Dear Ali M. M. I. Qureshy , it will depend on the application you want. From the spectra provided, it is clear than sample 3 shows a larger absorption at 400nm than samples 1 and 2, which show identical absorptions at 400nm. So if you want that your electrode absorbs more light (at 400nm) then sample 3 should be the best.

However, surface area should be a very important parameter, the larger the surface area, the larger the number of molecules /species that can be close to the electrode to experience any photoelectrochemical process.

Since you used three different sol-gels, probably you also obtained three different surfaces, and it would be useful to know what their appearances are. A SEM image or AFM would be helpful to asses this surface parameter.

Hope it helps.

Thank you, Dr. Manuel Gómez,

I am working on a photoelectrochemical cell for hydrogen production using wastewater. The surface area of the proposed electrode is 850 cm2.

Dear Ali M. M. I. Qureshy

I suppose Manuel Gómez was referring to the TiO2 sol-gel surface area (the smaller the Titania NPs the larger the surface area they expose at a fixed volume).

Dear Ali M. M. I. Qureshy , Giuseppe Curro is right, I was referring to TiO2 sol-gel surface area. Probably a SEM or AFM could give you a hint of the appearance of your TiO2 layers in each electrode, but to have information about their surface areas you could get a good estimation by using the Brunauer–Emmett–Teller (BET) method based on adsorption isotherms of nonreactive nitrogen or argon.

Hope it helps. And thanks Giuseppe for making this point clear!

I agree with dear Manuel Gomez. I also think your samples have different thickness and concentration. I am also currently working on Tio2. your third sample more thick (i guess)