*2.2. Deposition-Precipitation Method*

Deposition-precipitation is another facile way to synthesize g-C3N4/BiOCl binary heterojunction. According to the studies reviewed, most researchers adopted this method to fabricate the heterojunction as listed in Table 2. Qingbo Li and co-workers proved that facet control was still the main factor that affected the photoactivity of the heterojunction under visible light illumination, though proper way of exfoliation could also improve the photocatalytic performance [61]. They synthesized g-C3N4, BiOCl-010, and BiOCl-001, separately, before the construction of the heterojunction. It was obvious that the BiOCl-010 possessed shaper edges and smaller size, according to Figure 1, which means its surface area was larger.

**Figure 1.** SEM images of (**a**) g-C3N4/BiOCl-001 and (**b**) g-C3N4/BiOCl-010. Reproduced with permission from Iqbal W et al, Catalysis Science & Technology; published by Royal Society of Chemistry, 2018.

Furthermore, the unique hierarchical flowerlike morphology of BiOCl could improve the photoactivity of the heterojunction because of its enlarged surface area [6]. This was proved by Liwen Lei and coworkers through synthesizing flower-like BiOCl by using Arabic gum (AG) as a template [34]. The heterojunction using AG showed better photocatalytic performance than that without using the template. Weidong Hou and coworkers also synthesized a flower-like g-C3N4/BiOCl heterojunction employing a microwave-assisted method [62]. However, the function of microwave was to accelerate the reaction process and to enhance the purity of the heterojunction [63,64]. Therefore, the utilization of microwave could not control the morphology of the composite. Compared to the study completed by Weidong Hou and colleagues, the presence of ethylene glycol could facilitate the formation of flower-like heterojunction. Tiekun Jia and colleagues also constructed a flower-like heterojunction by using ethylene glycol and glycerine [65]. The presence of glycerine increased the surface area of the composite.


**Table 2.** G-C3N4/BiOCl heterojunctions synthesized by deposition–precipitation method.

Though it was observed that the presence of g-C3N4 during the synthesis of the BiOCl would make the morphology of BiOCl become thinner, the addition of some template could construct even thinner morphology, which means enlarged surface area. Yang Bai and coworkers synthesized a g-C3N4/BiOCl heterojunction by using cetyltrimethylammonium chloride (CTAC) as the template [66]. The size of the composite was about 10 nm. The template facilitated the formation of BiOCl nano-dots deposited on the surface of g-C3N4 as reported by Chun-zhi Zheng and colleagues [67]. However, if the heterojunction was synthesized without using any template, the morphology of the composite was supposed to be larger. Jiangbo Sun and coworkers constructed a g-C3N4/BiOCl-010 heterojunction just by adjusting the PH value [68]. The size of it was about 5 μm.

According to Lingjun Song and colleagues, the thickness of BiOCl decreased from 40 to 20 nm after combined with g-C3N4 [69]. The same phenomenon was observed by Weidong Hou and coworkers, the as-prepared flower-like BiOCl became thinner as the content of g-C3N4 increased [62]. The width and thickness of pristine BiOCl became smaller than 2 μm and 33.7 nm after the construction of the binary heterojunction, as demonstrated in Figure 2. Notably, among all the articles reviewed in this section, Shan Shi and colleagues employed NaBiO3 to synthesize the g-C3N4/BiOCl heterojunction instead of Bi(NO3)3·5H2O [21]. According to the study, though its size was about 1 μm, the BiOCl in the as-prepared heterojunction was 001 facets exposed, which could facilitate response to the UV light illumination. When it came to visible light photocatalytic reaction, the facile route adopted by Liwen Lei and coworkers could synthesize the heterojunction with larger surface area among all the deposition–precipitation methods reviewed.
