JP2016056040A - Surface modification method of titanium oxide and modified body thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface modification method of titanium oxide and a modified body thereof, which can improve light absorption characteristics in the visible-light and infrared regions.SOLUTION: Titanium oxide fine particles are doped with a dope material according to a plasma method in liquid using pulsed microwave as an energy source, by irradiating an aqueous solution of titanium oxide fine particles with plasma using an electrode composed of a dope material. Further, the titanium oxide fine particles are modified into those imparted with light absorption characteristics in a part of the visible-light region by irradiating an aqueous solution of titanium oxide fine particles with plasma using an electrode whose electrode core material is covered with an insulating metal oxide.SELECTED DRAWING: None

Description

  The present invention relates to a surface modification method for titanium oxide and a modified body thereof.

  Titanium oxide is the ninth most abundant element on earth, and is an inexpensive mineral that can be used for various purposes depending on the crystal structure. There are anatase, rutile, and brookite crystal structures, and anatase and rutile have the property of transmitting visible light. Particles of submicron or larger become white due to light scattering, and particularly the rutile type is often used as a white pigment because of its high thermal stability. The anatase type and rutile type are also used as building materials for exteriors due to anti-fogging and self-cleaning effects of antibacterial and superhydrophilic materials and glass due to the photocatalytic action of titanium oxide.

  In solar cells, when the specific surface area was increased (atomization) in order to increase the catalytic activity, light scattering was reduced and visible light was transmitted, so that the output as a battery was not obtained. In addition, Mr. Gretzel of Lausanne University of Technology in Switzerland confirmed that the output was dramatically increased by adsorbing a visible light absorbing dye (sensitizing dye) on the surface of the titanium oxide fine particles. This is called a dye-sensitized solar cell. As a general manufacturing method, titanium oxide fine particle ink is printed on a substrate on which a transparent conductive film is formed, and then the titanium oxide is formed on the substrate using a technique such as sintering. After fixing the fine particles, the dye is adsorbed on the titanium oxide surface by being immersed in a sensitizing dye solution. Thereafter, an electrolyte layer is formed between the counter electrode and bonded to form a solar battery cell. In this manufacturing method, since there is no vacuum or ultra-high temperature process (heating at about 500 ° C. is necessary with the formation of the sintered body), it is attracting attention as a practical low-cost solar cell, and research is progressing.

International Publication No. 2013/027838 JP 2010-121193 A JP 2011-056428 A JP 2011-058064 A JP 2012-036468 A JP2013-237582A

  As a photocatalyst, titanium oxide can be used only in the ultraviolet region in the irradiation energy of sunlight, due to its own visible light transmission characteristics. From the solar spectral energy distribution shown in Fig. 1 (ASTM G173-03), calculating the energy utilization rate with the ultraviolet region below 380 nm confirms that only 4.6% of the solar energy is used. . In addition, when used as an exterior building material having a photocatalytic function in an eco house recently recommended by the Ministry of the Environment, it is necessary to use a thermal barrier paint or the like because it transmits through the infrared region due to its characteristics. is there.

  As dye-sensitized solar cells being developed as titanium oxide solar cells, sensitizing dyes are roughly classified into organic dyes and inorganic dyes. Since organic dyes are inexpensive but have a problem in durability, generally highly stable inorganic dyes are used. However, inorganic dyes are composed of metal complexes based on noble metals such as Ru and Pt and are extremely expensive. In addition, the immersion solution used in the dye adsorption step requires an organic acid such as deoxycholic acid dissolved in an alcohol solvent in order to prevent the association of the metal complex in addition to the metal complex described above (Patent Literature). 1). For this reason, equipment such as a local exhaust device is required in this step.

  The present invention has been made in view of the circumstances as described above, and provides a surface modification method for titanium oxide and a modified body thereof that can improve the light absorption characteristics in the visible light and infrared regions. Is an issue.

  In order to solve the above-mentioned problem, the method for producing doped titanium oxide fine particles of the present invention uses an electrode composed of a dope material to form an aqueous solution of titanium oxide fine particles by an in-liquid plasma method using pulsed microwaves as an energy source. The titanium oxide fine particles are doped with the dope material by irradiating with plasma. In the method for producing the doped titanium oxide fine particles, the dope material is preferably tungsten. By this method for producing doped titanium oxide fine particles, doped titanium oxide fine particles having light absorption characteristics from visible light to infrared region can be obtained.

