The band gap of a semiconductor can be tuned by the quantum confinement effect, e.g. thin films or quantum dots exhibit a higher band gap than their bulk counterpart:

http://pubs.acs.org/doi/abs/10.1021/cm901228x

Thanks for your valuable answer to my question, Sir

But, As you said, In the case of TiO2 by doping/substitution higher band gap semiconductor (like ZnO) and forming a composite can we increase the band gap?

My general query- Can we increase the band gap of currently available so called maximum wide band gap (TiO2) to also make "absorb in the Visible radiation"?

Can you suggest any ideas/suggestion into this...

with regards

That's something great input, Sir!!

Thanking you

You must to select a light source corresponding with bang gap energy of photocatalyst.In case of  wide band gap photocatalysts such TiO2,ZnO,SiO2 ,a UV light sources(mercury lamp,xenon,detrium..etc) to be suitable. If you use a narrow ban gap photocatalyst such as CdS, CdSe .., visible light sources are suitable(tungsten).To increase bang gap of photocatalyst you must to doping a wide band gap semiconductor in your pristine photocatalyst such as ZnO/CuO, doped , TiO2/Ag2O, ZnO/Fe2O3,TiO2/Au2O3 …

Thanks Artur for valuable comment.

Thanks Artur for valuable comment.

Hope the attached articles help you.

https://www.sciencedirect.com/science/article/pii/S0921883117304260

https://www.sciencedirect.com/science/article/pii/S0925346717305505