JP5008433B2 - Method for producing zinc oxide crystals - Google Patents

Description

The present invention relates to a method for producing a zinc oxide crystal, which is expected to be used and applied in applications such as light emitters, dye-sensitized solar cells, and photocatalysts.

  Vapor deposition methods such as sputtering, molecular beam epitaxy, metal organic vapor phase epitaxy, ion plating, and reactive plasma deposition have been actively developed in recent years. All of them required special and very expensive equipment. In addition, many methods have problems from the viewpoint of cost and production efficiency in that the thin film preparation takes a long time.

  For this reason, research is being carried out on the liquid phase method, which is less expensive to produce and less burdensome on the environment than the gas phase method.

  For example, the method includes a step of holding a substrate in an electrolytic solution and a step of forming zinc oxide on the substrate by electrodeposition, and the electrolytic solution includes zinc ions and at least one or more additives. A method for producing a zinc oxide needle-like structure characterized by this is disclosed (for example, see Patent Document 1). Examples of the additive used in the method of Patent Document 1 include organic solvents, halides, monomers, monomer polymers, and surfactants.

In addition, a substrate having a crystal plane containing a metal-containing material having a regular crystal orientation structure in a fixed orientation is immersed in a reaction solution capable of depositing a metal oxide to oxidize the crystal plane containing the metal-containing material. A manufacturing method of a metal oxide structure characterized by depositing a physical crystal is disclosed (for example, see Patent Document 2). This Patent Document 2 describes that a metal oxide structure and metal oxide particles having either a needle shape or a rod shape can be produced efficiently and at low cost.
JP 2002-356400 A (Claims [Claim 1] and [Claim 2]) JP 2006-96591 A (Claims [Claim 1], Specification [0007])

  However, the method disclosed in Patent Document 1 requires special equipment such as electrolysis. In addition, in the method disclosed in Patent Document 2, since it is necessary to have a highly oriented crystal film as a substrate, there is a limitation on the substrate to be used, or it is necessary to form a seed layer on the substrate, and productivity is increased. Had low problems.

  On the other hand, when zinc oxide is applied to a dye-sensitized solar cell or a photocatalyst, it is required to have a large surface area. There has been no example in which zinc oxide having a sufficiently large surface area was obtained by the in-solution precipitation method.

An object of the present invention is to provide a method for producing a zinc oxide crystal having a large surface area.
The objective of this invention is providing the manufacturing method of the zinc oxide crystal | crystallization which does not require baking and can be manufactured simply at low temperature.

The invention according to claim 1 adjusts an aqueous solution containing at least a water-soluble polymer as a zinc ion, an amine compound and an additive and having a pH adjusted to 7 or higher to 50 ° C. to 90 ° C. Immerse the base material and leave it in the immersed state for a predetermined time, so that the crystal shape on the base material is composed of a plurality of petal-like parts and a central part surrounded by these petal-like parts. This zinc oxide crystal is precipitated in a self-organized manner.
In the invention according to claim 1, a zinc oxide crystal having a flower shape on the substrate can be produced by a simple process of immersing the substrate in an aqueous solution containing at least zinc ions and an amine compound. Moreover, the number of petal-like parts of flower-like zinc oxide crystals can be increased by including a water-soluble polymer as an additive in the aqueous solution and setting the temperature of the aqueous solution within the above range. The zinc oxide crystal obtained in the present invention does not need to be fired and has higher productivity than the conventional technique. The zinc oxide crystal thus obtained has a large surface area.

The invention according to claim 2 is the manufacturing method according to claim 1, wherein the amine compound is ammonia.
In the invention which concerns on Claim 2, if an amine compound is ammonia, the number of petal-like parts of a flower-like zinc oxide crystal can be increased.

The invention according to claim 3 is the method according to claim 1, wherein the water-soluble polymer is a polyol-based water-soluble polymer.
In the invention according to claim 3, if the water-soluble polymer is a polyol-based water-soluble polymer, the number of petal-like portions of flower-like zinc oxide crystals can be particularly increased.

The invention according to claim 4 is the manufacturing method according to any one of claims 1 to 3, wherein the zinc ion concentration in the aqueous solution is 0.005 to 0.50 mol / L.
In the invention which concerns on Claim 4, if the zinc ion density | concentration is in the said range, the number of petal-like parts of a flower-like zinc oxide crystal can be increased.

