The codoping will will change forbidden band and conduction band.

The codoping in the photocatalyst enhances the activity. In n-type semiconductor, electrons can be increased in the conduction band while in the p-type semiconductor, holes can be reduced at the valance band. These phenomena hinders the electron-hole recombination in the semiconductors. The enhancement in electrons or reduction in holes depends on the semiconductor type.

Thanks

Dear Dr. Malini

Doping by metal ions added additional levels below conduction bands: thus, resulting in lowering the band gap. Similarly, doping by non-metals adds levels above the valence band and as result, valence band is uplifted decreasing the band gap again. In case of codoping by metal and no--metals, both the levels may come closer than their original positions and will result in further lowering in the band gap. But, this is not always the case, some other factors may also play their own roles. This lowering in band gap of a semiconductor may make it active in visible range (red shift), provided it is not absorbing the visible radiations earlier.