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Article

Efficient Visible-Light-Responsive Ag3PO4/g-C3N4/Hydroxyapatite Photocatalyst (from Oyster Shells) for the Degradation of Methylene Blue: Preparation, Properties and Mechanism

1
Marine College, Shandong University, Weihai 264209, China
2
Rushan Huaxin Foods Group, Rushan 264509, China
3
Weihai Ecological Environment Monitoring Center of Shandong Province, Weihai 264299, China
*
Authors to whom correspondence should be addressed.
Submission received: 2 January 2022 / Revised: 13 January 2022 / Accepted: 16 January 2022 / Published: 19 January 2022

Abstract

:
A novel ternary Ag3PO4/g-C3N4/hydroxyapatite photocatalyst was prepared, and its morphology, composition and structure were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, and electron spin resonance, etc. The results show that g-C3N4 is evenly dispersed in the interior of hydroxyapatite, forming a homogeneous composite, and significantly improves the band gap structure of the material as a whole. Ag3PO4/g-C3N4/hydroxyapatite has good electron transfer ability and an appropriate energy band structure, which shows that the material has a good degradation effect and stability. Finally, based on the characterization and experimental results, a possible Z-scheme mechanism was proposed, and the active species involved in the reaction are mainly ·O2 and h+.

1. Introduction

Environmental pollution has become an important factor affecting global development and human health [1]. Among various forms of pollution, the discharge of sewage will directly lead to the destruction of the ecological environment and promote the accumulation and transfer of harmful substances in organisms. Therefore, the treatment of pollutants in water is particularly important [2]. At present, the widely used methods of wastewater treatment include adsorption [3], biological [4], membrane separation [5], and electrochemical methods [6], which have various degrees of effectiveness. However, these methods also have some disadvantages, such as a slow process, high cost, and high power consumption. In particular, some methods enrich pollutants, and there are secondary pollution problems. In recent decades, photocatalytic technology, as a green and sustainable wastewater treatment technology, has been introduced into water pollution treatment; its unique advantages [7] include a high treatment efficiency, green environmental characteristics and the ability to use renewable solar energy. Therefore, a variety of photocatalytic materials have been developed, such as TiO2 [8], CdS [9], Bi2MoO6 [10], and perovskite materials [11].
Among photocatalysts, silver catalysts are important, such as Ag3PO4 [12], AgVO3 [13], Ag2CO3 [14], and Ag2O [15]. Among them, Ag3PO4 has attracted much attention for its high quantum yield, but its photocorrosion phenomenon limits its stability [16]. Therefore, it is still a great challenge to find an appropriate cocatalyst to improve the photocatalytic activity and stability of Ag3PO4 [17]. In recent years, graphite-phase carbon nitride (g-C3N4) has become a strong candidate for water purification materials due to its simple preparation method, stable structure and good visible light response. However, the photocatalytic efficiency of g-C3N4 is not ideal due to the high recombination rate of photogenerated electrons and holes [18]. Therefore, many composite materials have been prepared, such as SnO2/g-C3N4 [19] and Pt/g-C3N4/BiOBr [20]. Among these materials, g-C3N4 can form a heterojunction structure with other components, thus improving the efficiency and stability of photocatalytic reactions.
Compared with Ag3PO4 and g-C3N4, the application of hydroxyapatite (HAP, Ca10(PO4)6(OH)2) materials in photocatalysis has received little attention. Although HAP has a wide band gap and poor visible light absorption performance, it can be used as an excellent carrier and cocatalyst [21], especially because it has excellent exchange performance, high adsorption performance and the same anion (PO43−) as Ag3PO4. All these characteristics provide a strong guarantee for the potential to combine silver phosphate and HAP, and the previous research results of our team also confirmed this view [22]. Although Ag3PO4 composite materials have been studied, ternary Ag3PO4, g-C3N4 and HAP composite materials have not been reported. In this study, a new Ag3PO4/g-C3N4/HAP composite was successfully synthesized by using discarded oyster shells (commonly found at the seaside) as raw materials. According to the structure and properties of the materials, the active species involved in the photocatalytic reaction were explored, and a possible mechanism was proposed. The experimental results showed that the Ag3PO4/g-C3N4/HAP composite exhibited good pollutant photodegradation performance due to its unique structure and properties. This work can provide some guidance for the effective treatment of industrial wastewater and the application of waste oyster shells.

