prepared. Sun et al. Also reported successful oxidative removal of carbon nanotubes. However, in these reports, no attempt is made to convert TiO ₂ to rutile.

  Varghese et al. Produced anatase nanotubes by anodizing titanium foil and studied the phase transition of the anatase nanotubes during annealing in an oxygen and argon atmosphere (Non-patent Document 39). According to this, at a temperature of about 620 ° C., the anatase phase was converted to a rutile phase, and at that temperature, the titania nanotube structure was already decomposed, and at best only a string-like substance was left. Furthermore, for such a phase transition, the crystal diameter is extremely important, and anatase crystals having a crystal diameter smaller than 14 nm are extremely stable and are not converted (Non-patent Document 40).

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  So far, nanotubes with only a pure rutile phase have not been produced. Therefore, a method for synthesizing rutile titanium oxide having a nanostructure is required.

According to a first aspect of the present invention, there is provided a method for synthesizing nanostructured titanium oxide, at least mostly consisting of a rutile phase,
a) providing a nanostructured template;
b) coating the template with a colloidal phase containing titanium oxide;
c) After the step b), a first heat treatment is performed on the template coated with titanium oxide to form a crystal phase consisting at least mostly of anatase phase; and d) after the step c), an oxidation is performed. A step of converting the anatase phase to the rutile phase by performing a second heat treatment on the titanium-coated mold;
A synthesis method is provided.

  The product obtained by the above synthesis method preferably consists of at least 90% rutile phase, more preferably at least 95%. In a preferred form, the product consists only of the rutile phase.

  The titanium nanostructure is considered to have a structure determined by the structure of the template. The titanium nanostructure preferably has nanotubes and / or nanoporous solids. The titanium nanostructure preferably has a dimension in at least one direction of 0.5 nm to 1000 nm.

  The mold is preferably a high aspect ratio cylinder or tube. The aspect ratio is particularly preferably between 100 and 500.

  Carbon is preferably added to the template. Further, nitrogen is preferably added to the template. However, the mold may consist of other materials, in particular materials that can be easily removed (for example by oxidation). Materials that can be used include nitrides, sulfides, carbides, or gallium arsenide.

  More preferably, the template has carbon nanotubes, carbon nanofibers, carbon herringbone, carbon onion, or a mixture thereof. Carbon nanotubes may be synthesized or functionalized by oxidation or acid treatment. In particular, it is preferred to use a template that has been functionalized to enhance the adhesion of the titanium oxide coating (eg, having aromatics).

It is particularly preferable that the template has single-walled carbon nanotubes, multi-walled carbon nanotubes, or a mixture thereof.
It is particularly preferable that the template has a specific structure. For example, the template may be a carbon nanotube mat, and the nanotubes may or may not be aligned. In this case, the titanium oxide coating preferably occupies substantially the entire space in the mold. After removing the template, a nanoporous structure of titanium oxide is formed.
However, the template need not have a specific structure. For example, it may be a carbon nanotube powder.

  The mold is preferably provided with a continuous or non-continuous coating material. The coating material is preferably a Group VIB transition metal, a Group VIIIB transition metal, a lanthanoid series metal, an actinide series metal, or a mixture thereof. Examples of coating materials include Ni, Pd, Pt, and Au.

  The template coated in this way may be effective in preparing a metal-supported catalyst having a metal on its inner surface. Examples of the metal include sputtering chemical modification (such as self-assembled monolayers such as thiols, alcohols, and carboxylic acids), wet impregnation, sputtering, atomic layer deposition, or reductive deposition of metal precursors. It can be arranged on the mold by any known technique. According to the present invention, the mold thus improved is coated with titanium oxide. When the template is removed, metal coated particles are placed on the internal surface of the titanium oxide nanostructure.

