The comparison of bandgap value of slab and bulk phase

I tried to calculate the bandgap of the TiO2-Rutile bulk phase, DFT(not +U) calculation shows the bandgap is about 1.8eV, which is roughly the same as the 1.78eV on the Material Project. When calculating the TiO2-Rutile -110 slab, whether +U or not +U, the bandgap is about 1.3eV, which is 0.5eV smaller than the bulk. After checking some literature, there are also many cases where the slab band gap is obviously smaller than bulk.

But for TiO2, a lot of literature show the bandgap of the slab is 2eV-3eV with the DFT+U method, and the U value given is not very large, between 4-6, but I can’t reproduce similar results. When I increase U to 6, the bandgap of Rutile-110-slab is still around 1.3,eV which has not increased significantly, let alone reached the experimental level of 3eV.

my question is:
Generally speaking, can the bandgap of slab be reduced by about 20%-40% compared to bulk?
Can +U make the bandgap change from 1.3eV to 3eV?
The slab calculation results given in the literature, using the DFT+U method to obtain a bandgap as large as 3eV, is it credible?

Hey Alma, just stumbled across this. If you’re still being confused about this problem consider surface states. Creating a slab will generally not shift the energy levels corresponding to the bulk valence and conduction bands very much, but what it can do is create new states from the surface’s dangling bonds. Those might be creating the new apparent band gap. Good luck!