Suppose I want to select a photocatalyst to degrade a dye or any other contaminant. Should I notice the max wavelength and peak of UV-Vis of the particular contaminant? Is there any relation between UV-Vis peak of the dye and bandgap of the photocatalyst?
I do not believe it is necessary to see it, but the chemical properties and reactivity between the photocatalyst and the dye, so that the reaction to take place. The wavelength if they are not generated colored intermediate compounds, remains the same and can evaluate the degradation kinetics.
If you will analyse the concentration of dye or contaminant using UV-Vis spectroscopy, then only max. wavelength and spectra are required. Otherwise, they are not required
The only important wavelength is related to the band gap of the catalyst; the absorption spectra of the dye or contaminant is immaterial.
You should notice to the peak of absorption of spectra because we measure the concentration of dye related to the absorption at the place of peak maximum
Yes, the absorption peak is determinant of the dye concentration, but not for choosing the optimum photocatalyst.
I would approach the problem this way: if TiO2 or other semiconductor is used it will not much matter what the dye spectrum looks like unless the dye absorbs over a significant portion of the semiconductor spectrum and effectively steals photons from the catalyst. If the spectrum of the dye is at longer wavelngth than the onset of semiconductor spectrum then it won't matter either. But if another sensitizer is used then energy transfer from sensitizer to dye will depend on the relative positions of the singlet transitions. Best advice is try a few experiments with 1-2% catalyst or sensitizer and without to see what you find.
For degradation of any dye or contaminant you do not need to worry about the absorption wavelength of particular dye or organic compound. The bandgap of the photocatalyst suggest you that which wavelength will be useful to excite your catalyst for the generation of electron hole pair. There is no relation between dye or contaminant absorption peak wavelength and bandgap of photocatalyst.
Dear Payam,
I would say that, yes, the absorption wavelength of the dye IS indeed important. This because you can either degrade the dye through a SEMICONDUCTOR SENSITISED PHOTOREACTION - i.e. photocatalyst (e.g. TiO2) absorbs light generating electron-hole pairs and in the presence of H2O and O2 forms highly oxidising species that degrade the dye - OR you can degrade the dye through a DYE SENSITISED PHOTOREACTION - i.e. the dye absorbs light (generally visible) and transitions HOMO to LUMO occur; the excited electrons are then injected into the conduction band of the semiconductor (given that the conduction band edge is lower in energy than the dye LUMO), leaving an oxidised form of the dye that either self-degrade further or it is degraded by oxidising species formed when the electron in the conduction band of the semiconductor (injected from the dye) are scavenged by O2 -. From the point of view of the semiconductor photocatalyst, the first mechanism (semiconductor sensitised photoreaction) is generally called SENSITISED PHOTOREACTION while the second mechanism (dye sensitised photoreaction) is also called CATALYSED PHOTOREACTION. Please check these papers to learn more:
1. Folli, A., Pade, C., Hansen, T. B., De Marco, T. & Macphee, D. E. TiO2 photocatalysis in cementitious systems: Insights into self-cleaning and depollution chemistry. Cem. Conc. Res. 42, 539–548 (2012).
2. Chen, F., Zhao, J. & Hidaka, H. Highly selective deethylation of rhodamine B: Adsorption and photooxidation pathways of the dye on the TiO2/SiO2 composite photocatalyst. Intern. J. Photoenergy 5, 209–217 (2003).
REMEMBER: The excitation of the dye (not the semiconductor) and further injection of the dye LUMO electrons in the conduction band of the semiconductor is also the first fundamental step in dye sensitised solar cells.
So, in conclusion, as long as you know the dye absorption wavelength, you can choose any semiconductor with conduction band edge lower than the dye LUMO level. If you do so, then you can degrade the dye by simply using light corresponding to the dye HOMO to LUMO transition (the band gap of the semiconductor photocatalyst does not matter much in this case),
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