Study on Challenges and Solutions for Enhancing Photocatalytic Activities of Ag3PO4 Nanoparticles for Degradation of Organic Dyes from Wastewater

Improper management of discharged waste material such as toxic organic dyes from industries causes severe health and economic problems because contaminated water with organic dyes is harmful to human being and aquatic ecosystems. The photocatalyst is a preferable wastewater treatment method because it is environmentally benign, cost-wise, and more efficient in degrading dyes. Most metal oxide semiconductors are photocatalytic materials, but nowadays, Ag3PO4 nanoparticles have attracted enormous attention due to their low bandgap and high absorption of visible light. However, Ag3PO4 nanoparticles have low performance due to photocorrosion, high recombination rate, and instability in the solution. Thus, in this review article, the mechanism to enhance the photocatalytic activities of Ag3PO4 nanoparticles was discussed, and the future perspective of the silver phosphate was also stated.


Introduction
Global widespread industrialization has brought a change in human life by giving vast opportunities to get employment. On the other hand, poor management of industrial wastes such as heavy metals, dyes, pharmaceutical waste, and agricultural wastes caused severe environmental pollution because most of these discharged compounds are stable so that they can persist for a long time [1][2][3]. Amazingly, the final destination of all industrial wastes including solid form, liquid, or gas form ended in the water body. As a result, the degree of environmental pollution by contaminated water is enormous. So, water pollution has tremendous negative consequences on the economy and health of human beings. Vanishing aquatic lives and deteriorating health of human beings are mainly the results of water pollution [4,5]. Wastewater treatment is crucial to get a healthy environment.
There are two types of wastewater treatment methods such as traditional and advanced wastewater treatment techniques. Conventional wastewater treatment methods are the following: coagulation, filtration, disinfection, sedimenta-tion, de-chlorination, adsorption onto activated carbon, screening, chemical precipitation [4][5][6], screening, and chemical precipitation. However, advanced techniques such as membrane filtration, reverse osmosis, ion exchange, electro-dialysis, adsorption, and advanced oxidation process have been used widely for wastewater treatment [7]. The advanced oxidation process (AOPs) is one of the recent methods for wastewater treatment due to degrading nonbiodegradable, chemically stable, persistent pollutants and its low operating cost, no chemical agent needed, and minimal sludge formation. Furthermore, the AOPs generated reactive oxygen species (ROS) that oxidize a broad range of organic pollutants inside wastewater [8].
Three common types of AOPs are photocatalyst, photolysis, and photo-Fenton which are light irradiated based [9]. Among those, photocatalysts gained tremendous attention because of the following reasons: (i) a small number of catalysts able to degrade a large number of dyes; (ii) no need for external force, environmentally benign; and (iii) costeffective technology, and the organic pollutants directly converted nonoffensive by-products such as CO 2 and H 2 O [10,11]. Metal oxide semiconductors ZnO [3] and TiO 2 are widely used photocatalytic materials owing to their low cost, strong redox capacity, and good thermal and chemical stability [12,13]. However, they operate only under UV light due to their wide bandgap (3-3.2 eV). The UV light reaches the earth's surface only 4-5% of the total solar spectrum, but the amount of visible light that arrives at the earth's surface is 45%. Doping of ZnO and TiO 2 reduces the bandgap of ZnO and TiO 2 . As a result, they can operate in the visible range. However, doping these metal oxides introduces a new energy level in the bandgap, which acts as the recombination center of photogenerated e --h + pairs. Such recombination of electrons-holes is the main factor to reduce the photocatalytic efficiency of TiO 2 and ZnO [14][15][16]. To harvest a large amount of visible light (45%), it needs to develop or use novel visible light-responsive photocatalytic materials.