  The titanium oxide fine particle reforming method of the present invention is a plasma in a titanium oxide fine particle aqueous solution using an electrode in which an electrode core material is coated with an insulating metal oxide by a submerged plasma method using pulsed microwaves as an energy source. Is characterized in that it is modified to titanium oxide fine particles having a light absorption property in a part of the visible light region.

  The titanium oxide fine particles of the present invention are obtained by any one of the methods described above, and have light absorption characteristics in a part of the visible light region or from the visible light to the infrared region.

  The absorption range of titanium oxide treated with submerged plasma shifts to the visible light side. When tungsten is doped, the absorption range is expanded from the visible light to the infrared range. As a result, solar energy irradiated on the earth can be used efficiently, and the photocatalytic activity is dramatically improved. In addition, when the obtained tungsten-doped titanium oxide is applied to building materials for exteriors, in addition to obtaining a self-cleaning effect by a stronger catalytic action than conventional, it is possible to shield the thermal energy of sunlight as an infrared shielding material It is possible to provide an eco-house function that prevents the indoor temperature from rising and suppresses the use of an air conditioner without preparing a separate thermal barrier paint. Moreover, since it has a visible light absorption characteristic, application to a solar cell without a sensitizing dye can be expected.

  When it is necessary to ensure visible light, such as when applied to window glass, titanium oxide modified by plasma treatment without doping is suitable. Although visible light transmission characteristics are slightly inferior to commercially available photocatalytic titanium oxide, it has been confirmed that the transmittance decreases to the same level as that of commercially available UV-cut glass, and it is considered that there is almost no substantial influence. In addition to being able to use much of the solar energy with a peak near 500 nm, this material can be expected to be applied to solar cells that transmit visible light.

It is the figure which showed the spectral energy characteristic of sunlight. It is the schematic of a plasma apparatus in a microwave liquid. It is a graph which shows the relationship between plasma processing time and a tungsten electrode elution amount. It is a chart which shows the XRD measurement result in the evaluation 1. It is a HAADF-STEM image in evaluation 2. It is a light absorption spectrum in evaluation 3 (Examples 1-6). It is a light absorption spectrum in evaluation 3 (Examples 7-11). It is a graph which shows the result of the catalyst activity measurement at the time of the high pressure mercury lamp irradiation in Evaluation 3. It is a spectrum of the high pressure mercury lamp used for the catalytic activity measurement.

  The present invention is described in detail below.

  In the present invention, the titanium oxide fine particles are modified by plasma irradiation. Specifically, pulse plasma is generated in an aqueous solution of titanium oxide fine particles using a microwave power source.

  When producing doped titanium oxide, a material serving as a doping source is used as the electrode material, and when performing simple plasma processing, an electrode in which the electrode core material is covered with a metal oxide is used. Since the metal oxide is an insulator but only covers the conductive core material, AC conductivity is ensured. In any method, hydrogen radicals generated in the plasma are thought to produce oxygen deficient products, and the electrode material is ionized in the plasma by configuring the electrode with the element to be introduced as a doping source. In the process of recondensing, the titanium oxide fine particles are doped.

  The absorption range of titanium oxide treated with submerged plasma shifts to the visible light side. When tungsten is doped, the absorption range is expanded from the visible light to the infrared range. As a result, solar energy irradiated on the earth can be used efficiently, and the photocatalytic activity is dramatically improved. In addition, absorption in the visible light region is possible without additives such as sensitizing dyes, so that application to solar cells can be expected.

  The raw material used for these treatments can use commercially available titanium oxide fine particles, and no special raw material is required.

  As a doping source, metallic tungsten is used as a plasma generating electrode and supplied from itself.

  Since water can be used as the solvent, it is possible to construct a very simple and safe system in which only titanium oxide fine particle powder to be modified and water are placed in the reaction vessel. In this system, since the dope material is supplied at a constant rate, it is considered that materials having different dope amounts can be produced by changing the plasma processing time from several minutes to several hours.

