Preparation and Characterization of Tubelike g-C3N4/ Ag3PO4 Heterojunction with Enhanced Visible-Light Photocatalytic Activity

Water pollution caused by dye wastewater is a potential threat to human health. Using photocatalysis technology to deal with dye wastewater has the advantages of strong purification and no secondary pollution, so it is greatly significant to look for new visible-light photocatalysts with high photocatalytic ability for dye wastewater degradation. Semiconductor photocatalyst silver phosphate (Ag3PO4) has high quantum efficiency and photocatalytic degradation activity. However, Ag3PO4 is prone to photoelectron corrosion and becomes unstable during photocatalysis, which severely limits its application in this field. In this study, a tubelike g-C3N4/Ag3PO4 heterojunction was constructed by the chemical precipitation method. An Ag3PO4 nanoparticle was loaded onto the surface of the tubelike g-C3N4, forming close contact. The photocatalytic activity of the photocatalyst was evaluated by the degradation of RhB under visible-light irradiation. The tubelike g-C3N4/Ag3PO4-5% heterojunction exhibited optimal photocatalytic performance. In an optimal process, the degradation rate of the RhB is 90% under visible-light irradiation for 40 min. The recycling experiment showed that there was no apparent decrease in the activity of tubelike g-C3N4/Ag3PO4-5% heterojunction after five consecutive runs. A possible Z-type mechanism is proposed to explain the high activity and stability of the heterojunction.


Introduction
With the rapid development of modern industry, the problem of water pollution is becoming increasingly serious, as it could harm human health [1][2][3][4]. Dye wastewater has large discharge, biological toxicity, complex composition, high content of organic matter, and poor biodegradability, which is a difficult problem in wastewater treatment. Semiconductor photocatalysis technology can transform solar into chemical energy and completely decompose organic matter under mild conditions. It shows great potential and good application prospects in solving environmental pollution and energy shortage [5,6]. The development of highly active photocatalysts is key to the application of photocatalysis technology. Various semiconductor photocatalysts, including TiO 2 , ZnO, and SnO 2 , were studied [7][8][9][10]. However, these conventional photocatalytic materials still have many problems, such as photocatalytic activity only under ultraviolet light, electron-hole recombination, and potential toxicity, which are primary obstacles to further application. Thereby, it is greatly significant to explore photocatalysts with high visible-light activity for the application of semiconductor photocatalysis.
In 2010, Ye et al. [11] reported a novel visible light photocatalyst, Ag 3 PO 4 , which showed very strong oxidation capacity and high visible-light catalytic activity. In addition, its quantum efficiency reached 90%, far higher than that of other metal oxide semiconductor photocatalysts [12][13][14][15]. However, Ag 3 PO 4 is prone to photoelectron corrosion and becomes

Synthesis of Tubelike g-C 3 N 4
The molten salt method was used to synthesize the tubelike g-C 3 N 4 [28,29]. First, 4.5 g of lithium chloride, 5.5 g of potassium chloride, and 1 g of melamine were ground together. The mixture in the crucible was heated in a muffle furnace at 550 • C for 4 h. After the reaction, lithium chloride and potassium chloride were removed by washing with deionized water. Lastly, the tubelike g-C 3 N 4 was collected and dried at 80 • C for 24 h. The sample was labeled as TCN.

Synthesis of Tubelike g-C 3 N 4 /Ag 3 PO 4 Heterojunction
The tubelike g-C 3 N 4 / Ag 3 PO 4 heterojunction was prepared by chemical precipitation. First, 0.17 g AgNO 3 was dissolved in deionized water, and a certain amount of tubelike g-C 3 N 4 was dispersed in the above solution. Then, 0.15 g Na 3 PO 4 was dissolved in deionized water and slowly dropped into the AgNO 3 solution. After magnetic stirring for 30 min, the solution was filtered, washed, and dried at 60 • C for 10 h. The tubelike g-C 3 N 4 /Ag 3 PO 4 heterojunctions with different tubelike g-C 3 N 4 mass contents of about 1, 3, 5, and 7% were labeled as TCN/Ag 3 PO 4 -1%, TCN/Ag 3 PO 4 -3%, TCN/Ag 3 PO 4 -5%, and TCN/Ag 3 PO 4 -7%, respectively. For comparison, the pure Ag 3 PO 4 nanoparticle was prepared by the chemical precipitation method without tubelike g-C 3 N 4 .