There are various types of metal oxide semiconductor that operate under visible light such as  [10,11,17,18]. Among these metal oxides, Ag 3 PO 4 is the preferable photocatalytic material because of its high photoefficiency than other aforementioned oxides. For instance, the quantum efficiency of Ag 3 PO 4 is up to 90% at a wavelength greater than 420 nm and its extremely high photooxidative ability to split water to the evolution of oxygen (O 2 ) and high organic dye removal under visible light irradiation [12,13]. The reasons why Ag 3 PO 4 have these mentioned benefits over other oxides are because of its novel atomic configurations and its metallic-like electronic structures. The reduction potential of H + /H 2 is more negative than the conduction band minimum of Ag 3 PO 4 . Consequently, Ag 3 PO 4 may produce oxygen, not hydrogen. As a result of mentioned novel properties of Ag 3 PO 4 , it has been chosen as a candidate material for photocatalytic properties than other narrow bandgap semiconductors. Moreover, Yi et.al reported the first Ag 3 PO 4 photocatalyst for degrading organic pollutants and splitting water [13,14]. The schematic diagram of a photocatalytic mechanism of Ag 3 PO 4 is shown in Figure 1. Figure 1 shows that the conduction band edge potential of Ag 3 PO 4 is more positive than the single-electron reduction process potential of -0.046 eV; thus, single-electron reduction for producing superoxide radicals ( • O 2 -) did not take place on the conduction band of Ag 3 PO 4 . Under visible light irradiation on the Ag 3 PO 4 catalyst, the electron-hole pairs were generated, and the electron was excited to the conduction band of Ag 3 PO 4 . The excess electron on the conduction band of Ag 3 PO 4 facilitated a multioxidation reaction (2e − + O 2 + 2H + = H 2 O 2 ), and it further react with single electros (H 2 O 2 + e − = OH o ) to form hydroxide radicals; those radicals reacted with methylene blue dyes and decomposed it into CO 2 and H 2 O. In addition to this, the combination of Ag + ions with single electrons forms a black Ag o nanoparticle which is the main reason for the instability and photocorrosion of Ag 3 PO 4 . In the second reaction, the hole reacted with H 2 O and formed OH o radicals while it has been reacted with dyes and decompose into carbon dioxide and water [15,16]. However, the photocatalytic activity of Ag 3 PO 4 is limited due to photocorrosion, high recombination rate, and instability in a solution [14,19,20]. In this review article, we discussed the possible means to enhance the degradation of organic pollutants from wastewater by using Ag 3 PO 4 . Finally, we discussed the future perspective of the Ag 3 PO 4 .
1.1. Structure of Ag 3 PO 4 for Photocatalysis Activity. Ma and coworkers reported the structural features and photocatalytic activity of the Ag 3 PO 4 by using first-principle density functional theory (DFT) [21]. They investigated the energy band structure and density of the state of the Ag 3 PO 4 , which were essential for photocatalysis to harness solar energy for oxidizing water and degrading pollutants inside wastewater. The crystal structure of Ag 3 PO 4 embraces isolated, regular PO 4 tetrahedral with a P-O distance of~1.539 angstroms (A°). The six Ag + ions dispersed among twelve sites of 2fold symmetry are shown in Figure 2. With the advancement of X-ray diffraction (XRD) technology, it established that it belongs to the body-centered cubic (bcc) structure type with a lattice parameter of 6.004 A°, and the space group is P4 3n [20]. The electronic structure was also studied by hybrid DFT to obtain the high photocatalytic efficiency of Ag 3 PO 4 .
The band structure edge had determined by Perdew-Burke-Ernzerhof's (PBE) generalized gradient approximation techniques, and its magnified view is nearly Fermilevel depicted in the following Figures 3(a) and 3(b). Both valance band edge maximum (VBEM) and conduction band edge minimum (CBEM) are located at point G. Figure 3(b) ascribes that the direct bandgap is equal to 2.61 eV for Ag 3 PO 4 . The indirect bandgap is equivalent to 2.43 eV found between points G and M. Therefore, the Ag 3 PO 4 is indirect band gap which is equal to 2.43 eV. As indicated in Figure 3(c), the valance band edge potential of Ag 3 PO 4 = 2:67 eV vs. NHE is more positive than the oxidation potential of water (H 2 O/O 2 (1.23 eV vs. NHE)), so it is attributed to the oxidation of water by a hole in the valance band to produce oxygen and oxidized organic pollutants. The conduction band edge minimum (CBEM) is 0.24 which is more positive than H + /H 2 and the single electron reaction process. As result, it is not produced hydrogen for energy production and superoxide radical (O -2 ) for photodegradation [22]. Umezawa and coworkers showed that the conduction band edge minimum (CBEM) was composed of hybridized Ag (5s, 5p) along with some P (3s) orbitals, while the valance band edge (EVB) was composed of the hybridized (4d) orbitals of Ag and 2p orbitals of oxygen as indicated in Figure 3(c) [23]. Owing to this unique structure and properties of Ag 3 PO 4 , it is regarded as a good photocatalytic activity in visible light.