  Components used to form the colloidal phase include inorganic titanium precursors such as titanium chloride, organic titanium precursors such as titanium isopropoxide (TTIP) and tetrabutyloxytitanate (TBOT), or a mixture thereof. Is preferably included. The components used to form the colloidal phase include water, alcohol (eg ethanol or benzyl alcohol), acetone, particle shaper (eg benzyl alcohol or acetylacetone), surfactant (eg sodium dodecylbenzene sulfate (SDBS) )), Or a mixture thereof.

In a preferred form, when the components used to form the colloidal phase are mixed, a sol (a colloidal mixture of solid and liquid) containing titanium oxide particles is formed to produce a template. Thereafter, a sol-gel transition is caused. This is done, for example, by adding water, acid (eg acetic acid) or alkali (eg ammonium hydroxide) to the colloidal phase. Sol-gel transition may be caused by neutralization.
The properties of the gel provide good adhesion to the mold. Preferred sols will be described in more detail in the following sols 1, 2 and 3.

  The coating method is particularly suitable for dip coating (particularly suitable for molds having a specific structure such as a carbon nanotube mat), or stirring coating (particularly for uniform molds such as carbon nanotube powder). .). The colloidal phase is preferably attached to the mold in the temperature range of 0 ° C to 90 ° C.

The molar ratio of water and titanium (atomic conversion) is preferably in the range of 0.1: 1 to 60: 1.
CNT: It is preferred ratio of TiO ₂ is 1% to 50%.
The coating is preferably dried at a temperature of room temperature (20 ° C.) to 150 ° C. in an oxidizing atmosphere or an inert atmosphere, and the drying time is preferably 10 minutes to 20 hours (becomes a xerogel). Furthermore, drying may be performed under vacuum or in a supercritical solvent (becomes an airgel), or may be performed by freeze-drying (becomes a cooled gel).

The first heat treatment is preferably performed in an oxidizing atmosphere, for example, in an air or oxygen atmosphere (calcination). The first heat treatment is preferably performed under atmospheric pressure. The humidity should be controlled during the first heat treatment as it can affect the gelation of the dip-coated material.
The first heat treatment is preferably performed at a temperature between 200 ° C and 500 ° C. The temperature of the first heat treatment depends on the composition of the coating sol. When TBOT and benzyl alcohol are contained in the sol (sol 3), 400 ° C. to 500 ° C. is required as the first heat treatment temperature.
The first heat treatment is preferably performed for 1 minute to 10 hours. The first heat treatment time depends on the type of coating process. In the case of dip coating, the first heat treatment time is suitably from 1 minute to 10 minutes. In the case of stirring coating, the first heat treatment time is suitably 1 minute to 10 hours.
Usually, after the first heat treatment, the crystal phase becomes a 100% anatase phase.

The second heat treatment is preferably performed at a temperature between 550 ° C and 950 ° C. This controls the surface morphology.
The second heat treatment is preferably performed in a non-oxidizing atmosphere. The non-oxidizing atmosphere in the heat treatment is more preferably an inert atmosphere, a non-inert atmosphere, or a mixed atmosphere of an inert atmosphere and a non-inert atmosphere. The atmosphere preferably contains nitrogen, an inert gas, or a mixture thereof. These are inert gases. On the other hand, it is preferable that the atmosphere contains hydrogen. This is a non-inert (reducing) gas.
The second heat treatment may be continuous or discontinuous.
The second heat treatment time is preferably 30 minutes to 20 hours, and more preferably 30 minutes to 4 hours.
The second heat treatment is preferably performed under atmospheric pressure. However, it may be performed under other pressure (for example, partial vacuum).

In the synthesis method, after the step d),
e) removing the mold;
Is preferably provided. The step e) is preferably performed by oxidation at a high temperature or by using an acid. For example, the removal of the mold is effected by heating at a temperature of 400 ° C. to 1000 ° C., more preferably 450 ° C. to 700 ° C., particularly preferably 520 ° C. to 560 ° C. under air, oxygen or water vapor. Anything can be used. Furthermore, the mold may be removed by a suitable oxygen plasma treatment at a low temperature (preferably room temperature).