Characterization
The crystal structure was determined by Philips X 'pert MPD (Almelo, The Netherlands). Cu target Kα was used as the radiation source (wavelength, 0.15418 nm), working voltage was 40 kV, working current was 20 mA, scanning range was 10-70 • , and scanning speed was 4 • min −1 . The surface morphology of the sample was observed by scanning electron microscope (S-4800; working voltage, 5 kV, Kyoto, Japan) and transmission electron microscopy (FEI Tecnai G20; working voltage, 200 kV, Hillsboro, OR, USA). X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Kratos Axis Ultra DLD detector (Manchester, UK). The photoluminescence (PL) spectra of the sample  The UVvisible diffuse reflectance spectra of the sample were measured with a Shimadzu UV-3600  spectrophotometer (UV-3600, Shimadzu, Kyoto, Japan).

Photocatalytic Activity Evaluation
We added 0.01 g of the catalyst to a 100 mL RhB solution with an initial concentration of 10 mg/L. In order to achieve adsorption-desorption equilibrium, the RhB solution (with the photocatalyst) was magnetically stirred in the dark for 30 min. Then, the solution was placed under a 300 W xenon lamp (filtering out ultraviolet light) for photocatalytic reaction. The RhB solution was taken every 5 min. After 30 min of centrifugation, the supernatant was removed, and the concentration of RhB solution was measured with a UV-vis spectrophotometer (721 G, Shanghai, China) at 664 nm absorbance value.

XRD and XPS Analysis
The XRD patterns of TCN, Ag 3 PO 4 , and the TCN/Ag 3 PO 4 heterojunction are shown in Figure 1 (110), (200), (210), (211), (220), (310), (222), (320), (321), (400) crystal planes corresponded to their crystal phase, respectively. This shows that the pure Ag 3 PO 4 phase was successfully prepared by chemical precipitation. The diffraction peak of the TCN/Ag 3 PO 4 heterojunction is mainly from Ag 3 PO 4 , while the characteristic diffraction peak of the tubelike g-C 3 N 4 did not appear because of the low content of tubelike g-C 3 N 4 in the heterojunction, which is difficult to be detected by X-ray diffraction.

Photocatalytic Activity Evaluation
We added 0.01 g of the catalyst to a 100 mL RhB solution with an initial concentration of 10 mg/L. In order to achieve adsorption-desorption equilibrium, the RhB solution (with the photocatalyst) was magnetically stirred in the dark for 30 min. Then, the solution was placed under a 300 W xenon lamp (filtering out ultraviolet light) for photocatalytic reaction. The RhB solution was taken every 5 min. After 30 min of centrifugation, the supernatant was removed, and the concentration of RhB solution was measured with a UV-vis spectrophotometer (721 G, Shanghai, China) at 664 nm absorbance value.

