*4.2. PCNB Heterojunction*

According to the theory of semiconductor physics about p-n junction, n-type semiconductor g-C3N4 combines with p-type semiconductor BiOCl to form one composite, which tends to have one single Fermi energy level under one certain circumstance. As mentioned above, the Fermi level of g-C3N4 is supposed to be higher than that of BiOCl. After the construction of PCNB heterojunction, band structures of the two materials were expected to be changed to align the Fermi levels (EF). The photocatalytic mechanism of PCNB system is shown in Figure 7.

**Figure 7.** The schematic mechanism of g-C3N4/BiOCl p-n junction.

Hybrid density-functional theory (DFT) calculation was used to anticipate the properties of the g-C3N4/BiOCl composite [51]. G-C3N4 and BiOCl could form a stable composite with a narrower bandgap (2.1 eV), then the absorption of visible light was enhanced. According to the values of work function (WF), g-C3N4 and BiOCl were supposed to be positively and negatively charged after the contact, respectively, then the built-in electric field between the two materials was formed. Photo-induced charge carriers' separation was greatly improved since the lifetime of them was prolonged, and the recombination of electrons and holes was hindered in the system of g-C3N4/BiOCl. By the values of the valence and conduction band offset (VBO and CBO), which were 0.69 and 1.78 eV, respectively. The band structures of g-C3N4 and BiOCl were changed after contact as shown in Figure 7. Just as depicted above, theoretically, p-n junction possesses one single Fermi energy level, so energy bands of p-type semiconductor tend to move upward, whereas that of n-type semiconductor tend to move downward. Then, the as-prepared heterojunction can still be defined as a type-II heterojunction. Compared to pure catalysts, the photo response of PCNB heterojunction can be expanded to visible light region, and separation of photo-induced charge carriers could be promoted.

Though the author of the article mentioned above anticipated the properties of the binary heterojunction by theoretical calculation, many studies of other researchers could provide proof of the results. For example, Xianlong Zhang and co-workers synthesized a PCNB heterojunction by using g-C3N4 and BiOCl in the absence of the surfactant [72]. XRD data of the samples indicated that the formation of heterojunction did not change the

structure of pristine g-C3N4 and BiOCl as shown in Figure 8a. XPS analysis was carried out to prove the strong interaction between g-C3N4 and BiOCl in the heterojunction. UV-Vis DRS spectrum (Figure 8b) indicated the wavelength of light response was expanded to visible light region. The recombination of electrons and holes was inhibited by the construction of heterojunction, which could be inferred from the photoluminescence spectra shown in Figure 8d. The results of the trapping experiments indicated that the dominant reactive species in the degradation of RhB are •O2 <sup>−</sup> and h+. The proposed degradation mechanism of RhB over the as-prepared catalysts was similar to that shown in Figure 7 which was different from that in Figure 5. The conduction band edge and valance band edge of BiOCl moved upward, while the band edges of g-C3N4 moved downward after the contact of the two materials. The experiments and tests conducted above verified the results of hybrid density-functional theory (DFT) calculation.

**Figure 8.** (**a**) XRD data, (**b**) UV-Vis DRS spectrum, (**c**) SEM, (**d**) photoluminescence spectra of all the samples. Reproduced with permission from Zhang X et al, Applied Surface Science; published by Elsevier BV, 2019.

Oxygen vacancies of PCNB were also introduced, which enhanced the photoactivity. Generally, oxygen vacancy is a common defect on oxide surfaces [90]. Oxygen vacancies could be introduced by certain methods. For example, removal of the surface oxygen atoms and elimination of the surface hydroxyl groups could be realized simultaneously through microwave irradiation and reaction between ethylene glycol and BiOCl [91]. However, there still is not a proper way to detect OVs quantitatively at present. The understanding of OVs is still infant. There are still many steps to take to elucidate the function of oxygen vacancy.

Qiao Wang and co-workers synthesized an oxygen vacancy-rich 2 D/2 D BiOCl/g-C3N4 p-n junction [71]. The efficiency of dechlorination and hydroxylation of 4-chlorophenol

over the as-prepared heterojunction were improved under the illumination of visible light. Notably, photoactivity of the catalyst was promoted greatly by introducing oxygen vacancies through the addition of the template of PVP. The presence of OVs could be detected by electron spin resonance (ESR) spectroscopy. Oxygen vacancy is a very common surface defect that exists on the surface of BiOCl, which could produce a new state in the bandgap and localize the photo-induced electrons [92]. The light absorption of BiOCl photocatalyst was expended to the visible light region in the presence of oxygen vacancies. Furthermore, OVs could facilitate the generation of superoxide radicals. Therefore, the introduction of OVs could improve the photoactivity of the catalyst. Other researchers used multiple methods to detect OVs. W. Hou and colleagues also synthesized another oxygen vacancyrich PCNB heterojunction [62]. They not only employed ESR spectroscopy to prove the presence of OVs on the surface of the catalyst, but also adopted O2-TPD profiles (Figure 9) to further detect the OVs. ESR and trapping experiments were also conducted to determine the main reactive species in the study, simultaneously.

**Figure 9.** The O2-TPD profiles of BiOCl, BOC/CN-5, BOC/CN-10, and BOC/CN-50. Reproduced with permission from Hou W et al, ChemistrySelect; published by John Wiley and Sons, 2020.

However, some researchers just adopted the mechanism depicted in Figure 7 to explain the photocatalytic reaction that happened over the g-C3N4/BiOCl heterojunction. For instance, Sheng Yin and co-workers synthesized a p-n junction g-C3N4/BiOCl with the assistance of ionic liquid [C16mim]Cl [74]. The characterization of as-prepared heterojunction showed the two materials constructed a stable heterojunction and stayed intact during the synthesis process. The photocatalytic performance of the composite was enhanced. However, the researchers did not use other methods, like trapping experiments, to prove the proposed mechanism.

In recent years, researchers have employed some advanced techniques to figure out the photocatalytic mechanism of the g-C3N4/BiOCl heterojunction. According to Z. Chen and co-workers, ultrafast transient absorption (TA) spectroscopy was adopted to test the mechanism [93]. TA spectroscopy provided more direct evidence of the PCNB heterojunction mechanism. Representative TA kinetic profiles indicated that the photoinduced electrons transferred to the CB of g-C3N4 in the binary heterojunction. If the band structures of the two photocatalysts stayed the same, the electrons were supposed to accumulate in the CB of BiOCl because of the more positive redox potential. It is reasonable to believe the results of TA spectroscopy could be the evidence of the PCNB mechanism.

Just like the study mentioned above [61], W. Cai and colleagues also adopted the valence band X-ray photoelectron spectroscopy (VB XPS) to determine the VB of BiOCl

and g-C3N4 [70]. However, the same technique was employed by both articles, which obtained different results. It was indicated that the alignment of the Fermi levels happened while preparing the g-C3N4/BiOCl composite. The photo-induced electrons gathered in the CB of g-C3N4, while the accumulation of the holes happened on the VB of BiOCl. The potential of the VB (1.58 eV) was not positive enough to produce •OH. It was reasonable to deduce that the superoxide radicals were the main reactive species.
