JP4789085B2 - Preparation of crystal-oriented sulfur-doped titanium dioxide film - Google Patents

Description

  The present invention relates to a method for producing a crystal-oriented thin film of a titanium dioxide photocatalyst doped with sulfur (S) as an impurity. More specifically, after sulfur implantation accelerated to several hundred keV (kilo elecroton bolt) is implanted into the titanium dioxide single crystal film, the crystal disorder accompanying the ion implantation is recovered by heat treatment to recover the crystal-oriented sulfur-added titanium dioxide. A film is prepared. The present invention also relates to a method of producing a sulfur-added titanium dioxide thin film crystallized by pulsed laser deposition using a target material obtained by firing titanium disulfide at a high temperature in air. A titanium dioxide film with a uniform film thickness of nm (nanometer) can be obtained, and it can be used as a photocatalyst film for environmental purification such as decomposition and removal of harmful gases such as nitrogen oxides and gas sensor elements .

  Titanium dioxide is widely used as a photocatalytic material utilizing clean light energy for air and water purification, deodorization, sterilization, antibacterial and the like. On the other hand, research is being actively conducted to further increase the efficiency of photocatalytic reactions and to develop visible light-responsive photocatalytic materials that can utilize sunlight. With regard to imparting visible light responsiveness to titanium dioxide, most attempts have been made to achieve it by modifying the electronic structure by adding metal ions. However, in most cases, since the added impurity ions act as carrier recombination centers, no photocatalytic activity is observed in the visible region, and even the intrinsic photocatalytic activity of titanium dioxide in the ultraviolet region is reduced.

  On the other hand, it is known that the photocatalytic performance is improved when nitrogen (N) or fluorine (F) of nonmetallic ions is added and substituted with oxygen (O) sites of titanium dioxide. The reason is that visible light responsiveness is imparted by adding N, and that titanium dioxide to which F is added improves the photoresponsiveness in the ultraviolet region. Furthermore, it has been reported that when S having a larger ionic radius than N and F is replaced with O, the electronic structure of titanium dioxide is significantly modified (Non-Patent Documents 1 to 3).

Regarding titanium dioxide having visible light responsiveness due to the addition of sulfur, there is a report on the production of a thin film having a polycrystalline structure containing many crystal defects (Patent Document 1). Moreover, although there is a report on the production of a titanium dioxide single crystal film by a pulse laser deposition method (Patent Document 2), there is no report on the production of a thin film in which sulfur is added and crystallized.
Visible light responsive photocatalyst: Sulfur-doped titanium dioxide Umebayashi Tetsu, Yakimaki Tetsuya, Yoshikawa Masato, Asai Yusuke Kogyo Kogyo (Nikkan Kogyo Shimbun), July issue (2003) 34-36. Sulfur-doping into rutile-titanium dioxide by ion implantation: photocurrent spectroscopy and first principles band calculation studies T. Umebayashi, T. Yamaki, S. Yamamoto, A. Miyashita, S. Tanaka, T. Sumita, and K. AsaiJournal of Applied Physics 93 (2003) 5156-5160. Visible light induced degradation of methylene blue on S-doped TiO2T.Umebayashi, T. Yamaki, S. Tanaka, and K. AsaiChemistry Letters 32 (2003) 330-331. Japanese Patent Application No. 2004-29331 JP 2002-020199 A

  An object of the present invention is to provide a method for forming a sulfur-added titanium dioxide film having a very thin film thickness of about several hundred nm (nanometers) and crystal defects reduced by crystal orientation.

  In order to solve the above-mentioned problems, the present invention is a method of forming a sulfur-added titanium dioxide film in which crystal is oriented by adding sulfur by ion implantation, and by compressing and firing a titanium disulfide powder. The above problem is solved by a method of forming a target material and forming a sulfur-added titanium dioxide film crystallized by laser vapor deposition.

BEST MODE FOR CARRYING OUT THE INVENTION

  As described above, the thin film according to the present invention is obtained by adding sulfur to a titanium dioxide single crystal film by an ion implantation method, recovering the crystal disorder caused by the ion implantation by a heat treatment after the ion implantation, and crystal-oriented sulfur-added dioxide A titanium film is obtained. As the manufacturing conditions, the heat treatment temperature after ion implantation is an important item.

  In addition, it is also possible to produce a crystal-oriented sulfur-added titanium dioxide film by performing pulsed laser deposition using a target material produced by firing titanium disulfide. As production conditions, the use of a target material obtained by compression-molding and firing titanium disulfide powder and the substrate temperature in pulsed laser deposition are important items.