XRD and XPS Analysis
The XRD patterns of TCN, Ag3PO4, and the TCN/Ag3PO4 heterojunction are shown in Figure 1. The diffraction peaks of TCN were at 11.7, 20.
, (400) crystal planes corresponded to their crystal phase, respectively. This shows that the pure Ag3PO4 phase was successfully prepared by chemical precipitation. The diffraction peak of the TCN/Ag3PO4 heterojunction is mainly from Ag3PO4, while the characteristic diffraction peak of the tubelike g-C3N4 did not appear because of the low content of tubelike g-C3N4 in the heterojunction, which is difficult to be detected by X-ray diffraction. The chemical composition of the TCN/Ag3PO4 heterojunction was characterized by XPS, as shown in Figure 2. Figure 2a is the survey spectrum of the TCN/Ag3PO4-5% heterojunction, which shows the presence of Ag, P, O, N, and C. The high-resolution XPS measurements of C 1s, N 1s, Ag 3d, O 1s, and P 2p are shown in Figure 2b-f. There are three peaks in Figure 2b. The peaks with binding energy at 284.6 and 287.5 eV were attributed to the C-C bonds and The chemical composition of the TCN/Ag 3 PO 4 heterojunction was characterized by XPS, as shown in Figure 2. Figure 2a is the survey spectrum of the TCN/Ag 3 PO 4 -5% heterojunction, which shows the presence of Ag, P, O, N, and C. The high-resolution XPS measurements of C 1s, N 1s, Ag 3d, O 1s, and P 2p are shown in Figure 2b-f. There are three peaks in Figure 2b. The peaks with binding energy at 284.6 and 287.5 eV were attributed to the C-C bonds and sp2 carbon in the triazine ring, respectively. The peaks with binding energy at 285.7 eV corresponded to C-O bonds in the hydroxyl state on the sample surface [31]. Results show that there were many hydrophilic hydroxyl groups on the surface of the heterojunction catalyst. These oxygen-containing carbons provided an active surface to form good contact with the Ag 3 PO 4 nanoparticle. Figure 2c shows two peaks in the N 1s spectra. The peak with binding energy at 398.2 eV is attributed to sp2 nitrogen in the triazine ring, and another peak with binding energy at 399.7 eV is ascribed to the terminal amino groups [32]. Results further confirmed the existence of g-C 3 N 4 in the TCN/Ag 3 PO 4 heterojunction. Figure 2d shows the high-resolution spectra of Ag 3d. Peaks with binding energy at 367.8 and 373.8 eV belonged to the 3d5/2 and 3d3/2 characteristic peaks, respectively, with a valence state of Ag + . Ag 3d is a single peak of splitting, indicating that the Ag element existed only in the form of Ag + [33]. Figure 2e shows the high-resolution spectrum of O 1s. The peak of O 1s could be fitted into two peaks, which were mainly the lattice oxygen of Ag 3 PO 4 with binding energy at 530.6 eV and the surface hydroxyl oxygen at 532.4 eV [34]. Figure 2f is the high-resolution spectrum of P 2p. The electron binding energy at the center of the characteristic single peak was located at 133.2 eV, suggesting that the P element was in a +5 valence state [35]. Results further confirmed the existence of Ag 3 PO 4 in the TCN/Ag 3 PO 4 heterojunction. sp2 carbon in the triazine ring, respectively. The peaks with binding energy at 285.7 eV corresponded to C-O bonds in the hydroxyl state on the sample surface [31]. Results show that there were many hydrophilic hydroxyl groups on the surface of the heterojunction catalyst. These oxygen-containing carbons provided an active surface to form good contact with the Ag3PO4 nanoparticle. Figure 2c shows two peaks in the N 1s spectra. The peak with binding energy at 398.2 eV is attributed to sp2 nitrogen in the triazine ring, and another peak with binding energy at 399.7 eV is ascribed to the terminal amino groups [32]. Results further confirmed the existence of g-C3N4 in the TCN/Ag3PO4 heterojunction. Figure 2d shows the highresolution spectra of Ag 3d. Peaks with binding energy at 367.8 and 373.8 eV belonged to the 3d5/2 and 3d3/2 characteristic peaks, respectively, with a valence state of Ag + . Ag 3d is a single peak of splitting, indicating that the Ag element existed only in the form of Ag + [33]. Figure 2e shows the high-resolution spectrum of O 1s. The peak of O 1s could be fitted into two peaks, which were mainly the lattice oxygen of Ag3PO4 with binding energy at 530.6 eV and the surface hydroxyl oxygen at 532.4 eV [34]. Figure 2f is the high-resolution spectrum of P 2p. The electron binding energy at the center of the characteristic single peak was located at 133.2 eV, suggesting that the P element was in a +5 valence state [35]. Results further confirmed the existence of Ag3PO4 in the TCN/Ag3PO4 heterojunction.   Figure 3 shows the SEM images of TCN, Ag 3 PO 4 , and the TCN/Ag 3 PO 4 -5% heterojunction, respectively. Figure 3A shows that TCN prepared by the molten salt method had a hollow tubelike shape, a square tube mouth with a smooth surface, and length distribution ranging from several microns to more than 10 microns. Figure 3B shows that Ag 3 PO 4 prepared by chemical precipitation was composed of many polycrystalline nanoparticles with an average size of 30-50 nm, and particles presented an obvious agglomeration phenomenon. Figure 3C is the SEM image of the TCN/Ag 3 PO 4 -5% heterojunction. Results show that the Ag 3 PO 4 nanoparticle was uniformly loaded onto the surface of tubelike g-C 3 N 4 . Figure 3D is the TEM image of TCN/Ag 3 PO 4 -5% heterojunction. The hollow tubelike g-C 3 N 4 structure and Ag 3 PO 4 nanoparticle were observed more clearly. Results showed that the tubelike g-C 3 N 4 both dispersed the Ag 3 PO 4 nanoparticle and formed the heterojunction. Figure 3 shows the SEM images of TCN, Ag3PO4, and the TCN/Ag3PO4-5% heterojunction, respectively. Figure 3A shows that TCN prepared by the molten salt method had a hollow tubelike shape, a square tube mouth with a smooth surface, and length distribution ranging from several microns to more than 10 microns. Figure 3B shows that Ag3PO4 prepared by chemical precipitation was composed of many polycrystalline nanoparticles with an average size of 30-50 nm, and particles presented an obvious agglomeration phenomenon. Figure 3C is the SEM image of the TCN/Ag3PO4-5% heterojunction. Results show that the Ag3PO4 nanoparticle was uniformly loaded onto the surface of tubelike g-C3N4. Figure 3D is the TEM image of TCN/Ag3PO4-5% heterojunction. The hollow tubelike g-C3N4 structure and Ag3PO4 nanoparticle were observed more clearly. Results showed that the tubelike g-C3N4 both dispersed the Ag3PO4 nanoparticle and formed the heterojunction.

