For determining the composition of the photocatalysts, X-ray photoelectron spectroscopy (XPS) is well known as a powerful tool, but another application of XPS that should be considered is to determine the band offset of heterojunctions . Comparing to the mentioned approach of Mott-Schottky, evaluating band shifting with XPS is more accurate because XPS records the elements' core levels, which are more stable than the surface value. In other words, the VB offset could be accurately determined when the deviation of the core levels before and after contact is measured. The VB offset is calculated as eqn. ( ) or (21), so the CB is calculated as eqn. ( ) . The exact position of band structure vs. NHE is useful in studying and applying for suitable applications, as mentioned above.