  Sulfur ion implantation is performed by irradiating the titanium dioxide film with a sulfur ion generated by an ion implantation apparatus in a vacuum. If the sulfur ions can be accelerated to several hundred keV (kilo elecroton volts), there are no special restrictions on the ion implantation apparatus. As the titanium dioxide used for ion implantation, a single crystal plate-like or single crystal thin-film single crystal is used. There is no particular limitation on the method for heating the ion-implanted thin film sample. Generally, the method is performed in air using an electric furnace equipped with a voltage power source and a temperature controller, but firing in an inert gas containing oxygen. But it doesn't matter. Usually, the heating temperature is 600 ° C. to 800 ° C. (preferably around 700 ° C.), and the time is 2 hours to 10 hours (preferably around 5 hours).

  In order to perform pulse laser deposition, the target material is produced by compression-molding titanium disulfide powder and firing it in air. There is no particular limitation on the method of synthesizing the titanium disulfide powder, and a high-purity powder is commercially available. There is no particular limitation on the method for forming the target material. Generally, the target material is formed by compression molding at a pressure of about 20 MPa (megapascal). There are no particular limitations on the method of firing the target material, and generally the firing is performed in air using an electric furnace equipped with a voltage power source and a temperature controller, but firing in an inert gas containing oxygen may also be used. . Usually, the firing temperature is 100 ° C. to 300 ° C. (preferably around 200 ° C.), and the time is 2 hours to 10 hours (preferably around 5 hours).

A sapphire (a-Al ₂ O ₃ ) single crystal substrate is used to form a sulfur-doped titanium dioxide film crystallized by pulsed laser deposition, and the substrate temperature is 350 ° C to 450 ° C (preferably around 400 ° C) ) To perform deposition in a vacuum. The laser used in the pulse laser deposition method may be any laser that can evaporate the target material, but is preferably an excimer laser (wavelength 248 nm).

  Hereinafter, the present invention will be described in more detail with reference to examples.

(Example 1)
The rutile-type titanium dioxide single crystal film was formed at a substrate temperature of 500 ° C. on an a-Al ₂ O ₃ (0001) single crystal substrate (10 mm × 10 mm) polished on both sides using a KrF excimer laser. . The produced rutile type titanium dioxide single crystal film had a thickness of about 300 nm. Sulfur ions were implanted into the prepared rutile-type titanium dioxide single crystal film at an acceleration energy of 150 keV, and sulfur was added to the titanium dioxide single crystal film. Thereafter, heat treatment was performed in air at a heating temperature of 650 ° C. for 6 hours, and the distribution of sulfur concentration added to the titanium dioxide film and the change in crystallinity of the film were evaluated by Rutherford backscattering (RBS) measurement. The RBS measurement results are shown in FIG. Fig. 1 (a) shows a rutile-type titanium dioxide single crystal film in which 150 keV sulfur is ion-implanted, and Fig. 1 (b) shows the RBS measurement results of the sample after heat treatment in air at 650 ° C for 6 hours. .

  As shown in Fig. 1 (a), from the side of the horizontal axis where the backscattering energy is high, a backscattering peak from Ti atoms contained in the titanium dioxide film, and subsequently a backscattering peak from ion-implanted sulfur atoms are observed. it can. This shows that sulfur is added in the titanium dioxide single crystal film. As a result of the evaluation, the sulfur concentration contained in the titanium dioxide film was found to be about 5 at.% (Atomic concentration).

Paying attention to the backscattering peak from Ti atoms contained in the titanium dioxide film, the analysis beam (2.7 MeV ⁴ He ⁺ ) is incident parallel to the TiO ₂ <100> axis and non-parallel ( Since the backscattering peaks of (random incidence) are almost the same, it can be confirmed that the crystal of the titanium dioxide film is disturbed by the ion implantation of sulfur.

As shown in FIG. 1B, when heat treatment is performed at 650 ° C., when the backscattering peak from titanium contained in the titanium dioxide film is incident in parallel to the TiO ₂ <100> axis direction, the backscattering yield is obtained. Can be confirmed from the fact that the disorder of the crystal of the titanium dioxide film is recovered. From the above, it can be seen that a sulfur-added titanium dioxide film having crystal orientation was obtained by heat-treating a titanium dioxide film into which sulfur ions were implanted.
(Comparative Example 1)
In the present invention, the heat treatment temperature of the titanium dioxide single crystal film into which sulfur ions are implanted is important. As a comparative example of Example 1, a titanium dioxide film sample into which sulfur ions were implanted was heat-treated in air at 500 ° C. for 6 hours and evaluated by the RBS method in the same manner as in Example 1. The crystal disorder of the crystal film did not recover. That is, it can be seen that the crystallized sulfur-added titanium dioxide film can be obtained by the heat treatment described in Example 1.
(Example 2)
Titanium disulfide powder (99.9%) was compression molded at a pressure of 20 MPa (megapascals) to produce a disk-shaped target material having a thickness of 3 mm and a diameter of 20 mm. Furthermore, the produced disk-shaped target material was baked with the electric furnace in the air. The firing temperature was 200 ° C., and the firing time was 5 hours. The composition ratio of sulfur (S) and titanium (Ti) in the produced target material was about 0.6 (S / Ti). Pulsed laser deposition is performed in a vacuum on a disk-shaped target material produced by focusing an excimer laser (wavelength 248 nm) with a frequency of 150 mJ per pulse and a repetition rate of 10 Hz onto an area of about 1 x 2 mm ² (~ 10 ^-5 Torr). A deposition substrate was placed at a distance of 7 cm from the target material, and a thin film was produced at a substrate temperature of 400 ° C. The thickness of the deposited film was about 52 nm. FIG. 2 shows the results of evaluating the crystal structure of the produced film by X-ray diffraction. From the result of the X-ray diffraction measurement, it can be confirmed that TiO ₂ (100) having a rutile structure is grown on the a-Al ₂ O ₃ (0001) substrate. That is, FIG. 2 is an X-ray diffraction pattern of a sulfur-added titanium dioxide film deposited by pulsed laser deposition on a sapphire single crystal substrate. Each peak is a rutile-type titanium dioxide and sapphire single crystal substrate peak, and it can be confirmed that a titanium dioxide film crystallized in the (100) crystal plane grows on the a-Al ₂ O ₃ (0001) substrate. .

