About photocatalyst

 Yesterday, I introduced photocatalyst, but the explanation was not over. So, here I want to continue explanation about photocatalyst. 
 Utilizing photocatalyst, we will be able to solve energy problem like energy depletion, and global warming. For example, it will be possible to reduce carbon dioxide, evolve hydrogen from water, and disrupt organic pollutant when light is irradiated to it. What are the mechanisms of these phenomenon? 
 There are three types of photocatalyst, semiconductor, organic semiconductor, and protein. 
 First, semiconductor has two different energy band inside the solid. The higher one is called conductance band and the lower one is called valence band. The gap between valence band and conductance band is called band gap. In case of no light irradiation, valence band is occupied by electrons. When semiconductor photocatalyst is exposed to light which have more energy than band gap, the light is absorbed, and electrons are excited from valence band to conductance band. Both electrons and holes in conductance band move to surface promptly. Excited electrons cause reduction and holes cause oxidation. Utilizing reduction, we can produce hydrogen and reduce carbon dioxide to methane. On the other hand, in oxidation, we can degrade organic pollutant. 
 These mechanism makes us possible to solve energy and environment problems. 
 Thank you for reading my poor sentence.