*2.1. Hydrothermal Method*

Generally, in the hydrothermal method, g-C3N4 powder-prepared through polymerizationwas dispersed in deionized water, and then the solution was stirred in order to prepare a suspension. Commonly, Bi (NO3)3·5H2O and KCl were added into the as-prepared suspension. Subsequently, the mixed suspension was transferred to an autoclave and heated to gain the BiOCl/g-C3N4. To date, three kinds of BiOCl/g-C3N4 heterojunction were prepared through the hydrothermal method (listed in Table 1).

**Table 1.** G-C3N4/BiOCl heterojunctions synthesized by hydrothermal method.


Just as analyzed above, morphology of BiOCl is the most important variable in the process. Generally speaking, BiOCl of the BiOCl/g-C3N4 system with larger surface area is expected to show better visible light absorption. So, the main purpose of this section is to find out which method could make the morphology of BiOCl become thinner.

L. Song and co-workers employed NH4Cl as blowing agent to make ultrathin g-C3N4 nanosheets (prepared by polymerization of melamine) in order to enhance the contact between the two materials [59]. In that article, two-dimensional g-C3N4/BiOCl heterojunctions were prepared through a facile hydrothermal method. The width of Pristine BiOCl is less than 3 μm and thicknesses is 50–60 nm. Notably, the construction of the binary heterojunction affected the morphology of the BiOCl. The thickness of nanodisc-like BiOCl was around 35–50 nm after loaded by ultrathin graphitic carbon nitride.

Yifan Yang and coworkers also adopted a hydrothermal method to synthesize the heterojunction [60]. Interestingly, before the construction of the g-C3N4/BiOCl heterojunction, they synthesized g-C3N4 and BiOCl separately. After the formation of the heterojunction, it was observed that the width of BiOCl was 2 μm and the thickness was about 200 nm. Obviously, the surface area of the heterojunction should be smaller than that synthesized in the study mentioned above, though it showed better photoactivity than pristine catalysts.

According to Yongkui Huang and colleagues, they used BiCl3 as a precursor to synthesize the heterojunction [58]. It was reported that nanoplate-shaped BiOCl decorated on the surface of g-C3N4 with thickness below 5 nm and widths of 20~30 nm. However, the result of X-ray powder diffraction (XRD) showed that the 001-facet exposed BiOCl and was the dominate, which is supposed to beneficial for the UV-light photocatalytic reaction.

Especially, when it comes to visible-light, among all the articles reviewed in this section, though heterojunction synthesized by using BiCl3 as precursor showed the thinnest structure, method employed by L. Song and co-workers should be the better way to synthesize g-C3N4/BiOCl heterojunction.
