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

Authors: Wegene Lelisa Beyecha; Misikir Tamiru Asefa; Gebisa Bekele Feyisa
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

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.


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## Body

## 1. 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 [1–3]. 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, sedimentation, de-chlorination, adsorption onto activated carbon, screening, chemical precipitation [4–6], 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) cost-effective technology, and the organic pollutants directly converted nonoffensive by-products such as CO2 and H2O [10, 11]. Metal oxide semiconductors ZnO [3] and TiO2 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 TiO2 reduces the bandgap of ZnO and TiO2. 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 TiO2 and ZnO [14–16]. 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 Fe2O3 (=2.1 eV), WO3 (=2.6 eV), SnO2 (=2.30 eV), Bi2O3 (=2.48 eV), Ag3PO4 (2.45 eV), CuO (indirectbandgap=1.4 eV), and Cu2O (2.17 eV) [10, 11, 17, 18]. Among these metal oxides, Ag3PO4 is the preferable photocatalytic material because of its high photoefficiency than other aforementioned oxides. For instance, the quantum efficiency of Ag3PO4 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 (O2) and high organic dye removal under visible light irradiation [12, 13]. The reasons why Ag3PO4 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+/H2 is more negative than the conduction band minimum of Ag3PO4. Consequently, Ag3PO4 may produce oxygen, not hydrogen. As a result of mentioned novel properties of Ag3PO4, it has been chosen as a candidate material for photocatalytic properties than other narrow bandgap semiconductors. Moreover, Yi et.al reported the first Ag3PO4 photocatalyst for degrading organic pollutants and splitting water [13, 14]. The schematic diagram of a photocatalytic mechanism of Ag3PO4 is shown in Figure 1.Figure 1
Photocatalyst mechanism of the Ag3PO4 for degradation of organic dye.Figure1 shows that the conduction band edge potential of Ag3PO4 is more positive than the single-electron reduction process potential of -0.046 eV; thus, single-electron reduction for producing superoxide radicals (•O2-) did not take place on the conduction band of Ag3PO4. Under visible light irradiation on the Ag3PO4 catalyst, the electron-hole pairs were generated, and the electron was excited to the conduction band of Ag3PO4. The excess electron on the conduction band of Ag3PO4 facilitated a multioxidation reaction (2e−+O2+2H+=H2O2), and it further react with single electros (H2O2+e−=OHo) to form hydroxide radicals; those radicals reacted with methylene blue dyes and decomposed it into CO2 and H2O. In addition to this, the combination of Ag+ ions with single electrons forms a black Ago nanoparticle which is the main reason for the instability and photocorrosion of Ag3PO4. In the second reaction, the hole reacted with H2O and formed OHo radicals while it has been reacted with dyes and decompose into carbon dioxide and water [15, 16]. However, the photocatalytic activity of Ag3PO4 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 Ag3PO4. Finally, we discussed the future perspective of the Ag3PO4.


### 1.1. Structure of Ag3PO4 for Photocatalysis Activity

Ma and coworkers reported the structural features and photocatalytic activity of the Ag3PO4 by using first-principle density functional theory (DFT) [21]. They investigated the energy band structure and density of the state of the Ag3PO4, which were essential for photocatalysis to harness solar energy for oxidizing water and degrading pollutants inside wastewater. The crystal structure of Ag3PO4 embraces isolated, regular PO4 tetrahedral with a P-O distance of ~1.539 angstroms (A°). The six Ag+ ions dispersed among twelve sites of 2-fold 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 P43¯n [20]. The electronic structure was also studied by hybrid DFT to obtain the high photocatalytic efficiency of Ag3PO4.Figure 2
XRD pattern for Ag3PO4 together with a simplified unit cell of Ag3PO4 (P: orange; O: red; Ag: blue) [20].The band structure edge had determined by Perdew-Burke-Ernzerhof’s (PBE) generalized gradient approximation techniques, and its magnified view is nearly Fermi-level depicted in the following Figures3(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 Ag3PO4. The indirect bandgap is equivalent to 2.43 eV found between points G and M. Therefore, the Ag3PO4 is indirect band gap which is equal to 2.43 eV. As indicated in Figure 3(c), the valance band edge potential of Ag3PO4=2.67 eV vs. NHE is more positive than the oxidation potential of water (H2O/O2 (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+/H2 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 Ag3PO4, it is regarded as a good photocatalytic activity in visible light.Figure 3
(a) Band structure of Ag3PO4 calculated using the PBE approach. (b) A magnified view of band structure near the Fermi level. (c) VBEM and CBEM potentials of Ag3PO4 [58].
(a)(b)(c)


### 1.2. Factors Affecting the Photocatalysis Activity of Ag3PO4

The size, shape, and exposed facet of Ag3PO4 affect the photodegradation of organic dyes from aqueous solutions [24, 25]. Therefore, controlling the morphology and crystal facet of Ag3PO4 during synthesis has improved its photocatalysis efficiency [25]. Dong and a coworker reported the effect of different morphology of Ag3PO4 on photocatalytic activities. These morphologies such as branched, tetra-pod, nanorod, and triangular were synthesized by the mixture of dimethyl flour (DMF) and water (H2O) 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 m2/g), tetrapod (3.35m2/g), nanorod shaped (14.9 m2/g), triangular-prism shaped (2.18m2/g), and irregular spherical Ag3PO4 (1.02m2/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].Figure 4
(a) Schematic diagram of growth and mechanism of formation of various Ag3PO4 crystals under different conditions. (b) SEM images of the branch (A), tetrapod (B), nanorod (C), and triangular prism (D) Ag3PO4 crystals [26].
(a)(b)Figure 5
Plot of C/C0 of MB and RhB for dye against irradiation time by branch (A), tetrapod (B), nanorod (C), triangular prism (D), and irregular spherical (E) [30].
(a)(b)Various shapes and facets of Ag3PO4 crystals have different photocatalysis efficiency. For example, Bi and coworkers synthesize a single crystalline Ag3PO4 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 CH3COOAg and Na2HPO4 while the cubic was formed by a reaction between [Ag(NH3)2] + and Na2HPO4. As revealed in Figure 6(b), an SEM image of dodecahedrons of Ag3PO4 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/m2) than of {100} facets (1.12 J/m2) which impart a high active site to adsorb organic dyes to degrade [27].Figure 6
(a) Schematic representation of Ag3PO4 has different morphologies. (b) SEM images of Ag3PO4 submicrocrystals with different morphologies: (A) rhombic dodecahedrons; (B) cubes. (c) Photocatalytic activities of Ag3PO4 rhombic dodecahedrons, cubes, and spheres and N-doped TiO2 for MO [27].
(a)(b)(c)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 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 Ag3PO4 is protonated and becomes positively charged since the initial Ag3pO4 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 Ag3PO4 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 Ag3PO4 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].


