English

• In recently years, water pollution caused by tetracycline (TC) and industry organism dye has induced serious threat to the health of human beings. Various technologies such as ion exchange, photocatalysis [1, 2], adsorption [3], have been used to deal with water pollution problems. Among them, photocatalytic technology has been widely concerned because of its simple and low-cost and potential applications in environmental protection and energy production [4, 5]. A large number of semiconductor metal oxide and sulphide photocatalysts, including TiO₂ [6, 7], ZnO [8, 9], WO₃ [10], CdS [11, 12], In₂S₃ [13, 14], Ag₃PO₄ [15], have been fabricated to develop efficient photocatalysts. As a photocatalyst, Ag₃PO₄ has been paid much more attention due to suitable band gap (2.36 eV) and the excellent photocatalytic property for the degradation of organic pollutants. However, some problems, including the high solubility, the photo-corrosion, instability and the fast recombination of photo-generated carriers, need to be solved. Many effective ways have been reported to improve the photocatalytic activity and stability of Ag₃PO₄, including size and morphology control, surface modification, constructing heterostructure/composite with other compounds [15, 16]. Heterogeneous photocatalytic systems, such as Ag₃PO₄/Ag₂S [15], Ag₃PO₄/In₂S₃ [17], Ag₃PO₄/PANI [18], Ag₃PO₄/CuBi₂O₄ [16], carbon dots [19], are considered as effective strategies for the dispose of TC and dyes, which can suppress the photocorrsion, prompt migration and separation of the photogenerated electrons and holes.

  Graphite carbon nitride (g-C₃N₄, 2.7 eV), as a nonmetallic polymeric material with high thermal, chemical stability and electronic properties, can have good visible light response [20, 21], which has much more promising for practical application. Xia et al. [22] fabricated the multifunctional photocatalyst of Mn/g-C₃N₄/ZSM-4, then combined with vacuum ultraviolet irradiation to eliminate O₃ byproduct and enhance toluene degradation via ozone assisted catalytic oxidation. Zhang et al. [23] tuned the dimensions and structures of nitrogen doped carbon nanomaterials derived from sacrificial g-C₃N₄/metal–organic frameworks to enhance electrocatalytic oxygen reduction. Moreover, g-C₃N₄ nanosheets can be used as a coating material due to its flexibility. Chen et al. [24] reported that the stability and photocatalytic performance of ZnO could be enhanced by coating g-C₃N₄ nanosheets as shell layer, which could be attributed to the hybridized effect and the core–shell structure induced by the match of lattice and energy level between the C₃N₄ shell and ZnO core. However, the photocatalytic efficiency of g-C₃N₄ is still not highly attributing to the high recombination rate of the photogenerated electron–hole pairs. To improve the separation efficiency of photogenerated carriers, heterojunction photocatalyst between g-C₃N₄ and other materials should be constructed. Considering the excellent photooxidation activity of Ag₃PO₄, g-C₃N₄/Ag₃PO₄ heterostructures have been fabricated and reported [25-29]. Liu et al. [29] reported the degradation of methylene blue (MB) over Ag₃PO₄@g-C₃N₄ hybrid core@shell composite, which presented the better photocatalytic activity. Ma et al. [30] reported nanocomposite of exfoliated bentonite/g-C₃N₄/Ag₃PO₄ for enhanced visible-light photocatalytic decomposition of RhB. However, the previously reported g-C₃N₄/Ag₃PO₄ composites still suffer the low efficiency and poor stability.

  In this work, 2D g-C₃N₄ nanosheets were served as support to fabricate g-C₃N₄/Ag₃PO₄ composite photocatalyst by calcination and solutions precipitating method. Their photocatalytic activities are evaluated by the degradation of three different kinds of pollutants (MO, RhB, TC) under visible light irradiation. Moreover, the effects of pH and coexisting ions are also investigated. The photocurrent response and electrochemical impedance spectra (EIS) performance of the g-C₃N₄/Ag₃PO₄ heterostructure is also discussed, and the possible mechanism of the photocatalytic activity is also proposed.

