Highly Efficient Ag3PO4/g-C3N4 Z-Scheme Photocatalyst for Its Enhanced Photocatalytic Performance in Degradation of Rhodamine B and Phenol

Ag3PO4/g-C3N4 heterojunctions, with different g-C3N4 dosages, were synthesized using an in situ deposition method, and the photocatalytic performance of g-C3N4/Ag3PO4 heterojunctions was studied under simulated sunlight conditions. The results revealed that Ag3PO4/g-C3N4 exhibited excellent photocatalytic degradation activity for rhodamine B (Rh B) and phenol under the same light conditions. When the dosage of g-C3N4 was 30%, the degradation rate of Rh B at 9 min and phenol at 30 min was found to be 99.4% and 97.3%, respectively. After five cycles of the degradation experiment for Rh B, g-C3N4/Ag3PO4 still demonstrated stable photodegradation characteristics. The significant improvement in the photocatalytic activity and stability of g-C3N4/Ag3PO4 was attributed to the rapid charge separation between g-C3N4 and Ag3PO4 during the Z-scheme charge transfer and recombination process.


Introduction
With the rapid development of industry, environmental pollution caused by industrial wastewater is becoming increasingly serious. Photocatalysis is an effective technology to degrade pollutants in water, which has been widely researched [1,2]. However, one-component semiconductor photocatalysts always face various defects, such as low visible-light availability and easy recombination of photogenerated charges. It has been proven that the construction of semiconductor heterostructures is an effective route to improve photocatalytic efficiency [3,4]. In recent years, an all-solid Z-scheme semiconductor composite photocatalyst has been applied in photocatalysis [5][6][7][8][9]. When Z-scheme photocatalysts are excited, h + from the valence band (VB) at a higher energy level can combine with e − from the conduction band (CB) at a lower energy level, while e − with a stronger reducing ability in CB at a higher energy level and h + with a stronger oxidation ability in lower VB at a lower energy level can participate in the reduction and oxidation processes during photocatalytic degradation, respectively. This method is conducive to obtain high charge separation efficiency and strong redox ability simultaneously, thus improving the photocatalytic efficiency [8,9].
Among the many types of pollutants, dyes and dangerous compounds are two main pollutants in industrial wastewater. Rh B and phenol are the typical substances of the two pollutants, respectively. Rh B is very harmful to human health. It can cause redness of skin and viscera, mild congestion of cerebral vascular, rupture of myocardial fiber, and other symptoms. Phenol has a strong corrosive effect on skin and mucous membrane, inhibiting the central nervous system and damaging the function of liver and kidney, etc. In addition, phenol is more difficult to degrade than other pollutants in water. Thus, they were chosen as the degradation object in photocatalytic experiments.
In this paper, we synthesized the Ag 3 PO 4 /g-C 3 N 4 Z-scheme heterojunction photocatalyst using the in situ deposition method and evaluated the photocatalytic activity by the degradation experiment for Rh B and phenol. The influence of g-C 3 N 4 and Ag 3 PO 4 on photocatalytic activity was studied in detail and the probable photocatalytic mechanism of Ag 3 PO 4 /g-C 3 N 4 was proposed.

Sample Preparation
Preparation of g-C 3 N 4 : A typical calcination method was used to prepare g-C 3 N 4 . Briefly, 10 g urea powder was placed in an alumina crucible with a lid. The crucible was heated in air at a heating rate of 2 • C·min −1 to 550 • C and, then held at this temperature for 2 h to obtain g-C 3 N 4 . Subsequently, the bulk g-C 3 N 4 was thermally exfoliated into g-C 3 N 4 nanosheets by calcination at 600 • C for 2 h in air. The light yellow product was collected and ground using an agate mortar for subsequent use.
Synthesis of Ag 3 PO 4 /g-C 3 N 4 : Fifty milligrams of g-C 3 N 4 nanosheets were dispersed in 80 mL of deionized water by ultrasonication. Silver ammonia solution (0.1 g·L −1 ) was dropped into the aqueous dispersion of g-C 3 N 4 nanosheets and, then magnetically stirred for 1 h to fully adsorb Ag(NH 3 ) 2+ ions on the surface of g-C 3 N 4 nanosheets. Then, the KH 2 PO 4 solution (0.1 g·L −1 ) was dropped into the above mixture under magnetic agitation and the mixture continued to be stirred for 1 h. The final product was collected by centrifugation, washed with deionized water and ethanol thrice, and dried at 70 • C for 1 h. Finally, the product was collected and ground with an agate mortar for further use. According to the theoretical dosage of g-C 3 N 4, the as-prepared samples were named Ag 3 PO 4 /g-C 3 N 4 -10 wt%, Ag 3 PO 4 /g-C 3 N 4 -20 wt%, Ag 3 PO 4 /g-C 3 N 4 -30 wt%, and Ag 3 PO 4 /g-C 3 N 4 -40 wt%. The actual dosage of g-C 3 N 4 detected by EDS were 9.2 wt%, 16.3 wt%, 27.7 wt%, and 41.8 wt%, respectively. In addition, the simple physical mixture of Ag 3 PO 4 and 30 wt% g-C 3 N 4 was named the Ag 3 PO 4 /g-C 3 N4-30% mixture .

