JP2011084442A - Fine wire-shaped titanium compound, titania electrode for dye-sensitized solar cell, and dye-sensitized solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fine wire-shaped titanium compound whose conversions into a fine wire as well as a long fiber are further accelerated and which exerts a favorable characteristic in the power generation efficiency when used as a material for a titania electrode after heat treatment, and further to provide a titania electrode for dye-sensitized solar cells utilizing the fine wire-shaped titanium compound, and also the dye-sensitized solar cell having such titania electrode. <P>SOLUTION: There is provided a TiO<SB>2</SB>(B) type fine wire-shaped titanium compound produced by the pressurizing-heating treatment of the mixture of titanate and an alkaline aqueous solution. The anatase type fine wire-shaped titanium compound is obtained by heat-treating at 900°C or higher, the TiO<SB>2</SB>(B) type compound in which the amount of impurity elements having a cation radius smaller than that of an alkali metal element in the TiO<SB>2</SB>(B) type titanate synthesized by the pressurizing-heating treatment is controlled to be 1/1,000 or less based on the mol number of the titanium element contained in the material. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention utilizes a thin-line titanium compound useful as a material for a titania electrode, a photocatalyst material, a self-cleaning paint, a fiber-like semiconductor material, etc. used in a dye-sensitized solar cell, and such a thin-line titanium compound. The present invention relates to a titania electrode for a dye-sensitized solar cell, a dye-sensitized solar cell having such a titania electrode, and the like.

  Fine-line titanium compounds (titanium oxide, titanic acid, titanate, etc .: including crystals) are used as materials for titania electrodes, photocatalytic materials, self-cleaning paints, fiber-like semiconductor materials, etc. used in dye-sensitized solar cells. Widely used.

  In particular, when a thin-line titanium compound is used for a titania electrode for a dye-sensitized solar cell, an increase in photovoltaic power generation efficiency can be expected. In this dye-sensitized solar cell, electrons excited by the dye move to the titanium oxide and also move to the transparent electrode collecting current, which promotes thinning and long fiber formation of the titanium oxide. If this is the case, the electrons can be prevented from coming into contact with recombination nuclei and rereaction sites, which are the dead and active points of electrons, and the power generation efficiency is increased.

  For example, Non-Patent Document 1 discloses that electric power generation is achieved by mixing a fine-line titanium compound with titania powder that has been conventionally used as a material for a titania electrode for a dye-sensitized solar cell, as compared with the case of titania powder alone. It has been shown that efficiency is improved.

Further, according to Non-Patent Document 1, nano-sized titanium oxide powder and 20 mol / L KOH aqueous solution are put into a “Teflon (registered trademark) centrifuge tube”, and are placed in a pressure-resistant glass container at 110 ° C. for 20 hours. It is described that long-fiber-like titanium oxide (titanium oxide nanowires) having a specific surface area exceeding 400 m ² / g can be obtained by pressure heating treatment (hydrothermal synthesis treatment).

  However, potassium hydroxide (KOH) has a problem that it is highly reactive and difficult to handle. In addition, the reactivity of potassium is high, and potassium hydroxide contains a large amount of potassium carbonate and the like, and it is generally difficult to obtain a high-purity reagent. For these reasons, it is desirable to synthesize with a hydroxide of Na in order to carry out a reaction for obtaining a highly pure product.

In addition, the fine-line titanium compound obtained by the above method (hydrothermal synthesis method) is generally heat-treated at a high temperature in order to remove impurities such as alkali metals in the compound and arrange the crystal lattice. However, when such heat treatment is performed, rearrangement of the crystal lattice occurs in the fine-line titanium compound, and there is a problem that the specific surface area is extremely reduced. That is, the thin linear titanium compound obtained by the above method has a limit in heat treatment temperature, and it is a fact that the desired properties cannot be exhibited. According to Non-Patent Document 1, it is shown that the heat treatment temperature is 500 ° C. and the specific surface area is 148.4 m ² / g, the heat treatment temperature is 600 ° C. and the specific surface area is 64.2 m ² / g. ing.

On the other hand, Non-Patent Document 2 discloses that TiO 2 is obtained by hydrothermal synthesis using natural rutile sand (titanium content 96%) as a raw material in a 10 mol / L NaOH aqueous solution at 150 ° C. for 72 hours. _{the 2} (B) type titanium oxide fine line product is obtained is disclosed. In this technique, the synthesized titanium oxide thin wire product is washed with hydrochloric acid (ion exchange), and then the sample is heated to 120 to 1000 ° C. in the atmosphere (heating time: 4 hours). System and specific surface area are also measured.

According to this technique, a TiO ₂ (B) type titanium oxide thin wire product is observed in heating at 300 to 500 ° C., but anatase is observed by heating at 500 to 900 ° C. Further, it has been observed that a part of anatase is phase-transformed into rutile by heating at 900 ° C. or higher. As for the specific surface area, after hydrothermal synthesis (before heat treatment), a specific surface area of 45 m ² / g is obtained, but it is also shown that the specific surface area gradually decreases with heating. .

  In particular, since this technology uses natural rutile sand (titanium content of 96%) as a raw material, the nanofiber contains a large amount of impurities. Such a material is used as a titania electrode for a dye-sensitized solar cell. When used as a material, there is a possibility of deteriorating electrical characteristics. Further, there is a problem that it is difficult to obtain a titanium oxide having a reduced diameter due to the impurities.

  In addition, with the titanium oxide compound obtained by this technique, it is expected that the heating temperature has a limit when the post-synthesis heat treatment is performed. That is, when a thin-line titanium compound is used as a material for a titania electrode for a dye-sensitized solar cell, the crystal system is preferably an anatase type. It has been reported that it disappears by heat treatment at 900 ° C. or higher, and the fiber diameter is remarkably increased.

Non-Patent Document 2 discloses that TiO ₂ (B) type titanium oxide is obtained by hydrothermal synthesis using commercially available titania powder as a raw material in a 10 mol / L NaOH aqueous solution at 150 ° C. for 72 hours. It is also disclosed that a system-like thin line product can be obtained. Further, in this technique, after the hydrothermally synthesized titanium oxide thin wire product is washed with hydrochloric acid (ion exchange), each sample is heated to 100 to 1000 ° C. in the atmosphere (heating time: 2 hours). It has also been measured for the crystal system. However, when commercially available titania powder is used as a raw material, there is no significant difference in characteristics from when natural rutile sand is used as a raw material, and the crystal system changes due to heat treatment at a relatively low temperature. (Phase transition) and a phenomenon that the diameter of the fiber becomes remarkably thick also appear.

  Non-Patent Document 3 discloses synthesizing a fine-line titanium compound by an alkoxide method. In addition, in this technique, when a commercially available titanium powder is used for a titania electrode, the short-circuit current Jsc is saturated even if the film thickness is increased, but when a thin-line titanium compound is used for a titania electrode. It is also shown that the phenomenon that the short-circuit current Jsc (“short-circuit current Jsc” will be described later) saturates is not observed.

