**3. Applications of g-C3N4/BiOCl Heterojunction**

To date, the g-C3N4/BiOCl heterojunction was mainly applied in the degradation of organic dyes, residual pharmaceutical agents, and plasticizers in aqueous solution. The heterojunction was not applied in fields of H2 generation, CO2 reduction, water splitting, or disinfection yet.

### *3.1. Dye Degradation*

Organic dyes are widely used, particularly azo dyes, and are very difficult to be degraded by conventional biological treatment because of the complex aromatic structure, which is highly hazardous to the human race [80,81]. For example, RhB is almost unable to be degraded under irradiation of visible light without the addition of a photocatalyst [34]. Degradation of rhodamine B was employed repeatedly to test the photocatalytic performance of the g-C3N4/BiOCl composite. Methyl orange (MO) and methylene blue (MB) were also used to test photoactivity of the binary heterojunction. All the heterojunctions applied to dye degradation were listed in Table 4.



**4.**G-C3N4/BiOClheterojunctionsappliedtodye

*Catalysts* **2021**, *11*, 1084


The main defect that influences the photoactivity of g-C3N4 is the low separation efficiency of the photo-induced charge carriers. It is believed the layered structure of [Bi2O2] 2+ blocks of bismuth oxyhalides (BiOX) could enhance the separation of the charge carriers [68]. This is why BiOX attracts the attentions of many researchers. Therefore, the introduction of BiOX could improve the photocatalytic performance of the binary heterojunction. Among heterojunctions of g-C3N4/BiOCl, g-C3N4/BiOBr, and g-C3N4/BiOI, g-C3N4/BiOCl composite showed the best visible light photocatalytic performance tested by degradation of RhB, according to J. Sun and colleagues [68]. So, the compositing of g-C3N4 and BiOCl could be a very efficient way to synthesize a better photocatalyst.

Obviously, the degradation rate of organic dyes was accelerated under visible-light illumination through the combination of the two pristine catalysts. Because of the dye sensitization, though RhB could be degraded over BiOCl under visible light irradiation, the introduction of g-C3N4 further enhanced the photocatalytic performance of the composite [67]. It was reported that the degradation efficiency could be about 25 times higher than pure g-C3N4 [6]. Y. Yang and colleagues introduced g-C3N4 into pristine BiOCl, then the degradation rate of MB was increased about 5.9 times higher under the illumination of visible light [60]. L. Song and coworkers demonstrated that the degradation rate of RhB reached 89 and 50% over pristine BiOCl and g-C3N4, respectively, while that of the as-prepared g-C3N4/BiOCl heterojunction reached almost 100% within 30 min of visible light irradiation [59].

As mentioned above, facet control could improve the photoactivity of the binary heterojunction greatly. The degradation rate of MO under visible light irradiation over ng-CN/BiOC-010 was about two times that over ng-CN/BiOC-001 [61,69]. Though prepared by different researchers, heterojunctions synthesized by 010 dominant BiOCl (010HB) showed improved photoactivity compared to 001 dominant BiOCl (001HB). According to the first ten heterojunctions listed in Table 4, though the concentration of RhB was different, 001HB could completely degrade RhB in about 50 min. Those prepared by 010HB showed better photoactivity. RhB could be degraded in about 30 min. The degradation rate of MB over 001HB was inferior to 010HB. 001HB could only remove MB in 2 h [60], while MB was degraded in 30 min over 010HB [65].

Just as discussed above, surface area is another crucial factor that influences the photoactivity of the heterojunction under visible light illumination. Yang Bai, Xianlong Zhang, and coworkers also synthesized 010HB [66,72], but the degradation of the model pollutant (RhB 10mg l-1) lasted for about more than an hour. Whereas, the flower-like structured heterojunction prepared by Liwen Lei and colleagues could remove RhB (20 mg/L) in 20 min [34]. The as-prepared catalyst showed the best photoactivity among all the heterojunctions applied to RhB degradation. Compared to the former two studies, the major difference was its enlarged surface area. Therefore, it is reasonable to believe that larger surface area could enhance the degradation of pollutant under visible light illumination.