Sample Characterization
The crystal structure was analyzed by a Bruker D8 X-ray diffractometer (XRD, Bruker, Germany), equipped with a Cu K α irradiation light source (λ = 0.154 nm). The microstructure was observed using a Tecnai G2 F20 transmission electron microscopy (TEM, FEI, Hillsboro, OR, USA). Room-temperature transient photoluminescence (PL) spectra were recorded using an FLS1000 spectrometer (EI, UK). UV-vis diffuse reflectance spectra (UV-Vis, Hitachi, Tokyo, Japan) were measured by using a UH4150 UV-Vis near-infrared spectrophotometer. The photocurrent response was measured using a CHI 760E electrochemical workstation (Chenhua, Shanghai, China).

Photocatalytic Activity Test
The photocatalytic activity was evaluated by the pollutant degradation experiments at room temperature. A Polfilet xenon lamp (300 W) with a 320-nm filter was used as the light source. The spectra of the xenon lamp are shown in Figure S1 and detailed experimental devices are shown in Figure S2. The reaction solution consisted of 50 mL of rhodamine B (Rh B, 5 mg·L −1 ) or 50 mL of phenol (10 mg·L −1 ), and the photocatalyst was 0.03 g Ag 3 PO 4 , g-C 3 N 4 , or Ag 3 PO 4 /g-C 3 N 4 . The photocatalyst was weighed and added to the reaction solution, and the reaction solution was continuously stirred in the dark for 30 min to achieve an adsorption-desorption balance between the photocatalytic material and pollutant. Subsequently, the solution was irradiated by a full-wavelength Xenon lamp, and the absorbance of the supernatant was measured at certain intervals. In the cyclic experiments, the photocatalyst was separated from the reaction system after each degradation experiment, washed with ethanol and deionized water, and re-dispersed in the newly-prepared reaction solution to repeat the degradation experiment. Figure 1 shows the XRD patterns of Ag 3 PO 4 , g-C 3 N 4 and Ag 3 PO 4 /g-C 3 N 4 -30 wt%. As shown in Figure 1, a strong peak appeared in the diffraction pattern of g-C 3 N 4 at 2θ = 26.5 • , corresponding to the (002) planes of g-C 3 N 4 (JCPDS card no. 87-1526), which is the characteristic interlayer stacking peak of g-C 3 N 4 [24]. The Ag 3 PO 4 and Ag 3 PO 4 /g-C 3 N 4 -30 wt% exhibited similar XRD patterns and all strong diffraction peaks corresponded to the cubic Ag 3 PO 4 phase (JCPDS card no. 06-0505). The inset provided the refined XRD patterns of Ag 3 PO 4 and Ag 3 PO 4 /g-C 3 N 4 -30 wt%. Compared with Ag 3 PO 4 , the XRD pattern of Ag 3 PO 4 /g-C 3 N 4 showed the characteristic peaks of g-C 3 N 4 ; however, the peak intensities were far weaker than that of Ag 3 PO 4 . This may be attributed to the inferior crystallinity and lower content of well-exfoliated g-C 3 N 4 .

