I am learning knowledge of bandgap engineering applied on wastewater treatment this semester. It seems that there are many properties of a semiconductor that influence photodegradation activity of pollutants but I am unclear which of them are responsible and how they work. I've already known that the bandgap, positions of conduction/valence band and properties of indirect/direct semiconductors are among them. Can someone tell me other semiconductor properties related to this?
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The particle size and specific surface area of the photocatalyst have a great influence on the photocatalytic activity. In addition, they must be stable in aqueous medium and under light irradiation. Defects present in the material as vacancies also affect the photocatalytic activity because they can increase or decrease the recombination of electron-hole pair.
Surface charge separation，surface area， Particle/crystal Size，Surface Defects or vacancies and most importantly the reaction medium as well as the stability of the catalysts in that medium.....
The bandgap of a semiconductor determines the range of light absorption. For e.g. if you are using zno nanoparticles (band gap>or= 3.37 eV) then the light it will absorb so to form the electron-hole pairs will be in the UV range. However, defects present in the material may also contribute towards the range of light absorption. As electron-hole pairs are further responsible for forming the reactive oxidative species required for carrying out the degradation process, the light used must be able to form the excitons.
Hope this helps.
Sometimes, photodegradation of an organic pollutant such as dye may be affected by the pH of dye solution itself (in addition to the suitable properties of semiconductor as explained above). Effective photodegradation of every dye solution occurs within its favourable pH range. My meaning is: although the semiconductor photocatalyst possesses most of the desired characteristics, the photodegradation of the pollutant may not be achieved if the reaction is not carried out within favourable pH range.
It is important to tailor the conduction and valence band edge position to match the redox potential of the polluntant you are investigating, otherwise the reaction will not proceed. To further enhance the activity of semiconductor, particle size, charge carrier traps/sinks and polymorphism among others, are parameters to consider.
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