**4. Mechanisms of the BiOCl/g-C3N4 Heterojunctions**

P-type photocatalyst BiOCl combined with typical n-type photocatalyst g-C3N4 could form a conventional type-II photocatalyst with a staggered-gap band structure. There are three different types of semiconductor heterojunctions overall as shown in Figure 4. In a type-I heterojunction, conduction band (CB) and valence band (VB) of the semiconductor (SC1) are higher and lower than that of the other semiconductor (SC2), respectively. When SC1 and SC2 construct type-II heterojunction, CB and VB of SC1 are higher than of SC2. Because of the built-in electric field formed inside of the composite, photo-induced electrons tend to migrate to the CB of SC2. At the same time, holes accumulate on VB of SC1 rapidly. Because the electrons and holes migrate to different semiconductors, charge separation is enhanced. The pattern of charge carriers' movement in type-III heterojunction is the same as in type-II heterojunction. The difference of the band structures of the two semiconductors is even larger than in the type-II heterojunction [52].

**Figure 4.** Three different types of semiconductor heterojunction.

Separation of electron-hole pairs could be prompted by the construction of the g-C3N4/BiOCl heterojunction. Solar energy utilization of the heterojunction was also more efficient since the wavelength of photo-response was broadened [8]. According to the articles reviewed, an interesting phenomenon is observed, some researchers believed that band structures of the two catalysts stayed unchanged after the combination of the two materials, whereas others thought that CB and VB of g-C3N4/BiOCl changed to align Fermi energy levels.

Theoretically, the Fermi energy level of an n-type catalyst is close to the bottom of CB. The Fermi energy level of a p-type catalysts is close to the top of VB [85]. After construction of heterojunction, the CB and VB of n-type catalyst tend to move downward, while those of p-type catalyst are moving upward. This kind of heterojunction based on g-C3N4 and BiOCl is labeled as PCNB in this article. The CB and VB of some g-C3N4/BiOCl heterojunctions stayed the same after the combination. This kind of heterojunction is denoted as the system of CNB here.