Shinpei and two others, “Synthesis of Titanium Oxide Nanowire and Dye-sensitized Solar Cell”, Chemical Industry, Vol. 60, pp. 796-800 (October 2004) Systhesis of titanate, TiO2 (B), and anatase TiO2 nano-fibers from natural rutile sand, Sorapong Pavasupree, Yosikazu Susuki, Susumu Yosikawa, Ryoji Kawahara, Journal of solid Stete Chemistry, 178 (2005), 3110-3116,2179-2185 Formation of Titania Nanotubes and Application for Dye-Sensitized solar Cells, Motonari Adachi, Yusuke Mata, Issei Okada and Susumu Yaku.

  The technologies that have been proposed so far have some problems with respect to the properties of the resulting thin-line titanium compound (power generation efficiency when used as a material for titania electrodes after thinning, long fiber, and heat treatment). There is room for improvement. Accordingly, an object of the present invention is to further promote the thinning and long fiber, and to produce a thin-line titanium compound that exhibits good characteristics in power generation efficiency when used as a material for a titania electrode after heat treatment, and such a thin-line shape It is an object of the present invention to provide a titania electrode for a dye-sensitized solar cell using a titanium compound and a dye-sensitized solar cell having such a titania electrode.

The fine-line titanium compound of the present invention capable of achieving the above object is a TiO ₂ (B) type fine-line titanium compound produced by subjecting a mixture of titanate and an aqueous alkali solution to pressure and heat treatment. The amount of the impurity element having a smaller cation radius than the alkali metal element in the TiO ₂ (B) type titanate synthesized by the pressure heat treatment is 1 / the number of moles of the titanium element contained in the raw material. A TiO ₂ (B) type fine wire titanium compound controlled to 1000 or less is then heat-treated at a temperature of 900 ° C. or higher, and has a gist in that it is anatase type.

  The thin-line titanium compound of the present invention includes a titania electrode used for a dye-sensitized solar cell, a photocatalyst material (for example, an air purifier or a vehicle deodorizing material), and a self-cleaning paint (for example, an outdoor paint or a building exterior wall material). It is useful as a material such as a fiber-like semiconductor material (for example, a constituent material of a bulk hetero structure of an organic solar cell), but particularly when used as a material for a titania electrode used in a dye-sensitized solar cell. , Will exhibit good power generation efficiency. As a specific configuration thereof, a titania electrode configured by mixing the fine wire titanium compound as described above and powdered titania can be cited. Moreover, it is preferable that the mixing rate of the fine wire titanium compound at this time is 0.1-20 mass% (ratio with respect to the total amount of a fine wire titanium compound and a powdery titania).

  By using the titania electrode as described above, a dye-sensitized solar cell that exhibits good power generation efficiency can be realized.

In this specification, “titanic acid” means a compound in which a metal M of titanate (xM ₂ O · yTiO ₂ ; M is an alkali metal, x and y are natural numbers) is replaced with a hydrogen atom H ((xH ₂ O.yTiO ₂ ); especially H ₂ Ti ₃ O ₇ .nH ₂ O (n is a natural number of 0 to 3) such as H ₂ Ti ₃ O ₇ .3H ₂ O), and water-containing titanium oxide ( H ₂ Ti ₃ O ₇ (x = 1, y = 3)).

Further, in this specification, the “titanium oxide compound” means titanium oxide (TiO ₂ ), hydrous titanium oxide (H ₂ Ti ₃ O ₇ ), and hydrates thereof (H ₂ Ti ₃ O ₇ .mH ₂ O). (M is a natural number of 1 to 3)). Further, in the present specification, the “titanium compound” is used to include all of the titanate, titanic acid, and titanium oxide compounds.

  According to the present invention, it is possible to further promote the thinning and lengthening of the titanium compound, and it is possible to realize a thin-line titanium compound that does not cause phase transition even by high-temperature heat treatment. When the compound is used as a material for a titania electrode for a dye-sensitized solar cell, an increase in power generation efficiency (photoelectric power generation efficiency) can be expected.

It is a schematic diagram for demonstrating that a titanate comprises a winding structure. It is an explanatory view conceptually showing the difference in the distortion of microscopic TiO ₆ crystal octahedron. It is a conceptual diagram which shows the distribution state of the impurity spread in two dimensions. It is a schematic sectional drawing for demonstrating an example of the manufacturing apparatus used by this invention. It is a scanning-type scanning electron micrograph substituted for drawing of the dry acicular compound obtained in the Example. It is a scanning substitute electron micrograph of the drawing of the dry acicular compound obtained by the comparative example. It is a graph which shows the Raman spectrum of the dry acicular compound obtained in the Example. It is a graph which shows the Raman spectrum of the dry acicular compound obtained by the comparative example. It is a graph which shows the XRD spectrum of a sample when heat-treating a dry acicular compound at various temperature. It is a scanning-type scanning electron micrograph in place of a drawing when the dried acicular compound was heat-treated at 400 ° C. for 5 hours. It is a scanning substitute electron micrograph of a drawing when a dry acicular compound is heat-treated at 900 ° C. for 24 hours. FIG. 2 is a drawing-substitute scanning electron micrograph when a dry acicular compound is heat-treated at 1050 ° C. for 24 hours. In Example 3, it is a bar graph which shows the relationship between the heating time and the value of each characteristic (all are evaluated by an absolute value) in each titania electrode. In Example 4, it is a bar graph which shows the relationship between the structure of each titania electrode, and the value of each characteristic (all are evaluated by an absolute value).

  The present inventors have been further researching from the viewpoint of further promoting the thinning and long fiber formation of the titanium compound. As part of that research, we have determined that certain impurities contained in the raw materials used in the production hinder the thinning and long fibers of titanium compounds. And when hydrothermal synthesis was performed with the amount of such impurities reduced as much as possible, it was found that thinning and long fibers of the titanium compound were effectively promoted, and its technical significance was recognized. (Japanese Patent Application No. 2008-269003).

  In the method for producing a fine-line titanium compound including a step of subjecting a mixture of titanate and an aqueous alkali solution to pressure-heat treatment and growing the titanate into a fine-line shape, the technique described above is titanium synthesized by the pressure-heat treatment. By controlling the number of impurity atoms with a smaller cation radius than the alkali metal element in the acid salt to 1/50 or less of the number of titanium element atoms contained in the raw material, the titanium compound is thinned. And a long fiber.

  The present inventors have made further studies to improve the properties of the fine-line titanium compound even after the above invention has been completed. As a result, the hydrothermal synthesis by further reducing the impurity elements contained in the raw material, followed by heat treatment at a temperature of 900 ° C. or higher surpasses that of the conventional fine-line titanium compound. As a result, the present invention was completed. That is, in the thin-line titanium compound of the present invention, the crystal system is mainly composed of anatase without causing phase transition even in heat treatment at about 900 ° C. And when such a thin-line titanium compound is used as a material for a titania electrode, the power generation efficiency of the dye-sensitized solar cell is further improved.