## 1.1. Structure of Ag3PO4 for Photocatalysis Activity

Ma and coworkers reported the structural features and photocatalytic activity of the Ag3PO4 by using first-principle density functional theory (DFT) [21]. They investigated the energy band structure and density of the state of the Ag3PO4, which were essential for photocatalysis to harness solar energy for oxidizing water and degrading pollutants inside wastewater. The crystal structure of Ag3PO4 embraces isolated, regular PO4 tetrahedral with a P-O distance of ~1.539 angstroms (A°). The six Ag+ ions dispersed among twelve sites of 2-fold 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 P43¯n [20]. The electronic structure was also studied by hybrid DFT to obtain the high photocatalytic efficiency of Ag3PO4.Figure 2
XRD pattern for Ag3PO4 together with a simplified unit cell of Ag3PO4 (P: orange; O: red; Ag: blue) [20].The band structure edge had determined by Perdew-Burke-Ernzerhof’s (PBE) generalized gradient approximation techniques, and its magnified view is nearly Fermi-level depicted in the following Figures3(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 Ag3PO4. The indirect bandgap is equivalent to 2.43 eV found between points G and M. Therefore, the Ag3PO4 is indirect band gap which is equal to 2.43 eV. As indicated in Figure 3(c), the valance band edge potential of Ag3PO4=2.67 eV vs. NHE is more positive than the oxidation potential of water (H2O/O2 (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+/H2 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 Ag3PO4, it is regarded as a good photocatalytic activity in visible light.Figure 3
(a) Band structure of Ag3PO4 calculated using the PBE approach. (b) A magnified view of band structure near the Fermi level. (c) VBEM and CBEM potentials of Ag3PO4 [58].
(a)(b)(c)


## 1.2. Factors Affecting the Photocatalysis Activity of Ag3PO4

The size, shape, and exposed facet of Ag3PO4 affect the photodegradation of organic dyes from aqueous solutions [24, 25]. Therefore, controlling the morphology and crystal facet of Ag3PO4 during synthesis has improved its photocatalysis efficiency [25]. Dong and a coworker reported the effect of different morphology of Ag3PO4 on photocatalytic activities. These morphologies such as branched, tetra-pod, nanorod, and triangular were synthesized by the mixture of dimethyl flour (DMF) and water (H2O) 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 m2/g), tetrapod (3.35m2/g), nanorod shaped (14.9 m2/g), triangular-prism shaped (2.18m2/g), and irregular spherical Ag3PO4 (1.02m2/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].Figure 4
(a) Schematic diagram of growth and mechanism of formation of various Ag3PO4 crystals under different conditions. (b) SEM images of the branch (A), tetrapod (B), nanorod (C), and triangular prism (D) Ag3PO4 crystals [26].
(a)(b)Figure 5
Plot of C/C0 of MB and RhB for dye against irradiation time by branch (A), tetrapod (B), nanorod (C), triangular prism (D), and irregular spherical (E) [30].
(a)(b)Various shapes and facets of Ag3PO4 crystals have different photocatalysis efficiency. For example, Bi and coworkers synthesize a single crystalline Ag3PO4 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 CH3COOAg and Na2HPO4 while the cubic was formed by a reaction between [Ag(NH3)2] + and Na2HPO4. As revealed in Figure 6(b), an SEM image of dodecahedrons of Ag3PO4 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/m2) than of {100} facets (1.12 J/m2) which impart a high active site to adsorb organic dyes to degrade [27].Figure 6
(a) Schematic representation of Ag3PO4 has different morphologies. (b) SEM images of Ag3PO4 submicrocrystals with different morphologies: (A) rhombic dodecahedrons; (B) cubes. (c) Photocatalytic activities of Ag3PO4 rhombic dodecahedrons, cubes, and spheres and N-doped TiO2 for MO [27].
(a)(b)(c)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 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 Ag3PO4 is protonated and becomes positively charged since the initial Ag3pO4 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 Ag3PO4 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 Ag3PO4 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].


## 2. Common Challenges to Enhancing Photocatalytic Activities of Ag3PO4 Nanoparticles

Figure7 shows the common challenges to promoting the dye degradation efficiency of Ag3PO4. Photocorrosion, instability, and high recombination rate are the main challenges to enhancing the degradation efficiency of Ag3PO4. Photocorrosion reduces photocatalytic activities of Ag3PO4 by covering the active sites of Ag3PO4 with black silver metals. These black silver metals formed when the silver ions in an interstitial site of Ag3PO4 accept the excited electrons and then reduced them to silver metals. Then, those silver metals can cover the surface of Ag3PO4. As a result, the photocatalyst of Ag3PO4 declined [15]. Instability of Ag3PO4 is related to a decomposing of Ag3PO4 in an aqueous solution, at high thermal heating and long irradiated light, so instability or photocorrosion of Ag3PO4 is limited the use of Ag3PO4 in practical application [31]. The recombination rates of photoexcited electrons and holes of Ag3PO4 also reduce the photocatalytic activity of Ag3PO4 owing to there being no scavenger of electrons and holes to suppress the reunion photogenerated charge carriers.Figure 7
Schematic diagrams of factors affecting photocatalytic activities of Ag3PO4.