  The XRD patterns of the as-prepared samples are displayed in Fig. 1. For pure g-C₃N₄ nanosheets, two characteristic diffraction peaks at 13.2° and 27.6° are observed, which correspond to the (100) and (002) planes of g-C₃N₄ [31], respectively. For the bare Ag₃PO₄, all the diffraction peaks can be assigned to the cubic phase of Ag₃PO₄ without any impurities (JCPDS: 06-0505). Compared with bare Ag₃PO₄, the weak characteristic diffraction peaks of g-C₃N₄ are found in the XRD pattern of g-C₃N₄/Ag₃PO₄ composite, which demonstrate the existence of both g-C₃N₄ and Ag₃PO₄, further indicating that the successful formation of g-C₃N₄/Ag₃PO₄ binary heterojunction composite.

  Figure 1

  

  Figure 1.  XRD patterns of the as-prepared g-C₃N₄ nanosheets, Ag₃PO₄, g-C₃N₄/Ag₃PO₄ composite.

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  Fig. 2a shows that the as-fabricated C₃N₄ has sheet morphology with large size and very small thickness. Form Fig. 2b, it can be observed that Ag₃PO₄ particles have an irregular spherical morphology with smooth surface and a tendency to aggregate, and the size of particles is about 0.1–0.7 μm. For g-C₃N₄/Ag₃PO₄ composite, Ag₃PO₄ particles are distinctly anchored on the surface of g-C₃N₄ nanosheets, also some particles are enwrapped with g-C₃N₄ nanosheets, as shown in Figs. 2c–e. The high-resolution TEM image of C₃N₄/Ag₃PO₄ composite in Fig. 2f clearly shows the crystalline lattice fringes with the interplanar distance of 0.239 nm, which corresponds to the (211) plane of cubic Ag₃PO₄ phase. Moreover, it confirms that g-C₃N₄ nanosheet can act as a support and surfactant to constrain Ag₃PO₄ particles in the composite. The elemental mappings in Fig. S1 (Supporting information) reveal the coexistence of five elements, including C, N, Ag, P and O. The uniform distributions of C and N elements among the sample indicate the presence of g-C₃N₄ nanosheets. Furthermore, the mapping images also verify that Ag, P and O elements form clusters with the uniform size and shape as SEM images confirmed, indicating that Ag₃PO₄ particles uniformly disperse on the surfaces of g-C₃N₄. The above-mentioned elements in g-C₃N₄/Ag₃PO₄ composite are also confirmed by EDX measurement (Fig. S1).

  Figure 2

  

  Figure 2.  SEM images of (a) bare g-C₃N₄ nanosheets, (b) Ag₃PO₄, (c, d) g-C₃N₄/Ag₃PO₄ composite. (e) TEM image and (f) high-resolution TEM image of g-C₃N₄/Ag₃PO₄ composite.

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  Five elements, C, N, Ag, P and O, can be observed in the survey spectrum in Fig. 3a, which is consistent with the result of EDS. For C 1s spectrum in Fig. 3b, two peaks at about 284.6 eV and 288.0 eV are present, which can be ascribed to sp² C-C bonds in graphitic or amorphous carbons and carbon atoms with three nitrogen atoms (C-N₃) bonds in g-C₃N₄, respectively [32]. The N 1s spectrum (Fig. 3c) can be deconvoluted into three peaks at 398.3 eV, 399.0 eV and 400.9 eV, respectively. The peak at 398.3 eV is attributed to sp²-hybridized nitrogen (C=N-C), suggesting the existent of g-C₃N₄. The other two peaks located at 399.0 eV, 400.9 eV are tertiary nitrogen (N(C)₃) groups and amino functions (C-N-H) [27, 33], respectively. Three peaks at 530.86 eV, 531.74 eV, 532.92eV for O 1s in Fig. 3d are ascribed to O₂- in Ag₃PO₄ [27], the surface OH-groups and the surface adsorbed H₂O molecules [19], respectively. Fig. 3e shows the binding energies of the Ag 3d_5/2 and 3d_3/2 peaks at 368.10 eV and 374.02 eV, which are usually ascribed to Ag⁺ in Ag₃PO₄. Furthermore, a broad peak centered at 132.69 eV of the P 2p spectrum (Fig. 3f) is observed for the composite, which is in consistent with P⁺⁵ from PO₄³⁻ [19].

  Figure 3

  

  Figure 3.  (a) XPS survey spectrum, XPS spectra of (b) C 1s, (c) N 1s, (d) O 1s, (e) Ag 3d and (f) P 2p for g-C₃N₄/Ag₃PO₄ composite.

