**5. Other Methods to Improve Photoactivity of Catalysts Based on CN and BOC**

There are many studies focused on how to further enhance the photoactivity of the g-C3N4/BiOCl system. For example, Chengyun Zhou and colleagues used carbondoped g-C3N4 (CCN) to combine with BiOCl. Then, the as-prepared p-n heterojunction showed improved photoactivity through degradation of tetracycline (TC) [7]. Notably, the bandgap of BiOCl was modulated by adjusting the ratio of Cl and O to synthesize a catalyst with a mild bandgap (2.33 eV), which was denoted as Bi12O17Cl2. The asprepared Bi12O17Cl2 was able to respond to visible light due to its narrowed band gap. Through integrating with carbon-doped g-C3N4, a heterojunction of CCN/ Bi12O17Cl2 was composited. CCN was integrated with Bi12O17Cl2 to form a stable heterojunction, and each of the elements were uniformly distributed on the surface of the photocatalyst. The absorption of visible light and separation of photo-induced charge carriers was enhanced by the coupling of the two materials. The degradation of TC over the heterojunction was improved greatly compared to pure catalysts. The results of ESR proved the presence of •OH and •O2 − during the degradation. Trapping experiments proved the presence of •OH, •O2 −, and holes affecting the degradation process. This study also

adopted the PCNB mechanism to explain the mineralization process. Those two materials formed a p-n junction after being integrated. The band structures were changed because of the construction of the composite. There are also many other ternary systems based on BiOCl/g-C3N4 that were synthesized, like BiOCl/Bi2MoO6/g-C3N4 [102], BiOCl/TiO2- C3N4 [103], g-C3N4@BiOCl/Bi12O17Cl2 [104], g-C3N4/oxygen-deficient BiOCl nanocomposite/graphene quantum dots [105], BiOCl/CdS/g-C3N4 [87], g-C3N4/BiOClxI1-x [106], g-C3N4/BiOClxBr1-x [107], BiOI/BiOCl/g-C3N4 [88], and Bi2S3/BiOCl/g-C3N4 [46]. Unlike the CCN/ Bi12O17Cl2 system, all the mechanisms of ternary systems mentioned above did not take the alignment of Fermi levels into account.

Ajay Kumara and co-workers also synthesized a new kind of heterostructure through constructing quaternary magnetic BiOCl/g-C3N4/Cu2O/Fe3O4 nano-heterojunction [108]. According to the vibrating sample magnetometry (VSM) studies, the addition of Fe3O4 endowed the heterojunction to be separated from liquid magnetically. The bandgap of the heterojunction was 2.58 eV, which indicated that the as-prepared catalyst could respond to visible light. The recombination of the photo-induced charge carriers was greatly inhibited. The degradation rate over the quaternary heterojunction was about 2.7 and 2.4 times higher as BiOCl and g-C3N4, respectively. It was even 0.5 times higher than BiOCl/g-C3N4 binary heterojunction. The main reactive species in the photocatalytic process were •OH and •O2 –, according to the results of trapping experiments. The mechanism was as depicted in Figure 12. The alignment of the Fermi energy levels was considered to happen during the preparation. P-n junctions were considered to be formed at the g-C3N4/BiOCl interface and the g-C3N4/Cu2O interface, respectively.

**Figure 12.** Proposed mechanism of quaternary heterojunction of BiOCl/g-C3N4/Cu2O/Fe3O4. Reproduced with permission from Kumar A et al, Chemical Engineering Journal; published by Elsevier BV, 2018.

### **6. Summary and Outlook**

According to the articles reviewed above, facet control and morphology of BiOCl were very important to the photoactivity of the heterojunctions. G-C3N4/BiOCl heterojunction with enlarged surface area accelerated the degradation of the azo dye. 010 facets exposed BiOCl of the heterojunction could enhance the absorption of the visible light.

BiOCl and g-C3N4 are both excellent photocatalysts despite their disadvantages, like fast photo-induced charge carriers' recombination and low efficient solar energy absorption. By coupling those two catalysts, we could get CNB heterojunction, PCNB heterojunction,

and Z-scheme heterojunction. A single catalyst cannot have all the advanced features simultaneously. The construction of catalysts based on g-C3N4/BiOCl is a good strategy to fabricate a perfect photocatalyst. Combining g-C3N4/BiOCl heterojunction with other materials could provide more active sites, and further improve its capability to respond to visible light or make the composite magnetic recyclable.

To date, the heterojunction based on BiOCl and g-C3N4 mainly used in the purification of water, according to the articles reviewed. The mechanisms used to explain the photocatalytic processes could be divided into three different scenarios. Though there were some researchers that employed certain advanced techniques to prove the source of the reactive species and the charge transfer over the as-prepared catalysts, there is still not enough direct evidence of the mechanisms. Gaining a clearer understanding of the charge transfer is very important for researchers to prepare better photocatalysts. The industrial application of photocatalysis will benefit from this direction of research. Nowadays, few researchers have focused on this direction. This article intends to inspire more studies to clarify the route of charge transfer.

**Author Contributions:** Conceptualization, investigation, resources, writing—original draft preparation, editing, Q.R.; writing—review, J.L.; supervision, Q.Y. and W.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