  The doping material may be other than tungsten, but the ionic radius of tungsten (VI) is 0.58mm, and the ionic radius of titanium (IV) is relatively close to 0.61mm (Handbook of Chem. & Phys., 79th Edition), because tungsten is easily replaced with titanium sites in the crystal, and since the doping material is used as an electrode, the electrode has a certain degree of conductivity and is a high melting point material, and it is excessive even in the heat in the plasma. Tungsten is preferable because it is required that no evaporation or ionization occurs.

  In addition, the microwave in-liquid plasma apparatus can be easily manufactured as a high-power system of kW class, and can be realized with a compact apparatus configuration that does not require a large-scale vacuum apparatus such as gas phase plasma, and has a great industrial advantage.

  In addition, since the pulsed plasma used this time is intermittently generated, the sample is not exposed to an unnecessarily high temperature because it immediately reaches a high temperature during ignition and is instantaneously cooled by an aqueous solution during fire extinguishing. The advantage is that the crystal structure does not change significantly before and after the plasma treatment. Moreover, in this invention, it is the simple structure which processes titanium oxide aqueous solution for several minutes to several hours by the plasma in a microwave liquid, and application as an industrial process is also easy.

  The inventors of the present invention have made various studies including the synthesis of nanoparticles on a submerged plasma method using pulsed microwaves as an energy source. In the present invention, the conventional technology such as the devices disclosed in Patent Documents 2 to 6 is referred to for the configuration of the “pulsed microwave plasma device”.

  The pulse microwave submerged plasma apparatus includes a microwave oscillator, a waveguide, a container for storing a liquid, and a submerged plasma source. Hereinafter, a description will be given with reference to the example of FIG.

  The microwave oscillator 10 generates and outputs a microwave, a power supply that supplies power to the magnetron for microwave generation, etc., and sends a signal to the microwave power supply to adjust the output of the microwave. It has a microwave power controller (function generator) to control.

  The waveguide 20 propagates the microwave output from the microwave oscillator 10 to the container 30. A three-dimensional circuit such as a power monitor that measures microwave power of each of emission and reflection and a tuner (such as a sleeve tuner) that matches load impedance can be attached to the waveguide 20. In addition, a terminal plunger or the like can be used for the waveguide 20 at the end opposite to the microwave oscillator 10. Furthermore, the waveguide 20 has a coaxial waveguide converter 21.

The container 30 is a container for storing a liquid (aqueous solution 31). Plasma is generated in the liquid stored in the container 30. In FIG. 2, reference numeral 32 denotes a quartz window through which the inside of the container 30 can be seen, 33 denotes a cooling rod for cooling the aqueous solution 31, and 34 denotes a thermometer such as a thermocouple.
The liquid is supplied with electric power for generating plasma in the liquid to modify the titanium oxide fine particles. This power is not a direct current pulse but a microwave having a single frequency spectrum such as 2.45 GHz, 5.8 GHz, and 9.5 GHz band. For this reason, high power supply efficiency becomes possible by optimizing the resonance structure and transmission line impedance, and the load on the electrodes can be reduced by using microwaves as the driving power. The microwave power is preferably pulsed with a plurality of periods as one pulse. When microwave power capable of steady plasma discharge is input to the plasma source, intense heat is generated by the power and the electrode is destroyed. However, the electric power for generating plasma is high, and in order to simultaneously realize these conflicting demands, the electric power supply needs to be a microwave pulse. On the other hand, if the pulse width of the microwave pulse is shortened, the plasma becomes corona discharge, that is, non-thermal equilibrium plasma, the temperature rise is suppressed, and electrode wear is remarkably reduced. However, since the energy given to the liquid becomes small, there is a concern that the reaction rate becomes slow.

  The in-liquid plasma source is a device for supplying the microwave propagating through the waveguide 20 to the liquid. This submerged plasma source has a coaxial tube 22 and an electrode 23.

  The coaxial tube 22 constitutes a part of the coaxial waveguide converter 21 and receives microwaves from the waveguide 20 and propagates them. In the coaxial waveguide converter 21, the waveguide 20 and the coaxial tube 22 are connected vertically. For this reason, when the microwave is transmitted from the waveguide 20 to the coaxial tube 22, the propagation direction is changed to the vertical direction. The coaxial tube 22 has a coaxial tube structure and includes a coaxial tube inner conductor 22a and a coaxial tube outer conductor 22b.