Factors
Affecting the Photocatalysis Activity of Ag 3 PO 4 . The size, shape, and exposed facet of Ag 3 PO 4 affect the photodegradation of organic dyes from aqueous solutions [24,25]. Therefore, controlling the morphology and crystal facet of Ag 3 PO 4 during synthesis has improved its photocatalysis efficiency [25]. Dong and a coworker reported the effect of different morphology of Ag 3 PO 4 on photocatalytic activities. These morphologies such as branched, tetra-pod, nanorod, and triangular were synthesized by the mixture of dimethyl flour (DMF) and water (H 2 O) solvent without using any kind of template materials under the different conditions as elucidated in Figure 4(a), and their SEM images revealed in Figure 4(b). The measured BET specific surfaces of different shapes are as follows: branched (18.9 m 2 /g), tetrapod (3.35m 2 /g), nanorod shaped (14.9 m 2 /g), triangularprism shaped (2.18m 2 /g), and irregular spherical Ag 3 PO 4 (1.02m 2 /g) surface area. The efficiency of degrading MB and RhB dyes was higher for branched ones since it has a high surface area to adsorb a large number of dyes which offered a high degradation ability of methylene blue (MB) and rhodamine blue (RhB) for branched as shown in Figures 5(a) and 5(b) [26].
Various shapes and facets of Ag 3 PO 4 crystals have different photocatalysis efficiency. For example, Bi and coworkers synthesize a single crystalline Ag 3 PO 4 dodecahedron with only {110} facets and cubes with {100} facets by facile precipitation methods as shown in Figure 6(a).The dodecahedron was synthesized by a reaction between CH 3 COOAg and Na 2 HPO 4 while the cubic was formed by a reaction between [Ag(NH 3 ) 2 ] + and Na 2 HPO 4 . As revealed in Figure 6(b), an SEM image of dodecahedrons of Ag 3 PO 4 microcrystals bounded by twelve (110) planes, but smooth corners and edges of cubic crystals enclosed by (100) planes. Furthermore, the evaluated photocatalytic performance of rhombic dodecahedrons was higher than cubes for the degradation of organic contaminants as shown in Figure 6(c), which may be due to the higher surface energy of (110) facets (1.31 J/m 2 ) than of {100} facets (1.12 J/m 2 ) which impart a high active site to adsorb organic dyes to degrade [27].
In addition to the above factors, catalyst dose, pH of the solution, and initial dye concentrations also have enormous impacts on photocatalysis efficiency [28]. The catalyst dose refers to the number of photocatalysts in the solutions. Thus, increasing catalyst dosage increases the active site on the surface of the catalyst which causes the formation of more     3 Journal of Nanomaterials reactive species. As a result, the photodegradation activities increase with an increase in catalyst dosage till the optimum amount is reached; beyond this, the solution becomes turbid and prevented the solar light that reaching the active site, and therefore, degradation efficiency starts to decrease [29]. Furthermore, the dye degradation efficiency depends on the pH values of wastewater. At low pH values, the surface of the Ag 3 PO 4 is protonated and becomes positively charged since the initial Ag 3 pO 4 has a negative surface charge according to the analysis of its negative zeta potential by Huang et al. [21], and it is easily protonated. As a result, it is efficient to degrade negatively charged organic dyes. Similarly, at high pH values of the solution, the surface of the Ag 3 PO 4 becomes negatively charged, and it is effective to degrade cationic dyes [22]. And also, an increase in initial dye concentration decreases the photocatalytic activities because covering the active site of Ag 3 PO 4 resulted in low solar light available on the surface of the catalyst, and therefore, low reactive species were formed, thus causing low degradation percentage [23,30]. As increasing irradiation times more electron-hole pairs were generated, and more reactive species were generated. As a result, the degradation efficiency is increasing with irradiation time [22].

