*4.1. CNB Heterojunction*

Generally, the proposed mechanism for the generation of reactive radicals on the surface of CNB heterojunction is shown in Figure 5. Photoexcited electrons firstly generated in the conduction band of g-C3N4 by irradiation of visible light because of its relatively mild band gap (2.7 eV). When it comes to dye degradation, photo-induced charge carriers also generated through dye sensitization. Then, electrons transferred to the conduction band of BiOCl because the conduction band of BiOCl is less negative than that of g-C3N4. Photogenerated electrons tend to transfer to a less negative conduction band. Electrons could react with O2 on the surface of CNB to generate superoxide radicals. At the same time, holes remaining in the valence band of g-C3N4 react with surface-absorbed H2O to generate hydroxyl radicals, so that separation of photo-generated charge carriers is improved and the catalyst can response to visible light. However, the redox ability of the heterojunction was sacrificed when photoactivity is improved, because the holes accumulated on VB of g-C3N4.

**Figure 5.** The mechanism for the generation of reactive radicals over CNB.

The mechanism mentioned above was adopted by Faisal Al Marzouqi and co-workers to explain the degradation of nizatidine over the BiOCl/g-C3N4 heterojunction [63]. The degradation efficiency of nizatidine was improved under the irradiation of visible light. According to the XRD pattern, the as-prepared catalyst was constructed by pure BiOCl and g-C3N4. The construction of the heterojunction was verified. As shown in the UV-vis diffuse reflectance spectra, the absorption edge of BiOCl was about 364 nm (in the UV range), and that of g-C3N4 was about 450 nm (in the visible range). After being combined, the absorption band edge of the heterojunction could be up to 476 nm. The photoactivity of the heterojunction was improved. The bandgap value for 10% BiOCl/g-C3N4 sample was 2.6 eV, which endowed the catalyst with the highest photoactivity among all the asprepared samples. Therefore, the bandgap of the composite was narrowed by combination of the two components. The degradation rate of nizatidine was enhanced by the construction of the heterojunction as shown in Figure 6b. This improvement was explained by the double-charge transfer mechanism as proposed in Figure 5. Obviously, the CB and VB of both pristine catalysts did not change. The generation of reactive radicals depicted in the article was the same as that in Figure 5. However, the article provided no further evidence to prove the main reactive radicals. The presence of hydroxyl radicals was supposed to be the main cause of the degradation of nizatidine in the article. But the study did not exclude the possibility that the hydroxyl radicals could be generated from superoxide radicals. Y. Yang and colleagues demonstrated hydroxyl and superoxide radicals were the main species during the photocatalytic oxidation of MB, too [60]. Hydroxyl radicals were supposed to be produced in the VB of g-C3N4.

**Figure 6.** (**a**) XPR pattern of as-prepared BiOCl/g-C3N4 samples; (**b**) Degradation rate of nizatidine at an initial concentration of 5 mg/L and pH = 5.6 with all the prepared samples; (**c**) SEM image of 10% BiOCl/g-C3N4 sample; (**d**) UV-vis diffuse reflectance spectra of the obtained samples. Reproduced with permission from Al Marzouqi F et al, ACS Omega; published by American Chemical Society, 2013.

To date, lots of CNB heterojunctions were reported. Wenwen Liu and colleagues constructed a 2-dimensional layered BiOCl/g-C3N4 composite, and the photodegradation of MO was greatly improved through constructing a CNB heterojunction [73]. When the mass ratio of BiOCl reached 70%, BiOCl/g-C3N4 heterojunction showed the highest photocatalytic performance. EIS images and PL spectra were carried out to prove that better charge separation was realized. The proposed mechanism was similar to that shown in Figure 5. Electrons generated in the conduction band of g-C3N4, and then transferred to the conduction band of BiOCl. As a result, superoxide radicals generated on the surface of the heterojunction. Holes in the valence band of C3N4 were accumulated to participate in the degradation of MO degradation. Trapping experiments exhibited •O2 − and holes were the main reactive species in the degradation of MO, which could be the evidence of the proposed mechanism. In this study, the VB and CB positions of BiOCl and g-C3N4 were determined by the Mott-Schottky curve. The alignment of band edges during the combination of the two materials was not taken into consideration, though the researchers did not directly adopt the standard values. The presence of the main reactive species was consistent with the proposed mechanism. Liwen Lei and co-workers prepared another heterostructure photocatalyst by combining BiOCl and g-C3N4 [34]. Arabic gum (AG) was added while synthesizing the heterojunction. They also proved that the superoxide and holes are the main reactive species through trapping experiments. The mechanism shown in Figure 5 was also adopted to explain the degradation of RhB over the composite.

