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Self-cleaning coating is a special nano photocatalyst coating combined by photocatalyst and nano technology.
Generally, detergents reduce the surface tension of water and the contact angle is lowered. When the surface of nano level photocatalytic film is exposed to light, the contact angle of the photocatalyst surface with water is reduced gradually. After enough exposure to light, the surface reaches super-hydrophilicity. In other words, it does not repel water at all, so water cannot exist in the shape of a drop, but spreads flatly on the substrate. The hydrophilic nature of titanium dioxide, coupled with the gravity, will enable the dust particles to be swept away following the water stream (rain), thus making the key feature of self-cleaning and easy-cleaning.

Features of Photocatalyst

The photocatalyst coating will show the super hydrophilicity feature under light irradiation. The contact angle of the surface will be reduced to <10 degree, which brings 5 benefits of this feature.

  • The water will not form a water drop on the surface when its contact angle is <10°, it will form a water film. The water will be in flat condition on the surface. This will help to reduce the water strain after rain wash.
  • Generally, detergents reduce the surface tension of water and the contact angle is lowered. We call it hydrophilicity feature. Nano-zone coating’s super-hydrophilicity simulates this feature, so that a single surface wash with water on can reach the same effect as a traditional cleaning with detergent. So after a rain wash, the surface will be cleaned like after using traditional wash with detergent.
  • The hydrophilic feature can keep the water on the surface and the entire surface can be covered with only a little water. This will prompt its transpiration. So if it is coated on a building wall, the building will need less energy to cool down in summer. If it is coated on a panel in the compressor of air condition, the air condition system will show better efficiency.
  • Since the dust in the air usually has a high contact angle, the low contact angle of the coated surface will avoid the dust adsorption.
  • The hydrophilicity can make the surface with no water drops while raining, so it looks cleaner in rain.

Fundamental concepts – what’s photocatalyst (photocatalysis)?

The word photocatalysis is a composite word which is composed of two parts, “photo” and “catalysis”.
Catalysis is the process where a substance participates in modifying the rate of a chemical transformation of the reactants without being altered or consumed in the end. This substance is known as the catalyst which increases the rate of a reaction by reducing the activation energy.

Generally speaking, photocatalysis is a reaction which uses light to activate a substance which modifies the rate of a chemical reaction without being involved itself. And the photocatalyst is the substance which can modify the rate of chemical reaction using light irradiation.
Chlorophyll of plants is a typical natural photocatalyst. The difference between chlorophyll photocatalyst to man-made nano TiO2 photocatalyst (here below mentioned as photocatalyst) is, usually chlorophyll captures sunlight to turn water and carbon dioxide into oxygen and glucose, but on the contrary photocatalyst creates strong oxidation agent and electronic holes to breakdown the organic matter to carbon dioxide and water in the presence of photocatalyst, light and water.

Photocatalysis Mechanism

When photocatalyst titanium dioxide (TiO2) absorbs Ultraviolet (UV) radiation from sunlight or illuminated light source (fluorescent lamps), it will produce pairs of electrons and holes.

The electron of the valence band of titanium dioxide becomes excited when illuminated by light. The excess energy of this excited electron promoted the electron to the conduction band of titanium dioxide therefore creating the negative-electron (e-) and positive-hole (h+) pair. This stage is referred as the semiconductor's 'photo-excitation' state. The energy difference between the valence band and the conduction band is known as the 'Band Gap'. Wavelength of the light necessary for photo-excitation is:

1240 (Planck's constant, h) / 3.2 eV (band gap energy) = 388 nm

The positive-hole of titanium dioxide breaks apart the water molecule to form hydrogen gas and hydroxyl radical. The negative-electron reacts with oxygen molecule to form super oxide anion. This cycle continues when light is available.