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Last updated

23-Aug-2009

What is Photocatalyst?

Photocatalyst has a prefix of Photo and a suffix of catalyst.

As the word implies, Photocatalyst is a material that use light to unlock its nature of being a catalyst. A catalyst is a substance that fasten the chemical reaction to enable it to occur in a lower temperature / energy level. (See the figure below) But at the end of the reaction, neither of its physical or chemical nature will be altered. ie. it can be used indefinitately.

Chemical reaction with/without catalyst

Photocatalyst has been documented in literature since 1930's. Commonly quoted materials include GdP、GdAs、ZnO (Zinc Oxide)etc. Among them, Titanium Dioxide (TiO2) carries most attention because its high Redox potential, chemical stability and being non-toxic。

In 1968, a Japanese scholar, 藤島昭, revealed the photocatalytic nature of TiO2. Since then, this material dominate the field. TiO2 is a semi-conductor, it has 3 crystalline structures, namely Anatase, Rutile and Brookite. Only Antase TiO2 has the photocatalytic property.

Application of photocatalyst is diverse. In the field of civilian usage, paint spray and thermal coating are the most common means of utilisation.

We provide Medical Grade Photocatalyst. It was developed in Japan by incorporating the nano-technology and is now manufactured in Taiwan. Once it is applied to a surface (wall, interior of car, furniture, ceiling, etc), it can remove the odour, being anti-bacterial, prevent mould from growing and remove dirt adhere on it (especially the outer wall of a building).

reaction

Mechanism of Photocatalyst

Being illuminated, electron (e-)of the photocatalyst will be excited and jump from the valence band to the conduction band and lost to the adjacent molecule (e.g. water or oxygen in the air). The site where the (e-) lost will be positively charged, named as electron hole(h+).

When water (H2O) contact the surface of the photocatalyst, it will (temproraily) dissociate into an opposing ions, Hydroxide ion(OH-)and Hydrogen ion(H+). The Hydrogen ion(H+)will bind with the "loosen" electron and form a neutrally charged "ion", Hydrogen free radicle(‧H). On the same token, the positively charged electron hole will bind with the Hydroxide ion, seize its "extra" electron and convert it into a neutrally charged "ion", Hydroxide free radicle(‧OH). As you might know, all these free radicles are very very chemically unstable.

When organic substance contact the surface of photocatalyst, it will also contact these free radicles. These free radicle will seize / give up their electrons and dissociate the covalent bonds of these organic materials. With further reactions, these organic substance will be converted into water and carbon dioxide ultimately. In academic terms, it is called Redox reaction.

In daily life, most household contaminant and pathogens are organic substance. Photocatalyst can decompose them into harmless water and carbon dioxide. As such, odour, dirt, bacteria, virus, and fungus will be decomposed and removed. We will have a microbial-free and clean environment at the end.

For last decades of technological advances, photocatalyst has been widely utilised in domestic, commerical and medical field. It was even been added on various products such as cloth and sock, for its merit of self-cleaning, odour-removal and germicidal nature.

Wow, isn't it too good to be true? if so, why is it not popular?

Just 2 reasons

  1. In order to excite the electrons of Titanium Dioxide, the incoming photons (light) must carries a sufficient amount of energy to bridge the energy band gap between the valence band and the conduction band. Energy of light, E, and the wavelength, lambda, has an inverse relationship, E = hC/λ.。 h is a constant, named Planck constant; C is the speed of light, another constant value. That is to say, if the wavelength is shorter (such as ultraviolet light), the energy of it is stronger. The band gap of Titanium Dioxide is 3.2 eV, matching the wavelenght of 380 nm, exactly fall in the ultraviolet-A band. In another word, if there is no special processing, visible light will not unveil the photocatalytic nature of Titanium Dioxide. Unfortunately, indoor environment is not supposed to have sunlight (the source of ultraviolet light). Therefore, it seems no point to use them in our daily life.
  2. $。Titanium is expensive, everybody knows. Titanium Dioxide, a product made from Titanium, will be more expensive. Antase Titanium Dioxide will undoubtedly very expensive. So what if the Antase Titanium Dioxide is made with nano-technology?

Nano-technology

A nanometer, nm, is just a unit of length. 1 meter has 100cm; 1cm has 10mm; 1mm has 1000micron; 1 micron has 1000nm. If you imagine the Earth is 1m, then a football is 1nm.

Nano-technology implies a technology that manufacture a product in size of less than 100nm and that is all it is about. However, in that size, many material will exhibit a unique behaviour / nature.

Main ingredient of Photocatalyst

The active ingredient is Antase Titanium Dioxide. When the granule of these Antase Titanium Dioxide were made in size of less than 7nm, their surface to volume ratio will be exponentially increased, so as light penetration rate. As such, the photocatalytic rate will be much faster.

Reference

1

Writer:藤島昭、橋本和仁、渡部俊也

Translator:王政友

Publisher:世茂

Published on:2006年04月27日

Language:Traditional Chinese

ISBN:9789577767660

Binding:paper back

2 Wikipedia http://zh.wikipedia.org

中文版本