The distance of the lamp to the substrate is mainly related to the light power, that means that as nearly the lamp is more photons per second may be absorbed by the substrate and therefore more electrons (e-) and holes (h+) are generated so that is expected that the photocatalytic reaction goes faster.
There are several studies in the literature that comment this. Under low light power conditions the reaction rate has a linear correlation with the light power, while at high light power conditions this relation gets square-root. But I must underline that other effects may affect the system analysis because photocatalytic reactions are quite complex.
I would suggest you to do some actionometry or easy power measurments of your lamp on different distances. This is one important information during the photocatalytic experiments.
totally agree with last post: closer the lamp more photons you have on your sample sourface. Try to measure with an energy metter.
Hi,
Mathias gives a good summary and helpful explanation.
One other thing you might want to check is if you can focus the light onto the TiO2 surface, since the light beam is likely to be divergent.
Best wishes
the closer distance, the higher light intensity. more holes will be generated under high intensities, which contribute greatly to the degradation of PPCPs or other organic molecular. I also believe that the photocatalytic efficiency of TiO2 may not increased with the light intensity when it reaches some limit, i.e. increasing the intensity, the holes may not increased at more.
Thanks to all of you for your valuable suggestions. I will bring lamp closer to the sample reactor having Titania film.
Have you measured the band gap of your TiO2 film?
I don't know the application you are aiming but I would suggest using another light source like a 365 nm lamp or a Xe lamp also.
No Mathias , i didnt measured the band gap of tiania film. I am using titania film for photocatalytic degradation of PPCPs.
You can measure the intensity of the lamp you are using by ferrioxalate actinometry. Photocatalytic degradation of a contaminant depends on the light dosage (Fluence rate x irradiation time). Fluence rate can be adjusted by adjusting the distance between sample and light source.
http://uqu.edu.sa/files2/tiny_mce/plugins/filemanager/files/4300270/DK3093ch12.pdf
Band gap calculations can be done from UV-vis spectra. Read the article below for more information:
A simple spectrophotometric method for determination of the optical constants and band gap energy of multiple layer TiO2 thin films
Monjoy Sreemany, Suchitra Sen,
http://dx.doi.org/10.1016/j.matchemphys.2003.09.030
If you are looking at the photocatalytic activity of TiO2 in a solution of PPCPs , you may need to consider also the optical depth of your reactor. In the following article there is a nice approach to design
G. Camera Roda, F. Santarelli, C. A. Martin, Design of photocatalytic reactors made easy by considering the photons as immaterial reactants., «SOLAR ENERGY», 2005, 79, pp. 343 - 352
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