*2.3. Solvent-Thermal Method*

The solvent thermal method is similar to the aforementioned hydrothermal method. The main difference is the starting materials of solvent thermal method are dissolved in some organic solvent while those of hydrothermal method dissolved in deionized water. All the g-C3N4/BiOCl heterojunctions synthesized by solvent thermal method are listed in Table 3.

**Figure 2.** SEM images of (**a**) flower-like BiOCl and (**b**) g-C3N4/BiOCl. Reproduced with permission from Hou W et al, ChemistrySelect; published by John Wiley and Sons, 2020.



Notably, the ionic liquid 1-hexyl-3-methylimidazolium chloride (IL: [HMIm]Cl) was used as one of the starting materials. It was used as a template and the source of chloride. For example, Xiao-jing Wang and colleagues prepared a novel g-C3N4/BiOCl heterojunction through a facile solvent thermal route [44]. According to the study, ionic liquid (IL: [HMIm]Cl) not only played an important role as the source of Cl, but also as a template to direct the growth of the nanoplate into flowerlike nanoparticles. The as-prepared g-C3N4/BiOCl composite was synthesized with larger surface area than the pristine catalysts and better photocatalytic performance. Ionic liquid was also adopted by other researchers, S. Yin and colleagues prepared a g-C3N4/BiOCl composite that employed the ionic liquid [C16mim]Cl as the source of Cl [74]. Like the study mentioned above, Bi(NO3)3·5H2O and ethylene glycol were used as starting materials. Similarly, the two composites showed a flower-like microsphere structure instead of the nano-slice structure. It is reasonable to believe that the presence of ionic liquid could direct the morphology of the g-C3N4/BiOCl heterojunction in these two studies. According to the XRD patterns of them, the BiOCl of the two heterojunctions were not 001 facet dominant like showed in Figure 3. This might mean the heterojunction could respond better to visible light than UV-light.

**Figure 3.** XRD patterns of BiOCl, C3N4, and g-C3N4/BiOCl heterojunction in different compositions. Reproduced with permission from Wang XJ et al, Chemical Engineering Journal; published by Elsevier BV, 2013.

Among all the heterojunctions synthesized through the solvent thermal method, only ionic liquid assisted routes could prepare flower-like g-C3N4/BiOCl heterojunctions. There were other templates that researchers employed to regulate the morphology of the heterojunction. For instance, cetyltrimethyl ammonium bromide (CTAB) and urea were used by Wei Cai and colleagues to prepare a nano-sliced g-C3N4/BiOCl heterojunction [70]. The construction of the heterojunction did not change the morphology of the BiOCl obviously. However, the presence of the template could not just direct the morphology. According to Qiao Wang and coworkers, the presence of the template could facilitate the generation of oxygen vacancies [71]. Unlike the aforementioned studies, NaCl was adopted as the source of Cl, and Polyvinylpyrrolidone (PVP) was used as the template. A facile solvent thermal method was adopted to prepare an ultrathin g-C3N4/BiOCl heterojunction. Just like reported by Xianlong Zhang and colleagues [72], the formation of the heterojunction could make the morphology of the BiOCl become thinner. It was proved that the absence of PVP lowered the photoactivity of the heterojunction. Oxygen vacancies were supposed to act as the trap of electrons and facilitated the generation of superoxide radicals. The result of the density-functional calculation indicated that the presence of the oxygen vacancies narrowed the bandgap, thus improving the absorption of light.

Though the addition of some templates could help to prepare heterojunctions with thinner structure and enhance the photoactivity, the presence of them might cause some environmental problems. So, scientists adopted some template-free solvent thermal methods to prepare the binary heterojunction. Xianlong Zhang and colleagues synthesized a 2 D/2 D g-C3N4/BiOCl composite via a solvent thermal method without the presence of any templates [72]. According to the study, HCl was used as the source of the Cl. The study indicated that ultrathin g-C3N4/BiOCl nanosheets were prepared without the presence of any templates. The two pristine catalysts were self-assembled to form a face-to-face contact, which facilitated the charge transfer and the light absorption. Wenwen Liu and coworkers prepared a square-like nanoplates heterojunction without using templates [73]. NaCl aqueous solution was the source of the Cl. Compared to the study mentioned before, HCl might help to enlarge the surface area of the heterojunction.

Therefore, the heterojunction prepared through the template-free solvent thermal method adopted by Xianlong Zhang and colleagues showed the thinnest structure among all the solvent thermal methods. However, the as-prepared BiOCl was 001 facet dominant. It is reasonable to believe that the as-prepared catalyst would exhibit better photoactivity

under UV-light illumination. According to the articles reviewed, all the heterojunction were not 001 facet exposed, except that prepared by Xianlong Zhang and colleagues. So, when it comes to visible light illumination, heterojunction synthesized by Qiao Wang and coworkers could exhibit better photocatalytic performance than other catalysts because of its enlarged surface area.