Structural Analysis and Microstructure
Molecules 2021, 26, x FOR PEER REVIEW 3 of 10 light source. The spectra of the xenon lamp are shown in Figure S1 and detailed experimental devices are shown in Figure S2. The reaction solution consisted of 50 mL of rhodamine B (Rh B, 5 mg·L −1 ) or 50 mL of phenol (10 mg·L −1 ), and the photocatalyst was 0.03 g Ag3PO4, g-C3N4, or Ag3PO4/g-C3N4. The photocatalyst was weighed and added to the reaction solution, and the reaction solution was continuously stirred in the dark for 30 min to achieve an adsorption-desorption balance between the photocatalytic material and pollutant. Subsequently, the solution was irradiated by a full-wavelength Xenon lamp, and the absorbance of the supernatant was measured at certain intervals. In the cyclic experiments, the photocatalyst was separated from the reaction system after each degradation experiment, washed with ethanol and deionized water, and re-dispersed in the newly-prepared reaction solution to repeat the degradation experiment. Figure 1 shows the XRD patterns of Ag3PO4, g-C3N4 and Ag3PO4/g-C3N4-30 wt%. As shown in Figure 1, a strong peak appeared in the diffraction pattern of g-C3N4 at 2θ = 26.5°, corresponding to the (002) planes of g-C3N4 (JCPDS card no. 87-1526), which is the characteristic interlayer stacking peak of g-C3N4 [24]. The Ag3PO4 and Ag3PO4/g-C3N4-30 wt% exhibited similar XRD patterns and all strong diffraction peaks corresponded to the cubic Ag3PO4 phase (JCPDS card no. 06-0505). The inset provided the refined XRD patterns of Ag3PO4 and Ag3PO4/g-C3N4-30 wt%. Compared with Ag3PO4, the XRD pattern of Ag3PO4/g-C3N4 showed the characteristic peaks of g-C3N4; however, the peak intensities were far weaker than that of Ag3PO4. This may be attributed to the inferior crystallinity and lower content of well-exfoliated g-C3N4.  Figure 2 shows TEM images of Ag3PO4, g-C3N4, and Ag3PO4/g-C3N4 photocatalysts. Figure 2a illustrates that Ag3PO4 consisted of approximately cubic particles with a size of 200-300 nm. As shown in Figure 2b, g-C3N4 presented thin wrinkled nanosheets. After thermal exfoliation, the specific surface area of g-C3N4 increased significantly, due to morphological changes. Figure 2c shows that the small-sized Ag3PO4 particles were attached to the surface of g-C3N4, forming a stable composite.  Figure 2 shows TEM images of Ag 3 PO 4 , g-C 3 N 4 , and Ag 3 PO 4 /g-C 3 N 4 photocatalysts. Figure 2a illustrates that Ag 3 PO 4 consisted of approximately cubic particles with a size of 200-300 nm. As shown in Figure 2b, g-C 3 N 4 presented thin wrinkled nanosheets. After thermal exfoliation, the specific surface area of g-C 3 N 4 increased significantly, due to morphological changes. Figure 2c shows that the small-sized Ag 3 PO 4 particles were attached to the surface of g-C 3 N 4 , forming a stable composite.  Figure 3 shows the UV-vis diffuse reflectance spectra of Ag3PO4, g-C3N4, and Ag3PO4/g-C3N4-30 wt% photocatalysts. As shown in Figure 3a, the absorption cutoff edges of Ag3PO4 and g-C3N4 were located at about 460 and 530 nm, respectively. Compared with Ag3PO4, the absorption edge of Ag3PO4/g-C3N4-30 wt% was basically unchanged. Based on the UV-vis absorption data, the bandgap width of the photocatalysts was calculated and results are shown in Figure 3b. The calculated bandgap width of g-C3N4 was about 2.78 eV, whereas the bandgap of Ag3PO4 and Ag3PO4/g-C3N4-30wt% decreased to 2.45 eV. By testing the photoelectrochemical properties of Ag3PO4, g-C3N4, and Ag3PO4/g-C3N4-30 wt% photocatalysts, the separation and transfer efficiency of photogenerated electron-hole pairs were studied and results are shown in Figure 4. Figure 4a presents the photoluminescence (PL) spectra of the as-synthesized photocatalysts. The PL emission peak of g-C3N4 was located at 460 nm, showing the highest PL intensity and indicating that the photogenerated charge of g-C3N4 exhibited high recombination efficiency. The PL emission peak of Ag3PO4 was located at 460 nm, showing a far lower PL intensity than g-C3N4. When Ag3PO4 was combined with g-C3N4, the location of the PL emission peak of Ag3PO4/g-C3N4-30 wt% was basically the same as Ag3PO4, but the PL peak intensity of Ag3PO4/g-C3N4-30 wt% was significantly lower than Ag3PO4. Among Ag3PO4, g-C3N4, and  Figure 3 shows the UV-vis diffuse reflectance spectra of Ag 3 PO 4 , g-C 3 N 4 , and Ag 3 PO 4 / g-C 3 N 4 -30 wt% photocatalysts. As shown in Figure 3a, the absorption cutoff edges of Ag 3 PO 4 and g-C 3 N 4 were located at about 460 and 530 nm, respectively. Compared with Ag 3 PO 4 , the absorption edge of Ag 3 PO 4 /g-C 3 N 4 -30 wt% was basically unchanged. Based on the UV-vis absorption data, the bandgap width of the photocatalysts was calculated and results are shown in Figure 3b. The calculated bandgap width of g-C 3 N 4 was about 2.78 eV, whereas the bandgap of Ag 3 PO 4 and Ag 3 PO 4 /g-C 3 N 4 -30wt% decreased to 2.45 eV.  Figure 3 shows the UV-vis diffuse reflectance spectra of Ag3PO4, g-C3N4, and Ag3PO4/g-C3N4-30 wt% photocatalysts. As shown in Figure 3a, the absorption cutoff edges of Ag3PO4 and g-C3N4 were located at about 460 and 530 nm, respectively. Compared with Ag3PO4, the absorption edge of Ag3PO4/g-C3N4-30 wt% was basically unchanged. Based on the UV-vis absorption data, the bandgap width of the photocatalysts was calculated and results are shown in Figure 3b. The calculated bandgap width of g-C3N4 was about 2.78 eV, whereas the bandgap of Ag3PO4 and Ag3PO4/g-C3N4-30wt% decreased to 2.45 eV. By testing the photoelectrochemical properties of Ag3PO4, g-C3N4, and Ag3PO4/g-C3N4-30 wt% photocatalysts, the separation and transfer efficiency of photogenerated electron-hole pairs were studied and results are shown in Figure 4. Figure 4a presents the photoluminescence (PL) spectra of the as-synthesized photocatalysts. The PL emission peak of g-C3N4 was located at 460 nm, showing the highest PL intensity and indicating that the photogenerated charge of g-C3N4 exhibited high recombination efficiency. The PL emission peak of Ag3PO4 was located at 460 nm, showing a far lower PL intensity than g-C3N4. When Ag3PO4 was combined with g-C3N4, the location of the PL emission peak of Ag3PO4/g-C3N4-30 wt% was basically the same as Ag3PO4, but the PL peak intensity of Ag3PO4/g-C3N4-30 wt% was significantly lower than Ag3PO4. Among Ag3PO4, g-C3N4, and By testing the photoelectrochemical properties of Ag 3 PO 4 , g-C 3 N 4 , and Ag 3 PO 4 /g-C 3 N 4 -30 wt% photocatalysts, the separation and transfer efficiency of photogenerated electron-hole pairs were studied and results are shown in Figure 4. Figure 4a presents the photoluminescence (PL) spectra of the as-synthesized photocatalysts. The PL emission peak of g-C 3 N 4 was located at 460 nm, showing the highest PL intensity and indicating that the photogenerated charge of g-C 3 N 4 exhibited high recombination efficiency. The PL emission peak of Ag 3 PO 4 was located at 460 nm, showing a far lower PL intensity than g-C 3 N 4 . When Ag 3 PO 4 was combined with g-C 3 N 4 , the location of the PL emission peak of Ag 3 PO 4 /g-C 3 N 4 -30 wt% was basically the same as Ag 3 PO 4 , but the PL peak intensity of Ag 3 PO 4 /g-C 3 N 4 -30 wt% was significantly lower than Ag 3 PO 4 . Among Ag 3 PO 4 , g-C 3 N 4 , and Ag 3 PO 4 /g-C 3 N 4 -30 wt%, Ag 3 PO 4 /g-C 3 N 4 exhibited the lowest PL peak intensity, which corresponded to the lowest recombination efficiency for photogenerated charges. As can be observed in Figure 4b, all photocatalyst electrodes exhibited rapid response when irradiated by a Xenon lamp (full wavelength). The Ag 3 PO 4 /g-C 3 N 4 -30 wt% showed the highest photocurrent response of about 16.35 µA·cm −2 , which was 2.79 times higher than Ag 3 PO 4 (5.87 µA·cm −2 ) and 21.8 times higher than g-C 3 N 4 (0.75 µA·cm −2 ). These results indicate that the combination of Ag 3 PO 4 and g-C 3 N 4 reduced the recombination efficiency of photogenerated electrons and holes, and accelerated the charges transfer, which is beneficial for photocatalysis. Ag3PO4/g-C3N4-30 wt%, Ag3PO4/g-C3N4 exhibited the lowest PL peak intensity, which corresponded to the lowest recombination efficiency for photogenerated charges. As can be observed in Figure 4b, all photocatalyst electrodes exhibited rapid response when irradiated by a Xenon lamp (full wavelength). The Ag3PO4/g-C3N4-30 wt% showed the highest photocurrent response of about 16.35 μA·cm −2 , which was 2.79 times higher than Ag3PO4 (5.87 μA·cm −2 ) and 21.8 times higher than g-C3N4 (0.75 μA·cm −2 ). These results indicate that the combination of Ag3PO4 and g-C3N4 reduced the recombination efficiency of photogenerated electrons and holes, and accelerated the charges transfer, which is beneficial for photocatalysis.