  Preferred acids for removing the template include hydrochloric acid, sulfuric acid, nitric acid, or a mixture of two or more thereof. It is particularly preferable to treat at a temperature of 50 ° C. to 130 ° C. using a mixture of sulfuric acid and nitric acid at a ratio of 3: 1.

After step d) or step e),
f) coating the surface of titanium oxide with a continuous or discontinuous coating material;
May optionally be provided. The coating is preferably performed before step e). This is because if the template is removed prior to coating, the coating may be deposited to some extent on the nanostructure internal surface and the amount and dispersion cannot be easily controlled. .

  Preferably, the coating material includes a Group VIB transition metal, a Group VIIIB transition metal, a lanthanoid series metal, an actinide series metal, or a mixture thereof.

  The nanostructure coated in this way has practical utility as a metal-supported catalyst having a metal on the outer surface. The metal can be placed on the mold using known techniques such as those described above in connection with wet impregnation, sputtering, atomic layer deposition, reductive deposition of metal precursors, or mold coating, for example. .

  Coating the nanostructure in this manner can be integrated with the mold coating described above. In this case, a nanostructure coated on the outer surface and the inner surface is obtained.

  The above synthesis method may optionally further include g) a step of removing the anatase phase.

  The thickness of the titanium dioxide nanostructure is preferably 1 to 20 crystal layers, more preferably 1 to 10 crystal layers, and particularly preferably 1 to 6 crystal layers.

  The second aspect of the present invention relates to nanostructured titanium synthesized by the above synthesis method.

  The third aspect of the present invention relates to a titanium oxide-containing colloidal phase formed from components including tetrabutyloxytitanate and / or titanium isopropoxide, benzyl alcohol and / or acetone, and a solvent.

  The solvent is preferably an alcohol such as ethanol. In a preferred form, the colloidal phase includes tetrabutyloxytitanate, benzyl alcohol, and ethanol.

  In this embodiment, a rutile single-phase nanotube is produced by coating a template of sacrificial carbon nanotubes with sol, causing a sol-gel transition and forming an anatase coating having a crystal diameter of 18 to 19 nm (sol 3). . After calcination following an appropriate heat treatment, the mold is thermally removed. The carbon nanotube serves as a support, prevents the anatase tube from collapsing, and provides a simple through hole for the rutile nanotube. The size of the rutile nanotube can be controlled by adjusting the size of the carbon nanotube template.

  Multi-walled carbon nanotubes and single-walled carbon nanotubes were used as templates. These nanotubes were synthesized, functionalized by oxidation or acid treatment, or coated with a metal (for example, Ni, Pd, or Au).

The said template was processed at the temperature between 0 degreeC-90 degreeC using the sol (colloid phase) shown below. CNT: The ratio of TiO ₂ was between 1% to 50%.
A predetermined amount of water, acid or alkali was added to promote nanotube TiO ₂ gelation and precipitation.

The following sol was prepared to coat the carbon nanotube template.
(Sol 1)
Sol 1 is a sol for carbon nanotube coating reported by Sun et al. (Non-patent Document 37). In the sol, an inorganic titanium salt is used as a precursor of the sol. TiCl ₄ was diluted to 0.3M with water in an ice bath. Thereafter, the aqueous solution was mixed with a (NH ₄ ) ₂ SO ₄ solution. The final molar ratio of Ti: SO ₄ ²⁻ was 1: 2. After the mixture was stirred at 90 ° C. for 1 hour, 2.5 M NH ₄ OH was added until the pH value became 7, and a precipitate was formed.
Thereafter, the precipitated product was washed with distilled water several times and dried at 110 ° C. (Non-patent Document 41: M. Niederberger, MH Bartl, GD Stucky, J. Am. Chem. Soc. 2002, 124, 13642). ).