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  The UV–vis diffuse reflectance spectra of g-C₃N₄, Ag₃PO₄ and g-C₃N₄/Ag₃PO₄ in Fig. S2a (Supporting information) shows that the light absorption edges of pure g-C₃N₄ and Ag₃PO₄ are located at about 430 nm and 530 nm, corresponding to band gap of 2.88 eV and 2.45 eV, in consistent with the reported result [19]. After the combination of g-C₃N₄ with Ag₃PO₄, the band gap absorption edge of composite displays red shifted compared with g-C₃N₄, implying the enhanced absorption ability of the composite in the visible light region. Therefore, more light will be absorbed and more electrons and holes are generated for the g-C₃N₄/Ag₃PO₄ composite. Fig. S2b (Supporting information) exhibits the PL spectra of g-C₃N₄ and the g-C₃N₄/Ag₃PO₄ composite with the excitation at 350 nm. The PL intensity of g-C₃N₄/Ag₃PO₄ composite is weaker than the pure g-C₃N₄, which suggests that the combination of g-C₃N₄ with Ag₃PO₄ can effectively inhibit recombination of photoexcited electron-holes pairs.

  To investigate the potential applicability of the composites, the photocatalytic properties of the samples were investigated by monitoring the degradation of MO, RhB and TC aqueous solution under visible light. As shown in Fig. 4a, the adsorption-desorption equilibrium between MO and photocatalysts will be reached within 90 min in the dark. 45.2% of MO (10 mg/L) will be adsorbed by g-C₃N₄/Ag₃PO₄ composites, which are far above those of pure g-C₃N₄ nanosheets and Ag₃PO₄ particles, suggesting that g-C₃N₄/Ag₃PO₄ composites have superior adsorptive ability. The blank experiment indicates that only little MO is degraded in the absence of photocatalyst, thus it can be negligible. Further, 95% of MO is degraded in the presence of g-C₃N₄/Ag₃PO₄ composite after 30 min. In comparison, only 10.0% and 39.1% are degraded for the g-C₃N₄ nanosheets and Ag₃PO₄ particles, respectively. After adsorption equilibrium for 60 min, more than 28.6% of RhB is adsorbed by g-C₃N₄/Ag₃PO₄ composite, as shown in Fig. 4c. The adsorptions of RhB on pure Ag₃PO₄ and pure g-C₃N₄ are 3.3% and 35.6%, respectively. More than 96% of RhB (10 mg/L) is degraded in the presence of g-C₃N₄/Ag₃PO₄ composite under visible irradiation for 15 min. The experiment results further prove that the g-C₃N₄/Ag₃PO₄ composites have superior adsorptive and photodegradation ability compared with the pure Ag₃PO₄ and pure g-C₃N₄ nanosheets. To further understand the reaction kinetics of the MO and RhB degradation catalyzed by various photocatalysts, the experimental data are fitted by a pseudo-first-order kinetics model as follows [16]:

  Figure 4

  

  Figure 4.  (a) Relative concentration (C/C₀) of MO (10 mg/L) versus time under visible light irradiation. (b) The corresponding plots of ln(C/C₀) versus time of (a). (c) Relative concentration (C/C₀) of RhB (10 mg/L) versus time. (d) the corresponding plots of ln(C/C₀) versus time of (c). (e) Photocatalytic degradation of 50 mg/L TC by as-fabricated photocatalysts. (f) The pseudo-first-order reaction kinetics for the degradation of 50 mg/L TC with prepared samples.