Photocatalysis Species
In order to identify the active species during the photocatalytic process, free radical capture experiments were carried out using Rh B as a target pollutant. EDTA-2Na, p-benzoquinone (BZQ), and tert-butanol were introduced during the photocatalytic process as h + , ·O2 − , and OH − inhibitors, respectively, and the results are shown in Figure 7. The introduction of tert-butanol during the photocatalytic process of Ag3PO4/g-C3N4-30 wt% rendered no influence on the photodegradation efficiency of Rh B, whereas EDTA-2Na and BZQ both significantly reduced the degradation efficiency of Rh B with a degradation rate of 4.4% and 12.4%, respectively. These results indicate that h + and O 2-are the main active species in Ag3PO4/g-C3N4-30 wt%.  Figure 8 presents the Z-scheme charge transfer pathway of the Ag3PO4/g-C3N4 composite photocatalyst for the degradation of organic pollutants. The bandgap of g-C3N4 was 2.7 eV with the VB potential of ~ 1.4 eV and CB potential of ~ −1.3 eV [27,28]. The potential of e − on the CB of g-C3N4 was −1.3 eV, which can reduce the molecular oxygen O2 to·O 2 because the potential of O2/·O2 -was −0.44 eV vs. NHE. Therefore, O 2-was the main active substance during the photocatalytic process by g-C3N4. The bandgap of Ag3PO4 was 2.45