  The coaxial pipe inner conductor 22a is disposed coaxially with the coaxial pipe outer conductor 22b in the hollow of the coaxial pipe outer conductor 22b. The coaxial tube inner conductor 22a has a barrel portion formed in a cylindrical shape, one end thereof connected to the coaxial waveguide converter 21, and the other end formed in a conical shape. 23. By exposing the tip of the electrode 23 in the aqueous solution 31 of titanium oxide, plasma 35 can be generated in this portion.

  For example, when tungsten is used for the electrode 23, a tungsten wire may be inserted into the coaxial tube inner conductor 22 a from the coaxial waveguide converter 21 side and extended from the tip as the electrode 23. As described above, when the electrode 23 is constituted by a movable tungsten rod that can move in the axial direction in the coaxial tube inner conductor 22a, even if the electrode 23 is consumed due to a reaction, the electrode 23 is externally connected. It can be sent out into the system by the operation of. Further, as described above, the electrode 23 in which the electrode core material is coated with the insulating metal oxide can be formed by coating the conical tip portion with the metal oxide. In FIG. 2, the code | symbol 24 is a refrigerant | coolant for cooling the coaxial pipe | tube internal conductor 22a.

  As described above, the method of the present invention includes a container 30 that contains a liquid, a microwave oscillator 10 that outputs a microwave, and an electrode 23 that applies a microwave to the liquid to excite plasma in the liquid. It is performed using a pulsed microwave plasma apparatus equipped with.

  After putting the aqueous solution 31 of titanium oxide fine particles into the container 30 of the pulsed microwave plasma device, the power of the microwave oscillator 10 of the pulsed microwave plasma device is turned on, and the microwave is output from the magnetron. The microwave propagates through the waveguide 20 and is given to the aqueous solution 31 through the coaxial waveguide converter 21 and the in-liquid plasma source. Thereby, plasma 35 is generated in the aqueous solution 31. Then, by using the electrode 23 made of tungsten, this tungsten can be eluted into the aqueous solution 31 and doped into the titanium oxide fine particles, whereby doped oxidation giving light absorption characteristics from the visible light to the infrared region. Titanium fine particles can be obtained. Further, by using the electrode 23 in which the electrode core material is coated with an insulating metal oxide, it can be modified into titanium oxide fine particles having a light absorption characteristic in a part of the visible light region.

  Since the titanium oxide fine particles obtained by the present invention have a light absorption property in a part of the visible light region or from the visible light to the infrared region, a photocatalyst, an ultraviolet blocking material, an infrared blocking material, a white pigment, Applications in fields such as insulating paints and solar cells are expected.

10 Microwave Oscillator 20 Waveguide 21 Coaxial Waveguide Converter 22 Coaxial Tube 22a Inner Conductor 22b Outer Conductor 23 Electrode 24 Refrigerant 30 Container 31 Aqueous Solution 32 Quartz Window 33 Cooling Rod 34 Thermometer 35 Liquid Plasma

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

Example 1
An outline of the microwave plasma device is shown in FIG. Conducting a 2.45GHz pulsed microwave generated by the microelectronic 1.5kW oscillator UM-1500EC-B to the reaction vessel through a waveguide, and concentrating the microwave energy at the tip of the tungsten electrode by coaxial waveguide conversion Thus, a plasma state is generated in the liquid. It can be confirmed from the experimental results shown in FIG. 3 that the tungsten electrode evaporates and ionizes into the solution and elutes as tungsten or tungsten oxide in the underwater plasma in which no raw material is charged. Since the elution amount is proportional to the plasma processing time, it is considered that the dope amount can be controlled by the processing time. The raw material used was ST-01 manufactured by Ishihara Sangyo, which is commercially available as photocatalytic titanium oxide.

  The plasma treatment is performed by the following method. Add 1g of raw titanium oxide to 500mL of ultrapure water and stir for 16 hours or more to make a well-dispersed titanium oxide aqueous solution, transfer this solution to the reaction vessel, and average power of 700W under atmospheric pressure for 46 minutes The plasma treatment was performed. In order to remove the tungsten oxide derived from the electrode, the obtained solution was washed in an alkaline aqueous solution adjusted to pH 13 with sodium hydroxide, filtered and dried, and recovered as a tungsten-doped titanium oxide powder.

