If within a few hours?

I think the complete degration may need long time due to the intrinsic band of ZnO.

I am not familiar with this part, why you want to degrade RhB?

Yes, degradation is possible -in a time span depending on intensity of sunlight and morphology of the catalyst.

Anybody is likely to provide with some references on the degradation of RhB by pure ZnO nanocrystal? Thanks a lot.

The ratio of Uv in the sun iight is small.  As the ZnO  was familiar with uv light, so the complete degradation is very difficult 

you may have better results when a metaloxide is doping with Zno