Photocatalysis Species
In order to identify the active species during the photocatalytic process, free radical capture experiments were carried out using Rh B as a target pollutant. EDTA-2Na, pbenzoquinone (BZQ), and tert-butanol were introduced during the photocatalytic process as h + , ·O 2 − , and OH − inhibitors, respectively, and the results are shown in Figure 7. The introduction of tert-butanol during the photocatalytic process of Ag 3 PO 4 /g-C 3 N 4 -30 wt% rendered no influence on the photodegradation efficiency of Rh B, whereas EDTA-2Na and BZQ both significantly reduced the degradation efficiency of Rh B with a degradation rate of 4.4% and 12.4%, respectively. These results indicate that h + and O 2− are the main active species in Ag 3 PO 4 /g-C 3 N 4-30 wt%.

Photocatalysis Species
In order to identify the active species during the photocatalytic process, free radical capture experiments were carried out using Rh B as a target pollutant. EDTA-2Na, p-benzoquinone (BZQ), and tert-butanol were introduced during the photocatalytic process as h + , ·O2 − , and OH − inhibitors, respectively, and the results are shown in Figure 7. The introduction of tert-butanol during the photocatalytic process of Ag3PO4/g-C3N4-30 wt% rendered no influence on the photodegradation efficiency of Rh B, whereas EDTA-2Na and BZQ both significantly reduced the degradation efficiency of Rh B with a degradation rate of 4.4% and 12.4%, respectively. These results indicate that h + and O 2-are the main active species in Ag3PO4/g-C3N4-30 wt%.  Figure 8 presents the Z-scheme charge transfer pathway of the Ag3PO4/g-C3N4 composite photocatalyst for the degradation of organic pollutants. The bandgap of g-C3N4 was 2.7 eV with the VB potential of ~ 1.4 eV and CB potential of ~ −1.3 eV [27,28]. The potential of e − on the CB of g-C3N4 was −1.3 eV, which can reduce the molecular oxygen O2 to·O 2 because the potential of O2/·O2 -was −0.44 eV vs. NHE. Therefore, O 2-was the main active substance during the photocatalytic process by g-C3N4. The bandgap of Ag3PO4 was 2.45  Figure 8 presents the Z-scheme charge transfer pathway of the Ag 3 PO 4 /g-C 3 N 4 composite photocatalyst for the degradation of organic pollutants. The bandgap of g-C 3 N 4 was 2.7 eV with the VB potential of~1.4 eV and CB potential of~−1.3 eV [27,28]. The potential of e − on the CB of g-C 3 N 4 was −1.3 eV, which can reduce the molecular oxygen O 2 to·O 2 because the potential of O 2 /·O 2 was −0.44 eV vs. NHE. Therefore, O 2− was the main active substance during the photocatalytic process by g-C 3 N 4 . The bandgap of Ag 3 PO 4 was 2.45 eV with a VB potential of~2.9 eV and CB potential of~0.45 eV [29].