  The invention according to claim 5 includes immersing the substrate in a solution that includes at least a water-soluble polymer as a zinc ion, an amine compound, and an additive, has a pH adjusted to 7 or more, and is adjusted to a temperature of 50 ° C. to 90 ° C. By leaving it in a dipped state for a predetermined time, a flower-like zinc oxide crystal whose crystal shape is composed of a plurality of petal-like parts and a central part surrounded by these petal-like parts on a substrate is obtained. It is the aqueous solution for zinc oxide crystal preparation used for the manufacturing method of Claim 1 made to precipitate in a self-organizing manner.

  The invention according to claim 6 is the aqueous solution according to claim 5, wherein the amine compound is ammonia.

  The invention according to claim 7 is the invention according to claim 5, wherein the water-soluble polymer is an aqueous solution of a polyol-based water-soluble polymer.

  The invention according to claim 8 is the invention according to any one of claims 5 to 7, wherein the aqueous zinc solution has a zinc ion concentration of 0.005 to 0.50 mol / L.

  The method for producing a zinc oxide crystal according to the present invention comprises preparing an aqueous solution containing at least a water-soluble polymer as a zinc ion, an amine compound and an additive and having a pH adjusted to 7 or more. The substrate is immersed in the adjusted and temperature-adjusted aqueous solution, and left in the immersed state for a predetermined time, so that the crystal shape is surrounded by a plurality of petal-like portions and these petal-like portions on the substrate. The flower-like zinc oxide crystal formed by the central part is deposited in a self-organized manner. Cost of depositing the substrate directly in the solution in a simple process of immersing the substrate in an aqueous solution containing at least zinc ions and an amine compound, without firing at a high temperature, and without using an expensive or complicated device. In particular, it is possible to produce a zinc oxide crystal having a flower shape by a very advantageous method. Moreover, the number of petal-like parts of flower-like zinc oxide crystals can be increased by including a water-soluble polymer as an additive in the aqueous solution and setting the temperature of the aqueous solution within the above range.

Next, the best mode for carrying out the present invention will be described.
The method for producing a zinc oxide crystal according to the present invention comprises preparing an aqueous solution containing at least zinc ions and an amine compound and having a pH adjusted to 7 or more, and immersing the substrate in this aqueous solution, thereby forming a crystal shape on the substrate. Is characterized by precipitating flower-like zinc oxide crystals in a self-organized manner. Cost of depositing the substrate directly in the solution in a simple process of immersing the substrate in an aqueous solution containing at least zinc ions and an amine compound, without firing at a high temperature, and without using an expensive or complicated device. In particular, it is possible to produce a zinc oxide crystal having a flower shape on a substrate by a very advantageous method.

  The aqueous solution used for precipitation of zinc oxide crystals is prepared by dissolving zinc ions and an amine compound in water and adjusting the pH. Examples of the compound capable of forming zinc ions in an aqueous solution include zinc nitrate, zinc sulfate, zinc acetate, and zinc chloride.

  The zinc ion concentration in the aqueous solution is adjusted within the range of 0.005 to 0.50 mol / L. Here, the zinc ion concentration in the aqueous solution was in the range of 0.005 to 0.50 mol / L because if the zinc ion concentration was less than 0.005 mol / L, the concentration was too low and crystals were formed on the substrate. This is because when the zinc ion concentration exceeds 0.50 mol / L, the number of petal-like parts of the flower-like zinc oxide crystal is reduced and the surface area is reduced. Among these, the ratio that the zinc ion concentration in the aqueous solution is 0.01 to 0.20 mol / L tends to decrease the rate of crystal precipitation when it is less than 0.01 mol / L. Since exceeding the crystal deposition rate tends to not increase even if the value is exceeded, it is particularly preferable.

  The amine compound is an essential component for producing a zinc oxide crystal having a flower shape, but also functions as pH adjustment of the aqueous solution. Examples of amine compounds include ammonia, ethylamine, diethylamine, triethylamine, hexamethylenetetramine and the like. Among these, ammonia is particularly preferable because the number of petal-like portions of flower-like zinc oxide crystals can be increased.

  Here, the reason why the pH of the aqueous solution is set to 7 or more is that when the pH is less than 7, the film does not precipitate on the substrate. Among these, the ratio that the pH of the aqueous solution is 8 to 11 is particularly preferable from the viewpoint of the precipitation rate of crystals.