UV-vis Diffuse Reflectance Spectra and Photoluminescence Spectra Analysis
The UV-vis DRS of TCN, Ag3PO4, and the TCN/Ag3PO4 heterojunction are shown in Figure 4a. The Ag3PO4 showed obvious absorption from the ultraviolet to the visible region, and its band edge absorption was 532 nm [36]. The band edge absorption of TCN was 460 nm, indicating a certain absorption of visible light. With the addition of TCN, the absorption spectra of TCN/Ag3PO4 heterojunction had a red shift compared to those of TCN and Ag3PO4. The shift is ascribed to the interaction between TCN and Ag3PO4. This result implied that more visible light could be harvested by the TCN/Ag3PO4 heterojunction.
The separation efficiency of a photoelectron-hole pair greatly affects the performance of the photocatalyst [37]. In Ag3PO4 systems, the rapid separation of photogenerated electrons is beneficial to reduce the photocurrent corrosion of Ag3PO4. Figure 4b shows the photoluminescence spectra of Ag3PO4 and the TCN/Ag3PO4 heterojunction, with an excitation wavelength of 365 nm. The emission peak of pure Ag3PO4 was obvious at 535 nm, indicating that the electron-hole pair in Ag3PO4 had a strong recombination rate. With the addition of TCN, the peak intensity of the TCN/Ag3PO4 heterojunction gradually decreased. The peak intensity of the TCN/Ag3PO4-5% heterojunction was the lowest, indicating a low recombination rate of charges. Therefore, the formation of the TCN/Ag3PO4 heterojunction is beneficial to the separation of electrons and holes, thus effectively improving its catalytic performance.

