Introduction to photocatalysis

The word "photocatalyst" has become commonly heard. What is a photocatalyst? In short, a photocatalyst is a "substance that facilitates chemical reactions by photoirradiation without becoming transformed". Photosynthesis in plants is a well-known example of photocatalysis, in which chlorophyll acts as the photocatalyst.
Organometallic complexes (pigments) and semiconductors are photocatalytic substances. Semiconductor photocatalysts work to promote chemical reactions resulting from photoirradiation. Their general functions include deodorizing, sterilizing, antibacterial effect, antifouling and removal of toxic substances. Research and development of photocatalysts is conducted to make use of these functions in daily life.

While titanium dioxide has only recently come to be known, other substances have been used as photocatalysts. Although the historical origin is not very well known, scientific literature from around 1930 mentions the photocatalytic reaction of zinc oxide. Titanium dioxide began to attract attention when Mr. Akira Fujishima (then a graduate student at Tokyo University) conducted research in 1968 as part of a basic study on oxide semiconductors that responded to light for use in electrophotographic imaging. It was known at that time that if photoirradiation were conducted in a solution with zinc oxide and cadmium sulfide as electrodes, electric current would flow in proportion to the intensity of light. Research was also actively conducted in Germany and the United States. Mr. Fujishima happened to obtain a single crystal of titanium oxide. He sliced the crystal with a diamond cutter and used it as an electrode. He made a closed circuit using a platinum electrode as the counter electrode , and exposed titanium oxide to the light of a xenon lamp. To his surprise, gas bubbles came out from both the titanium oxide and platinum electrodes. He immediately collected these gases and examined them by gas chromatography. Oxygen had been generated from the platinum by the titanium oxide and hydrogen. It was found that water had broken down into oxygen and hydrogen by photoirradiation. Moreover, the titanium oxide had not broken down, and no change was observed in its surface characteristics even after several days of continuous photoirradiation. This phenomenon is very similar to photosynthesis in plants.
This was made known to the world as the "Honda-Fujishima effect", and no semiconductor photocatalyst has been found to surpass titanium oxide.
Mr. Akira Fujishima (now a Doctor of Engineering and a professor at the Graduate School of Engineering, University of Tokyo) thus discovered semiconductor photocatalysis in Japan, which has become advanced in the field.

fig 1

Now, let us explain light, which plays the main supporting role in photocatalysis of titanium dioxide. UV rays are necessary for titanium dioxide to act as a photocatalyst. UV rays in sunlight or indoor fluorescent light are clean energy sources that are always found where we live and do need to be generated by special energy sources.
While Professor Fujishima used titanium dioxide with a rutile-type crystal structure for his photocatalytic experiment, the anatase type, which works more efficiently as a photocatalyst, is mainly used at present. Titanium dioxide is an n-type semiconductor, which conducts electricity by electrons. When titanium dioxide is exposed to UV rays with more than a certain amount of energy, valence-band electrons become excited and move into on an upper-level valence band. This is the photoexcited state of semiconductor. The difference in energy between the valence and conduction bands is called band-gap energy. The band gap of anatase-type titanium dioxide is 3.2eV.x.

fig 2

The energy of light can be converted into a wavelength of 387.5 nm using the equation above. This means that UV rays with a wavelength of approximately 388 nm are necessary. When electrons are photoexcited into the conduction band, gaps form in the valence band where the electrons are removed. The removed electrons and gaps induce photocatalysis.

fig 3