Common Challenges to Enhancing
Photocatalytic Activities of Ag 3 PO 4 Nanoparticles Figure 7 shows the common challenges to promoting the dye degradation efficiency of Ag 3 PO 4 . Photocorrosion, instability, and high recombination rate are the main challenges to enhancing the degradation efficiency of Ag 3 PO 4 . Photocorrosion reduces photocatalytic activities of Ag 3 PO 4 by covering the active sites of Ag 3 PO 4 with black silver metals. These black silver metals formed when the silver ions in an interstitial site of Ag 3 PO 4 accept the excited electrons and then reduced them to silver metals. Then, those silver metals can cover the surface of Ag 3 PO 4 . As a result, the photocatalyst of Ag 3 PO 4 declined [15]. Instability of Ag 3 PO 4 is related to a decomposing of Ag 3 PO 4 in an aqueous solution, at high thermal heating and long irradiated light, so instability or photocorrosion of Ag 3 PO 4 is limited the use of Ag 3 PO 4 in practical application [31]. The recombination rates of photoexcited electrons and holes of Ag 3 PO 4 also reduce the photocatalytic activity of Ag 3 PO 4 owing to there being no scavenger of electrons and holes to suppress the reunion photogenerated charge carriers.    Loading noble metal on the surface of Ag 3 PO 4 is the method to enhance the photocatalytic activities of Ag 3 PO 4 because the noble metal loaded on the surface of Ag 3 PO 4 acts as an electron scavenger [32]. This means the excited electrons from the valence band are removed from the Ag 3 PO 4 by noble metal so that there is no further recombination of charge carriers and suppresses photocorrosion because the excited electron does not get interstitial Ag + ions to form black Ag metal covers the active sites of Ag 3 PO 4 which is reducing the absorption of light and adsorption of dyes to the active site of Ag 3 PO 4 and leads to depletion of photocatalytic activity to degradation dyes from aqueous solution [33]. For instance, Figure 8 shows that decorating of platinum (pt) on the surface of Ag 3 PO 4 highly enhances the stability of Ag 3 PO 4 which has exhibited better efficiency when it is used for six consecutive runs [34]. However, the performance of neat Ag 3 PO 4 deteriorated when it had been used for a six repeating runs due to the gradual decomposition of Ag 3 PO 4 to weakly active Ag nanoparticle which is the main cause for instability and photocorrosion of Ag 3 PO 4 [34,35].
The aforementioned advantages were accomplished as follows. The loaded noble metals on Ag 3 PO 4 formed Schottky-type heterostructures as depicted in Figure 9(a). Since the contact between metal and semiconductor forms the Schottky barrier at their interface owing to the difference in work functions [36], the work function of Ag 3 PO 4 conduction band edge (CBE), Au, Pd, and Pt is 4.95, 5.1, 5.12, and 5.65 eV, respectively [34,37] since Ag 3 PO 4 n-type semiconductor and its Fermi level are close to the conduction band edge. When noble metal contacts with Ag 3 PO 4 , the electrons flow from semiconductor to metals to attain two systems at the equilibrium, and a new Fermi level is established, and the internal electric field directed from Ag 3 PO 4 to metal was formed. When the (Au, Pd, Pt)/ Ag 3 PO 4 heterostructure is imposed to visible light, the electron in the VB excited to CB and by forming a hole at VB, and electrons in the CB easily flow to metals owing to CB of Ag 3 PO 4 at higher potential and also an internal electric field-assisted the flow electron from Ag 3 PO 4 to metals. As result, the charge recombination rate and the instability of Ag 3 PO 4 are effectively suppressed [34]. The electron reacted with oxgen and form O 2reactive species that able to decompose organic pollutants. The holes left in VB have proceeded an oxidization reaction to decompose organic dyes as depicted in Figure 9(b).