## 3. Methods for Enhancing Photocatalysis Activity of Ag3PO4

### 3.1. Loading Noble Metal on the Surface of Ag3PO4

Loading noble metal on the surface of Ag3PO4 is the method to enhance the photocatalytic activities of Ag3PO4 because the noble metal loaded on the surface of Ag3PO4 acts as an electron scavenger [32]. This means the excited electrons from the valence band are removed from the Ag3PO4 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 Ag3PO4 which is reducing the absorption of light and adsorption of dyes to the active site of Ag3PO4 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 Ag3PO4 highly enhances the stability of Ag3PO4 which has exhibited better efficiency when it is used for six consecutive runs [34]. However, the performance of neat Ag3PO4 deteriorated when it had been used for a six repeating runs due to the gradual decomposition of Ag3PO4 to weakly active Ag nanoparticle which is the main cause for instability and photocorrosion of Ag3PO4 [34, 35].Figure 8
The recycled photocatalytic degradation curves of MO Ag3PO4 and Pt/Ag3PO4 [34].
(a)(b)(c)(d)(e)(f)The aforementioned advantages were accomplished as follows. The loaded noble metals on Ag3PO4 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 Ag3PO4 conduction band edge (CBE), Au, Pd, and Pt is 4.95, 5.1, 5.12, and 5.65 eV, respectively [34, 37] since Ag3PO4 n-type semiconductor and its Fermi level are close to the conduction band edge. When noble metal contacts with Ag3PO4, 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 Ag3PO4 to metal was formed. When the (Au, Pd, Pt)/Ag3PO4 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 Ag3PO4 at higher potential and also an internal electric field-assisted the flow electron from Ag3PO4 to metals. As result, the charge recombination rate and the instability of Ag3PO4 are effectively suppressed [34]. The electron reacted with oxgen and form O2- reactive 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).Figure 9
(a) Schottky-type heterostructures contact with noble metals. (b) Mechanism of electron transfer between Ag3PO4 and noble metals under visible light in organic solution [34].
(a)(b)In addition to those benefits, noble metal nanoparticles loaded on the surface of Ag3PO4 revealed remarkable enhancement absorption of both UV and visible light than pure Ag3PO4 as indicated in Figure 10. This can be the attribution of localized surface plasmon resonance effect (LSPR) of noble metal loaded on the surface of Ag3PO4 in which a vibrational frequency of coming electromagnetic radiation matches with the vibration frequency of the electrons in the noble metals which help to harvest a large amount of coming light [38]. These have improved the photocatalytic activity of Ag3PO4 [34]. As shown in Table 1, many reported works on the loading of noble metal on surface Ag3PO4 to enhance the photocatalytic activities of Ag3PO4 are listed.Figure 10
UV-vis diffuse reflectance spectra of Ag3PO4 and M/Ag3PO4 [34].Table 1
Summary of the reported on loading noble metal on the surface of Ag3PO4.
Loaded noble metalsTypes of dyesEfficiencyInitial dye Conc. (CO)Illumination lightReferencePt/Ag3PO4MO100% (15 min)10 ppmVisible light[34]Au /Ag3PO4MO88% (15 min)10 ppmVisible light[34]Pd/Ag3PO4Mo86 (15 min)10 ppmVisible light[34]Ag/Ag3PO4RhB and MB40 min (100%)10 mg/LVisible light[60]Ag/Ag3PO4RhB2 min (100%)10 mg/LVisible light[33]Different noble metals have a different effects on enhancing photocatalytic activities of Ag3PO4 because of their various specific properties. Figure 11(a) shows the enhancement of photocatalytic activities of the Ag3PO4 by loading noble metals, and a comparative study of the photocatalytic activities of various noble metals loaded on the surface of Ag3PO4 and with other photocatalyst materials. As indicated in this Figure 11(a), different noble metals loaded on Ag3PO4 show unequal performances because they do have not equal work functions. The large differences in work function between the noble metals and Ag3PO4 are necessary for a better photocatalytic process, because it facilitates a transfer of a photogenerated electron from Ag3PO4 to noble metals. So, the degrading ability of dyes by Pt/Ag3PO4 is larger than the other two noble metals due to Pt having a large work function difference with the conduction band edge of Ag3PO4 as shown in Figure 9(a) which highly assists the electron transfer from Ag3PO4 to Pt. As a result, the photodegradation Pt loaded on Ag3PO4 is higher than the other two noble metals and bare Ag3PO4 [39]. In general, as indicated in Figure 11(a), the degradation of the dye by noble metal loaded on the surface of Ag3PO4 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 Ag3PO4 negative due to it has plenty OH-(hydroxyl group) [41] which helped to adsorb highly cationic dyes.Figure 11
(a) Photocatalytic degradation curves of MO. (b) Photocatalytic degradation curves and TOC (insert) of MB, RhB, and MO over Pt/Ag3PO4 photocatalyst [34].
(a)(b)