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  where C₀ represents the initial concentration of dye and C refers to the concentration of dye at different irradiation times t, and k is the reaction rate constant. Obviously, the calculated k for the degradation of MO and RhB over g-C₃N₄/Ag₃PO₄ composite is larger than those calculated for the reactions over pure g-C₃N₄ nanosheets and Ag₃PO₄, as shown in Figs. 4b and d. Especially, the k value for the degradation of MO over g-C₃N₄/Ag₃PO₄ composite (0.1187 min⁻¹) is about 7.67 folds compared with that of Ag₃PO₄ (0.0155 min⁻¹). Likewise, the k value for the degradation of RhB over g-C₃N₄/Ag₃PO₄ composite (0.18482 min⁻¹) is about 2.20 folds compared with that of Ag₃PO₄ (0.08405 min⁻¹). To further understand the photocatalytic activity of the fabricated samples, the degradations of TC are performed. As seen from Fig. 4e, the adsorption-desorption equilibrium between TC and photocatalysts can be achieved within 60 min in the dark. Meanwhile, the adsorption ability of g-C₃N₄/Ag₃PO₄ composite exhibits higher compared with that of pure g-C₃N₄ nanosheets and Ag₃PO₄ particles, which is consistent with the result of degradations of MO and RhB. Moreover, the blank experiment suggests that the self-degradation of TC can be ignored, and pure g-C₃N₄ nanosheets and Ag₃PO₄ show poor photocatalytic activities, in which merely about 12.6% and 65.8% of TC are degraded in 30 min under visible light irradiation, respectively. While for g-C₃N₄/Ag₃PO₄ composite, nearly 80% of TC can be degraded within 30 min. Fig. 4f shows the preudo-first-order reaction kinetics for the degradation of 50 mg/L TC with prepared samples, which display that the k value for the degradation of TC over g-C₃N₄/Ag₃PO₄ composite (0.0897 min⁻¹) is about 1.9 folds and 21.8 folds compared with those of Ag₃PO₄ (0.0473 min⁻¹) and g-C₃N₄ nanosheets (0.0041 min⁻¹), respectively. The experimental results suggest that the introduction of the Ag₃PO₄ on the g-C₃N₄ nanosheets facilitates photogenerated carrier, thus significantly improving the photocatalytic activity.

  In the actual wastewater system, there are some coexisting ions, and the pH value are different, so the impact of inorganic ions and pH values on the degradation performance should be considered [34, 35]. For mimic natural wastewater, various inorganic ions with the concentration of 0.1 mol/L, including Na⁺, Cl⁻, SO₄^2-, NO₃⁻, CO₃^2-, are added to the TC solution to investigate the catalytic efficiency of g-C₃N₄/Ag₃PO₄ catalytic system, and the results are shown in Fig. 5a. No obvious difference can be observed in the degradation curves of the TC solutions containing Na₂CO₃, NaNO₃, Na₂SO₄, suggesting the universal photocatalytic effects of g-C₃N₄/Ag₃PO₄ composite in different environment. However, the catalytic efficiency toward TC degradation decreases a little in the presence of NaCl. The photocatalyst g-C₃N₄/Ag₃PO₄ after reaction is further characterized with XRD as displayed in Fig. S3 (Supporting information), in which no diffraction peaks of g-C₃N₄/Ag₃PO₄ can be observed, suggesting that the transformation of g-C₃N₄/Ag₃PO₄ composite photocatalyst to other substances with the introduction of NaCl. The appearance of characteristic peaks of AgCl (JCPDS: 31-1238) imply that the crystal structure of g-C₃N₄/Ag₃PO₄ has been transformed into AgCl. Therefore, photocatalytic activity is reduced because of the destroyed crystal structure of Ag₃PO₄ [36]. The influence of pH values on the degradation toward TC is displayed in Fig. 5b, when the pH values are within the range of 2.87–9.24, the g-C₃N₄/Ag₃PO₄ composite show excellent photodegradation activities towards TC, only the photodegradation efficiency at the pH values of 9.24 reduces a little. The results suggest that g-C₃N₄/Ag₃PO₄ can be used as an efficient photocatalyst for practical application in wastewater treatment.

  Figure 5

  

  Figure 5.  (a) Photodegradation curves of TC (50 mg/L) with different inorganic salts. (b) Photodegradation curves of TC at different pH condition using g-C₃N₄/Ag₃PO₄ composite as photocatalysts.