In addition to those benefits, noble metal nanoparticles loaded on the surface of Ag 3 PO 4 revealed remarkable enhancement absorption of both UV and visible light than pure Ag 3 PO 4 as indicated in Figure 10. This can be the attribution of localized surface plasmon resonance effect (LSPR)  [38]. These have improved the photocatalytic activity of Ag 3 PO 4 [34]. As shown in Table 1, many reported works on the loading of noble metal on surface Ag 3 PO 4 to enhance the photocatalytic activities of Ag 3 PO 4 are listed. Different noble metals have a different effects on enhancing photocatalytic activities of Ag 3 PO 4 because of their various specific properties. Figure 11(a) shows the enhancement of photocatalytic activities of the Ag 3 PO 4 by loading noble metals, and a comparative study of the photocatalytic activities of various noble metals loaded on the surface of Ag 3 PO 4 and with other photocatalyst materials. As indicated in this Figure 11(a), different noble metals loaded on Ag 3 PO 4 show unequal performances because they do have not equal work functions. The large differences in work function between the noble metals and Ag 3 PO 4 are necessary for a better photocatalytic process, because it facilitates a transfer of a     Journal of Nanomaterials photogenerated electron from Ag 3 PO 4 to noble metals. So, the degrading ability of dyes by Pt/Ag 3 PO 4 is larger than the other two noble metals due to Pt having a large work function difference with the conduction band edge of Ag 3 PO 4 as shown in Figure 9(a) which highly assists the electron transfer from Ag 3 PO 4 to Pt. As a result, the photodegradation Pt loaded on Ag 3 PO 4 is higher than the other two noble metals and bare Ag 3 PO 4 [39]. In general, as indicated in Figure 11(a), the degradation of the dye by noble metal loaded on the surface of Ag 3 PO 4 was better when compared to the bare one due to the aforementioned reason [34]. Moreover, Figure 11(b) shows that the efficiency of degrading different types of dyes by identical photocatalysts is not the same owing to the structure, and the surface charge difference has existed between the given dyes [40], and the surface charge of Ag 3 PO 4 negative due to it has plenty OH -(hydroxyl group) [41] which helped to adsorb highly cationic dyes.       There are different methods to enhance the photocatalytic activity of a given semiconductor among as forming heterojunctions with another semiconductor [42], which is provided the following advantages to improve photocatalysis performance such as (1) reducing the recombination of photogenerated charge carriers by transferring electrons and holes between a junction, and as result, the formation of reactive oxidative species increased which was decontaminated organic dyes from wastewater [39]; (2) extending the absorption of solar light for large bandgap semiconductors when combined with small bandgap semiconductors [43]; (3) improve stability issue of a single component; and (4) forming nanoheterostructure semiconductors has a large surface area and enough active sites to adsorbed organic dyes. Therefore, coupling Ag 3 PO 4 with other semiconductors is among the methods to enhance the photocatalytic activities of Ag 3 PO 4 . The main advantageous form heterojunctions with Ag 3 PO 4 are as follows: (i) solving instability issues and reducing the photocorrosion, (ii) improving the absorption of solar light in the visible range, and (iii) sup-pressing the recombination of charge carriers [39,44,45]. For instance, Figure 12 reveals the composite of Co 3 O 4 / Ag 3 PO 4 in which a heterojunction was formed between Co 3 O 4 and Ag loaded on Ag 3 PO 4 in which silver metal appeared on the surface of Ag 3 PO 4 during the photocatalysis process [44]. As indicated in this Figure 12, the photoexcited electrons and holes transferred Ag and Co 3 O 4 , respectively. As a result, there is no recombination of holes-electrons and photocorrosion because both charge carriers have not stayed on the Ag 3 PO 4 after photoexcitation has been done. The solar absorption of mentioned heterojunction is better than a single component.
Bi and coworkers decorated the silver halides as coreshell on the surface of rhombic dodecahedral Ag 3 PO 4 by the ion change method, and as we knew, the Ag 3 PO 4 has rhombic dodecahedral morphology that has high surface energy which makes it dissolve in solution [45]. The intention of decorating Ag 3 PO 4 is to prevent dissolution and enhance the stability of Ag 3 PO 4 during photocatalyst activity since silver halides have lower solubility in aqueous solution than pristine Ag 3 PO 4 . As well as, the valance band and conduction band of all halides are less negative than Ag 3 PO 4  11 Journal of Nanomaterials except Cl, so that the separation of charge carriers increases, and the recombination rate may be significantly reduced. In addition to this, the photocorrosion was inhibited for Ag (Br, I)/Ag 3 PO 4 by facilitated multiple-electron reduction reaction of oxygen (O 2 + 2H + + 2e − = H 2 O 2 ) [46] at Ag 3 PO 4 without photoreduction of silver ions into black silver metals. As a result, photodegradation of organic dyes by Ag 3 PO 4 was enhanced as shown in Figure 13.