  The principle that thinning and lengthening of the titanium compound are effectively promoted by reducing the impurity elements can be considered as follows. In general, it is known that the ionic radius varies depending on the element, the valence of the ion, and the coordination number. In the present invention, the valence is the valence of ions that are stable in titanate. That is, K and Na are monovalent cations, Ca and Zn are divalent cations, Al is a trivalent cation, and Si is a tetravalent cation. The ionic radius should be discussed with the radii of ions in titanate (for example, “Butler / Harrod Inorganic Chemistry (above)” (RD Sharnnon Acta Crystallographica, A32, 751 (1976): Reference: However, you may discuss the ionic radius by comparing the ionic radii relative to the ionic radius of the same coordination number. In addition, from the mechanism described later, the element that affects the thinning of the titanium compound is a cation radius when comparing the cation radius as described above rather than the alkali metal element constituting the alkaline aqueous solution as a raw material. Is required to be a small element.

In the titanic acid metal salt such as Na ₂ Ti ₃ O ₇ , titanic acid and the metal element constitute a layered structure, and the element ratio thereof constitutes a compound according to a constant integer ratio. A step structure according to the ratio of a certain element is formed between the layers, and the step structure grows a roll-like needle crystal in a self-aligned manner during pressure heating treatment by a hydrothermal synthesis method. It is believed that (for _{example, the Soft Chemical Systhesis of TiO 2} (B) from layered Titanatees, THOMAS P.FEIST aND PETER K.DAVIEST). Such a winding structure will be described with reference to the drawings.

FIG. 1 is a schematic diagram for explaining that a titanate (for example, sodium titanate: Na ₂ Ti ₃ O ₇ ) constitutes a wound structure. In the figure, 1 indicates a TiO ₆ crystal octahedron. ing. The TiO ₆ crystal octahedron 1 has steps at three consecutive locations, and in order to finally obtain the shape of the needle-like compound by hydrothermal synthesis, the corners of the TiO ₆ crystal octahedron 1 are crystallized. Since the TiO ₆ crystal octahedrons 1 are distorted at a constant rate and accumulated over a long period of time, the winding structure is finally shown. It is considered to constitute an acicular compound having a one-dimensional structure.

FIG. 2 conceptually shows the difference in strain of the microscopic TiO ₆ crystal octahedron 1 when the radii of metal element ions (cation radii) are different, such as Na cation and K cation. It is explanatory drawing. FIG. 2 (a) shows the case containing Na cation (indicated by 2 in the figure) (that is, when NaOH is used as the alkaline aqueous solution), and FIG. 2 (b) shows K cation (in the figure, 3) (that is, when KOH is used as an alkaline aqueous solution).

For example, when a K cation is present as an alkali metal ion [FIG. 2 (b)], the plane interval constituted by the TiO ₆ crystal octahedron 1 is widened, and the planes constituted by the TiO ₆ crystal octahedron 1 are Less geometric interference. Then, since the geometrical interference between the planes is reduced, it is considered that the distortion at the step on the plane of the TiO ₆ crystal octahedron 1 is easily increased. In addition, it is considered that K and Na, which are cations, supply electrons and affect the orbital of oxygen molecules at the corners of the TiO ₆ crystal octahedron 1.

  From this principle, it can be considered that the step on the plane of titanate is influenced by the ionic radius (cation radius) of the alkali metal element used. The radius of the Na cation is 0.116 nm, whereas the radius of the K cation is 0.152 nm. For example, “Butler / Harod Inorganic Chemistry (above)” (RD Sharnon Acta Crystallographica, A32 751 (1976): Reference).

  Accordingly, when KOH is used as the alkaline aqueous solution (that is, when K is present as the alkali metal atom), the winding is more difficult than when NaOH is used (that is, when Na is present as the alkali metal atom). It is expected that the radius of curvature of the structure will be reduced, and the resulting titania compound is considered to exhibit a large specific surface area due to finer crystals.

In contrast to the above mechanism, when a certain amount or more of impurity elements are present in the synthesis atmosphere, the long-term winding structure formed by accumulation of distortion of the impurity elements is disturbed locally. Since the radius of curvature is changed to hinder the configuration of the winding structure, it is considered that the radius of curvature increases. That is, it is considered that the impurity element disturbs the consistency of the macroscopic winding structure by disturbing the strain of the plane constituted by the long-term TiO ₆ crystal octahedron at regular intervals. As described above, Ca, Al, Zn, Si, Mg, and the like can be considered as impurities mixed in the hydrothermal synthesis environment. These impurities are generally ionic radii as compared with K cations and Na cations. The (cation radius) is small and the valence of electrons is large. For example, according to the above references, the cation radii of these elements are Ca ²⁺ : 0.114 nm, Al ³⁺ : 0.068 nm, Zn ²⁺ : 0.088 nm, Si ⁴⁺ : 0.054 nm, Mg ²⁺ : 0.086 nm In addition, even if it is the same element, although an ion radius changes with the valence and coordination number of an ion, it is set as the ion radius in the valence of a cation when the said element combines as a titanate here.

Therefore, when the impurity element ions are substituted between the planes formed by the TiO ₆ crystal octahedron, K ions and Na ions having the same valence of electrons are eliminated, so that the number of atoms is small. However, it is considered that the winding structure of the crystal is easily broken by the impurity element. Here, since the alkali metal element (K or Na) entering between the planes constituted by the TiO ₆ crystal octahedron has a two-dimensional distribution, the impurity atom is 1/100 of the alkali metal atom. As shown in FIG. 3 (conceptual diagram showing the distribution state of impurities spread in two dimensions) (in the figure, 4 indicates an alkali metal element, 5 indicates an impurity element), one-dimensional (linear) Since the impurity element is present at a ratio of 1 to 10, the influence on the winding structure cannot be ignored.

In the above-described winding structure, the sheet-like H ₂ Ti ₃ O ₇ molecule is denatured in the alkaline solution in relation to the electronic state of the ions present in the periphery, and therefore has a constant curvature. It is also explained that the sheet-like H ₂ Ti ₃ O ₇ molecule is stabilized (for example, “Formation Mechanism of H ₂ Ti ₃ O ₇ Nanotubes” S. Zhang, LM Peng, Q. Chen). , GH Du, G. Dawson and W. Z. Zhou, The American Physical Society Vol. 91, No. 25 (2003) 256103-1.

As shown in the prior art (see Non-Patent Document 1), the titania compound obtained by hydrothermal synthesis in the KOH system has an extremely low specific surface area due to subsequent heat treatment. Such a phenomenon is also observed by NaOH-based hydrothermal synthesis, and it has been reported that the shape of the titania compound itself collapses at 1000 ° C., and the fiber becomes thicker due to atomic position exchange. It can be explained that these phenomena are caused by a change in the curvature of the small TiO ₆ crystal octahedron constituting the above-described winding structure.

