hi every one,
I have a porphyrin-based compound which I need to find its bandgap. since it is not soluble in water I mixed it with TiO2 and then dispersed in water and ethanol. I prepared a thin film on quartz (the film is not transparent). I took its UV-vis but the result is weird (the photo below). what is the problem? I would appreciate it if anyone can help me.
Dear Yasaman,
The absorbance spectra looks really weird and in my opinion, it is due to probable errors occurring during the measurement. Actually the sample preparation for absorbance is very crucial for recording a correct UV-Vis absorption spectrum. In my research tenure I have come across such problems several times. Two factors should be kept in mind while measuring the UV-Vis absorption spectra-
1. For liquid samples, a good stable dispersion/solubility is very important for obtaining correct data. If the samples settle down with time in the dispersion medium, error occurs in the measurement.
2. To counter this problem, if thin film is prepared on any substrate, the thickness of the film should be very less. Otherwise, thick films may result erroneous spectra.
Now, by reading your problem, it seems that you have prepared thick film (as you mentioned that “the film is not transparent”) and that may be the origin of your problem. To overcome this, I would recommend to measure the absorbance by Diffuse Reflectance Spectroscopy (DRS). The factors like solubility and film thickness are not active in DRS and it is very suitable for the samples which are poorly dispersed in any liquid medium.
Best regards,
SS
Dear Yasaman,
Did porphyrin get completely soluble in the solvent you used ? The error might be because of solvent / solubility. You can try CHCl3 and CH2Cl2 or DMF solvents and check how it works.
Regards,
Jeevitha
Subhajit Saha thank you Sir for your thorough answer. For UV-vis, thin film has to be transparent? because the only available device is UV-vis and I should make it with this unfortunately.
Jeevitha Dhanapal thank You for your answer. Unfortunately my sample is a polymer network which is not soluble in any solvents. I just can disperse it in water with help of TiO2
Yasaman Sheidaei , can you please explain how TiO2 helps to disperse the polymer in water? Because TiO2 itself is poorly dispersed in water. Another point I want to mention is that, DRS is an attachment that is provided with any standard commercial UV-Vis spectrophotometer (but optional). At first please be sure that your instrument does not have the DRS attachment.
If you really dont have DRS attachment, you can apply a trick. Just peel of the film surface with the help of scotch tape, repeat the step 2/3 times. This will make your film more transparent and then you can check the absorbance. If this process does not work, its better to go for DRS mode.
Subhajit Saha I tried to disperse my sample in water but it did not work at all. then I grind it with TiO2 then sonicated it in water for 2 hours and it worked. but it can be poorly dispersed. (e.g. after one hour that it is not moved, they will all sediment)
Hello Yasaman Sheidaei
To find the bandgap please do the solid UV i.e. UV-vis DRS study of your sample in the reflactance mode. Then form the UV data you can obtain the bandgap of your sample using the Tauc method.
Regards
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