Supporting materials such as cellulose, graphene, and other porous materials have been used with Ag 3 PO 4 . These supporting materials increases the photocatalytic activities of Ag 3 PO 4 by improving the surface area, morphology and reducing agglomerations of Ag 3 PO 4 . Based on this concept, Yang and coworkers synthesized Ag 3 PO 4 /graphene oxide nanocomposite by simple precipitation methods and subsequently reduced graphene oxide to reduce graphene to form Ag 3 PO 4 /reduced graphene oxide nanocomposite. As a result, the morphology and size were controlled according to synthesis techniques illustrated in Figure 14(a). In addition to these, the conductive property of graphene inhibited recombination e --h + pairs by charge transferring between Ag 3 PO 4 and graphene as indicated in Figure 14(c). Due to the above reason, the photodegradation of organic dyes by Ag 3 PO 4 /graphene-based nanocomposite is excellent when compared to bare Ag 3 PO 4 as shown in Figure 14(b). To check the stability of Ag 3 PO 4 and Ag 3 PO 4 /GR, the XRD characterization was recorded according to Figure 14(d) after a photocatalysis activity. It is observed that Ag metals appeared for Ag 3 PO 4 which is the main cause for photocorrosion but not appeared for composite owing to the electron in an Ag 3 PO 4 transferring to graphene sheet as indicated in Figure 14(c) before it combined with Ag + ions to form black silver metals. Therefore, combining Ag 3 PO 4 with graphenebased materials has reduced photocorrosion to a large extent [47]. Further, Table 2 shows the lists of published works on Ag 3 PO 4 /metal oxide semiconductor or supporting materials for dye degradation.
The photocatalytic activity toward degradation of organic dyes from aqueous solution was also enhanced when Ag 3 PO 4 was combined within a different dimension (1D, 2D, 3D) semiconductor materials [48] which were reviewed in the following Table 3. Various morphology of semiconductors offers special properties for the materials such as increased surface area by reducing agglomeration, increasing charge transferring between the junction for transporting and separation photogenerated electron-hole pairs, and serving as a superior template for hetero growth of Ag 3 PO 4 [1,49,50].

Ag 3 PO 4 -Based Ternary Nanocomposite for Degrading
Dyes from Wastewater. Ternary nanocomposite types of the composite are formed when three semiconductors that have matched band structures are combined, and also, ternary composite is formed when the binary composite is clumped together with supporting materials like graphene, cellulose, etc. In ternary composite, the synthesized materials have synergetic performance and properties of three single components thus resulting in highly inhibited recombination of charge carriers since the time of recombination has elongated by easily transferring charge carriers between the junction of three semiconductors. In addition to these, the stability and absorption of light are highly enhanced than single and binary components, and therefore, the photodegradation of organic dyes by ternary is higher than in single and binary materials [51].
So, forming Ag 3 PO 4 containing ternary nanocomposites improved the degradation efficiency of Ag 3 PO 4 . Among these materials, one is the Ag 3 PO 4 . And other two materials are used to enhance the photostability and photocatalytic activities of Ag 3 PO 4 . Using ternary nanocomposites has resulted in a reduction in the consumption of expensive noble metals. Among the three ternary nanocomposites, one of them may be magnetic or supporting materials. The magnetic material enhances the photocatalytic activities as well as the recoverability of Ag 3 PO 4 . A typical example of the photocatalytic activities of ternary nanocomposites is shown in Figure 15. As revealed in Figure 15, the conduction band edge and valance band edge of WO 3 are more positive than the conduction band edge and valance band edge of Ag 3 Figure 12: Electrons transfer mechanism in heterostructure photocatalyst [44].      Figure 16(a) [53]. As explained in Figure 16(b) the photocatalytic mechanism of ternary is like the following. Firstly, the composite was irradiated with visible light; electrons were excited from VB to the CB in both CoFe 2 Figure 15: Schematic diagram of Ag 3 PO 4 -based ternary nanocomposite [52]. 15 Journal of Nanomaterials separation after being used for photocatalytic activity. The performance achieved by this material is around 97% toward degradation of AO7 dyes [55]. Li et al. reported the ZnFe 2 O 4 -ZnO-Ag 3 PO 4 hollow nanospheres synthesized by using phenolic formaldehyde microspheres as a template via an impregnating-calcination process. And it shows good photocatalytic activity toward mineralization of organic pollutants inside wastewater owing to the high surface area; small particle size is achieved. As shown in Figure 17, cascade electron transfer from CB of ZnFe 3 O 4 to ZnO to Ag 3 PO 4 is enhanced charge separation. And due to the small particle size that is difficult to separate after being used for photocatalysis activity, so by using external magnetic, it is easily separated because ZnFe 3 O 4 has a magnetic property [56].