  The titania compound of the present invention is a material that is hydrothermally synthesized using raw materials with reduced impurity elements. However, such a titania compound constitutes an ideal winding structure, and as a result, the compound Since the crystal diameter is reduced and the crystal structure is ideal, the lattice distortion is less relaxed by the subsequent heat treatment, and the temperature of the phase transition of the crystal is about 200 ° C. compared with that shown in the prior art. It is thought that it moves to the high temperature side.

  In general, when a crystal is heat-treated at a high temperature, atoms vibrate due to thermal energy. When there is an incomplete crystal portion such as a defect in the crystal, the atomic position moves due to vibration caused by heat and bonds with a large number of atoms are temporarily performed (electron sharing states are different). Finally, the most stable state is obtained by taking the most stable bond. Such a phenomenon is well known as an effect of heat treatment (annealing treatment) for eliminating imperfect portions of the crystal. In addition to eliminating strain in the crystal lattice as described above, for example, when impurities exist in the crystal, the position of the impurity element changes due to thermal vibration, and the interface or grain boundary can capture the impurities. There is also a possibility that the impurities move to the surface, or the impurities are inactivated and further discharged out of the crystal.

  As described above, the heating at a high temperature (for example, 400 ° C. or more) at which the position of atoms in the crystal is converted is repaired, and the impurity element in the crystal is moved and is not transferred. May be activated and generally works in the direction of improving the electrical properties of the material, but if the crystal undergoes a phase transition, it is below the temperature at which the phase transition occurs in order to maintain an optimal crystal system Heat treatment at temperature is necessary. From such a viewpoint, the titania compound obtained by the conventional technique has a limit in improving material properties.

The titania compound used in the present invention is hydrothermally synthesized using a raw material in which impurity elements are reduced as much as possible. The amount of the impurity element at this time needs to be controlled to 1/1000 or less with respect to the number of moles of titanium element contained in the raw material. In addition, the titania compound (thin linear titania compound) at the stage of hydrothermal synthesis has a crystal system of TiO ₂ (B) type, but becomes anatase type by subsequent heat treatment. The heat treatment temperature at this time needs to be 900 ° C. or higher (preferably 1000 ° C. or higher, more preferably 1050 ° C. or higher). However, if the heat treatment temperature becomes too high, the crystal system transitions to the rutile type, and therefore, it is preferable to set the temperature below 1100 ° C. (however, when kept for 24 hours).

  Further, the time for performing the heat treatment (treatment time) is not limited at all, but from the viewpoint of eliminating lattice distortion over the entire thin-line titania crystal, it is 0.5 hours or more necessary for lattice relaxation. It is preferable to do. However, if the heat treatment is continued at a constant temperature even if the treatment time is too long, a heat history is accumulated, and a phenomenon equivalent to that when heated to a higher temperature appears, so that an undesirable transition may occur. Because it only increases the cost industrially, it should be within 48 hours.

  In addition, about the atmosphere at the time of the said heating, although a special installation is not required and it can implement in air | atmosphere, you may heat in an oxygen atmosphere from a viewpoint of supplementing the oxygen deficiency in titania. Further, in the case where mixing of nitrogen or oxygen in the atmosphere becomes a problem, heating may be performed in an inert gas atmosphere or a vacuum atmosphere. In addition, when it is desired to control the characteristics of a semiconductor by intentionally generating oxygen defects, heating may be performed in a reducing gas atmosphere such as hydrogen. The higher the heating rate during the heating, the better, for example, 20 ° C./min or more. By controlling the heating rate in this way, the special properties of the wound structure crystal are revealed in order to reduce the possibility that the structure unique to the wound structure collapses during dehydration and assumes the properties of conventional crystals. The effect of making it appear.

  In order to obtain the titania compound as described above, it is important to suppress the concentration of impurities mixed in the raw material. However, as a means for reducing the concentration of impurities, as a means for reducing the concentration of impurities, an instrument for preparing a strong alkali solution is made of glass. It is fundamental not to use equipment that reacts with alkali such as. In the dissolution of sodium hydroxide, the heat of dissolution is large and accompanied by heat generation, and the viscosity of the solution is high. Further, when the solution is diluted, the volume thereof is reduced. Therefore, when the evaluation is made with a volumetric flask, it is necessary to fill the pure water several times and repeat the stirring. Therefore, when prepared in a volumetric flask, it is necessary to cool the hot solution to adjust the rating, and to leave the solution for a certain period of time to remove bubbles in the solution, and to accurately adjust the rating. It is necessary to dilute several times.

  In addition, before using the NaOH standard solution, the impurity concentration is measured by ICP (Inductively Coupled Plasma) emission spectroscopic analysis, and it is 1/1000 or less with respect to the number of moles of titanium element contained in the raw material. It is also useful to perform hydrothermal synthesis after confirming the above.

  It is desirable to use a coating material or resin with low reactivity such as Teflon coating as the instrument used in the above process. Further, industrially, a method of preliminarily determining the ratio of raw materials to be mixed and mixing in consideration of volume reduction can be considered. In addition, as alkaline aqueous solution used for hydrothermal synthesis, although KOH and NaOH are mentioned as a typical thing, it is preferable to use NaOH from a viewpoint that a reagent with few impurities can be obtained.

  The fine-line titanium compound used in the present invention is produced by hydrothermal synthesis. Hydrothermal synthesis refers to a reaction in which a liquid containing titanate is subjected to pressure and heat treatment to grow a titanate compound (including crystals) or a titanate compound (including crystals). As a method for preparing a liquid containing a titanate, various conventionally known methods can be employed. For example, the titanate may be directly dissolved or dispersed in water, but preferably a titanium oxide compound (particularly titanium oxide). And alkaline aqueous solution. In a mixture of a titanium oxide compound and an aqueous alkali solution, hydrothermal synthesis proceeds rapidly.

  Titanium oxide may be of any crystal system as long as it is of high purity, and may be anatase type, rutile type, brookite type, and other reported sri-lankite type, high pressure type, and type II. When synthetic titanium oxide is used, impurities mixed in the product (thin wire titanium compound) can be further reduced, and the electrical characteristics of the product can be enhanced. In the examples, the anatase type was used.

The fine linear titanium compound after hydrothermal synthesis has a specific surface area (BET specific surface area) of about 47 to 81 m ² / g. In addition, this fine-line titanium compound has a crystal system mainly composed of a TiO ₂ (B) structure.

  As an aqueous alkali solution used for hydrothermal synthesis, an aqueous solution of an alkali metal hydroxide such as an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution can be used. This alkaline aqueous solution is generally a high concentration alkali, and the concentration of the alkali metal is, for example, about 1 to 20 mol / L, preferably about 2 to 17 mol / L, and more preferably about 5 to 15 mol / L.

  The amount of the titanium oxide-based compound is, for example, about 1 to 30 g, preferably about 3 to 20 g, and more preferably about 5 to 15 g with respect to 100 mL of the alkaline aqueous solution.