(Sol 2)
For comparison, an organic titanium precursor and sol-gel method for producing titanium oxide nanotubes on a porous alumina template reported by Zhang et al. (Non-patent Document 34) were used. Titanium isopropoxide (TI) was used with acetylacetone (ACAC), ethanol, and water in a TI: ACAC: H ₂ O: EtOH ratio of 1: 2: 3: 20.

(Sol 3)
The sol 3 is mainly used in this research field, and is a sol composed mainly of an organic titanium precursor such as tetrabutyloxytitanate (TBOT) or titanium isopropoxide (TI). It is dissolved in and mixed with benzyl alcohol (BA) or acetone. Benzyl alcohol is known to play a role as a surface shaping agent, and keeps the particle size small (Non-Patent Document 42: S. Vargas, R. Arroyo, E. Haro, R. Rodriguez, J. Mater. Res., 14/10 (1999), 3932-3937). After adding the CNT suspension, gelation was started by adding a small amount of water or acetic acid dropwise. The reaction temperature was changed from 0 ° C. to 90 ° C. and the mixture was stirred for 10 minutes to 3 hours. The resulting gel was stored dry at room temperature in an air atmosphere, dried at 100 ° C., or filtered and calcined at various temperatures.

  Each obtained composition was collected by filtration, washed with ethanol or water, and dried in an air atmosphere. And it calcined at 400 degreeC under the air atmosphere for 4 hours, and the coating material was crystallized as an anatase phase.

  Thereafter, the calcined material is heat-treated at a temperature of 650 ° C. to 950 ° C. for 2 to 4 hours under a nitrogen flow excluding oxygen to transfer the anatase phase to the rutile phase, while the rutile phase Was present as a continuous phase on the carbon nanotubes.

The final oxidation was performed in an air atmosphere or an oxygen atmosphere at a temperature of 520 ° C. to 560 ° C. to remove the carbon nanotube template and to leave the titanium oxide nanotubes made of the rutile phase. The titanium oxide nanotube was a rutile single phase as a result of measurement by electron diffraction and X-ray diffraction. These were stable even at least above 700 ° C. Nitrogen physisorption measurements by BET (Brunauer Emmett Teller), the specific surface area is large in desirable ^is between 20m ² / g~120m 2 / g, the size of the specific surface area applied process parameters (e.g., particles formed The concentration of the agent, the titanium concentration, and the conditions of the second heat treatment).

  FIG. 1 is a scanning electron microscopy (SEM) photograph of the rutile phase nanotube product after removal of the template. It can be seen that the rutile product is a distinct hollow nanotube. The product consists of two forms: an uneven and thick structure (upper SEM photograph) and a smooth and thin structure (lower SEM photograph). The form can be controlled by processing parameters such as the concentration of titanium precursor and the concentration of water.

  FIG. 2 shows XRD (x-ray diffraction) data for the rutile phase nanotube product of FIG. 1 (a) before template removal and (b) after template removal. It can be seen that the anatase phase does not remain after removing the template.

  Heat treatment sequence used in the preferred embodiment (calcined at 400 ° C. in air or oxygen atmosphere, then heat treated at 800 ° C. in nitrogen or other non-oxidizing atmosphere, and then at a temperature exceeding 400 ° C. in air or oxygen atmosphere In this case, the desired rutile phase nanostructure can be obtained.

Upon calcination, the anatase phase crystallizes from the amorphous titanium oxide phase.
The anatase phase is converted to the rutile phase by the heat treatment step. By being in a non-oxidizing atmosphere, the template is maintained as it is, and the nanostructure does not collapse.
The final template removal step is performed after the rutile phase titanium oxide nanostructure is formed.

  Although the invention has been described with reference to preferred embodiments, it is to be understood that various modifications are possible within the scope of the invention.

2 shows a scanning electron microscopy (SEM) photograph of the rutile phase nanotube product after template removal. 2 shows XRD (x-ray diffraction) data for the rutile phase nanotube product of FIG. 1 (a) before template removal and (b) after template removal.