Energy Band Structure and Photocatalytic Mechanism
The generated electrons (e − ) in the CB of Ag 3 PO 4 are insufficient to reduce O 2 into O 2− . Therefore, holes (h + ) play a major role during the photocatalytic degradation of organic matter by Ag 3 PO 4 . eV with a VB potential of ~2.9 eV and CB potential of ~0.45 eV [29]. The generated electrons (e − ) in the CB of Ag3PO4 are insufficient to reduce O2 into O 2− . Therefore, holes (h + ) play a major role during the photocatalytic degradation of organic matter by Ag3PO4.
Based on the energy band analysis, it can be inferred that the photogenerated ein the CB of Ag3PO4 can combine with h + in the VB of g-C3N4 due to the formation of a heterojunction interface between Ag3PO4 particles and g-C3N4 nanosheets, resulting in the accumulation of ein the CB of g-C3N4 and h + in VB of Ag3PO4. The h + in the VB of Ag3PO4 can directly react with pollutants, whereas the electrons in CB of g-C3N4 can reduce O2 into O 2− , which reacts with pollutants. The Z-scheme charge transfer mechanism promotes the separation of electron-hole pairs, slows down the photocorrosion of Ag + , and improves photocatalyst activity and stability.

Conclusions
In summary, the Z-scheme heterojunction Ag3PO4/g-C3N4 photocatalyst was synthesized using an in situ deposition method and exhibited excellent photocatalytic degradation activity for Rh B and phenol under Xenon lamp irradiation. The observed rate constant (k) for the degradation of Rh B by Ag3PO4/g-C3N4 was found to be 0.4227 min −1 , which was 4.09 and 20.24 times higher than pure Ag3PO4 and g-C3N4, respectively. Moreover, the k value for the degradation of phenol by Ag3PO4/g-C3N4 was 0.0540 min −1 , which was 5.35 and 20.00 times higher than pure Ag3PO4 and g-C3N4, respectively. Overall, the formation of the Z-scheme heterojunction hindered the recombination of photogenerated electrons and holes, and accelerated the electron transfer, thus improving the activity and stability of photocatalysts.
Supplementary Materials: Figure S1: The spectra of xenon lamp, Figure S2: The picture of experimental setup. Based on the energy band analysis, it can be inferred that the photogenerated e − in the CB of Ag 3 PO 4 can combine with h + in the VB of g-C 3 N 4 due to the formation of a heterojunction interface between Ag 3 PO 4 particles and g-C 3 N 4 nanosheets, resulting in the accumulation of e − in the CB of g-C 3 N 4 and h + in VB of Ag 3 PO 4 . The h + in the VB of Ag 3 PO 4 can directly react with pollutants, whereas the electrons in CB of g-C 3 N 4 can reduce O 2 into O 2− , which reacts with pollutants. The Z-scheme charge transfer mechanism promotes the separation of electron-hole pairs, slows down the photocorrosion of Ag + , and improves photocatalyst activity and stability.

Conclusions
In summary, the Z-scheme heterojunction Ag 3 PO 4 /g-C 3 N 4 photocatalyst was synthesized using an in situ deposition method and exhibited excellent photocatalytic degradation activity for Rh B and phenol under Xenon lamp irradiation. The observed rate constant (k) for the degradation of Rh B by Ag 3 PO 4 /g-C 3 N 4 was found to be 0.4227 min −1 , which was 4.09 and 20.24 times higher than pure Ag 3 PO 4 and g-C 3 N 4 , respectively. Moreover, the k value for the degradation of phenol by Ag 3 PO 4 /g-C 3 N 4 was 0.0540 min −1 , which was 5.35 and 20.00 times higher than pure Ag 3 PO 4 and g-C 3 N 4 , respectively. Overall, the formation of the Z-scheme heterojunction hindered the recombination of photogenerated electrons and holes, and accelerated the electron transfer, thus improving the activity and stability of photocatalysts.
Supplementary Materials: Figure S1: The spectra of xenon lamp, Figure S2: The picture of experimental setup.