**2. Synthesis of g-C3N4/BiOCl Heterojunction**

In the field of photocatalysis, g-C3N4 has been the focus of research in the past decade. Because the photo-induced electrons of g-C3N4 recombines with the hole very quickly. Though visible light accounts for more than 40% of solar light, this feature limits its utilization as a perfect photocatalyst.

The indirect band structure of BiOCl means photo-induced electron-hole pairs cannot recombine very quickly [8]. Furthermore, its layered structure facilitates the generation of the reactive species. The biggest problem is that this material only responds to UV-light, which merely accounts for 5% of solar light. This means BiOCl cannot make use of solar energy sufficiently.

In order to overcome the shortcomings of the semiconductors mentioned above, metal and carbonaceous materials were introduced [52]. Doping of those materials could improve charge separation and light absorption of the pristine catalysts. For example, noble metal with high electric conductivity and different Fermi level energy were doped into the semiconductor. The metal particles acted as the electron reservoirs or acceptors on the surface of the catalyst. However, holes generated on the semiconductor cannot migrate to noble metal particles because of the existence of the Schottky barrier. Thus, charge separation is realized through elemental doping [22]. However, doping of other materials has its limitations as mentioned above. None-mental heterojunctions are becoming more attractive. The semiconductor-semiconductor hybridization is generally supposed to be an effective way to improve the photoactivity of the semiconductors. Composite of BiOCl and g-C3N4 is proved to be an excellent form of a heterojunction photocatalyst.

As it is known to us all, efficient methods to enhance the activity of photocatalyst can be divided into two types: (1) facilitating photo-induced charge separation; (2) improving the efficiency of solar energy utilization. There is no doubt that making a heterojunction between two different semiconductors is an excellent way to realize the two aims at the same time. Compositing BiOCl and g-C3N4 can not only improve charge carriers' separation, but also make the composite respond to visible light.

Heterojunctions of g-C3N4/BiOCl could be synthesized by many methods. According to the articles reviewed, there are three different methods overall employed by researchers, hydrothermal, deposition–precipitation, and solvent thermal. It is hard to say which one of them is the best. Generally, catalysts with larger surface area and thinner morphology could facilitate the degradation of pollutants. All the heterojunctions reviewed showed enhanced visible light absorption. Thus, it is reasonable to say the photoactivity of the catalysts was improved through the construction of the heterojunction.

Graphitic carbon nitride synthesized through calcination of urea showed higher surface area and better photoactivity [22]. Considering the theoretical specific surface area of perfect monolayer g-C3N4 is 2500 m2 g−1, there is still a long distance to cover [53]. However, it is not the main reason that explains the photocatalytic improvement of the composite based on the system of g-C3N4/BiOCl. The BET surface area of pristine g-C3N4 could be 150.10 m2 g−1, but the photoactivity is much lower than the hybrid. The surface area of BiOCl-CNs-3% is much lower than BiOCl-CNs-5%, whereas the photoactivity is much higher [6].

The mass ratio of the heterojunction is a very important factor. There is no doubt that the addition of BiOCl can prompt the photo-induced charge separation, and C3N4 can enhance the absorption of visible light in the systems. Theoretically, higher mass ratio of BiOCl means more chances for charge carriers to be separated, which could improve the photoactivity of the composite. When the mass ratio of one certain component increases, the photoactivity of the composite will not stop being enhanced until the generation of

and separation of photo-induced charge carriers reaches a certain balance. However, higher ratio of BiOCl does not always mean higher photoactivity. In the system of (20%) g-C3N4/BiOCl when the mass ratio of g-C3N4 reached 20%, the catalyst showed the best photocatalysis performance [21]. When the mass ratio was below 20%, increasing the ratio of g-C3N4 led to the photocatalytic enhancement of the heterojunction. If the mass ratio increased further, the activity started to decrease. Because too much g-C3N4 tends to agglomerate with itself, the contact between BiOCl and g-C3N4 was weakened [54]. The same thing happened when the mass ratio of BiOCl increased.

Generally, the morphology of g-C3N4 is hard to control unless using a certain template or different precursor [55]. Direct calcination of the precursors could only produce bulk g-C3N4. In order to improve the connection between the g-C3N4 and the BiOCl, exfoliation of the bulk g-C3N4 was adopted by many researchers. There are mainly three exfoliation methods that were adopted, chemical blowing, thermal, and liquid exfoliation. Previous researchers have proved that g-C3N4 exfoliated by thermal exfoliation method exhibited better photocatalytic performance than others through comparative experiments [56]. According to the articles reviewed, g-C3N4 commonly synthesized before the construction of the binary heterojunctions, so the construction of the heterojunction could not affect the morphology of g-C3N4. However, the presence of g-C3N4 could influence the morphology of BiOCl in the binary heterojunction. Therefore, the morphology control of BiOCl should be a very important factor that influences the photocatalytic performance of the binary heterojunction system under visible light irradiation.

Moreover, facet control of BiOCl is also a very efficient method to enhance photoactivity of the heterojunction. BiOCl with exposed 001 and 010 facets could be synthesized through PH value adjustment during the process of the construction route [57]. The better visible-light photocatalytic performance of BiOCl-010 could be attributed to the larger surface area, which could also beneficial for the construction of the heterojunction.