  In addition, it is preferable to include a water-soluble polymer as an additive as a raw material for an aqueous solution, since a flower-like zinc oxide crystal having a particularly large surface area, that is, having a large number of petal-like parts is obtained. Examples of the water-soluble polymer include polyol-based water-soluble polymers. If the water-soluble polymer is a polyol-based water-soluble polymer, the number of petal-like parts of flower-like zinc oxide crystals can be particularly increased. Examples of the polyol-based water-soluble polymer include polyethylene glycol, polypropylene glycol, copolymers and derivatives thereof. For example, when polyethylene glycol (PEG) is used as the water-soluble polymer, the C—O—C group (hydrophilic portion) in PEG is partially modified on the surface of zinc oxide, so that the site for crystal growth is partially Thus, it is considered that a flower-like zinc oxide crystal having a large number of petal-like parts is obtained.

  The temperature of the aqueous solution is adjusted when the substrate is immersed in the aqueous solution. The temperature of the aqueous solution when depositing zinc oxide crystals is 50 to 90 ° C. When the temperature of the aqueous solution is within the above range, the number of petal-like parts of flower-like zinc oxide crystals can be increased. Here, the temperature of the aqueous solution is set to 50 to 90 ° C., because the reaction does not proceed at a temperature lower than 50 ° C., and when the temperature exceeds 90 ° C., the water evaporates quickly and the zinc ion aqueous solution becomes dry. . Among these, the temperature of the aqueous solution is particularly preferably 60 to 80 ° C. from the viewpoint of the deposition rate of the film.

  Any substrate may be used as long as the pH is 7 or more and the surface is negatively charged. In particular, a base material made of a metal oxide is preferable. For example, a base material having a negatively charged surface can be obtained by immersing a silicon wafer or a glass substrate in an aqueous solution having a pH of 7.0 or more.

By immersing the base material in the prepared aqueous solution and allowing to stand for a predetermined time, a zinc oxide crystal having a flower shape is deposited on the base material in a self-organized manner. For example, when ammonia is used as an amine compound and a substrate having a negatively charged surface is immersed in an aqueous solution, zinc ions are converted into ammonia in the aqueous solution as shown in the following formulas (1) to (2). Then, an ammine complex ion that is positively charged gathers on the substrate surface due to the interaction with the negative charge existing on the substrate, and the collected ammine complex reacts with OH ⁻ in the aqueous solution. Thus, it is assumed that ZnO nucleation occurs, nucleus growth proceeds rapidly from that point, and a zinc oxide crystal having a flower-like shape is formed on the substrate.

Zn ²⁺ + 4NH ₃ → Zn (NH ₃ ) ₄ ²⁺ (1)
Zn (NH ₃ ) ₄ ²⁺ + 2OH ⁻ → ZnO + H ₂ O + 4NH ₃ (2)
In addition, it is known that zinc oxide is amphoteric if the aqueous solution is simply allowed to stand without immersing the base material, and therefore, when the pH of the aqueous solution is high, it does not precipitate while dissolved in the aqueous solution. Yes.

  The time for immersing the substrate in the aqueous solution is preferably 2 hours or less. Here, the reason why the immersion time is set to 2 hours or less is that when the immersion time is too long, the number of the petal-like parts formed tends to decrease. Among these, the immersion time is particularly preferably 2 hours or less.

  In addition, what kind of thing may be used for the container at the time of immersing a base material in aqueous solution and depositing a zinc oxide crystal | crystallization. It is preferable to use an open container or a closed container in accordance with various conditions for depositing crystals.

  The zinc oxide crystal manufactured by the above manufacturing method has a diameter in the range of 1 to 20 μm and forms a flower-like crystal shape. The flower-like zinc oxide crystal is composed of a plurality of petal-like parts and a central part surrounded by these petal-like parts. One end of the petal-like part is pointed, the other end of the petal-like part is located on the outer peripheral part of the center part, and the lateral width of the petal-like part is 0.1 to 1 μm and the length is 0.2 to 5 μm. The central portion is composed of one or two or more secondary aggregates in which fine particles are aggregated, and the size of the central portion is 1 to 5 μm. A zinc oxide crystal having such a shape has a large surface area.

