Firstly, you can comfirm the position of fermi level of your material using M-S test. Then with the help of VB XPS, you will get the difference between the fermi level and the top of VB, so the top of VB can be calculated. At last, you can use UV-vis to get the band gap of your material. Naturally, on the basis of the top of VB and the band gap, the bottom of CB can be calculated.