2. Results and Discussion

2.1. XRD Analysis

Figure 1 shows the XRD patterns of different samples. Figure 1a shows the diffraction patterns of HAP, CN, and their composites. A distinct diffraction peak appears at 27.4°, corresponding to the characteristic (002) plane from the interlayer-stacking of conjugated aromatic systems in graphitic carbon nitride [23]. The diffraction peak of the (211) crystal plane of HAP was observed at 31.77°, and the whole spectrum corresponds to the HAP standard spectrum (JCPDS PDF #09-0432) [24,25] indicating that HAP with high purity can be successfully synthesized from oyster shells. The main inorganic component in oyster shells is CaCO3, which accounts for more than 90% of the total mass of oyster shells. In the reaction mentioned in this paper, calcium carbonate in oyster shell is dissolved by acetic acid to form calcium acetate solution. After the addition of disodium hydrogen phosphate, ionized Ca2+ and PO43− existed in the solution, which formed hydroxyapatite precipitation under certain weak alkaline conditions. When g-C3N4 is combined with HAP, the diffraction pattern of the composite is basically consistent with that of HAP. Only in CNH(IV) with a high g-C3N4 content did a weak g-C3N4 characteristic peak appear. This indicates that carbon nitride does not destroy the HAP crystal structure, and the amount of carbon nitride on the surface of the material is very small. However, the characteristic diffraction peaks of silver phosphate (JCPDS PDF #06-0505) appear in all the composites after the addition of silver ions (Figure 1b). For example, strong diffraction peaks appear at 2θ = 33.29° and 36.59°, corresponding to the (210) and (211) crystal planes of Ag3PO4, respectively [26,27]. However, only a weak signal of HAP was observed at 31.77°. This shows that silver ions can fully exchange with Ca2+ in the CNH material, and silver phosphate is successfully obtained. With increasing silver content, the characteristic peak of silver phosphate gradually increases, indicating that more silver is loaded in the CNH materials. Figure 1 shows that the preparation method designed in this paper can successfully prepare CN, HAP, CNH and Ag/CNH materials with high purity.

2.2. SEM and HRTEM Images

The morphologies of the different materials are shown in Figure 2. Figure 2a shows SEM images of CN. CN is mainly composed of sheet structures, which are stacked with each other to form a large number of pore structures. This excellent structure is likely to give the sample a large specific surface area and good adsorption performance. SEM images of HAP and CNH are shown in Figure 2b–e. All four materials show rod structures at the nanometer level. Even in the CNH(IV) sample with a high CN content, the material maintained a uniform morphology and did not show a sheet structure. Combined with the XRD results, we speculated that CN is embedded inside HAP in the CNH composite so that the CNH composite retains the nanorod-like structure of HAP. Figure 2f–h shows SEM images of three Ag/CNH materials with different loading ratios of CN. The three Ag/CNH materials have little difference in surface morphology, showing an elongated rod structure. However, compared with materials not loaded with silver, the particle length-to-width ratio is larger, and the rods appear to be more elongated. The energy dispersive X-ray spectroscopy (EDX) diagram of 3:1-Ag/CNH(III) is shown in Figure 2i, indicating that the main elements in the material are Ag, P, Ca and O, and no impurity elements exist.
To further clarify the morphology and element distribution of the materials, HRTEM tests were carried out on CNH(III) and Ag/CNH(III), and the results are shown in Figure 3. The morphology of CNH(III) observed in Figure 3a is consistent with that in Figure 2d. In addition, the internal composition of the material is uniform, and no other structures are found. In Figure 3b, the 0.817 nm wide lattice fringe corresponds to the (100) crystal plane of HAP. Figure 3c shows the diffraction spots of CNH(III), which include diffraction spots of the HAP (210), (212), and (002) planes. According to the results of Figure 1a, Figure 2a–e and Figure 3a–d, we can confirm that carbon nitrides were uniformly dispersed into HAP crystals without changing the short rod structure of HAP. The HRTEM results for Ag/CNH(III) are shown in Figure 3d–f. In Figure 3d, it can be clearly observed that silver phosphate is loaded on CNH and mainly exists at the outer boundaries of particles. This distribution helps silver phosphate make adequate contact with reactive molecules. Furthermore, silver phosphate is dispersed on CNH particles without aggregation. Figure 3e shows a magnified image of the diffraction patterns of different components. The diffraction patterns at 0.344 nm and 0.245 nm in the composite correspond to the (002) crystal plane of HAP and (211) crystal plane of Ag3PO4, respectively, which is also consistent with the conclusion of XRD. Figure 3f shows the element distribution mapping results. It can be seen from the figure that Ag, P, Ca, O, and N are evenly dispersed in the material. In particular, N, which represents carbon nitride, was rarely detected by XRD and SEM, but its uniform distribution can be clearly seen in Figure 3f, confirming once again the speculation that CN is dispersed in HAP. In other words, when CN is added to HAP, the two do not simply mix, but form an inseparable whole. Therefore, in the subsequent discussion, we will analyze CNH as a whole.