  Since the zinc oxide crystal has a light emission wavelength in at least one wavelength region of the ultraviolet and green regions, it can be used for a light emitting body such as an LED. Moreover, since the said zinc oxide crystal has a large surface area, it can be used for a dye-sensitized solar cell. Furthermore, the zinc oxide crystal has a photocatalytic function and exhibits antifouling, antifogging, superhydrophilic and antibacterial effects, and is therefore effective for use as a photocatalyst.

Next, examples of the present invention will be described in detail together with comparative examples.
<Example 1>
Zinc nitrate hexahydrate was prepared as a zinc source, and polyethylene glycol having a molecular weight of about 1000 was prepared as an additive. Moreover, ammonia was prepared as an alkali source for pH adjustment. Moreover, the silicon substrate was prepared as a base material. First, a 1 mol / L aqueous ammonia solution containing 0.05% by mass of polyethylene glycol having a molecular weight of about 1000 in 50 ml of an aqueous solution in which 0.027 mol / L zinc nitrate hexahydrate is dissolved has a pH of 10.6. And an aqueous solution was prepared by stirring at room temperature for 30 minutes. The zinc ion concentration in the prepared aqueous solution was adjusted to 0.019 mol / L. Next, the prepared aqueous solution is put into a beaker, the temperature of the aqueous solution is raised to 70 ° C., the silicon substrate is immersed therein, and the solution is allowed to stand while maintaining the aqueous solution temperature at 70 ° C. After 6 hours, the silicon substrate is removed from the aqueous solution. I took it out. Crystals were deposited on the extracted silicon substrate. Then, after washing | cleaning the silicon substrate in which the crystal | crystallization precipitated, it dried at 60 degreeC.

  When the obtained crystal was observed with a scanning electron microscope (SEM), as shown in FIGS. 1 (a) and 1 (b), a flower-like crystal grew on the silicon substrate. It was confirmed that Further, as a result of measurement by X-ray diffraction (XRD, X-ray Diffractometer), as shown in FIG. 2, the XRD pattern of the crystal coincides with zinc oxide (JCPDS card No. 36-1451) and is deposited on the surface of the silicon substrate. It was confirmed that the crystal having a flower shape was a zinc oxide crystal.

<Example 2>
Crystals were deposited on the slide glass by the same method as in Example 1 except that the substrate was changed from a silicon substrate to a slide glass. When SEM observation and XRD measurement were performed in the same manner as in Example 1, it was confirmed that the crystals deposited on the slide glass were flower-like zinc oxide crystals.

<Example 3>
Crystals were deposited on the ITO film-coated glass substrate in the same manner as in Example 1 except that the substrate was changed from the silicon substrate to the ITO film-coated glass substrate. When SEM observation and XRD measurement were performed in the same manner as in Example 1, it was confirmed that the crystals deposited on the glass substrate with the ITO film were flower-like zinc oxide crystals.

<Reference Example 1>
Crystals were deposited on the silicon substrate by the same method as in Example 1 except that no additive was added when preparing the aqueous solution. When observed by SEM in the same manner as in Example 1, it was confirmed that crystals having a flower shape grew on the silicon substrate, as shown in FIGS. 3 (a) and 3 (b). When the crystal obtained in Reference Example 1 was compared with the crystal obtained in Example 1, the number of petal-like parts provided in each flower-like zinc oxide crystal was small. Further, when XRD measurement was performed in the same manner as in Example 1, it was confirmed that the crystals deposited on the silicon substrate were zinc oxide crystals. From this result, it can be seen that regardless of the presence or absence of the additive dissolved in the aqueous solution, the flower-shaped zinc oxide crystal precipitates, and the presence of the additive determines the number of petal-like parts of the flower-like zinc oxide crystal. It was confirmed that it can be increased.

<Comparative Example 1>
The silicon substrate was immersed in the aqueous solution by the same method as in Example 1 except that sodium hydroxide was used instead of ammonia, but no crystals were deposited on the silicon substrate. No particles were generated in the aqueous solution.

<Comparative example 2>
The silicon substrate was immersed in the aqueous solution by the same method as in Example 1 except that the pH of the aqueous solution was changed from 10.6 to 6.8, but no crystals were deposited on the silicon substrate. In addition, the particle | grains were produced | generated in aqueous solution.