UV-vis Diffuse Reflectance Spectra and Photoluminescence Spectra Analysis
The UV-vis DRS of TCN, Ag 3 PO 4 , and the TCN/Ag 3 PO 4 heterojunction are shown in Figure 4a. The Ag 3 PO 4 showed obvious absorption from the ultraviolet to the visible region, and its band edge absorption was 532 nm [36]. The band edge absorption of TCN was 460 nm, indicating a certain absorption of visible light. With the addition of TCN, the absorption spectra of TCN/Ag 3 PO 4 heterojunction had a red shift compared to those of TCN and Ag 3 PO 4 . The shift is ascribed to the interaction between TCN and Ag 3 PO 4 . This result implied that more visible light could be harvested by the TCN/Ag 3 PO 4 heterojunction.
The separation efficiency of a photoelectron-hole pair greatly affects the performance of the photocatalyst [37]. In Ag 3 PO 4 systems, the rapid separation of photogenerated electrons is beneficial to reduce the photocurrent corrosion of Ag 3 PO 4 . Figure 4b shows the photoluminescence spectra of Ag 3 PO 4 and the TCN/Ag 3 PO 4 heterojunction, with an excitation wavelength of 365 nm. The emission peak of pure Ag 3 PO 4 was obvious at 535 nm, indicating that the electron-hole pair in Ag 3 PO 4 had a strong recombination rate. With the addition of TCN, the peak intensity of the TCN/Ag 3 PO 4 heterojunction gradually decreased. The peak intensity of the TCN/Ag 3 PO 4 -5% heterojunction was the lowest, indicating a low recombination rate of charges. Therefore, the formation of the TCN/Ag 3 PO 4 heterojunction is beneficial to the separation of electrons and holes, thus effectively improving its catalytic performance.

Photocatalytic Activities Analyses
The catalytic activity of different catalysts was investigated by using RhB as a simulated degradation target, as shown in Figure 5a. In the absence of a catalyst, RhB hardly degraded under visible-light irradiation. The TCN and Ag3PO4 samples exhibited visiblelight photocatalytic activity, and the degradation rate of RhB was 25 and 44%, respectively after 40 min visible light irradiation. Compared with the TCN and Ag3PO4, the TCN/Ag3PO4 heterojunction exhibited enhanced photocatalytic activity. The TCN/Ag3PO4-5% heterojunction resulted in 90% degradation of RhB within 40 min. It is an important factor for the formation of heterojunction with the appropriate amount of TCN. When the amount of TCN is low, Ag3PO4 particles cannot be fully loaded and thus cannot effectively form the heterojunction. On the other hand, excessive TCN absorbs the visible light and forms a masking effect on the heterojunction, thus decreasing its catalytic efficiency. For low-concentration pollutants, the kinetic behaviors for the degradation reaction can be studied with the following equation: The kinetic constants (k) of different samples were calculated and are shown in Figure 5b. The kinetic constant of RhB degradation with TCN/Ag3PO4-5% (0.0077 min −1 ) was about 5.9 and 3.8 times as high as that of TCN (0.0013 min −1 ) and Ag3PO4 (0.0020 min −1 ), respectively. This result shows that the formation of the TCN/Ag3PO4 heterojunction could efficiently enhance the photocatalytic performances of Ag3PO4. To investigate the stability of Ag3PO4 and the TCN/Ag3PO4 heterojunction, the photocatalyst was recovered and dried for the next cycle of degradation. Degradation conditions remained the same as before, and there were 5 cycles. Experimental results are

Photocatalytic Activities Analyses
The catalytic activity of different catalysts was investigated by using RhB as a simulated degradation target, as shown in Figure 5a