### 3.2. Forming Binary Ag3PO4 Composite with Semiconductor and Supported Material

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 Ag3PO4 with other semiconductors is among the methods to enhance the photocatalytic activities of Ag3PO4. The main advantageous form heterojunctions with Ag3PO4 are as follows: (i) solving instability issues and reducing the photocorrosion, (ii) improving the absorption of solar light in the visible range, and (iii) suppressing the recombination of charge carriers [39, 44, 45]. For instance, Figure 12 reveals the composite of Co3O4/Ag3PO4 in which a heterojunction was formed between Co3O4 and Ag loaded on Ag3PO4 in which silver metal appeared on the surface of Ag3PO4 during the photocatalysis process [44]. As indicated in this Figure 12, the photoexcited electrons and holes transferred Ag and Co3O4, respectively. As a result, there is no recombination of holes-electrons and photocorrosion because both charge carriers have not stayed on the Ag3PO4 after photoexcitation has been done. The solar absorption of mentioned heterojunction is better than a single component.Figure 12
Electrons transfer mechanism in heterostructure photocatalyst [44].Bi and coworkers decorated the silver halides as core-shell on the surface of rhombic dodecahedral Ag3PO4 by the ion change method, and as we knew, the Ag3PO4 has rhombic dodecahedral morphology that has high surface energy which makes it dissolve in solution [45]. The intention of decorating Ag3PO4 is to prevent dissolution and enhance the stability of Ag3PO4 during photocatalyst activity since silver halides have lower solubility in aqueous solution than pristine Ag3PO4. As well as, the valance band and conduction band of all halides are less negative than Ag3PO4 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)/Ag3PO4 by facilitated multiple-electron reduction reaction of oxygen (O2+2H++2e−=H2O2) [46] at Ag3PO4 without photoreduction of silver ions into black silver metals. As a result, photodegradation of organic dyes by Ag3PO4 was enhanced as shown in Figure 13.Figure 13
Photocatalytic activities of N-doped TiO2 and AgX/Ag3PO4 [45].Supporting materials such as cellulose, graphene, and other porous materials have been used with Ag3PO4. These supporting materials increases the photocatalytic activities of Ag3PO4 by improving the surface area, morphology and reducing agglomerations of Ag3PO4. Based on this concept, Yang and coworkers synthesized Ag3PO4/graphene oxide nanocomposite by simple precipitation methods and subsequently reduced graphene oxide to reduce graphene to form Ag3PO4/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 Ag3PO4 and graphene as indicated in Figure 14(c). Due to the above reason, the photodegradation of organic dyes by Ag3PO4/graphene-based nanocomposite is excellent when compared to bare Ag3PO4 as shown in Figure 14(b). To check the stability of Ag3PO4 and Ag3PO4/GR, the XRD characterization was recorded according to Figure 14(d) after a photocatalysis activity. It is observed that Ag metals appeared for Ag3PO4 which is the main cause for photocorrosion but not appeared for composite owing to the electron in an Ag3PO4 transferring to graphene sheet as indicated in Figure 14(c) before it combined with Ag+ ions to form black silver metals. Therefore, combining Ag3PO4 with graphene-based materials has reduced photocorrosion to a large extent [47]. Further, Table 2 shows the lists of published works on Ag3PO4/metal oxide semiconductor or supporting materials for dye degradation.Figure 14
(a) Schematic illustration of the synthesis of Ag3PO4 and Ag3PO4/graphene-based composite. (b) C/Co vs. time photodegradation RhB synthesized materials. (c) Photodegradation mechanisms in an Ag3PO4/GR composite under visible light. (d) XRD spectra of Ag3PO4 and Ag3PO4/GR after photo degradation [47].
(a)(b)(c)(d)Table 2
Summary of reported Ag3PO4/semiconductor or supporting materials for dye degradation.
CatalystTypes of dyesEfficiency (%)Initial dye Conc.Illumination lightReferencesAg3PO4/ZnOMB98.16% (30 min)_Visible light[39]Ag3PO4/celluloseMO90% under sun light (80 min)0.2 g/LSun light[61]Ag3PO4/grapheneMB90% (20 min)2.5×10−5 MVisible light[62]Ag3PO4/TNSRhB99.11% (50 min)50 mg/LSun light[63]Co3O4/Ag3PO4MB100% (12 min)10 mg/LVisible light[44]N-doped carbon quantum dot/Ag3PO4MB100% (70 min)20 mg/LVisible light[64]g-C3N4/Ag3PO4MB96% (10 min)10 mg/LVisible light[65]The photocatalytic activity toward degradation of organic dyes from aqueous solution was also enhanced when Ag3PO4 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 Ag3PO4 [1, 49, 50].Table 3
List of published work on the Ag3PO4 within 1D, 2D, and 3D morphology semiconductor.
CatalystTypes of dyesDegradation efficiencyDimension of semiconductor combined with Ag3PO4Initial dye Conc. (Co)Illumination lightReferenceAg3PO4/MoS2RhB90% (8 min)2D graphene nanosheets20 mg/LVisible light[49]Ag3PO4/TO2RhB100% (7.5 min)1D (nanofibers)10 mg/LVisible light[66]Ag3PO4/La2Ti2O7RhB100% (8 min)2D (nanosheets)10 mg/LUV/visible light[67]Ag3PO4/CuORhB100% (75 min)3D (carambola-like nanostructure)1×10−5 mol/LVisible light[68]Ag3PO4/TNSRhB99.11% (50 min)2D (TiO2 nanosheets)10 mg/LVisible light[63]g-C3N4/Ag3PO4MO3 times (Ag3PO4)2D (sheets)10 mg/LVisible light[69]5 times (g-C3N4)Ag3PO4/TiO2Black liquor60.4% (300 min)1D (nanofibers)30 mg/LVisible light[70]Ag3PO4/PANRhB100% (16 min)1D (nanofibers)—Visible light[71]