The concentration distribution of sulfur added to the film and the change in crystallinity of the film were evaluated by Rutherford backscattering (RBS) measurement. The RBS measurement results are shown in FIG. As shown in FIG. 3, from the higher backscattering energy on the horizontal axis, the backscattering peak from Ti atoms contained in the titanium dioxide film and the backscattering beak from added sulfur atoms can be confirmed. It can be confirmed that sulfur is added in the film. As a result of evaluating the sulfur concentration in the film by RBS, it was found that about 2 at.% (Atomic number concentration) of sulfur was added. Focusing on the backscattering peak from Ti atoms contained in the titanium dioxide film, the backscattering yield when the analysis beam (2.0 MeV ⁴ He ⁺ ) is incident parallel to the TiO ₂ <100> axis direction is It can be confirmed that the titanium dioxide film is crystallized in the (100) plane because the backscattering peak yield is reduced to less than half of the case of non-parallel incidence (random incidence).
(Comparative Example 2)
In the present invention, the substrate temperature in pulsed laser deposition is important. As a comparative example of Example 2, thin film samples were prepared by changing the substrate temperature in pulse laser deposition under the same conditions as in Example 2, and the crystal structure of the film was evaluated by X-ray diffraction and RBS methods. In the film produced at a substrate temperature of 300 ° C., the addition of sulfur could be confirmed, but the crystal orientation could not be confirmed. In the film produced at the substrate temperature of 500 ° C., the crystal orientation could be confirmed, but the presence of sulfur in the film could not be confirmed. That is, it can be seen that a crystal-oriented sulfur-added titanium dioxide film can be obtained depending on the substrate temperature during vapor deposition described in Example 2.
[The invention's effect]

  As described above in detail, the present invention makes it possible to produce a crystal-oriented sulfur-added titanium dioxide thin film that can reduce crystal defects and can be expected to increase the efficiency of the photocatalytic reaction. It is extremely effective as a method for forming a novel photocatalytic thin film having visible light responsiveness.

  Titanium dioxide is widely used as a photocatalytic material utilizing clean light energy for air and water purification, deodorization, sterilization, antibacterial, etc., but sulfur obtained by the ion implantation method or pulsed laser deposition method of the present invention. The crystal-oriented titanium dioxide to which is added can be applied to environmental purification such as decomposition and removal of harmful gases such as nitrogen oxide as a photocatalytic film, and to gas sensor elements.

FIG. 1 (a) shows the results of Rutherford backscattering (RBS) measurement of the thin film sample shown in Example 1. FIG. 1 (a) is a rutile type titanium dioxide single crystal film in which 150 keV sulfur is ion-implanted, and FIG. The RBS measurement results of the sample after heat treatment in air at 650 ° C for 6 hours after injection are shown. 4 is an X-ray diffraction pattern of a sulfur-added titanium dioxide film obtained by pulsed laser deposition on a sapphire single crystal substrate shown in Example 2. FIG. The Rutherford backscattering (RBS) measurement result of the thin film sample shown in Example 2 is shown.

Claims (4)

The titanium disulfide to prepare the data Getto material was calcined in air, Ri by the pulsed laser deposition method using the target material, by controlling the substrate temperature at 350 ° C. to 450 ° C., crystals oriented sulfur addition A method for producing a titanium dioxide film.   The method for producing a thin film according to claim 1, wherein the target material is produced by firing compression-molded titanium disulfide in air at a temperature of 100C to 300C.   The method for producing a thin film according to claim 1, wherein the composition ratio of sulfur and titanium in the target material is 0.6 (= S / Ti).   2. The method for producing a thin film according to claim 1, wherein a sulfur-added titanium dioxide film is formed on a (0001) plane of a sapphire single crystal substrate.

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