Claims (29)

A method for synthesizing nanostructured titanium oxide, at least mostly consisting of a rutile phase,
a) providing a nanostructured template;
b) coating the template with a colloidal phase containing titanium oxide;
c) after the step b), a first heat treatment is performed on the template coated with titanium oxide to form a crystal phase consisting at least mostly of anatase phase; and d) after the step c), A step of converting the anatase phase into a rutile phase by subjecting the template coated with titanium oxide to a second heat treatment;
A synthesis method comprising: The synthesis method according to claim 1, characterized in that the product consists only of a rutile phase. The synthesis method according to claim 1, wherein the mold has a high aspect ratio cylinder or tube. The synthesis method according to any one of claims 1 to 3, wherein any one of carbon, nitride, sulfide, carbide, or gallium arsenide is added to the template. The synthesis method according to claim 1, wherein nitrogen is added to the template. The synthesis method according to claim 4 or 5, wherein the template has carbon nanotubes, carbon nanofibers, carbon herringbone, carbon onion, or a mixture thereof. The synthesis method according to claim 6, wherein the template includes single-walled carbon nanotubes, multi-walled carbon nanotubes, or a mixture thereof. The synthesis method according to claim 1, wherein the mold is provided with a continuous or non-continuous coating layer of a coating material. 9. The method of claim 8, wherein the coating material comprises a Group VIB transition metal, a Group VIIIB transition metal, a lanthanoid series metal, an actinide series metal, or a mixture thereof. The synthesis method according to claim 1, wherein the template has a specific structure. The synthesis method according to claim 8, wherein the template is a carbon nanotube mat. The component used to form the colloidal phase includes titanium chloride, titanium isopropoxide, tetrabutyloxytitanate, or a mixture thereof. The synthesis method described in 1. 13. Synthesis according to any one of the preceding claims, characterized in that the components used to form the colloidal phase comprise water, alcohol, acetone, organic particle shaping agents, or mixtures thereof. Method. The synthesis method according to claim 1, wherein the step b) includes a step of causing a sol-gel transition of the colloidal phase. The synthesis method according to any one of claims 1 to 14, wherein the first heat treatment is performed within a range of 200 ° C to 500 ° C. The synthesis method according to claim 1, wherein the first heat treatment is performed in an oxidizing atmosphere. The synthesis method according to any one of claims 1 to 16, wherein the second heat treatment is performed within a range of 550C to 950C. The synthesis method according to claim 1, wherein the second heat treatment is performed in a non-oxidizing atmosphere. The synthesis method according to claim 18, wherein the non-oxidizing atmosphere includes nitrogen, an inert gas, or a mixture thereof. The synthesis method according to claim 18, wherein the non-oxidizing atmosphere contains hydrogen. 21. The synthesis method according to claim 1, wherein the second heat treatment is continuous or discontinuous. After the step d),
e) removing the mold;
The synthesis method according to any one of claims 1 to 21, characterized by comprising: 23. The synthesis method according to claim 22, wherein the step e) is performed by oxidation at a high temperature or by using an acid. After step d) or step e),
f) coating the surface of the titanium oxide with a continuous or discontinuous coating material;
The synthesis method according to claim 1, comprising: 25. The method of claim 24, wherein the coating material comprises a Group VIB transition metal, a Group VIIIB transition metal, a lanthanoid series metal, an actinide series metal, or a mixture thereof. Furthermore, g) The process of removing an anatase phase is provided, The synthesis method of any one of Claims 1-25 characterized by the above-mentioned. 27. The synthesis method according to claim 1, wherein the nanostructured titanium oxide has a thickness of 1 to 20 crystal layers. A nanostructured titanium oxide synthesized by the synthesis method according to claim 1. A colloidal phase containing titanium oxide, characterized in that it is formed from a component comprising tetrabutyloxytitanate and / or titanium isopropoxide, benzyl alcohol and / or acetone, and a solvent.

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