how can a semiconductor show better photocatalytic activity under solar irradiation rather than uv light if we consider that semiconductor has large band gap + no impurities ?
The very fundamental necessity for a semiconductor to work effectively under solar light is, it should have a smaller band gap, which would facilitate absorption of lower energy / longer wavelength of light. So, if a semiconductor has a large bandgap (example TiO2, 3.2 eV), it would absorb only higher energy light i.e. UV. So, its only ~4% of solar light. Therefore, it is necessary to dope the semiconductor, thus reducing its bandgap, and making it to absorb more solar light, generate enough e-h pairs, and give higher photocatalytic activity.
Second case can be, to make p-n heterojunction of large bandgap material with smaller bandgap material. This would make smaller bandgap semiconductor to absorb more solar light, and the generated electrons can be transferred to larger bandgap semiconductor.
I agree with you but may be a better effeciency in the separation on e/h could lead to a good photocatalytic activity. What do you think?
Yes, better charge separation, long diffusion length would make a photocatalyst more efficient.
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