The similar results could be observed by other researchers. According to Qingbo Li, Wenwen Liu, and their colleagues [61,73], the as-prepared 010HB could remove MO in about three hours under visible light irradiation. Whereas, 001HB synthesized by Wenjie Shan and coworkers degraded MO in 5 h under similar conditions [75]. However, surface area might be the main factor that influences the degradation of MO under visible light illumination. The flower-like 001HB constructed by Xiaojing Wang and colleagues could degrade MO in 80 min [44], though it was 001 facet dominant. As all the experiments were conducted under similar conditions, it is reasonable to believe the enlarged surface area improved the photocatalytic performance of 001HB greatly.

Furthermore, the photodegradation rate of MB over pure g-C3N4 was higher than pure BiOCl, according to T. Jia and colleagues [65]. The removal efficiency of MB over as-prepared heterojunction was about two times higher than the pristine catalysts. The main reason might be that MB could not be photosensitized. The light absorption rate of the catalysts might play a more important role. S. Shi and colleagues also observed a

similar phenomenon, and the MB degradation rate over the as-prepared heterojunction of the two catalysts was about four times higher than the pristine materials [21].

Therefore, according to the articles discussed in this section, g-C3N4/BiOCl heterojunctions with 001 facet dominant BiOCl and enlarged surface area could appear superior in terms of dye degradation.

### *3.2. Other Applications*

Besides dye degradation, other applications of the g-C3N4/BiOCl composite were seldom reported, as according to Table 5.


**Table 5.** Other applications of g-C3N4/BiOCl heterojunctions.

One of the most important applications is the removal of recalcitrant industrial materials. 4-chlorophenol (4-CP) is an important material widely used in many areas. It could be used to manufacture sanitizers, germicides, precursors of pesticides, and dyes [82]. Just like other endocrine disruptors (bisphenol A (BPA), bisphenol S (BPS), and bisphenol F (BPF)), which are hazardous to the environment, they were employed by Q. Wang and co-workers to test the photocatalytic performance of the g-C3N4/BiOCl heterojunction [71]. It was indicated that the refractory pollutants could be mostly degraded within 2 h, while these endocrine disruptors were impossible to remove by conventional wastewater treatment.

As one of the plasticizers, about 60% of Dibutyl phthalate (DBP) was photodegraded over the g-C3N4/BiOCl catalyst within 300 min of visible light irradiation, according to W. Shan and colleagues [75]. Plasticizers, such as DBP, could adversely affect the neurodevelopment of infant and child, which is very hard to be degraded by normal wastewater treatment plants [83]. Photocatalysis is supposed to be an effective way to remove DBP from aqueous solution.

Degradation of the residual pharmaceutical agents is another application of the g-C3N4/BiOCl binary heterojunction. For example, carbamazepine, as a psychotropic and antiepileptic drug, is a recalcitrant and toxic chemical, which was adopted by researchers to test the photoactivity of the g-C3N4/BiOCl heterojunction [62]. It was indicated that the binary heterojunction showed more superior photoactivity than pure catalysts.

The contamination of antibiotics has drawn lots of attention in recent years. The amine-based pharmaceutical nizatidine, which could cause environmental problems, was applied in the field of photocatalysis by some researchers [63]. The result indicated that the degradation rate of the recalcitrant pollutant was accelerated over the g-C3N4/BiOCl photocatalyst. Application of antibiotic degradation, as one of the important applications of photocatalysis, is expected to have a bright future.

Moreover, according to B. Zhang and co-workers [84], g-C3N4/BiOCl could be used to modify an ITO electrode. That is the only application reported recently other than the degradation of recalcitrant pollutants.

However, it is not clear that if the same factors influenced the dye degradation could improve the removal of other chemicals, because there is still not enough research to be compared with.

According to Tables 4 and 5, the g-C3N4/BiOCl heterojunctions reviewed showed excellent stability and reusability. Almost every as-prepared catalyst exhibited stable photoactivity within at least five recycles. Furthermore, XRD patterns of the catalysts after several cycles showed that the crystal phase of the materials still stayed intact [21,58,61,71].