Photocatalytic Activities Analyses
The catalytic activity of different catalysts was investigated by using RhB as a simulated degradation target, as shown in Figure 5a. In the absence of a catalyst, RhB hardly degraded under visible-light irradiation. The TCN and Ag3PO4 samples exhibited visiblelight photocatalytic activity, and the degradation rate of RhB was 25 and 44%, respectively after 40 min visible light irradiation. Compared with the TCN and Ag3PO4, the TCN/Ag3PO4 heterojunction exhibited enhanced photocatalytic activity. The TCN/Ag3PO4-5% heterojunction resulted in 90% degradation of RhB within 40 min. It is an important factor for the formation of heterojunction with the appropriate amount of TCN. When the amount of TCN is low, Ag3PO4 particles cannot be fully loaded and thus cannot effectively form the heterojunction. On the other hand, excessive TCN absorbs the visible light and forms a masking effect on the heterojunction, thus decreasing its catalytic efficiency. For low-concentration pollutants, the kinetic behaviors for the degradation reaction can be studied with the following equation: The kinetic constants (k) of different samples were calculated and are shown in Figure 5b. The kinetic constant of RhB degradation with TCN/Ag3PO4-5% (0.0077 min −1 ) was about 5.9 and 3.8 times as high as that of TCN (0.0013 min −1 ) and Ag3PO4 (0.0020 min −1 ), respectively. This result shows that the formation of the TCN/Ag3PO4 heterojunction could efficiently enhance the photocatalytic performances of Ag3PO4. To investigate the stability of Ag3PO4 and the TCN/Ag3PO4 heterojunction, the photocatalyst was recovered and dried for the next cycle of degradation. Degradation conditions remained the same as before, and there were 5 cycles. Experimental results are The kinetic constants (k) of different samples were calculated and are shown in Figure 5b. The kinetic constant of RhB degradation with TCN/Ag 3 PO 4 -5% (0.0077 min −1 ) was about 5.9 and 3.8 times as high as that of TCN (0.0013 min −1 ) and Ag 3 PO 4 (0.0020 min −1 ), respectively. This result shows that the formation of the TCN/Ag 3 PO 4 heterojunction could efficiently enhance the photocatalytic performances of Ag 3 PO 4 .
To investigate the stability of Ag 3 PO 4 and the TCN/Ag 3 PO 4 heterojunction, the photocatalyst was recovered and dried for the next cycle of degradation. Degradation conditions remained the same as before, and there were 5 cycles. Experimental results are shown in Figure 6a. In the cyclic experiment, the photocatalytic performance of Ag 3 PO 4 obviously decreased, and the degradation rate of RhB for the fifth time was only 15%. The degradation rate of RhB by the TCN/Ag 3 PO 4 heterojunction for the fifth time was 85%. Results showed that the TCN/Ag 3 PO 4 heterojunction exhibited excellent stability. The samples of Ag 3 PO 4 and the TCN/Ag 3 PO 4 heterojunction after the cycle experiment were analyzed by X-ray diffraction, and results are shown in Figure 6b. In the XRD of Ag 3 PO 4 , a new diffraction peak appeared at 38.1 • , which is the (111) plane diffraction peak of Ag [38]. Results showed that Ag 3 PO 4 was prone to photoelectron corrosion and became unstable during photocatalytic reaction. There was no new diffraction peak in the XRD of the TCN/Ag 3 PO 4 heterojunction. Results showed that the TCN/Ag 3 PO 4 heterojunction maintained good stability in the cyclic experiment. shown in Figure 6a. In the cyclic experiment, the photocatalytic performance of Ag3PO4 obviously decreased, and the degradation rate of RhB for the fifth time was only 15%. The degradation rate of RhB by the TCN/Ag3PO4 heterojunction for the fifth time was 85%. Results showed that the TCN/Ag3PO4 heterojunction exhibited excellent stability. The samples of Ag3PO4 and the TCN/Ag3PO4 heterojunction after the cycle experiment were analyzed by X-ray diffraction, and results are shown in Figure 6b. In the XRD of Ag3PO4, a new diffraction peak appeared at 38.1°, which is the (111) plane diffraction peak of Ag [38]. Results showed that Ag3PO4 was prone to photoelectron corrosion and became unstable during photocatalytic reaction. There was no new diffraction peak in the XRD of the TCN/Ag3PO4 heterojunction. Results showed that the TCN/Ag3PO4 heterojunction maintained good stability in the cyclic experiment.

