**1. Introduction**

With the development of human civilization, many refractory pollutants were discharged to the environment, which are hard to be degraded by traditional purification methods. For example, pollutants like tetracycline [1], bisphenol A [2], Astrazone Black [3], estriol [4], and tetraethylated rhodamine [5] can hardly be degraded by normal waste-water treatment plants. Generally, the degradation of recalcitrant organic pollutants relies on the consumption of energy, such as the Fenton process. Photocatalysis has drawn much attention because of the utilization of solar energy and friendliness to the environment.

In photocatalysis, the heterojunction of two different materials is deemed as an efficient strategy to develop diverse hybrid composites with multiple functionalities [6]. Photocatalysts normally possess some defects that restrict the utilization of the material, for example, ultrafast recombination of photo-induced charge carriers [7], low efficient utilization of sunlight, and wide bandgap [8]. Hybridization of two different catalysts would be an excellent way to improve the photoactivity. Lots of photocatalysts were synthesized

**Citation:** Ren, Q.; Liu, J.; Yang, Q.; Shen, W. A Review: Photocatalysts Based on BiOCl and g-C3N4 for Water Purification. *Catalysts* **2021**, *11*, 1084. https://doi.org/10.3390/catal11091084

Academic Editors: Ioan Balint and Monica Pavel

Received: 19 August 2021 Accepted: 6 September 2021 Published: 8 September 2021

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in order to enhance the photoactivity of the catalysts, such as MoS2/g-C3N4 [9], CuInS2/g-C3N4 [10], Ag2O/g-C3N4 [11], Ag2O/TiO2 [12], AgI/CuBi2O4 [13], CuS/BiVO4 [14], Ag3PO4/MoS2 [15], g-C3N4/SiO2 [16], ZnFe2O4/TiO2 [17], LaFeO3/SnS2 [18], Bi2O3/g-C3N4 [19], and Ag2O/Bi5O7I [20]. Recently, heterojunctions based on BiOCl and g-C3N4 drew much attention because of the abundance of the materials in the environment.

Graphitic carbon nitride is regarded as a nice photocatalyst because of its nontoxicity, stability in pH over a broad range (0–14), easy to prepare, and the narrow bandgap [21]. After being first synthesized in 1834 [22], carbon nitride has been used in many areas, such as virus inactivation [23], activation of benzene [24], H2 revolution [25,26], fuel cells [27], CO2 reduction [28], and organic pollutants degradation [29]. However, because of its high recombination rate of the photogenerated charge carriers and low BET surface area, the application of g-C3N4 is restricted. Recently, many studies focused on building heterojunctions to improve its photoactivity, such as the system of WO3/g-C3N4 [30], ZnWO4/g-C3N4 [31], and In2S3/g-C3N4 [32]. According to these studies, coupling graphitic carbon nitride with other kinds of semiconductors could construct better photocatalysts by reducing the recombination rate of the photogenerated charge carriers or increasing the surface area.

On the contrary, layered structure of BiOCl facilitates the photogenerated charge carriers' separation and endows it with a strong ability to degrade organic pollutants [33]. Morphology control was employed by many researchers to improve the pristine catalyst [34]. For example, according to E. Ramírez Meneses and co-workers [35], the addition of capping agents could affect the morphology of BiOCl. However, the as-prepared catalysts were unable to be excited by visible light. In order to expand the light absorption range of BiOCl, many researchers synthesized heterostructures like carbon dots/BiOCl [36], BiOCl/g-C3N4 [37], WO3/BiOCl [38], Bi2MoO6-BiOCl [33], BiOCl/BiOBr [39], m-Bi2O4/BiOCl [40], BiOCl/BiVO4 [41], Bi2O2CO3/BiOCl [42], and BiOI/BiOCl [43].

Among them, the composition of BiOCl and g-C3N4 is considered as an excellent combination. The heterojunction could enhance the separation of the photo-induced charge carriers and enable the catalyst to respond to visible light [43,44]. Noble metal doping is also considered as a good method to improve the semiconductor. However, the high cost of noble metal doping restricts its utilization. If a noble metal doping catalyst is used repeatedly, the catalyst will be eroded, and perhaps generate new pollutants [45]. Heterojunctions of semiconductors is friendly to the environment, stable, and abundant in nature. Especially, the system of g-C3N4/BiOCl could be used repeatedly and facile to be produced.

Some researchers also found the photoactivity of g-C3N4/BiOCl heterojunctions could be further enhanced by combining them with other materials. For example, the systems of Bi2S3/BiOCl/g-C3N4 [46], BiOCl/g-C3N4/kaolinite [47], and g-C3N4/CDs (carbon dots)/BiOCl [48]. Notably, through the addition of mediators, Z-scheme catalysts can be synthesized, such as the systems of g-C3N4/Au/BiOCl [49] and BiOCl/RGO/protonated g-C3N4 [50]. However, few researchers have focused on figuring out which method could prompt the photoactivity of the binary heterojunction. According to all the articles reviewed here, analysis of the proposed mechanism was an important section. Based on the adopted characterizations and experiments, the mechanism was discussed to help readers to understand the whole photocatalysis process.

First, in this article, the methods of preparation and the applications of the g-C3N4/BiOCl heterojunctions are reviewed. Then, the binary heterojunctions mentioned in this article were classified into three types according to the proposed mechanisms. The major difference between them is whether the alignment of band structures was taken into account after the syntheses. Many researchers carried out some experiments to prove their mechanisms. For instance, ESR and trapping experiments could demonstrate the main reactive species during the reaction. DFT (density functional theory) calculation was also adopted to anticipate the band structures and main reactive species [51]. However, all the methods seemed still not enough to directly prove the mechanism. Some of the studies seem controversial to each other. All three types of proposed mechanisms will be discussed in this article while

trying to find some patterns. Though dye sensitization happened when the heterojunctions were used to degrade some pollutant, this article tries to discuss whether the alignment of the band structure should be taken into account. At last, in the section of summary and outlook, the direction of the study on the mechanism is proposed.