Example 2
Add 2g of raw titanium oxide to 500mL of ultrapure water and stir for 16 hours or more to make a well-dispersed titanium oxide aqueous solution, transfer this solution to the reaction vessel, below 20kPa-abs. Plasma treatment was performed for 30 minutes under reduced pressure. It is characterized in that a stable plasma state can be obtained even under low input power under reduced pressure. A diaphragm pump was used for decompression. In order to remove the tungsten oxide derived from the electrode, the obtained solution was washed in an alkaline aqueous solution adjusted to pH 13 with sodium hydroxide, filtered and dried, and recovered as a tungsten-doped titanium oxide powder. The raw materials used and the generation principle of plasma are the same as in the first embodiment.

Example 3
The treatment was performed in the same manner as in Example 2 with a plasma treatment time of 60 minutes.

Example 4
The treatment was performed in the same manner as in Example 2 with a plasma treatment time of 90 minutes.

Example 5
The treatment was performed in the same manner as in Example 2 with a plasma treatment time of 130 minutes.

Example 6
The treatment was performed in the same manner as in Example 2 with a plasma treatment time of 180 minutes.

Example 7
The experiment was conducted using an electrode in which a 0.5 mm thick yttrium oxide film was formed on a stainless steel core material instead of the tungsten electrode as a doping source. In this method, the plasma energy density is low due to the electrode shape, and the dope material does not elute, so that the superiority in the case of performing the treatment purely by plasma irradiation can be confirmed.

  Add 2g of raw material titanium oxide to 500mL of ultrapure water and stir for 16 hours or more to make a well-dispersed titanium oxide aqueous solution, transfer this solution to the reaction vessel, and lower than 20kPa-abs. Plasma treatment was performed for 5 minutes under reduced pressure. In this method, since an insulating film is formed on the electrode, some input power is required even under reduced pressure. The obtained solution was filtered and dried, and recovered as plasma-treated titanium oxide powder.

Example 8
The treatment was performed in the same manner as in Example 7 with a plasma treatment time of 10 minutes.

Example 9
The treatment was performed in the same manner as in Example 7 with a plasma treatment time of 20 minutes.

Example 10
The treatment was performed in the same manner as in Example 7 with a plasma treatment time of 40 minutes.

Example 11
The treatment was performed in the same manner as in Example 7 with a plasma treatment time of 80 minutes.

The evaluation shown below was performed using untreated Ishihara Sangyo photocatalyst titanium oxide ST-01 as a comparative example raw material titanium oxide.

Evaluation 1 (XRD analysis)
FIG. 4 shows a result of comparison by XRD analysis of the titanium oxide powder subjected to the plasma treatment. It was confirmed that the comparative example was anatase-type titanium oxide, and the same anatase-type was shown in Example 1 where the plasma treatment was performed. From this result, it can be confirmed that the crystal structure does not change even when the plasma is irradiated.

Evaluation 2 (electron microscope observation)
FIG. 5 shows the result of comparison between the comparative example and the titanium oxide powder of Example 1 subjected to the plasma treatment by HAADF-STEM images. Since the HAADF-STEM image can obtain a contrast proportional to the square of the atomic number, it is characterized in that the portion with a high atomic number is observed white. Since the atomic numbers of titanium and tungsten are 22, 74, respectively, if tungsten is doped, an image in which the doped portion becomes a white bright spot should be obtained. As a result of the observation, in Example 1 after the plasma treatment, it has been confirmed that there are bright spots that are not seen in the comparative example, and it is considered that titanium oxide is doped with tungsten.

Evaluation 3 (Optical characteristic evaluation)
The light absorption spectrum of the sample was calculated from the reflectance obtained from the diffuse reflection spectrum using an ultraviolet-visible spectrophotometer V-670 manufactured by JASCO. As a result, as shown in FIG. 6, in the wavelength range of visible light of 380 to 750 nm, the comparative example transmits almost, whereas the tungsten-doped titanium oxides of Examples 1 to 6 show absorption. In Examples 7 to 11 in which only plasma irradiation was performed without doping tungsten, absorption was observed in the region of 380 to 500 nm as shown in FIG. Absorption at 380-500 nm increases with increasing processing time. This is considered to be because oxygen deficiency of titanium oxide occurs due to exposure to plasma under reduced pressure as the treatment time increases.