Photocatalytic Mechanism of TCN/Ag3PO4 Heterojunction
To further investigate the photocatalytic reaction mechanism of the TCN/Ag3PO4 heterojunction, the TCN/Ag3PO4-5% heterojunction was used as photocatalyst, and different scavengers were selected to carry out the photodegradation experiment. Among them, ethylenediaminetetraacetic acid (EDTA) was the electron hole (h + ) scavenger [39], benzoquinone (BQ) was the superoxide radical ( • O2 − ) scavenger [40], and isopropanol (IPA) was the hydroxyl radical (•OH) scavenger [41], with a concentration of 1 mmol/L. The effects of various scavengers on the degradation rate of RhB are shown in Figure 7.

Photocatalytic Mechanism of TCN/Ag 3 PO 4 Heterojunction
To further investigate the photocatalytic reaction mechanism of the TCN/Ag 3 PO 4 heterojunction, the TCN/Ag 3 PO 4 -5% heterojunction was used as photocatalyst, and different scavengers were selected to carry out the photodegradation experiment. Among them, ethylenediaminetetraacetic acid (EDTA) was the electron hole (h + ) scavenger [39], benzoquinone (BQ) was the superoxide radical ( • O 2 − ) scavenger [40], and isopropanol (IPA) was the hydroxyl radical (•OH) scavenger [41], with a concentration of 1 mmol/L. The effects of various scavengers on the degradation rate of RhB are shown in Figure 7. shown in Figure 6a. In the cyclic experiment, the photocatalytic performance of Ag3PO4 obviously decreased, and the degradation rate of RhB for the fifth time was only 15%. The degradation rate of RhB by the TCN/Ag3PO4 heterojunction for the fifth time was 85%. Results showed that the TCN/Ag3PO4 heterojunction exhibited excellent stability. The samples of Ag3PO4 and the TCN/Ag3PO4 heterojunction after the cycle experiment were analyzed by X-ray diffraction, and results are shown in Figure 6b. In the XRD of Ag3PO4, a new diffraction peak appeared at 38.1°, which is the (111) plane diffraction peak of Ag [38]. Results showed that Ag3PO4 was prone to photoelectron corrosion and became unstable during photocatalytic reaction. There was no new diffraction peak in the XRD of the TCN/Ag3PO4 heterojunction. Results showed that the TCN/Ag3PO4 heterojunction maintained good stability in the cyclic experiment.