However, the BiOCl/g-C3N4 heterojunction prepared by Xiaojing Wang and colleagues showed a different result [44]. Like the studies mentioned above [73], XPR, FT-IR spectroscopy, and PL emission spectra were carried out to demonstrate the formation of the heterojunction. The light response wavelength of BiOCl was broadened, while the charge separation was enhanced. Trapping experiments were also carried out to detect the main reactive species in the photocatalytic process. It turned out that •O2 − was not the main reactive species, whereas holes played an important role during the degradation of MO.

Why the hydroxyl radicals were not generally supposed to generate during the photocatalytic reaction was not mentioned in the above studies. Zhang Sai and co-workers explained the reason in their study [86], the standard CB and VB potentials of g-C3N4 are approximately −1.3 and 1.40 eV, respectively. The standard redox potential of •O2 −/O2 is −0.13 eV (vs. NHE), which is more positive than the CB potential of g-C3N4. So, it is very easy for e- on the CB of g-C3N4 to generate superoxide radicals. The VB potential of g-C3N4 is less positive than the standard potential of •OH/OH−, which is +1.99 eV (vs. NHE). This makes holes on the VB of the catalyst and cannot be captured and to produce •OH radicals. If the CB and VB of the g-C3N4/BiOCl catalysts stay unchanged after the construction of the type-II heterojunction, electrons accumulate on the CB of BiOCl (−1.1 eV) [8] to form •O2 −. Holes migrate to the VB of g-C3N4, but cannot generate hydroxyl radicals. Therefore, superoxide radicals and holes are the main reactive species in the systems of BiOCl/g-C3N4. This theory is consistent with the results mentioned above. The work of L. Song and co-workers also suggested that the standard redox potential of the VB of g-C3N4 was not positive enough to generate •OH groups [69]. J. Sun and colleagues directly used the standard potentials of the pristine catalysts to describe the mechanism without taking the alignment of the Fermi energy level into account [68].

Q. Li and co-workers employed the result of X-ray photoelectron spectroscopy (VB XPS) spectra to determine the VB of pure g-C3N4, which was 1.44 eV NHE [61]. Compared to the standard potential of •OH/OH−, the generation of •OH was not expected to happen on the VB of g-C3N4. The result of trapping experiments suggested that •O2 − and holes were the dominant reactive species during the degradation of MO.

Some other researchers did not only adopt trapping experiments to determine the main species, for example, L. Song and co-workers also adopted ESR spectra and trapping experiments to find out the main reactive species [59]. The presence of superoxide radicals was directly proved by the ESR test. The generation of hydroxyl radicals was not detected. Trapping experiments proved holes also played an important role during the oxidation of RhB.

Just like the aforementioned study of Xiaojing Wang and colleagues [44], T. Jia and colleagues determined the CB and VB potentials of BiOCl and g-C3N4 by using theoretical calculation, then holes were proved to be the main reactive species during the oxidation of MB through trapping experiments [65].

There are some other studies that adopted a similar mechanism to explain the degradation of pollutants over ternary catalysts based on the system of BiOCl/g-C3N4, like systems of BiOCl/g-C3N4/kaolinite [47], g-C3N4/CDs/BiOCl [48], BiOCl/CdS/g-C3N4 [87], and BiOI-BiOCl/C3N4 [88].

However, Xiaojuan Bai and colleagues demonstrated that hydroxyl radicals were still produced, though the VB of g-C3N4 was not positive enough [89]. They synthesized a kind of photocatalyst by modifying g-C3N4 with fullerene. After the modification, the degradation rate of MB was improved. Trapping and ESR experiments proved that holes and •OH were the main reactive species in the photodegradation of MB. After the modification, the VB of C60/g-C3N4 was more positive by 0.17 eV. Considering the theory depicted above, that was not positive enough to generate •OH directly on the VB of g-C3N4. The mechanism was further researched by adding N2 to create an anoxic suspension. The degradation of MB was almost unchanged in the presence of N2, which indicated that the •OH was generated on the surface of the composite, but not through the reaction induced

by electrons on the CB of g-C3N4. This study seems contradictory to the theory described above that the VB of g-C3N4 was not positive enough to produce •OH [86].

The CNB system is a typical type-II heterojunction due to the band structures of the two materials. Trapping experiments were carried out to clarify the main reactive species, which proved to be superoxide radicals and holes. However, according to the study discussed above [86], there is still something unclear about the mechanism depicted in this section. Some more works are required to elucidate the reaction that happened over the heterojunction of CNB.