  Moreover, about the Examples 1-6 which performed tungsten dope, the photocatalytic activity was evaluated with the following method. After putting a 50 mg sample, 5 mL of 5 vol% acetic acid aqueous solution and a stirrer into a reinforced hard test tube P18 made by Nippon Denka Glass, this test tube was put on a test table controlled at 25 ° C. After irradiating light with a high pressure mercury lamp for 20 minutes, the gas inside was collected, and the amount of carbon dioxide generated by a GC-8A gas chromatograph manufactured by Shimadzu was compared. The relative photocatalytic activity when the comparative example is 1 is shown in FIG. As a result, it was confirmed that the catalytic activity of tungsten-doped titanium oxide produced by plasma treatment exceeded that of the comparative example in almost all samples. The high-pressure mercury lamp used for the evaluation is mainly composed of an ultraviolet light component of 380 nm or less as shown in FIG. 9, but also contains a considerable amount of visible light component. It is considered that the visible light absorption characteristic developed by tungsten doping contributes to the improvement of the catalytic activity, that is, the experimental result is a slight visible light component of 405, 408, 436, 546, 577, 579 nm contained in the mercury lamp. On the other hand, it is considered that the catalyst activity is improved by up to 73% compared with the conventional catalyst. In the sunlight supplied in a continuous spectrum (Fig. 1), the catalyst activity is much higher than the experimental result. It is thought to show. In Examples 7 to 11, photocatalyst measurement was not performed because there were only three visible light absorption regions 405, 408, and 436 corresponding to the spectrum of the mercury lamp, but the same effect was obtained from the above results. Can be guessed.

  From the above, the tungsten-doped titanium oxide of Examples 1 to 6 obtained by the plasma treatment has sufficient activity as an ultraviolet / visible light reaction photocatalyst, and has a characteristic that the amount of absorption increases from the visible light toward the infrared region. Confirmed. When this material is applied to exterior building materials, it exhibits a self-cleaning effect with a stronger catalytic action than before, and also prevents the rise of indoor temperature by shielding the thermal energy of sunlight as an infrared shielding material. It is possible to provide a function of suppressing the use of an air conditioner or the like. Further, this tungsten-doped titanium oxide has a visible light absorption characteristic, and can be expected to be applied to a solar cell without a sensitizing dye.

  On the other hand, when it is necessary to secure visible light such as application to window glass, Examples 7 to 11 in which plasma treatment is performed without doping are desirable. Although absorption is seen up to the region of 380 to 500 nm compared with the comparative example, the transmittance of the same region of the commercially available UV cut glass is about 60 to 80%, so the light absorption in this range is substantially There seems to be little influence. Solar energy is known to have an energy peak around 500 nm (FIG. 1), and in this sample, this large irradiation energy can be used to dramatically improve the catalytic activity.

Claims (5)

  Doped titanium oxide in which titanium oxide fine particles are doped by irradiating plasma to a titanium oxide fine particle aqueous solution using an electrode composed of the dope material by an in-liquid plasma method using pulsed microwave as an energy source A method for producing fine particles.   The manufacturing method of the dope titanium oxide microparticles | fine-particles of Claim 1 whose said dope material is tungsten.   The manufacturing method of the doped titanium oxide fine particle of Claim 1 or 2 which obtains the doped titanium oxide fine particle which provided the light absorption characteristic from visible light to the infrared region.   Part of the visible light region is obtained by irradiating plasma to a titanium oxide fine particle aqueous solution using an electrode whose electrode core is coated with an insulating metal oxide by an in-liquid plasma method using pulsed microwaves as an energy source. A modification method of titanium oxide fine particles, which is modified to titanium oxide fine particles imparted with light absorption characteristics.   Titanium oxide fine particles obtained by the method according to any one of claims 1 to 4 and having a light absorption property in a part of a visible light region or in a visible light region to an infrared region.