The pressurization and heating conditions for hydrothermal synthesis are, for example, as follows.
Gauge pressure: 0.2-2 MPa, preferably 0.3-1.5 MPa, more preferably 0.4-1.2 MPa (particularly 0.5-1.0 MPa)
Reaction temperature: 110 ° C. or higher, preferably 120 ° C. or higher, more preferably 130 to 200 ° C. (especially 160 to 190 ° C.)

The titania compound obtained by the hydrothermal synthesis is a fine line compound in which a titanate and titanium oxide laminated in a layer form are wound. The wound thin linear compound is desirably washed (especially washed with water) if necessary, and then contacted with a protonic acid to deionize or desorb ions, and then dehydrated. By contacting with a protonic acid, the titanate (xM ₂ O · yTiO ₂ ) moiety is converted into titanic acid (eg, H ₂ Ti ₃ O ₇ .nH ₂ O. Particularly H ₂ Ti ₃ O ₇ .3H ₂ O). Become. Further, the titanic acid becomes hydrous titanium oxide (H ₂ Ti ₃ O ₇ ) by the drying treatment and becomes titanium oxide (TiO ₂ ) as the drying proceeds further. Even if these post-treatments are carried out, the wound thin linear structure is maintained, and various wound thin linear titanium compounds can be produced depending on the stage of the post-treatment. However, when drying progresses and the initial titanate portion becomes titanium oxide, the whole becomes titanium oxide, and at the compound level, the winding structure disappears and becomes a TiO ₂ (B) structure.

  In addition, since the cohesive force of the wound thin linear compound is relatively weak, the aggregates bonded with a weak cohesive force are maintained in any of the removal of the reaction solution, the removal of the protonic acid solution, and the removal of the washing solution. In order to achieve this, it is recommended that the liquid portion be separated and removed without applying an external force strongly. As a means for separating and removing the liquid portion, centrifugal separation may be employed, but it is preferable to employ natural sedimentation (decantation), filtration, or the like. In the present invention, the wound thin linear compound has high handling properties and high sedimentation properties, and separation and recovery from a liquid can be performed very easily even when natural sedimentation (decantation) or filtration is employed.

  As the protonic acid, for example, a strong acid (mineral acid) such as hydrochloric acid or sulfuric acid is generally used. The proton concentration when contacting with a protonic acid is, for example, 0.1 mol / L or more, preferably 0.3 mol / L or more, and more preferably 0.5 mol / L or more. When the proton acid concentration is too low, the efficiency of proton exchange with titanate decreases. The upper limit of the proton acid concentration is not particularly limited, but an upper limit may be set from the viewpoint of manufacturing cost. If the proton acid concentration is high, the titanate may be partially dissolved. The proton concentration may be, for example, 6 mol / L or less, preferably 5 mol / L or less, and more preferably 3 mol / L or less.

  The hydrothermally synthesized titania compound is first subjected to a drying treatment, but the method of the drying treatment at that time is not particularly limited, and freeze drying, heat drying, vacuum drying (vacuum drying), air drying, and a combination of these as appropriate. However, it is desirable to use a vacuum dryer with good airtightness in order to prevent the nano-like substance from diffusing into the external environment. The titanic acid can be made into hydrous titanium oxide under relatively mild drying conditions (for example, heating conditions of about 70 to 150 ° C. when heating and drying at normal pressure), but hydrous titanium oxide is oxidized. To make titanium, relatively strong drying conditions (for example, heating conditions of about 300 to 600 ° C. in the case of heat drying at normal pressure) are required.

  The thin-line titanium compound obtained as described above has a structure in which thinning and long fiber formation are further promoted. Since such a titanium compound has promoted thinning of the titanium compound and thus promotes aggregation in the solution, the BET specific surface area is also characterized. When the surface area per unit volume is calculated with a simple cylinder model, it can be calculated that in a region where the aspect ratio is larger than 1, such as a needle-shaped crystal, the aspect ratio increases as the value of the BET specific surface area increases. If crystallization (acicular crystals) is promoted, the value increases. Therefore, it is desirable that needle crystals have a large BET specific surface area as an indicator of thinning.

In the case of constituting a titania electrode for a dye-sensitized solar cell, if the BET specific surface area is large, the adsorbing ability of the dye is increased, which is desirable. In the titania compound obtained above, the BET specific surface area is, for example, 48 m ² / g or more, preferably about 81 m ² / g. In a dye-sensitized solar cell, photoelectrons excited by the dye move to titanium oxide and also to a transparent electrode collecting current, which can promote thinning and long fiber formation of titanium oxide. For example, during the movement of the photoelectrons, it is possible to prevent the photoelectrons from coming into contact with the recombination nuclei and rereaction sites that are the dead and active points of the electrons.

  A titania electrode for a dye-sensitized solar cell has generally been composed of a titania powder (mainly anatase crystal). In such a titania electrode, when the electrons propagate in the titania electrode, the crystal orientation of the porous titania electrode is not uniform, and electrons that trap lattice defects and the like at places where the crystal orientation intersects, grain boundaries, etc. It seems that there are many capture sites. In addition, when electrons propagate through the crystal, the crystal orientation is not uniform, so the direction of electron movement is frequently changed and reaches the titania electrode surface with a certain probability and re-reacts with the redox solution (Redox solution). This can cause power generation efficiency to decrease.

  In order to solve these problems, the loss of electrons is reduced by providing a so-called bypass capable of transporting electrons along the crystal orientation of the fine-line titania compound by blending the fine-line titania compound with the titania powder. be able to. Increasing the power generation efficiency of a titania electrode by blending a titania powder with a fine-line titania compound is already a known phenomenon (for example, Non-Patent Document 1), but has been proposed and developed so far. In the fine-line titania compound, even when mixed with the titania powder, there is a problem that the shrinkage is large during the firing process of the electrode and it is peeled off from the substrate.

  On the other hand, in the case of using the thin-line titanium compound (impurities reduced and processed at a higher temperature) obtained as described above and a titania powder as a material for a titania electrode, titania The power generation efficiency of the electrode can be greatly improved. In order to exert such an effect, the blending amount of the fine wire titanium compound (ratio to the total amount of the fine wire titanium compound and the powdered titania) is preferably 0.1% by mass or more. However, if this amount is excessive, the amount of dye adsorbed by the titania nanofiber itself is small, and the amount of relative dye adsorbed decreases. Therefore, the amount is preferably 20% by mass or less. The more preferable lower limit of the compounding amount of the thin linear titanium compound is 1% by mass (more preferably 2% by mass), and the more preferable upper limit is 10% by mass (more preferably 6% by mass).

  The fine linear titanium compound used in the present invention is obtained by hydrothermal synthesis, and a conventionally known hydrothermal reaction apparatus can be used as an apparatus for carrying out such hydrothermal synthesis. Carry out using the reactor described. FIG. 4 is a schematic cross-sectional view showing an example of a reaction apparatus used in the present invention.