2.3. XPS Analysis

To further confirm the existence forms and valence states of each element, the samples were characterized by XPS, and the results are shown in Figure 4a–e. Figure 4a shows the full spectra of several materials. The characteristic peaks of Ag, Ca, P, and O appear in Ag/CNH(III), which is consistent with the results in Figure 2i and Figure 3. Figure 4b,c shows the characteristic curves of C1s and N1s in CN. In Figure 4b, two peaks at 284.8 and 287.9 eV can be attributed to contaminated carbon and N–C–N coordination in g-C3N4 [28,29]. In addition, four peaks at 398.03 eV, 399.41 eV, 400.63 eV, and 403.72 eV appear in the N1s spectrum in Figure 4c. The peak at 398.03 eV is considered to be CN=C in sp2 triazine rings [30,31], and the peaks at 399.41 eV and 400.63 eV belong to the tertiary N group N-(C)3 and terminal amino groups with hydrogen [32,33]. The peak at 403.72 eV is likely caused by π−π* excitation between stacked interlayers [34,35,36]. In Figure 4d, there are two obvious peaks at 367.87 eV and 373.87 eV in the Ag 3d region, which are generated by Ag+ 3d5/2 and Ag+ 3d3/2, respectively. Their appearance is due to Ag+ in the complex, indicating that Ag exists in the form of Ag+ in the material [37,38]. The O 1s peaks at 530.7 eV and 531.9 eV in Figure 4e are generated by lattice oxygen and hydroxyl oxygen in the composite material, respectively [39]. This shows that elements such as Ag, O, C, and N exist in the material in combined states, which again confirms the conclusions of XRD analysis.

2.4. Optical Properties and Band Structure

To investigate the light absorption capacity of the five different materials in this paper, UV–VIS analysis was carried out, and the results are shown in Figure 5a. As seen from the figure, HAP has an absorption peak at wavelengths lower than 350 nm, but there is no absorption peak in the visible light region, indicating that HAP cannot absorb visible light. However, CN and CNH have a strong absorption peak at 400 nm, and the absorption edge extends to approximately 500 nm. Therefore, when CN is doped into HAP to form CNH, although the basic morphology of HAP is maintained, the light absorption performance of HAP, especially the absorption of visible light, is greatly improved. When silver phosphate is added, the absorption capacity of visible light is improved to a greater extent, and the material shows two relatively strong absorption peaks at 420 nm and 460 nm and maintains a certain absorption capacity in the range of 500~720 nm. Therefore, in the visible light range, the response width of Ag/CNH(III)is better than that of HAP, CN and CNH. Through the UV–VIS absorption spectra, (Ahv)1/2~hv can be obtained through calculation, as shown in Figure 5b. As seen from the figure, the band gap widths of HAP, CN, CNH, Ag3PO4 and Ag/CNH(III) are 3.3 eV, 2.70 eV, 2.56 eV, 2.35 eV, and 2.38 eV, respectively, which is consistent with the conclusions in the literature [40,41]. The N2 adsorption–desorption properties of the different materials were also tested to compare their specific surface areas, and the results are shown in Figure 5c. All the materials present hysteresis loops, indicating that mesopores account for a large proportion of the pore size distribution [42]. When CN is combined with HAP and silver phosphate, the specific surface area of the material becomes smaller. The material with the largest specific surface area is CN, whose specific surface area is 62.92 m2·g−1, which is probably related to the low density of CN and its surface structure.

