Photocatalyst

Introduction Environmental contamination, which is growing around the world or in our daily home life, is a serious social problem not to be neglected. Examples of such contamination can be endlessly listed as follows: — Water pollution caused by industrial and household wastes — Respiratory diseases caused by air pollutants such as SOx or NOx — Room air contamination caused by organic compounds emerging from newly developed building materials — Dioxin emerging from resin materials during incineration The fact that using energy to eliminate such environmental contamination increases emission of CO2 resulting in more global warming, however, leads us to a dilemma not to use energy to achieve our anti-pollution goal. Under such circumstances, we have come to the conclusion that we need a new material that can gently harmonize the contaminated environment to restore original conditions by using natural energy which is a part of the environment and low-cost energy supplied to our daily home life. One solution to that problem is our proposal, Photo-catalyst.

What is Photo-catalyst? Photo-catalyst produces surface oxidation to eliminate harmful substances such as organic compounds or nearby bacteria, when it is exposed to the sun or fluorescent lamp. By applying this principle to water treatment, dissolving NOx in the air, or room air purification, photo-catalyst can be used for various steps in purifying a contaminated environment. The function of the photo-catalyst can be divided into five major categories as follows:

1. 2. 3. 4. 5.

Purifying water Preventing contamination Anti-bacteria Deodorizing Purifying the air (dissolving NOx)

It might be well understood that the functions listed above are those which amplify or accelerate the functions of the sun, or ultra-violet radiation. In this sense, it is not strange to regard titanium dioxide as a photo-catalyst from the viewpoint that it works as the catalyst in accelerating the functions of the light.

What kind of light is necessary for the photo-catalyst? As we explained, the photo-catalyst can be activated by light, so what kind of light is necessary for the photo-catalyst? There are various sources of light such as the sun, incandescent lamps, fluorescent lamps, light traps, disinfectant light, and so on. Those sources emit lights with different wavelengths necessary for their specific purposes.

TiO2 is a semiconductor which turns to a high-energy state by receiving light energy, and releases electrons from its illuminated surface. If the energy received at this stage is high enough, electrons that were initially located in the so-called ‘valence band’ all jump up to the ‘conduction band’. Thus, the energy that makes electrons jump up is provided by light, and this light energy is believed to be the energy of the light’s wavelengths. Therefore, calculating from the height that the electrons have to jump up, this light should have the same wavelength as ultraviolet light. E = hv E : energy h : Plank’s constant v : frequency ν = c / λ c : light speed λ : wavelength Therefore, E = hc /λ Here, E is titanium dioxide 3.2 eV (3.2 eV = 3.2 × 1.6 × 10-19J), and if you substitute the determinate values （c: 3.0 × 108m/s, h: 6.63 × 10-34J・s）, you will find out that the necessary wavelength is approx. 380 nm, which tells us that the light needed to activate Photocatalyst is ultraviolet light.

Thin-film Photocatalyst Titanium dioxide (TiO2) is a harmless substance widely applied in various fields such as cosmetics, toothpaste, extenders for medicines, and coating. For these uses, TiO2 is usually supplied in the form of powder. But in order to use TiO2 as an effective photo-catalyst for the 5 functions, powder is not an appropriate form, for it may be blown off by wind or washed out by water, and when used to purify water, it has to be separated from the water. Thus, a method to fix the powder has long been considered. It is easy to fix powdery TiO2 with a binder, but if an organic binder is used, the photo-catalytic reaction will destroy the binder itself. Inorganic binder is not influenced by photo-catalytic reaction, but only the powder exposed on the surface can work effectively despite the total amount of powder contained in the binder.

In collaboration with National Industrial Research Institute of Nagoya, we succeeded in developing the coating technology of photo-catalytic thin-film that can cover all surface only with TiO2, instead of using powderyTiO2. This method, called the Sol-Gel method, uses titanium alkoxide as a starting material. It is hydrolyzed to obtain sol, and the sol is applied to coat substrate through such coating methods as the dip-coating method, to form a film. At this stage, the film is not the film of TiO2. So, the sol is sintered along with the substrate to be crystallized, and thus the film of titanium dioxide is formed.

Supercritical Technology The Sol-Gel method, however has some weaknesses. It is not effective for granular carriers or carriers with minute cracks because parts of sol fluid get caught due to the capillary phenomenon. The fluid left in minute pores or cracks turns into powder after being sintered, which cannot form a film. To overcome this weakness, the supercritical sol-gel method was developed. When a fluid reaches a certain level of temperature and pressure, the boundary between liquid phase and vapor phase is lost. This point is called the critical point, and the area that exceeds this point is called the supercritical region. In the supercritical region, a fluid holds qualities of both liquid and gas, which enables the fluid to move

freely in any minute spaces. In this region, even a viscous fluid like sol can be crystallized as thin-film in any minute spaces, without causing the capillary phenomenon. When discharged from a nozzle into atmosphere pressure, titanium dioxide dissolved in supercritical fluid bursts into deposition. The resulting deposited substance is usually ultra-fine TiO2 particles. By providing them with appropriate dissolution and deposition conditions to escape through minute spaces, we succeeded in coating fibrous and microscopic substances with thin-films.

Sol-Gel method Titania is caught in minute cracks between fibers.

Supercritical sol-gel method Fiber surface is coated with titania film and nothing remains in spaces between fibers.

Dye Sensitizing Titanium Dioxide Solar Battery Supercritical fluid technology has been considered for application to the instantaneous decomposition of persistent substances, as demonstrated with supercritical water. But because a great amount of energy is consumed in this application, how this technology can be applied to new materials is being considered today. Under these conditions, we have worked on the development of both titanium dioxide thin-film and supercritical fluid in association with various national and public research institutions, and succeeded in producing unprecedented hybrid thin-film by combining the most advanced features of the two technologies. As an example of application of this technology, we are now considering applying it to the production of dye sensitizing titanium dioxide solar batteries. The dye sensitizing titanium dioxide solar battery, which can be priced low considering its conversion efficiency, is now being developed as a new-generation solar battery, aiming to be applied to exterior glass or house windows.