  In the manufacturing apparatus shown in FIG. 4, a sealed pressurizing container 44 is disposed in a bore of a central cylindrical space (room temperature bore) 42 (diameter 100 mm in the illustrated example) of a cylindrical electromagnet (superconducting magnet in the illustrated example) 41. It is set. The pressurization container 44 can be heated by a heater 46, and the pressurization container 44 contains a sealed reaction container 45 containing a liquid containing titanate and water 43. The sealed pressurizing container 44 is a pressure-resistant rigid container, while the reaction container 45 is a flexible container capable of pressurizing the contents by external pressure. If such an apparatus is used, the water 43 in the pressurizing container 44 is heated by the heater 46 and the internal pressure of the container 44 rises, so that the liquid containing titanate in the reaction container 45 is pressurized.

  In the apparatus configuration shown in FIG. 4, a magnetic field can be applied from an electromagnet (superconducting magnet) 41. However, the thin-line titanium compound of the present invention can basically be manufactured without applying a magnetic field. is there. However, if necessary, hydrothermal synthesis may be performed while applying a magnetic field.

  Also, in the illustrated example, the sealed pressurizing container 44 is composed of a lid body portion 44b and a main body portion 44a, and by opening and closing these with a screw-in type, the contents can be taken out while maintaining pressure resistance. However, the opening / closing method of the container 44 is not limited to the screw-in type, and various known methods can be adopted.

  In the illustrated example, the reaction vessel 45 is located at the magnet center (where the magnetic field is maximum), but may be removed from the magnet center. Further, a plurality of reaction containers 45 may be accommodated in the pressurizing container 44. When a plurality of reaction containers 45 are accommodated, they may be accommodated side by side or may be accommodated by being stacked vertically. The reason why the water 43 and the reaction vessel 45 are placed in the pressurizing vessel 44 is that the pressure required for hydrothermal synthesis can be achieved at a relatively low temperature by using the water 43 as a pressure medium. This is in order to avoid the direct contact between the strong alkali and the apparatus, thereby prolonging the apparatus life. However, the reaction vessel 45 is not essential, and the hydrothermal synthesis is performed by directly putting the liquid containing titanate into the pressurizing vessel 44. May be performed.

  When the heater 46 is disposed in the magnet center, even if it is a weak magnetic material heater, the heater is attracted to the magnet center by the attractive force of the magnet, so that it is difficult to directly heat the place away from the magnet center. Therefore, it is recommended to quickly transfer the heat of the heater 46 by disposing a heat transfer plate, for example, a copper plate 50, at a place away from the magnet center. In addition, it is desirable to cover a portion of the outer surface of the pressurizing container 44 that is not directly heated by the heater 46 (in the illustrated example, a portion covered with the heat transfer plate 50 and a lid portion) with a heat insulating material 71. .

  Furthermore, in the apparatus configuration shown in FIG. 4, a cooling system 51 is installed. In this cooling system 51, a vacuum layer and a cooling water layer are formed on the inner peripheral surface of the room temperature bore 42, and the thermal influence of the heater 46 is prevented from reaching the superconducting magnet 41. The cooling system 51 is not limited to the illustrated one, and a known cooling means can be used alone or in combination as appropriate. When the magnetic field generating means 41 is a normal electromagnet or permanent magnet, the cooling system 51 is not necessary. For example, when a normal conducting coil is used, a large amount of cooling water is used to cool Joule heat generated in the coil. Is required.

  In the illustrated example, the heating temperature can be controlled while being monitored by a heating thermometer 62 in the vicinity of the heater, and the pressure is monitored by a pressure sensor 49 communicating with the inside of the pressurizing container 44 via the piping 47 and the piping joint 48a. It can be controlled.

  Furthermore, in the apparatus configuration shown in FIG. 4, a thermocouple internal thermometer 61 for measuring the temperature of the gas phase part of the pressurizing container 44 is provided. The internal thermometer 61 is inserted into the pressurizing container 44 from a tightening joint 48b attached to a pipe 47 extending from the lid portion of the pressurizing container 44, and the hermeticity of the container 44 is maintained. Yes. Further, by measuring the gas phase temperature while maintaining the hermeticity in the pressurizing container 44, it is possible to indirectly measure the pressure change in the pressurizing container 44, and to detect the pressure fluctuation of the pressurizing container 44. It can be used as a sensor (particularly a leak sensor). That is, since the pressurization container 44 is kept hermetically sealed, it has a slow thermal balance with the outside world and can be regarded as an adiabatic system in a short time. When the contents leak in such a pressurizing container 44, the gas phase portion adiabatically expands and the temperature decreases. On the other hand, when pressurized gas is fed, the gas phase portion is substantially adiabatically compressed and the temperature rises until mixing of the pressurized gas and contents occurs. The pressure change by a normal pressure gauge measures the amount of strain of the diaphragm and is insensitive and shows a change over time, whereas the temperature change in the gas phase part shows a differential change with respect to the pressure change. Shown and extremely sensitive. Therefore, it can be used as a sensitive pressure fluctuation sensor (particularly a leak sensor).

  When viewed from the viewpoint of the length of the heat conduction route, the tip of the thermocouple internal thermometer 61 is far from the portion (heating portion) of the pressurizing container 44 that is directly heated by the heater 46. Is desirable. The illustrated heat conduction route is based on the heating part, the non-heating part on the upper part of the pressurizing container body 44a, the pressurizing container lid part, the pipe 47, the pipe joint 48a, the tightening type joint 48b, and the thermocouple sheath part. It has reached the front-end | tip part of the thermocouple type internal thermometer 61 through this, and it can be said that the distance is long. It is recommended that the shaft part (sheath part, etc.) of the thermocouple internal thermometer 61 is not in direct contact with the heating part. For example, the shaft part is a non-heating part or a joint part extending from the non-heating part. It is desirable to be in contact.

  When a sheath thermocouple is used for the thermocouple internal thermometer 61, the protective tube (pressure partition) is made of a material having heat resistance and thermal conductivity, such as noble metal, nickel, stainless steel, graphite, diamond-like carbon, diamond. It is desirable to use ceramics. The thermocouple internal thermometer 61 does not need to be inserted into the pressurizing container 44 as long as the gas phase temperature can be measured.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Example 1
Using the production apparatus shown in FIG. 4, a fine-line titanium compound was produced as follows. First, 208.4 g (NaOH molecular weight: 40.0) of sodium hydroxide (manufactured by Nakarai Tesque, reagent grade: 96% purity) was quickly weighed, and the whole amount was transferred to a beaker. At this time, NaOH adhering to the medicine spoon was also washed away with water. Then, taking care that the total amount of the solution does not exceed 500 mL, all NaOH was mixed with water in a beaker to obtain an aqueous sodium hydroxide solution. Water (pure water) used for dissolution at this time is pure water for electronics industry having a specific resistance of 19 MΩ · cm or more.