### 3.3. Ag3PO4-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 Ag3PO4 containing ternary nanocomposites improved the degradation efficiency of Ag3PO4. Among these materials, one is the Ag3PO4. And other two materials are used to enhance the photostability and photocatalytic activities of Ag3PO4. 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 Ag3PO4. 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 WO3 are more positive than the conduction band edge and valance band edge of Ag3PO4, and their bandgap is 2.44 eV and 2.35 eV for WO3 and Ag3PO4, respectively. So, electron-hole pairs were generated under visible light for both semiconductors, and also, the electron migrates from Ag3PO4 CB(conduction band) to WO3 CB (conduction band), and the hole migrated from the valance band (VB) of WO3 to the valance band (VB) of Ag3PO4. After that, the holes oxidized organic dyes adsorbed on the surface of Ag3PO4, and the electrons were consumed via proceeding multioxidation reaction with oxygen O2+2H+ + 2e− ⟶ H2O2, E0=+0.682 V vs. NHE. Further, the produced H2O2 reacted with electrons and produced hydroxyl radical ̊OH (H2O2 + e− ⟶ OH− + ̊OH) which participated degradation of organic dyes from aqueous solution, and the Fe2O3 in the composite acts as a mediator for electron transfer from Ag3PO3 to WO3 and also its scale number of recyclability of catalyst for practical application [52]. Moreover, some of the lists of published articles on Ag3PO4 based on ternary nanocomposite for degradation of dyes are explained in Table 4.Figure 15
Schematic diagram of Ag3PO4-based ternary nanocomposite [52].Table 4
Lists of published articles on Ag3PO4-based ternary nanocomposite for degradation of dyes.
CatalystTypes dyesEfficiency (%)Initial dye Conc. (Co)Illumination lightReferenceTiO2/Ag3PO4/GO(0.6)Acid blue 25 dye98% (20 min)18μmol/LVisible light[72]Ag3PO4/CoFe2O4/GOMB100% (30 min)10 mg/LVisible light[54]Fe3O4/C/Ag3PO4RhB100% (40 min)5×10−5 MVisible light[73]Ag3PO4/TiO2/Fe3O4AO7 dyes97% (30 min)15μMVisible light[55]ZnFe2O4/ZnO/Ag3PO4RhB100% (240 min)10 mg/LSimulated solar light[74]Ag3PO4/AgBr/agMB92.02% (75 min)15 mg/LVisible light[75]CuO/ZnO/Ag3PO4MO81.50% (18 min)10 mg/LVisible light[22]Ag3PO4/ZnO/Fe3O4Congo red97% (90 min)10 mg/LVisible light[76]Ag3PO4/g-C3N4/ZnOTetracycline hydrochloride (TC)88% (180 min)30 mg/LVisible light lamp[51]And also, the magnetically separable Ag3PO4/CoFe2O4/GO composite reported by Chao and his coworkers was synthesized by an efficient precipitation method with different mass ratios of Ag3PO4 to CoFe2O4/GO. The Ag3PO4/MG-15% photocatalysts showed better photocatalytic activity than bare Ag3PO4; under Ag3PO4/MG-15%, 91% of MO is degraded, while under the same condition, only 36% of MO was degraded by the bare Ag3PO4 after 15 min of light irradiation, and also, Ag3PO4/CoFe2O4/GO shows good recyclability when compared to bare Ag3PO4 as evidenced in 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 CoFe2O4. At the same time, electrons transferred from CB of CoFe2O4 to CB of Ag3PO4 and reduced the oxygen adsorbed on the surface of the catalyst and formed O-o2 radical, and holes transferred from the valance band of Ag3PO4 to the valance band of CoFe2O4. Superoxide (O-o2) radical degrades MO dyes. The holes simultaneous oxidize MO directly into carbon dioxide and water molecules and the photoreaction; the e--h+ pairs could be separated with ease due to the high electron transfer capacity of GO [54].Figure 16
(a) Repeated photocatalytic experiments. (b) Proposed mechanism for degradation of MO dye with Ag3PO4/CoFe2O4/GO composite [53].
(a)(b)The hierarchical Ag3PO3/TiO2/Fe3O4 nanocomposite structure shows high photocatalytic activity, high stability, and good recyclability than Ag3PO4 toward degradation of AO7 dyes from wastewater due to high photogenerated charge separation which attribution of TiO2 and Ag+ ions that produced by photoinduced electron and inhibit the further decomposition of Ag3PO4. The Fe3O4 helps for 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 ZnFe2O4-ZnO-Ag3PO4 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 ZnFe3O4 to ZnO to Ag3PO4 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 ZnFe3O4 has a magnetic property [56].Figure 17
The cascade electron transfer from ZnFe3O4/ZnO/Ag3PO4 [56].


#### 3.3.1. 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 and hole reacted with oxygen and water to form superoxide radicals (O2-o) and hydroxyl radicals (oOH), respectively, according to Equations (2) and (4). Further superoxide radicals reacted with H2O and form hydroperoxyl radicals (HO2o) according to (Equation (3)). Thirdly, these formed superoxide radicals (O2-o), hydroxyl radicals (oOH), and hydroperoxyl radicals (HO2o) oxidized organic dyes and converted them into CO2 and H2O which is not a secondary pollutant. The overall mechanisms of photodegradation of organic dyes are elucidated in Figure 18 [57].
(1)Semiconductor+hveV⟶e−+h+(2)O2+e−⟶O2−o(3)O2−o+H2O⟶HO2o(4)H2O/OH−+h+⟶OHo(5)O2−o/HO2o,OHo+OPs⟶CO2+H2OFigure 18
Schematic overview of the photocatalytic degradation process of organic [59].


#### 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 Ag3PO4 which are photocorrosion, instability, and high recombination rate. However, they have not stated the cost of a precursor used to synthesize Ag3PO4, but the precursor used for fabricated Ag3PO4 is expensive. So, we will suggest that replacing the Ag3PO4 precursor with a less-cost precursor will improve the availability and efficiency of Ag3PO4 for photocatalysis applications. Especially, extracting a precursor from waste by-products and naturally occurring materials enhances the nobility and application of Ag3PO4 for photodegradation. For instance, eggshells (ES) and bones are rich in phosphate so we can use them as precursors for synthesizing Ag3PO4. Synthesizing of Ag3PO4/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 TiO2 is crucial to reducing the wideband to narrow bandgap that enables them to harvest the visible light while a couple with Ag3PO4. Thus, the doped wide-bandgap oxides not only reduce the limitation of the Ag3PO4 but also actively participate in the degradation of the organic dyes in the wastewater.