Photocatalytic Mechanism of TCN/Ag3PO4 Heterojunction
To further investigate the photocatalytic reaction mechanism of the TCN/Ag3PO4 heterojunction, the TCN/Ag3PO4-5% heterojunction was used as photocatalyst, and different scavengers were selected to carry out the photodegradation experiment. Among them, ethylenediaminetetraacetic acid (EDTA) was the electron hole (h + ) scavenger [39], benzoquinone (BQ) was the superoxide radical ( • O2 − ) scavenger [40], and isopropanol (IPA) was the hydroxyl radical (•OH) scavenger [41], with a concentration of 1 mmol/L. The effects of various scavengers on the degradation rate of RhB are shown in Figure 7.   The photocatalytic efficiency of RhB decreased significantly after the addition of EDTA and BQ, and the degradation rate of RhB was 39% and 45% after 40 min reaction, respectively. Results showed that h + and •O 2 − were captured by EDTA and BQ, respectively, so the number of h + and •O 2 − in the photocatalytic reaction was greatly reduced, resulting in a lower degradation rate of RhB. When IPA was added, the degradation rate of RhB was 80% after 40 min reaction. The IPA mainly captured •OH, thus slightly decreasing the degradation rate of RhB. It is clear that h + and •O 2 − were the main active species for RhB degradation, while •OH only played a minor role. According to previous reports [42,43], the valence band (VB) and conduction band (CB) potentials of Ag 3 PO 4 were about 2.69 and 0.24 eV, which are both more positive than those of g-C 3 N 4 (−1.15 and 1.60 eV), respectively. According to traditional heterojunction theory, the CB of g-C 3 N 4 is more negative than that of Ag 3 PO 4 , and the electrons generated after photoexcitation migrate from the CB of g-C 3 N 4 to the CB of Ag 3 PO 4 . The VB of Ag 3 PO 4 is more positive than that of g-C 3 N 4 , and holes generated after photoexcitation migrate from the VB of Ag 3 PO 4 to the VB of g-C 3  , the holes that migrated to CB of g-C 3 N 4 could not oxidize water to produce •OH. This is not consistent with the results of photocapture experiments in which •OH is also an active species. According to the experimental results of the stability and capture of the TCN/Ag 3 PO 4 heterojunction, a possible Z-type photocatalytic mechanism is proposed, as shown in Figure 8. The potential position of the VB of g-C 3 N 4 was close to the CB of Ag 3 PO 4 , so it was easy to combine the holes in the VB and the electrons in the CB. Therefore, the electrons in the VB of g-C 3 N 4 and the holes in the VB of Ag 3 PO 4 were effectively separated. The electrons in the CB of g-C 3 N 4 would react with the dissolved O 2 to produce •O 2 − , which could directly oxidize RhB. Meanwhile, the holes in the VB of Ag 3 PO 4 can also directly oxidize RhB. Its high valence band potential of +2.69 eV could oxidize water to produce •OH, and •OH also had a strong oxidation capacity and can oxidize RhB. The Z-type mechanism is consistent with the experimental results of the photostabilization and photocapture of the heterojunction catalyst. The photocatalytic efficiency of RhB decreased significantly after the addition of EDTA and BQ, and the degradation rate of RhB was 39% and 45% after 40 min reaction, respectively. Results showed that h + and •O2 − were captured by EDTA and BQ, respectively, so the number of h + and •O2 − in the photocatalytic reaction was greatly reduced, resulting in a lower degradation rate of RhB. When IPA was added, the degradation rate of RhB was 80% after 40 min reaction. The IPA mainly captured •OH, thus slightly decreasing the degradation rate of RhB. It is clear that h + and •O2 − were the main active species for RhB degradation, while •OH only played a minor role. According to previous reports [42,43], the valence band (VB) and conduction band (CB) potentials of Ag3PO4 were about 2.69 and 0.24 eV, which are both more positive than those of g-C3N4 (−1.15 and 1.60 eV), respectively. According to traditional heterojunction theory, the CB of g-C3N4 is more negative than that of Ag3PO4, and the electrons generated after photoexcitation migrate from the CB of g-C3N4 to the CB of Ag3PO4. The VB of Ag3PO4 is more positive than that of g-C3N4, and holes generated after photoexcitation migrate from the VB of Ag3PO4 to the VB of g-C3N4. •O2 − /O2 potential (-0.33 eV) is more negative than that CB of Ag3PO4, so electrons migrating to the CB of Ag3PO4 cannot react with oxygen to produce •O2 − . This is not consistent with the results of photocapture experiments in which •O2 − is the main active species. A large number of electrons gathering on the crystal surface of Ag3PO4 lead to the photocurrent corrosion of Ag3PO4. This is inconsistent with the experimental results of the stability of the TCN/Ag3PO4 heterojunction. Second, because the potential of the VB of g-C3N4 was more negative than the potential of •OH/H2O (2.27 eV), the holes that migrated to CB of g-C3N4 could not oxidize water to produce •OH. This is not consistent with the results of photocapture experiments in which •OH is also an active species. According to the experimental results of the stability and capture of the TCN/Ag3PO4 heterojunction, a possible Z-type photocatalytic mechanism is proposed, as shown in Figure 8. The potential position of the VB of g-C3N4 was close to the CB of Ag3PO4, so it was easy to combine the holes in the VB and the electrons in the CB. Therefore, the electrons in the VB of g-C3N4 and the holes in the VB of Ag3PO4 were effectively separated. The electrons in the CB of g-C3N4 would react with the dissolved O2 to produce •O2 − , which could directly oxidize RhB. Meanwhile, the holes in the VB of Ag3PO4 can also directly oxidize RhB. Its high valence band potential of +2.69 eV could oxidize water to produce •OH, and •OH also had a strong oxidation capacity and can oxidize RhB. The Z-type mechanism is consistent with the experimental results of the photostabilization and photocapture of the heterojunction catalyst.