Photocatalysis Mechanisms in a Semiconductor to
Decontaminate Organic Dyes. Photoreaction is essential to complete mineralization of organic pollutants from an aqueous solution at ambient temperature and pressure without introducing secondary pollutants. Under this part, we discussed the photoreaction pathway for any semiconductor materials used for photodegradation. The photodegradation of organic dyes is a redox reaction in which reduction reaction and oxidation reaction take place in the conduction and valance band, respectively. The process of photoreaction is divided into three steps. First, the electron (e -) and hole (h + ) pairs are generated by photon energy greater than or equal to the bandgap of the semiconductor, and the electrons transfer from VB to CB by leaving a vacant electron site (hole) in a VB. Second, the photo-induced electron   (3)). Thirdly, these formed superoxide radicals (O 2 -o ), hydroxyl radicals ( o OH), and hydroperoxyl radicals (HO 2 o ) oxidized organic dyes and converted them into CO 2 and H 2 O which is not a secondary pollutant. The overall mechanisms of photodegradation of organic dyes are elucidated in Figure 18 [57].
3.3.2. Future Perspective and the Opinion of the Reviewer. All the mentioned methods had been used as supportive material to reduce the limitation of the Ag 3 PO 4 which are photocorrosion, instability, and high recombination rate. However, they have not stated the cost of a precursor used to synthesize Ag 3 PO 4 , but the precursor used for fabricated Ag 3 PO 4 is expensive. So, we will suggest that replacing the Ag 3 PO 4 precursor with a less-cost precursor will improve the availability and efficiency of Ag 3 PO 4 for photocatalysis applications. Especially, extracting a precursor from waste by-products and naturally occurring materials enhances the nobility and application of Ag 3 PO 4 for photodegradation. For instance, eggshells (ES) and bones are rich in phosphate so we can use them as precursors for synthesizing Ag 3 PO 4 . Synthesizing of Ag 3 PO 4 /supporting materials used to reduce agglomeration. For instance, silicate can be extracted from bagasse ash which is a by-product of sugar cane because it is cost-wise and available everywhere in the sugar industry. Doping of large bandgap semiconductors such as ZnO and TiO 2 is crucial to reducing the wideband to narrow bandgap that enables them to harvest the visible light while a couple with Ag 3 PO 4 . Thus, the doped widebandgap oxides not only reduce the limitation of the Ag 3 PO 4 but also actively participate in the degradation of the organic dyes in the wastewater.

Conclusion
Nano-scaled metal oxide semiconductors have many applications due to their unique properties. The photocatalyst is among the wastewater treatment methods, where nanoscaled semiconductors have been used as a catalyst. Among the metal oxide semiconductors, Ag 3 PO 4 gain significant attention due to its lower bandgap and visible lightabsorbing efficiency of up to 95%. However, due to high photocorrosion, instability, and high recombination rate, the photocatalytic activities of Ag 3 PO 4 are getting lower. Such factors which hinder the photocatalytic activity of Ag 3 PO 4 can be overcommed by using Noble metal loaded Ag 3 PO 4 , forming binary composition such as Ag 3 PO 4 /metal oxides or supporting matarials, Ag 3 PO 4 -based ternary nanocomposite,and changing the morphology of Ag 3 PO 4 . From all the possible approaches, the mentioned limitations of Ag 3 PO 4 were minimized, and the degradation efficiency of the organic dyes in the wastewater was enhanced. Therefore, the Ag 3 PO 4 with reduced photocorrosion, instability, and recombination rate is a promising agent to degrade organic dye from wastewater.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.