*2.4. Calcination Method*

Besides the methods mentioned above, Wenjie Shan and colleagues synthesized the g-C3N4/BiOCl heterojunction through an in-situ calcination method [75]. They obtained a composite with enhanced photoactivity through the direct calcination of Bi2O3 and guanidine hydrochloride. Despite the different starting materials, the as-prepared composite showed superior catalytic performance and enlarged surface area (25.50 m<sup>2</sup> g−1), similar to the flower-like structured g-C3N4/BiOCl heterojunctions synthesized through other methods. Notably, the 001 facet of as-prepared catalyst showed very strong signal. The heterojunction might show better photocatalytic performance under UV-light illumination.

Therefore, the methods employed to synthesize the g-C3N4/BiOCl binary photocatalyst mainly focused on making a thinner and hierarchical structure. Facilitating the charge transfer between the two components is one of the main purposes. The larger surface area might be beneficial for the enlarged interfacial contact area, which is why the thinner structured heterojunction exhibited obviously enhanced photoactivity compared to the pristine catalysts. It is believed larger surface area of one certain material could provide more active sites for the absorption of organic matters and photocatalytic reactions [45,76,77]. On one hand, higher surface area means higher mass to surface area rate and more area for other catalysts to deposit on it. On the other hand, enlarged surface area endows photocatalysts to absorb visible light more efficiently [41]. Among the articles reviewed, the exfoliation and introduction of organic template produced the thinner structured BiOCl heterojunction that possessed larger face-to-face interface with g-C3N4 and facilitated the efficient charge separation of light-induced charge carriers of the heterojunction system [65,71]. A similar pattern could be observed in other heterojunction systems [7,34,44,49,78]. However, whether a certain structure is beneficial for photoactivity generally depends on the circumstance [79]. The function of morphology still needs to be studied further to reveal a clearer pattern.
