Fermi energy level (EF) is an important part of the band structure; it helps determine the semiconductor type of photocatalysts and predict the transport of electrons in heterojunctions. Ultraviolet photoelectron spectroscopy (UPS) is the technique of recording VB and EF of photocatalysts. For UPS results, the EF is located at 0 eV, and the intersection of Ox and spectra tangent is relative VB position. Combining with the above techniques, the band structure of photocatalyst with CB, VB, EF, trapping states vs. NHE can be revealed. In addition, the work function () can be directly determined by eqn. ( ) . The work function equals the difference between vacuum energy (Evacuum, at 4.5 V vs. NHE) and EF (eqn. (24). Thus, UPS can be used to confirm consistency between the results. In addition, when photocatalysts are combined to fabricate heterojunction photocatalysts, there are the transports of electrons from photocatalysts with lower work function to photocatalysts with higher work function. This transport can induce internal electric fields to support or prevent the further transport of excited electrons of heterojunction photocatalysts under light irradiation. This is the explanation for the existence of type-II and Z-scheme photocatalysts.