Firstly, are you sure that this is "true"" photocatalytic activity and not just adsorption or photolysis? I suggest to perform an experiment with another substance instead of dye (i.e phenol for example)

I can think of three reasons for your observation:

1) Your light source used is not exclusively blue light but has parts of ultraviolet light as well. If you use a broadband light source with a filter keep in mind that the cutoff given for the filter is not a hard cutoff but rather a range so a 400nm cutoff filter will still leave some light below 400nm. Its best to use a lamp that does not emit any ultraviolet light in the first place, e.g. an LED with a narrow emission band.

2) As already mentioned your dye might be absorbing the light resulting in photolysis of the dye compound. It might also be a synergistic effect with the photocatalyst as in dye-sensitization. I would recommend using an alternative degradation test that is not based on a dye or at least a dye that does not have any absorption in the wavelength region your lamp is emitting.

3) You might form surface complexes that absorb visible light and induce a photocatalytic reaction similar to dye-sensitization. See for example doi: 10.1007/s10563-011-9126-8.

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Question

Why is TiO2 active under visible light?

dear all, thanks for your reply, we have thoroughly checked that it is photocatalytic degradation. The adsorption of the dye on the catalyst and connecting tube is around 25 % in dark. After irradiation of light it reaches to above 90 %. Another thing thier is also reduction in the COD was observed.

I conclude that it is because of the synergic effect of dye and catalyst which lead to the degradation of adsorbed dye.

I also agree with that the presence of photocatalytic activity of certain dyes via TiO2 under visible-light irradiation is most likely due to the synergistic effects of dye and TiO2, dye sensitization. However, the visible light-deriven TiO2 showed unsatisfactory photocatalytic results for the degradation of other terget compounds than dyes, such as bezene, toluene, ethyl benzene, and xylenes.

dear Rajesh,

can you give more infos:

- about the kind of the dye (p.e. thiazine oder acridine oder xanthene dyes, phorphyrines... ) but not the dye you are using if it is secret prior to a future paper.

- About the conditions:

P-25 e.g. TiO2 contains :

1/ The TiO2 powder (Degussa, P-25), which is a standard material in the field of photocatalytic reactions, contains anatase and rutile phases in a ratio of about 3 : 1.

2// dye and catalyst mixed without solvent or mixed in solvent and then evaporated?

3/ / If you use a solvent like water, is your suspension aereted oder deareted with nitrogen bubling?

3/ Are you using organic solvent?

Please give more infos about the experimental part.

have you isolated the degradated products of your dye?

JRG

May be it is adsorption, or light is not purely from 450-700 nm

Check the filter