A catalyst changes the rate of chemical reactions, while it is not consumed by the reaction itself.
(Through a chemical reaction, molecules and atoms form bonds or disconnect; e.g. combustion. For example, alcohol ignites without fire in the presence of platinum powder. This is because platinum behaves as a catalyst.)

A photocatalyst is a substance that generates catalyst activity using energy from light. Titanium dioxide is the typical material. As well as accelerating chemical reactions like a regular catalyst, the surface of the titanium dioxide photocatalyst has the characteristic of becoming hydrophilic due to the reaction with light.
For further information, please refer to Photocatalyst Introduction on this web site.

Titanium dioxide is a white powder that is most widely used as a pigment, as well as being used in cosmetic products and paper. Titanium atoms are relatively abundant on the earth’s surface (10th most abundant element), so there are few resource or environmental issues related to it.
Through producing it as a fine powder or controlling its crystalline shape, titanium dioxides for photocatalytic use have increased catalytic capacity.

Photocatalyst gains catalytic ability by absorbing light, so it needs light to absorb.
Titanium dioxide does not absorb visible light, and it needs ultraviolet light with a shorter wavelength than 400 nm. For many antibacterial products, antibacterial agents (e.g. silver) are added to maintain effectiveness in places with less light.

Photocatalyst causes oxidative decomposition of matter on the surface, so they can remove organic taints (e.g. oil, cigarette stains) or odorous components that have reached its surface. They can inhibit the growth of germs on the surface (antibacterial effect) as well. However, since they only work at the surface, they cannot decompose/remove things that are not close to the surface. The effect of photocatalyst is generally proportional to the amount of light (ultraviolet) and area which receives light.
Moreover, using the hydrophilicity of photocatalyst, waterdrops can be prevented from fogging mirrors and windows (anti-fogging effect).

This function keeps the surface clean by decomposing organic dirt adhering to it or washing away dirt with water (e.g. rain) by its hydrophilicity.

The titanium dioxide photocatalyst itself has a semi-permanent duration since it is stable and does not react, but it may lose effectiveness if undecomposable substances (inorganic material or too much organic dirt) adhere to the surface.
(In this case, it will recover by washing with water and then exposing to sunlight).
There may be cases where the resin that fixes the photocatalyst deteriorates or comes off resulting in the loss of the titanium dioxide.
For details, please read the product manufacturer’s description.

Most cases with no effect are due to not enough light (ultraviolet). Please use near a window or lamp where light is abundant. Most self-cleaning depends on the material’s hydrophilic nature. Please use where it is exposed to both light and water at the same time.
There are photocatalysts used to decompose odors and poisonous gases with adsorbents such as activated carbon which are effective even without enough light. They may regain effectiveness when exposed to sunlight.

The ingredient titanium dioxide is approved as a food additive, and is widely used in food and cosmetic products. Most titanium dioxide used as a photocatalyst is in the form of fine powder.
Safety information regarding photocatalyst products, including data such as oral toxicity, skin irritation, mutagenicity and skin sensitization, is scheduled to be disclosed for each product under the rules of the industry association.

Materials with catalytic effect absorbing light with a wavelength longer than 400 nm have been developed by adding additives to titanium oxide, and products using those materials are on sale.

Photocatalytic technology began with the discovery that titanium dioxide can decompose water into oxygen and hydrogen (Honda-Fujishima effect). Research of photocatalysis as a clean energy source is continuing, but the lack of efficiency is a major bottleneck for practical application, so it may take longer to realize.