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  In order to study the photocatalytic stability of the as-fabricated samples, recycle experiments of MO degradation over g-C₃N₄ nanosheets, Ag₃PO₄ and the g-C₃N₄/Ag₃PO₄ composite were carried out, as shown in Fig. 6a. For the bare Ag₃PO₄, the photocatalytic efficiency decreases from 64.5% to 29.1% after five cycles, indicating the bare Ag₃PO₄ photocatalyst is unstable. The significant reduction of the photocatalytic performance of Ag₃PO₄ can be attributed to the partial reduction of Ag⁺ into Ag⁰ by photogenerated electrons [35]. In contrast, the degradation of g-C₃N₄/Ag₃PO₄ composite remains high (~93%) across the repeated cycles, with a slight decrease to 88% observed after five recycling runs. The effective energy level structure facilitates the separation of the electron-hole pairs quickly, thus obviously improving the photoactivity and photostability of catalyst. The result is in accord with the analysis of PL (Fig. S2 in Supporting information), transient photocurrent responses (Fig. S4a in Supporting information) and EIS measurement (Fig. S4b in Supporting information). Furthermore, the XRD of the as-fabricated is compared with that used g-C₃N₄/Ag₃PO₄ composite after 5 cycles, as Fig. 6b shows. After photocatalytic reaction, no crystal structure variation of the composite is observed. A rather weak characteristic diffraction peaks at 38.1° is observed and attributed to the (111) crystal planes of metallic silver, indicating that only a small quantity of Ag generated in the photodegradation process. The appearance of a small amount of Ag serves as the transmission bridge of carrier, thus the carrier transfer rate and the separation of electron-hole pairs can be improved by the Schottky barriers at the metal-semiconductor interfaces. Furthermore, the localized surface plasmon resonance of Ag nanoparticles on catalyst can also promote the photocatalytic efficiency. The results indicate that high photocatalytic efficiency and stability of the g-C₃N₄/Ag₃PO₄ composite, which are important attribution for their practical applications in real-life for eliminating organic pollutants from wastewater.

  Figure 6

  

  Figure 6.  (a) Recycling tests for the photocatalytic MO degradation in the presence of g-C₃N₄/Ag₃PO₄ composite photocatalyst under visible-light irradiation. (b) XRD pattern of the as-fabricated and the used g-C₃N₄/Ag₃PO₄ composite.

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  As is known that the three major reactive species, superoxide radical (^.O₂⁻), hole (h⁺) and hydroxyl radical (^.OH) play important role in photocatalytic reaction process. In order to investigate the dominant active species in the photocatalytic process, the trapping experiments were carried out. As displayed in Fig. S5 (Supporting information), the photocatalytic activities of g-C₃N₄/Ag₃PO₄ composite toward the degradation of MO are investigated in the presence of benzoquinone (BZQ), ethylenediaminetetraacetic acid (EDTA) and isopropanol (IPA) employed to act as ^.O₂⁻, h⁺ and ^.OH scavengers, respectively. Obviously, the introduction of BZQ and EDTA into the photocatalyst can basically affect the photodegradation efficiencies of MO, the photocatalytic degradation is greatly quenched, which suggests that ^.O₂^- and h⁺ are the main reactive species in the g-C₃N₄/Ag₃PO₄ composite photocatalytic process. In contrast, the photocatalytic degradation of MO is inhibited slightly by introducing IPA, suggesting that the ^.OH also has small effect on the photocatalytic efficiency.

  On the basis of the above experimental results and characterization analysis, a possible photocatalytic mechanism of the g-C₃N₄/Ag₃PO₄ composite photocatalyst is proposed to follow a Z-scheme mechanism, as illustrated in Fig. 7. According to the previous reports [16, 28, 29, 37, 38], the valence band (VB) and conduction band (CB) potentials of Ag₃PO₄ are about 2.67 and 0.27 eV, which is both more positive than that of g-C₃N₄ (1.60 and −1.15 eV), respectively. Under visible light irradiation, the electrons in the VB of both g-C₃N₄ and Ag₃PO₄ are excited to the CB, leaving holes in the VB. The CB of Ag₃PO₄ is more negative than the Fermi level of metallic Ag, and the VB of g-C₃N₄ is more positive than the Fermi level of Ag, thus the photo-excited electrons in the CB of Ag₃PO₄ is more likely to be transferred to the metallic silver. Meanwhile, the photogenerated holes in VB of g-C₃N₄ are transferred to Ag and combined with electrons. Then, the electrons left in CB of g-C₃N₄ can combine with the dissolved oxygen molecules and produce the superoxide radical anions (^.O₂⁻), which play an important role in overall photocatalytic reaction. In addition, the photogenerated holes in VB of Ag₃PO₄ could directly oxidize dye. Of course, part of photogenerated holes can combine with OH⁻ to produce active ^.OH. Thus, the photocatalytic activity of g-C₃N₄/Ag₃PO₄ composite are mainly attributed to the generated ^.O₂⁻ and h⁺ active species, meanwhile, parts of ^.OH can also degrade dye and TC. Therefore, Ag nanoparticles existed in the interface of g-C₃N₄ and Ag₃PO₄ might act as a charge transmission bridge to efficiently improve the separation of electron-hole pairs. Considering strong absorption of the degradation substrates (MO and RhB) in the region of the illumination, the photosensitization also plays certain function in the photocatalytic process. Dyes can be excited by visible light and produce the excited state of the dye*, electron will inject from the adsorbed dye* species to the semiconductor conduction band (and/or surface states), also the oxidized dye species (dye+) can be produced when electrons are lost. The injected electron can combine with the surface-adsorbed O₂ molecules to produce the ^.O₂⁻, in competition with back electron transfer to the dye⁺ radical cation. However, the former process is usually two/three-order faster [[39], [40], [41]]. The superoxide radical anions can be involved in photocatalytic degradation, thus the photosensitization partially works in the photocatalytic process.