<Example 4>
The silicon substrate with zinc oxide crystal produced in Example 1 was subjected to fluorescence measurement. As a result, as shown in FIG. 4, ultraviolet emission having a wavelength of about 385 nm and green emission having a peak top at about 550 nm were detected in the emission spectrum, and the glass substrate with zinc oxide crystals produced in Example 1 of the present invention. The zinc oxide crystals were shown to be good light emitters.

<Example 5>
A ruthenium complex (Ru (dcbpy) ₂ (NCS) ₂ ; manufactured by Kojima Chemical Co., Ltd.) was dissolved in a mixed solvent consisting of acetonitrile and ethanol to prepare a dye solution. The glass substrate with the ITO film on which the zinc oxide crystals prepared in Example 3 were deposited was immersed in this dye solution overnight, and the dye was adsorbed to obtain a dye electrode. On the other hand, a platinum electrode coated with platinum on a conductive glass substrate is prepared, and the platinum electrode and the dye electrode are fixed so that their active surfaces face each other at intervals of 20 μm, and an electrolytic solution is injected therebetween. A dye-sensitized solar cell was produced. In the injected electrolyte solution, a solution prepared by dissolving 0.1 mol / L lithium iodide, 0.05 mol / L iodine and 0.5 mol / L tetrabutylammonium iodine salt in acetonitrile, respectively. Using. When the produced dye-sensitized solar cell was exposed outdoors during the day, the expression of electromotive force was confirmed.

<Example 6>
The photocatalytic activity of the silicon substrate with zinc oxide crystals produced in Example 1 was evaluated. The photocatalytic activity was evaluated according to the following procedure.

First, a silicon substrate with a zinc oxide crystal was placed in a 1 L glass container, and the container was sealed. Next, 350 ppm (initial concentration) of acetaldehyde was introduced into the container. Next, the container into which this acetaldehyde was introduced was irradiated with an ultraviolet lamp having an irradiation amount of 1.2 mW / cm ² for 1 hour. The acetaldehyde concentration inside the container after irradiation was measured with a gas detector tube (manufactured by Gastec Corporation), and the removal rate was determined based on the formula (1) shown below.

Removal rate [%] = [(initial density−density after light irradiation) ÷ initial density] × 100 (1)
As a result of the evaluation, a high acetaldehyde removal rate of 98% was obtained. From this result, it was shown that the zinc oxide crystal of the silicon substrate with a zinc oxide crystal of the present invention has a good photocatalytic function.

The figure which shows the SEM image of the zinc oxide crystal whose crystal shape produced in Example 1 is a flower shape. The figure which shows the XRD pattern of the zinc oxide crystal whose crystal shape produced in Example 1 is a flower shape. The figure which shows the SEM image of the zinc oxide crystal whose crystal shape produced in Reference Example 1 was a flower shape. FIG. 3 shows an emission spectrum of a zinc oxide crystal having a flower shape produced in Example 1.

Claims (8)

  An aqueous solution containing at least a water-soluble polymer as a zinc ion, an amine compound and an additive and having a pH adjusted to 7 or higher was adjusted to 50 ° C. to 90 ° C., and the substrate was immersed in the aqueous solution adjusted in temperature. By standing for a predetermined time in a state, a self-organization of flower-like zinc oxide crystals composed of a plurality of petal-like parts and a central part surrounded by these petal-like parts on the base material A method for producing a zinc oxide crystal, characterized in that the precipitation is carried out.   The process according to claim 1, wherein the amine compound is ammonia.   The production method according to claim 1, wherein the water-soluble polymer is a polyol-based water-soluble polymer.   The manufacturing method according to any one of claims 1 to 3, wherein a zinc ion concentration in the aqueous solution is 0.005 to 0.50 mol / L.   The base material is immersed in a solution containing at least a water-soluble polymer as a zinc ion, an amine compound and an additive and having a pH adjusted to 7 or higher and adjusted to a temperature of 50 ° C to 90 ° C. By allowing it to stand, a crystal-like zinc oxide crystal composed of a plurality of petal-like parts and a central part surrounded by these petal-like parts is deposited on the base material in a self-organized manner. An aqueous solution for producing zinc oxide crystals used in the production method according to claim 1.   The aqueous solution according to claim 5, wherein the amine compound is ammonia.   The aqueous solution according to claim 5, wherein the water-soluble polymer is a polyol-based water-soluble polymer.   The aqueous solution according to claim 5, wherein the zinc ion concentration is 0.005 to 0.50 mol / L.