## 3.1. Loading Noble Metal on the Surface of Ag3PO4

Loading noble metal on the surface of Ag3PO4 is the method to enhance the photocatalytic activities of Ag3PO4 because the noble metal loaded on the surface of Ag3PO4 acts as an electron scavenger [32]. This means the excited electrons from the valence band are removed from the Ag3PO4 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 Ag3PO4 which is reducing the absorption of light and adsorption of dyes to the active site of Ag3PO4 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 Ag3PO4 highly enhances the stability of Ag3PO4 which has exhibited better efficiency when it is used for six consecutive runs [34]. However, the performance of neat Ag3PO4 deteriorated when it had been used for a six repeating runs due to the gradual decomposition of Ag3PO4 to weakly active Ag nanoparticle which is the main cause for instability and photocorrosion of Ag3PO4 [34, 35].Figure 8
The recycled photocatalytic degradation curves of MO Ag3PO4 and Pt/Ag3PO4 [34].
(a)(b)(c)(d)(e)(f)The aforementioned advantages were accomplished as follows. The loaded noble metals on Ag3PO4 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 Ag3PO4 conduction band edge (CBE), Au, Pd, and Pt is 4.95, 5.1, 5.12, and 5.65 eV, respectively [34, 37] since Ag3PO4 n-type semiconductor and its Fermi level are close to the conduction band edge. When noble metal contacts with Ag3PO4, 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 Ag3PO4 to metal was formed. When the (Au, Pd, Pt)/Ag3PO4 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 Ag3PO4 at higher potential and also an internal electric field-assisted the flow electron from Ag3PO4 to metals. As result, the charge recombination rate and the instability of Ag3PO4 are effectively suppressed [34]. The electron reacted with oxgen and form O2- reactive 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).Figure 9
(a) Schottky-type heterostructures contact with noble metals. (b) Mechanism of electron transfer between Ag3PO4 and noble metals under visible light in organic solution [34].
(a)(b)In addition to those benefits, noble metal nanoparticles loaded on the surface of Ag3PO4 revealed remarkable enhancement absorption of both UV and visible light than pure Ag3PO4 as indicated in Figure 10. This can be the attribution of localized surface plasmon resonance effect (LSPR) of noble metal loaded on the surface of Ag3PO4 in which a vibrational frequency of coming electromagnetic radiation matches with the vibration frequency of the electrons in the noble metals which help to harvest a large amount of coming light [38]. These have improved the photocatalytic activity of Ag3PO4 [34]. As shown in Table 1, many reported works on the loading of noble metal on surface Ag3PO4 to enhance the photocatalytic activities of Ag3PO4 are listed.Figure 10
UV-vis diffuse reflectance spectra of Ag3PO4 and M/Ag3PO4 [34].Table 1
Summary of the reported on loading noble metal on the surface of Ag3PO4.
Loaded noble metalsTypes of dyesEfficiencyInitial dye Conc. (CO)Illumination lightReferencePt/Ag3PO4MO100% (15 min)10 ppmVisible light[34]Au /Ag3PO4MO88% (15 min)10 ppmVisible light[34]Pd/Ag3PO4Mo86 (15 min)10 ppmVisible light[34]Ag/Ag3PO4RhB and MB40 min (100%)10 mg/LVisible light[60]Ag/Ag3PO4RhB2 min (100%)10 mg/LVisible light[33]Different noble metals have a different effects on enhancing photocatalytic activities of Ag3PO4 because of their various specific properties. Figure 11(a) shows the enhancement of photocatalytic activities of the Ag3PO4 by loading noble metals, and a comparative study of the photocatalytic activities of various noble metals loaded on the surface of Ag3PO4 and with other photocatalyst materials. As indicated in this Figure 11(a), different noble metals loaded on Ag3PO4 show unequal performances because they do have not equal work functions. The large differences in work function between the noble metals and Ag3PO4 are necessary for a better photocatalytic process, because it facilitates a transfer of a photogenerated electron from Ag3PO4 to noble metals. So, the degrading ability of dyes by Pt/Ag3PO4 is larger than the other two noble metals due to Pt having a large work function difference with the conduction band edge of Ag3PO4 as shown in Figure 9(a) which highly assists the electron transfer from Ag3PO4 to Pt. As a result, the photodegradation Pt loaded on Ag3PO4 is higher than the other two noble metals and bare Ag3PO4 [39]. In general, as indicated in Figure 11(a), the degradation of the dye by noble metal loaded on the surface of Ag3PO4 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 Ag3PO4 negative due to it has plenty OH-(hydroxyl group) [41] which helped to adsorb highly cationic dyes.Figure 11
(a) Photocatalytic degradation curves of MO. (b) Photocatalytic degradation curves and TOC (insert) of MB, RhB, and MO over Pt/Ag3PO4 photocatalyst [34].
(a)(b)


## 3.2. Forming Binary Ag3PO4 Composite with Semiconductor and Supported Material

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 Ag3PO4 with other semiconductors is among the methods to enhance the photocatalytic activities of Ag3PO4. The main advantageous form heterojunctions with Ag3PO4 are as follows: (i) solving instability issues and reducing the photocorrosion, (ii) improving the absorption of solar light in the visible range, and (iii) suppressing the recombination of charge carriers [39, 44, 45]. For instance, Figure 12 reveals the composite of Co3O4/Ag3PO4 in which a heterojunction was formed between Co3O4 and Ag loaded on Ag3PO4 in which silver metal appeared on the surface of Ag3PO4 during the photocatalysis process [44]. As indicated in this Figure 12, the photoexcited electrons and holes transferred Ag and Co3O4, respectively. As a result, there is no recombination of holes-electrons and photocorrosion because both charge carriers have not stayed on the Ag3PO4 after photoexcitation has been done. The solar absorption of mentioned heterojunction is better than a single component.Figure 12
Electrons transfer mechanism in heterostructure photocatalyst [44].Bi and coworkers decorated the silver halides as core-shell on the surface of rhombic dodecahedral Ag3PO4 by the ion change method, and as we knew, the Ag3PO4 has rhombic dodecahedral morphology that has high surface energy which makes it dissolve in solution [45]. The intention of decorating Ag3PO4 is to prevent dissolution and enhance the stability of Ag3PO4 during photocatalyst activity since silver halides have lower solubility in aqueous solution than pristine Ag3PO4. As well as, the valance band and conduction band of all halides are less negative than Ag3PO4 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)/Ag3PO4 by facilitated multiple-electron reduction reaction of oxygen (O2+2H++2e−=H2O2) [46] at Ag3PO4 without photoreduction of silver ions into black silver metals. As a result, photodegradation of organic dyes by Ag3PO4 was enhanced as shown in Figure 13.Figure 13
Photocatalytic activities of N-doped TiO2 and AgX/Ag3PO4 [45].Supporting materials such as cellulose, graphene, and other porous materials have been used with Ag3PO4. These supporting materials increases the photocatalytic activities of Ag3PO4 by improving the surface area, morphology and reducing agglomerations of Ag3PO4. Based on this concept, Yang and coworkers synthesized Ag3PO4/graphene oxide nanocomposite by simple precipitation methods and subsequently reduced graphene oxide to reduce graphene to form Ag3PO4/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 Ag3PO4 and graphene as indicated in Figure 14(c). Due to the above reason, the photodegradation of organic dyes by Ag3PO4/graphene-based nanocomposite is excellent when compared to bare Ag3PO4 as shown in Figure 14(b). To check the stability of Ag3PO4 and Ag3PO4/GR, the XRD characterization was recorded according to Figure 14(d) after a photocatalysis activity. It is observed that Ag metals appeared for Ag3PO4 which is the main cause for photocorrosion but not appeared for composite owing to the electron in an Ag3PO4 transferring to graphene sheet as indicated in Figure 14(c) before it combined with Ag+ ions to form black silver metals. Therefore, combining Ag3PO4 with graphene-based materials has reduced photocorrosion to a large extent [47]. Further, Table 2 shows the lists of published works on Ag3PO4/metal oxide semiconductor or supporting materials for dye degradation.Figure 14
(a) Schematic illustration of the synthesis of Ag3PO4 and Ag3PO4/graphene-based composite. (b) C/Co vs. time photodegradation RhB synthesized materials. (c) Photodegradation mechanisms in an Ag3PO4/GR composite under visible light. (d) XRD spectra of Ag3PO4 and Ag3PO4/GR after photo degradation [47].
(a)(b)(c)(d)Table 2
Summary of reported Ag3PO4/semiconductor or supporting materials for dye degradation.
CatalystTypes of dyesEfficiency (%)Initial dye Conc.Illumination lightReferencesAg3PO4/ZnOMB98.16% (30 min)_Visible light[39]Ag3PO4/celluloseMO90% under sun light (80 min)0.2 g/LSun light[61]Ag3PO4/grapheneMB90% (20 min)2.5×10−5 MVisible light[62]Ag3PO4/TNSRhB99.11% (50 min)50 mg/LSun light[63]Co3O4/Ag3PO4MB100% (12 min)10 mg/LVisible light[44]N-doped carbon quantum dot/Ag3PO4MB100% (70 min)20 mg/LVisible light[64]g-C3N4/Ag3PO4MB96% (10 min)10 mg/LVisible light[65]The photocatalytic activity toward degradation of organic dyes from aqueous solution was also enhanced when Ag3PO4 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 Ag3PO4 [1, 49, 50].Table 3
List of published work on the Ag3PO4 within 1D, 2D, and 3D morphology semiconductor.
CatalystTypes of dyesDegradation efficiencyDimension of semiconductor combined with Ag3PO4Initial dye Conc. (Co)Illumination lightReferenceAg3PO4/MoS2RhB90% (8 min)2D graphene nanosheets20 mg/LVisible light[49]Ag3PO4/TO2RhB100% (7.5 min)1D (nanofibers)10 mg/LVisible light[66]Ag3PO4/La2Ti2O7RhB100% (8 min)2D (nanosheets)10 mg/LUV/visible light[67]Ag3PO4/CuORhB100% (75 min)3D (carambola-like nanostructure)1×10−5 mol/LVisible light[68]Ag3PO4/TNSRhB99.11% (50 min)2D (TiO2 nanosheets)10 mg/LVisible light[63]g-C3N4/Ag3PO4MO3 times (Ag3PO4)2D (sheets)10 mg/LVisible light[69]5 times (g-C3N4)Ag3PO4/TiO2Black liquor60.4% (300 min)1D (nanofibers)30 mg/LVisible light[70]Ag3PO4/PANRhB100% (16 min)1D (nanofibers)—Visible light[71]


