hello researchers;
Generally, the oxide compound semiconductors such as TiO2; ZnO, Fe2O3...;are more preferable for the photocatalytic activity under UV light but MnO2 is more important under visible light ; Does anyone explain me why MnO2 is more active under visible light?
Manganese dioxide (MnO2) (Eg: 1.30eV) is one of the visible light response semiconductor photocatalysts. Extending the photoresponse of MnO2 to the visible light region is related with narrowing of its band gap .
Just to add to comment of Hassan, choice of light source is largely dependent on the band gap of a photocatalyst. For TiO2 etc. which you mentioned, when you do a UV-vis test you discover it absorbs around 390-400 nm. Using the appropriate equation e.g. eV=1260/wavelength, when you convert this to electron volt (eV) that is the unit for band gap you discover this is about 3.2 eV. From the understanding of spectrum of light UV light absorbs within 200-390 there about while 400+ is for visible light. Therefore you should check the wavelength range of your material or catalyst to determine the appropriate light source to use. Modification by doping, sensitization etc. can of course shift the band gap and improve light absorption. I hope this guides you, you can check some of my references as well as those by Tahir et al to gain more understanding.
The band gap was estimated using the transition rate equation for direct band gap
semiconductor. The absorption coefficient for direct transition is given by the equation:
alpha (hv)=A(hv - Eg) n
where hv= photon energy,a= absorption coefficient (a=4*3.14*k / wavelength; k is the absorption index or absorbance), Eg is the band gap energy. A= constant, n= ½ for the allowed direct band. The exponent ‘n’ depends on the type of transition and it may have values 1/2, 2, 3/2 and 3 corresponding to the allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively
Ref: Journal of Alloys and Compounds 539 (2012) 174–178
In the Tauc equation of determination of band gap of materials how can I calculate the value of \alpha (as a function of frequency)? - ResearchGate. Available from: https://www.researchgate.net/post/In_the_Tauc_equation_of_determination_of_band_gap_of_materials_how_can_I_calculate_the_value_of_alpha_as_a_function_of_frequency [accessed Mar 13, 2017].
All of the comments are correct. But, they are not all of things about heterogeneous photocatalytic activity. Photocatalytic activity of semiconductors depends on many parameters such as production of electron-hole pairs, speed of charge transfer in the semiconductors, adsorption constant of the pollutant over the photocatalyst, separation of charge carriers, VB and CB energy positions, and so on. Hence, we cannot simply predict photocatalytic activity of a semiconductor by its band gap.
I think it needs lower energy (longer wavelength) to be affected by light which is comparable with the wavelength of visible light (380 to 700 nanometers.). See the link that could help you : Article Photocatalytic activity of β-MnO 2 nanotubes grown on PET fi...
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