  Since NaOH generates heat of dissolution, stirring was performed with a magnetic stirrer and the beaker was cooled. Incidentally, the raw material NaOH includes Mg (5 ppm), Ca (0.002 mass%), Zn (0.001 mass%) and Al (0.002 mass%) as impurities. The number of element atoms was about 5 ppm for Mg, 0.001 atom% for Ca, 3 ppm for Zn, and about 0.002 atom% for Al with respect to the number of Na atoms. In addition to the impurity elements, impurities (cations) such as K (0.05% by mass), Pb (5 ppm), Fe (5 ppm) and Ni (0.001% by mass) are formed in the raw material NaOH. Element)), but this amount was so small that the effect of the present invention was not affected.

  The aqueous sodium hydroxide solution obtained as described above was transferred to a 500 mL volumetric flask. At this time, it was necessary to wash away the remaining aqueous sodium hydroxide solution, but it was washed away with care so as not to exceed the rating line. The aqueous sodium hydroxide solution after dissolution has heat, high viscosity, and bubbles do not easily escape, so the aqueous solution was left for about one night. After that, water was added to match the rating line. After adjusting to the rating line of the volumetric flask, the volumetric flask lid was firmly held, and the mixture was turned upside down and stirred for a while. At this time, since the volume was reduced by dissolution of water, the above process was repeated to fill the water up to the evaluation line, and a standard solution (10 mol / L) of an aqueous sodium hydroxide solution was obtained.

  10 g of anatase-type titanium oxide powder (manufactured by Wako Pure Chemical Industries, Ltd., Titanium (IV) Oxide, Anatase Form 5 μm) with respect to 100 mL of the standard solution (10 mol / L) of the aqueous sodium hydroxide solution obtained as described above: Were mixed in a beaker and stirred with a magnetic stirrer for 30 minutes or more to prepare a raw material solution in which anatase was well dispersed.

Put the above raw material solution into a polytetrafluoroethylene reaction vessel (length: 10 cm) 45, wrap a polytetrafluoroethylene tape around the screw part, and fill the raw material solution with 8.5% of the reaction vessel 45 with the solution. The airtight seal was made with some air left. The reaction vessel 45 was placed on the bottom of the pressurization vessel 44, and water was poured from the lid 44b in the pressurization vessel 44 to a position of 7 cm, and then the lid 44b of the pressurization vessel 44 was closed and sealed. The water 43 in the pressurizing container 44 was heated using the heater 46 to pressurize and heat the reaction container 45 (temperature: 185 ° C., gauge pressure: 0.95 MPa). After pressurization and heating were continued for 66 hours, the reaction vessel 45 was taken out and cooled to room temperature. In the obtained reaction solution, an aggregate of a soft and bulky titania compound (acicular compound) having a wound structure of Na ₂ Ti ₃ O ₇ was precipitated without precipitation.

A soft and bulky aggregate of needle-like compounds (needle compounds having a wound structure of Na ₂ Ti ₃ O) is washed and filtered with pure water, and Na ions and hydrogen ions are replaced with hydrochloric acid. After obtaining the gaseous compound, the sample was heated and dehydrated (dried) at 400 ° C. A scanning electron micrograph (photographing magnification: 5000 times) of the dried acicular compound (Example) is shown in FIG. Further, FIG. 6 shows a scanning electron micrograph (photographing magnification: 5000 times) of a needle-like compound (comparative example) obtained by synthesis with ordinary purity (other synthesis conditions are basically the same as those described above). .

  As is apparent from these results, it can be seen that the needle-like compound of the present invention (FIG. 5) has a finer form and longer fibers (high aspect ratio). On the other hand, in the acicular compound of the comparative example (FIG. 6), it can be seen that a certain amount of thinning is observed, but the fiber length is relatively short.

The acicular compound (Example) obtained above was lightly pulverized and subjected to Raman spectroscopic analysis to investigate its crystallinity. The measurement conditions at this time are as follows.
[Raman spectroscopy measurement conditions]
Apparatus: “NR-1000 laser Raman spectrophotometer” (trade name: manufactured by JASCO Corporation)
Excitation light source: Argon ion laser Excitation wavelength: 514.5 nm
Excitation light output: 100 mW
Measurement method: 90 degree scattering method Measurement beam diameter: about 200 μmφ
Integration count: 2 times

FIG. 7 shows the Raman spectrum of the needle compound of the example, and FIG. 8 shows the Raman spectrum of the needle compound of the comparative example. As is clear from these results, in the needle-shaped compound of the present invention using a high-purity raw material (FIG. 7), a part of the anatase crystal system coexists (peak of Raman shift 151 cm ⁻¹ ). It can be seen that a titanium compound mainly composed of TiO ₂ (B) type showing high crystallinity is formed. On the other hand, in the acicular compound of the comparative example (FIG. 8), it can be seen that the Raman spectrum peak spreads and the crystallinity is low.

The acicular compound (Example) obtained above was lightly pulverized and the BET specific surface area was measured using an “automatic specific surface area / pore distribution measuring device Tristar 3000” manufactured by Shimadzu Corporation. It was 0 m ² / g. Moreover, it was 37.9 m < ² > / g when the BET specific surface area in the acicular compound of the above-mentioned comparative example was measured.

(Example 2)
When various titania nanofibers (dried needle-like compounds) obtained in Example 1 were heat-treated at 600 ° C., 800 ° C., 900 ° C., 1000 ° C., 1050 ° C., and 1100 ° C. in an air atmosphere (the heating time was any 9 shows the X-ray diffraction (XRD) spectrum of the sample for 24 hours. As is apparent from this result, no rutile was observed in the sample even when the sample was heated at 1000 ° C. for 24 hours, which is usually reported to be a transition temperature to rutile, and anatase. It can be seen that only is observed.

  FIG. 10 shows a scanning electron micrograph (SEM photograph) obtained when the titania nanofiber obtained in Example 1 was dehydrated at 400 ° C. for 5 hours, when heated at 900 ° C. for 24 hours. A scanning micrograph (SEM photograph) is shown in FIG. 11 (photographed with a scanning electron microscope “S-800” manufactured by Hitachi, Ltd.). Scanning electron micrograph (SEM photograph) when heated at 1050 ° C. for 24 hours. Are shown in FIG. 12 (photographed with a scanning electron microscope “S-4000” manufactured by Hitachi, Ltd.).

  As is apparent from this result, it is understood that there is no significant difference in fiber shape between the two. As shown in Example 1, titania nanofibers synthesized by a high-purity hydrothermal synthesis method are thinned and have high crystallinity, and the phase transition temperature is about 200 ° C. in the heating step after the treatment. It turns out that it has shifted to the high temperature side.

(Example 3)
A slurry is prepared by adding polyethylene glycol (Poly-ethylene-glycol: PEG) as a binder and pure water as a solvent to the various titania nanofibers obtained in Example 1, and fluorine-doped indium oxide. It was applied on a transparent electrode (FTO electrode) by a blade method to form a titania electrode. At this time, the mixing ratio of titania nanofibers, PEG and water (titania nanofibers: PEG: pure water) is 1: 2.2: 31.7 (mass ratio).