## 3.3. Ag3PO4-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 Ag3PO4 containing ternary nanocomposites improved the degradation efficiency of Ag3PO4. Among these materials, one is the Ag3PO4. And other two materials are used to enhance the photostability and photocatalytic activities of Ag3PO4. 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 Ag3PO4. 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 WO3 are more positive than the conduction band edge and valance band edge of Ag3PO4, and their bandgap is 2.44 eV and 2.35 eV for WO3 and Ag3PO4, respectively. So, electron-hole pairs were generated under visible light for both semiconductors, and also, the electron migrates from Ag3PO4 CB(conduction band) to WO3 CB (conduction band), and the hole migrated from the valance band (VB) of WO3 to the valance band (VB) of Ag3PO4. After that, the holes oxidized organic dyes adsorbed on the surface of Ag3PO4, and the electrons were consumed via proceeding multioxidation reaction with oxygen O2+2H+ + 2e− ⟶ H2O2, E0=+0.682 V vs. NHE. Further, the produced H2O2 reacted with electrons and produced hydroxyl radical ̊OH (H2O2 + e− ⟶ OH− + ̊OH) which participated degradation of organic dyes from aqueous solution, and the Fe2O3 in the composite acts as a mediator for electron transfer from Ag3PO3 to WO3 and also its scale number of recyclability of catalyst for practical application [52]. Moreover, some of the lists of published articles on Ag3PO4 based on ternary nanocomposite for degradation of dyes are explained in Table 4.Figure 15
Schematic diagram of Ag3PO4-based ternary nanocomposite [52].Table 4
Lists of published articles on Ag3PO4-based ternary nanocomposite for degradation of dyes.
CatalystTypes dyesEfficiency (%)Initial dye Conc. (Co)Illumination lightReferenceTiO2/Ag3PO4/GO(0.6)Acid blue 25 dye98% (20 min)18μmol/LVisible light[72]Ag3PO4/CoFe2O4/GOMB100% (30 min)10 mg/LVisible light[54]Fe3O4/C/Ag3PO4RhB100% (40 min)5×10−5 MVisible light[73]Ag3PO4/TiO2/Fe3O4AO7 dyes97% (30 min)15μMVisible light[55]ZnFe2O4/ZnO/Ag3PO4RhB100% (240 min)10 mg/LSimulated solar light[74]Ag3PO4/AgBr/agMB92.02% (75 min)15 mg/LVisible light[75]CuO/ZnO/Ag3PO4MO81.50% (18 min)10 mg/LVisible light[22]Ag3PO4/ZnO/Fe3O4Congo red97% (90 min)10 mg/LVisible light[76]Ag3PO4/g-C3N4/ZnOTetracycline hydrochloride (TC)88% (180 min)30 mg/LVisible light lamp[51]And also, the magnetically separable Ag3PO4/CoFe2O4/GO composite reported by Chao and his coworkers was synthesized by an efficient precipitation method with different mass ratios of Ag3PO4 to CoFe2O4/GO. The Ag3PO4/MG-15% photocatalysts showed better photocatalytic activity than bare Ag3PO4; under Ag3PO4/MG-15%, 91% of MO is degraded, while under the same condition, only 36% of MO was degraded by the bare Ag3PO4 after 15 min of light irradiation, and also, Ag3PO4/CoFe2O4/GO shows good recyclability when compared to bare Ag3PO4 as evidenced in 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 CoFe2O4. At the same time, electrons transferred from CB of CoFe2O4 to CB of Ag3PO4 and reduced the oxygen adsorbed on the surface of the catalyst and formed O-o2 radical, and holes transferred from the valance band of Ag3PO4 to the valance band of CoFe2O4. Superoxide (O-o2) radical degrades MO dyes. The holes simultaneous oxidize MO directly into carbon dioxide and water molecules and the photoreaction; the e--h+ pairs could be separated with ease due to the high electron transfer capacity of GO [54].Figure 16
(a) Repeated photocatalytic experiments. (b) Proposed mechanism for degradation of MO dye with Ag3PO4/CoFe2O4/GO composite [53].
(a)(b)The hierarchical Ag3PO3/TiO2/Fe3O4 nanocomposite structure shows high photocatalytic activity, high stability, and good recyclability than Ag3PO4 toward degradation of AO7 dyes from wastewater due to high photogenerated charge separation which attribution of TiO2 and Ag+ ions that produced by photoinduced electron and inhibit the further decomposition of Ag3PO4. The Fe3O4 helps for 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 ZnFe2O4-ZnO-Ag3PO4 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 ZnFe3O4 to ZnO to Ag3PO4 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 ZnFe3O4 has a magnetic property [56].Figure 17
The cascade electron transfer from ZnFe3O4/ZnO/Ag3PO4 [56].