  Figure 7

  

  Figure 7.  Schematic diagram of photogenerated electron-hole separation process between g-C₃N₄ and Ag₃PO₄.

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  In summary, g-C₃N₄/Ag₃PO₄ photocatalysts were successfully constructed by in situ deposition of Ag₃PO₄ onto 2D g-C₃N₄ nanosheets through precipitating method. The photocatalytic results showed that the g-C₃N₄/Ag₃PO₄ composite had obviously superior photocatalytic activity and photostability over that of pure Ag₃PO₄ particles and g-C₃N₄ nanosheets for the photodegradation of MO, RhB and TC under visible light irradiation. The improved photocatalytic activity can be ascribed to the efficient separation of photoinduced electron–hole pairs. The photocatalytic process of the g-C₃N₄/Ag₃PO₄ photocatalysts were further supported by transient photocurrent response, PL spectrum, active species trapping experiment results. The work could supply insight into fabricating novel photocatalysts with high efficiency and reusability for important applications in elimination organic pollutants from wastewater.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 61504048, 51672109, 21707043), Natural Science Foundation of Shandong Province for Excellent Young Scholars (Nos. ZR2016JL015, ZR2017BEE005).

  Appendix A. Supplementary data

  Supplementary material related to this article canbefound, in the online version, at doi: https://doi.org/10.1016/j.cclet.2019.05.029.

  
  
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  Figure 1  XRD patterns of the as-prepared g-C₃N₄ nanosheets, Ag₃PO₄, g-C₃N₄/Ag₃PO₄ composite.

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  Figure 2  SEM images of (a) bare g-C₃N₄ nanosheets, (b) Ag₃PO₄, (c, d) g-C₃N₄/Ag₃PO₄ composite. (e) TEM image and (f) high-resolution TEM image of g-C₃N₄/Ag₃PO₄ composite.

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  Figure 3  (a) XPS survey spectrum, XPS spectra of (b) C 1s, (c) N 1s, (d) O 1s, (e) Ag 3d and (f) P 2p for g-C₃N₄/Ag₃PO₄ composite.

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  Figure 4  (a) Relative concentration (C/C₀) of MO (10 mg/L) versus time under visible light irradiation. (b) The corresponding plots of ln(C/C₀) versus time of (a). (c) Relative concentration (C/C₀) of RhB (10 mg/L) versus time. (d) the corresponding plots of ln(C/C₀) versus time of (c). (e) Photocatalytic degradation of 50 mg/L TC by as-fabricated photocatalysts. (f) The pseudo-first-order reaction kinetics for the degradation of 50 mg/L TC with prepared samples.

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  Figure 5  (a) Photodegradation curves of TC (50 mg/L) with different inorganic salts. (b) Photodegradation curves of TC at different pH condition using g-C₃N₄/Ag₃PO₄ composite as photocatalysts.

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  Figure 6  (a) Recycling tests for the photocatalytic MO degradation in the presence of g-C₃N₄/Ag₃PO₄ composite photocatalyst under visible-light irradiation. (b) XRD pattern of the as-fabricated and the used g-C₃N₄/Ag₃PO₄ composite.

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  Figure 7  Schematic diagram of photogenerated electron-hole separation process between g-C₃N₄ and Ag₃PO₄.

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