  After applying the slurry, the step of firing the titania electrode at 400 ° C. was repeated to produce a three-layer titania nanofiber electrode, and dye N-719 (ruthenium dye) was adsorbed on the surface to constitute a solar cell. . Using this solar cell, various power generation characteristics [short circuit current (Jsc), open circuit voltage (Voc), power generation efficiency (η), fill factor (FF)] were measured.

  The short-circuit current (Jsc), open-circuit voltage (Voc), power generation efficiency (η), and fill factor (FF) are defined as follows. A voltmeter is connected between the front and back electrodes of the solar cell, the generated voltage measured by applying light is called an open circuit voltage (Voc), and the current measured by connecting to an ammeter is called a short circuit current (Jsc). These values vary depending on the type of semiconductor. In principle, the open circuit voltage (Voc) that is generated even if the cell area is different does not change, but the short circuit current (Jsc) is proportional to the area. Also, the maximum generated power Wm divided by the open circuit voltage (Voc) and the short circuit current (Jsc) is called the fill factor (FF). The closer this value is to 1, the better the performance of the solar cell. Means.

  Further, the ratio of the light energy hitting the solar cell, which indicates how much it is converted into electric energy, is referred to as the total conversion efficiency of the solar cell electrode [corresponding to the power generation efficiency (η) above], and the fill factor (FF) is shown. ) Can be expressed as a function of the short-circuit current (Jsc), the open-circuit voltage (Voc), and the fill factor (FF) (η = Jsc × Voc × FF). The measurement results are shown in Table 1 below. Based on this result, FIG. 13 shows the relationship between the heating time of the fine-line titania and the values of each characteristic (all evaluated with absolute values).

  From this result, we could consider as follows. The absolute values of the power generation efficiency are all low, but it is understood that the short-circuit current Jsc value is improved in the case of using the titania nanofiber heated at a high temperature. That is, it can be seen that when titania nanofibers are used as the electrode material, it is preferable to perform annealing at as high a temperature and as long as possible.

  The reason why such a phenomenon occurs has not been fully clarified, but could probably be considered as follows. That is, by heating the titania nanofibers at a high temperature for a long time, the “crystal defects” in the crystal lattice are repaired, and Na atoms remaining as impurities are diffused into inactive sites such as surfaces and grain boundaries. It can be considered that the efficiency of electron propagation is improved by trapping or discharging into the atmosphere and reducing the number of electron trap sites in the titania nanofiber crystal.

Example 4
As described above, the use of titania nanofibers heated at a high temperature for a long time improves the total conversion efficiency of the solar cell electrode [corresponding to the power generation efficiency (η) above]. In a dye-sensitized solar cell using a solar cell electrode made of a single fiber, practical high power generation efficiency cannot be secured. The reason for this was considered to be due to the extremely low dye adsorption ability of the dye N-719, which is a ruthenium dye. In this regard, the color of the titania nanofibers is apparent even when the dye is adsorbed at the same time as compared with the titania electrode produced from titania powder [“AEROXIDE TiO ₂ P25” Nippon Aerosil Co., Ltd. (Degussa)]. A white color could be visually confirmed.

For this reason, further studies were made on the structure of the titania electrode that can ensure high power generation efficiency using the synthesized titania nanofibers.

First, titania powder [80% by mass of anatase type crystal, 20% by mass of rutile type crystal: “AEROXIDE TiO ₂ P25” Nippon Aerosil Co., Ltd. (Degussa): hereinafter simply referred to as “P25”] and the above titania nano The composition ratio (mass ratio) of the fiber is changed in a range of 99: 1 to 90:10 (P25: titania nanofiber) to form a titania electrode having three layers, and the power generation efficiency of the dye-sensitized solar cell investigated. The results are shown in Table 2 below. Table 2 below also shows a dye-sensitized solar cell when the titania nanofiber compounding amount is fixed to 1% by mass and the titania electrode layers are thickened to 3 layers and 6 layers. The power generation efficiency (η) was also measured. Further, based on this result, the relationship between the configuration of the titania electrode and the value of each characteristic (all are evaluated as absolute values) is shown in FIG. 14 (bar graph). In Table 2, “3 layers (99: 1)” means that the compounding ratio (P25: titania nanofibers) is 99: 1 and that there are 3 layers (others are equivalent to this). ). In addition, what is described as three-layer P25 indicates an existing titania electrode (the electrode is composed of only the titania powder “P25”).

Table 3 below shows the relationship between the composition ratio of titania nanofibers and the power generation efficiency η for the three-layer titania electrode among the electrodes shown in Table 2. At this time, the ratio (η / η ₁ ) when the average value (η ₁ = 2.384) of the power generation efficiency η of the existing titania electrode “three-layer P25” is 1.0.
Is shown as an improvement rate.

  As is clear from this result, it is understood that the power generation efficiency η by the combination of titania nanofibers is improved in all the mixture ratios (99: 1 to 90:10). There are experimental variations and measurement errors in the improvement of power generation efficiency, and it is not possible to specify which blending ratio range is the optimum range, but in the range of the above blending ratio, the power generation efficiency is 1.2 times as a whole. Therefore, it can be judged that the ratio is improved 1.5 times.

  From the above considerations, it was found that the composition of titania nanofibers improves the power generation efficiency η. However, in the case of 1% by mass with the smallest blending ratio of titania nanofibers (the blending ratio is 99: 1). When the titania electrode is increased from 3 layers to 6 layers, the power generation efficiency η shows 5.9 (%), which indicates that the power generation efficiency η is the best.

1 TiO ₆ crystal octahedron 2 Na cation 3 K cation 4 Alkali metal element 5 Impurity element 41 Magnet 44 Sealed pressurization vessel 46 Heater 45 Sealed reaction vessel 61 Thermocouple internal thermometer

Claims (4)

A TiO ₂ (B) type fine wire titanium compound produced by subjecting a mixture of titanate and an aqueous alkali solution to pressure and heat treatment, wherein the TiO ₂ (B) type titanic acid is synthesized by the pressure and heat treatment A TiO ₂ (B) type fine wire titanium compound in which the amount of impurity element having a smaller cation radius than the alkali metal element in the salt is controlled to 1/1000 or less with respect to the number of moles of titanium element contained in the raw material. Then, an anatase type fine wire titanium compound, which has been heat-treated at a temperature of 900 ° C. or higher.   It is a titania electrode used for a dye-sensitized solar cell, Comprising: The titania electrode comprised by mixing the fine wire titanium compound of Claim 1, and a powdery titania.   The titania electrode according to claim 2, wherein a mixing ratio of the fine-line titanium compound is 0.1 to 20% by mass.   A dye-sensitized solar cell using the titania electrode according to claim 2.