### 3.3.1. 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 and hole reacted with oxygen and water to form superoxide radicals (O2-o) and hydroxyl radicals (oOH), respectively, according to Equations (2) and (4). Further superoxide radicals reacted with H2O and form hydroperoxyl radicals (HO2o) according to (Equation (3)). Thirdly, these formed superoxide radicals (O2-o), hydroxyl radicals (oOH), and hydroperoxyl radicals (HO2o) oxidized organic dyes and converted them into CO2 and H2O which is not a secondary pollutant. The overall mechanisms of photodegradation of organic dyes are elucidated in Figure 18 [57].
(1)Semiconductor+hveV⟶e−+h+(2)O2+e−⟶O2−o(3)O2−o+H2O⟶HO2o(4)H2O/OH−+h+⟶OHo(5)O2−o/HO2o,OHo+OPs⟶CO2+H2OFigure 18
Schematic overview of the photocatalytic degradation process of organic [59].


### 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 Ag3PO4 which are photocorrosion, instability, and high recombination rate. However, they have not stated the cost of a precursor used to synthesize Ag3PO4, but the precursor used for fabricated Ag3PO4 is expensive. So, we will suggest that replacing the Ag3PO4 precursor with a less-cost precursor will improve the availability and efficiency of Ag3PO4 for photocatalysis applications. Especially, extracting a precursor from waste by-products and naturally occurring materials enhances the nobility and application of Ag3PO4 for photodegradation. For instance, eggshells (ES) and bones are rich in phosphate so we can use them as precursors for synthesizing Ag3PO4. Synthesizing of Ag3PO4/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 TiO2 is crucial to reducing the wideband to narrow bandgap that enables them to harvest the visible light while a couple with Ag3PO4. Thus, the doped wide-bandgap oxides not only reduce the limitation of the Ag3PO4 but also actively participate in the degradation of the organic dyes in the wastewater.


## 3.3.1. 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 and hole reacted with oxygen and water to form superoxide radicals (O2-o) and hydroxyl radicals (oOH), respectively, according to Equations (2) and (4). Further superoxide radicals reacted with H2O and form hydroperoxyl radicals (HO2o) according to (Equation (3)). Thirdly, these formed superoxide radicals (O2-o), hydroxyl radicals (oOH), and hydroperoxyl radicals (HO2o) oxidized organic dyes and converted them into CO2 and H2O which is not a secondary pollutant. The overall mechanisms of photodegradation of organic dyes are elucidated in Figure 18 [57].
(1)Semiconductor+hveV⟶e−+h+(2)O2+e−⟶O2−o(3)O2−o+H2O⟶HO2o(4)H2O/OH−+h+⟶OHo(5)O2−o/HO2o,OHo+OPs⟶CO2+H2OFigure 18
Schematic overview of the photocatalytic degradation process of organic [59].


## 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 Ag3PO4 which are photocorrosion, instability, and high recombination rate. However, they have not stated the cost of a precursor used to synthesize Ag3PO4, but the precursor used for fabricated Ag3PO4 is expensive. So, we will suggest that replacing the Ag3PO4 precursor with a less-cost precursor will improve the availability and efficiency of Ag3PO4 for photocatalysis applications. Especially, extracting a precursor from waste by-products and naturally occurring materials enhances the nobility and application of Ag3PO4 for photodegradation. For instance, eggshells (ES) and bones are rich in phosphate so we can use them as precursors for synthesizing Ag3PO4. Synthesizing of Ag3PO4/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 TiO2 is crucial to reducing the wideband to narrow bandgap that enables them to harvest the visible light while a couple with Ag3PO4. Thus, the doped wide-bandgap oxides not only reduce the limitation of the Ag3PO4 but also actively participate in the degradation of the organic dyes in the wastewater.


## 4. Conclusion

Nano-scaled metal oxide semiconductors have many applications due to their unique properties. The photocatalyst is among the wastewater treatment methods, where nano-scaled semiconductors have been used as a catalyst. Among the metal oxide semiconductors, Ag3PO4 gain significant attention due to its lower bandgap and visible light-absorbing efficiency of up to 95%. However, due to high photocorrosion, instability, and high recombination rate, the photocatalytic activities of Ag3PO4 are getting lower. Such factors which hinder the photocatalytic activity of Ag3PO4 can be overcommed by using Noble metal loaded Ag3PO4, forming binary composition such as Ag3PO4/metal oxides or supporting matarials, Ag3PO4-based ternary nanocomposite,and changing the morphology of Ag3PO4. From all the possible approaches, the mentioned limitations of Ag3PO4 were minimized, and the degradation efficiency of the organic dyes in the wastewater was enhanced. Therefore, the Ag3PO4 with reduced photocorrosion, instability, and recombination rate is a promising agent to degrade organic dye from wastewater.



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*Source: 5564667-2022-06-06.xml*