2.5. Fluorescence Spectroscopy and Electrochemical Analysis

Photoluminescence (PL), photocurrent response and electrochemical impedance spectra of CNH, Ag3PO4 and Ag/CNH(III) are shown in Figure 6. As shown in Figure 6a, Ag/CNH(III) has the lowest photoelectric hole recombination efficiency, which is more favorable for the improvement of photocatalytic performance [43,44]. Figure 6b shows the photocurrent intensity of each material when the light source was turned on and off. At the moments when the lamp was turned on and off, the photoresponse reaction rates follow the order Ag/CNH(III) > Ag3PO4 > CNH. This indicates that Ag/CNH(III) plays a key role in electron transfer and can effectively prevent electron–hole recombination and improve the separation efficiency of photogenerated carriers [45,46]. Moreover, electrochemical impedance spectroscopy (EIS) was used to evaluate the internal resistance during charge transfer, as shown in Figure 6c. As seen from the figure, the curve radius of Ag/CNH(III) is the smallest, which indicates that the material has a small interfacial charge transfer resistance, which can improve the transfer process of carriers, again verifying the excellent charge transfer ability of the material.

2.6. Activity and Stability Test

We conducted a simulation test on the pollutant degradation performance of different materials under simulated sunlight, and the results are shown in Figure 7. To better compare the effects of different components on photocatalytic performance, 3:1-Ag/CN and 3:1-Ag/HAP were also prepared for comparison, as shown in Figure 7a. As seen from the figure, HAP has almost no degradation activity for MB under simulated sunlight. In contrast, CN shows a strong adsorption capacity in the dark treatment stage, adsorbing approximately 30% MB in 40 min, which is related to the large specific surface area of the material itself (shown in Figure 5). However, under light, the degradation ability of CN is not good, and the degradation rate is approximately 50% after 100 min. Similarly, CNH and Ag/CN both show similar performance. However, silver phosphate and its composite materials show strong photodegradation after the light is turned on. Among them, the Ag/CNH(III) composite has the best performance and can degrade 60% MB in 20 min and 94% in 60 min. Therefore, it can be seen from Figure 5a to Figure 7a that the light absorption capacity of materials plays a more important role than their specific surface area in determining the catalytic performance. We also tested the activity of materials with different proportions of carbon nitride and silver, and the results are shown in Figure 7b. The degradation effect of the 3:1-Ag/CNH(III) composite is considerably better than that of the other four materials. The same conclusion can be drawn from the kinetic curve of degradation (Figure 7c). Figure 7d shows the UV–VIS absorption spectrum of MB solution degraded by 3:1-Ag/CNH(III). It can be seen from the figure that the MB absorption peak at 664 nm gradually weakens until it disappears, indicating that MB has been completely degraded In addition to activity, the stability of photocatalysts is an important factor in their practical application. As shown in Figure 7e, the catalyst was separated by centrifugation and used three consecutive times. After three cycles, the properties of the material decreased (the degradation rate at 60 min decreased from 94 to 60%), but the degradation effect remained above 60%, indicating that the material has good stability but still has the potential for further improvement. To determine the reason for the performance decline, the used composite was analyzed by XRD, and the results are shown in Figure 7f. A weak peak at 38.07° appears in the XRD spectrum, which is the characteristic peak corresponding to Ag2O (JCPDS PDF #43-0997). This indicates that after repeated use of the catalyst, a small amount of Ag3PO4 is converted into Ag2O, resulting in a reduction in catalytic activity.

2.7. Capture Experiment and Reaction Mechanism

To clarify the types of free radicals involved in the reaction, we carried out free radical trapping experiments on Ag/CNH(III). Figure 8a shows a comparison of the degradation rates of materials without and with different trapping agents. IPA, TEMPOL, and EDTA-2Na were used to capture ·OH, ·O2, and h+ generated during photocatalysis, respectively [47,48]. After adding IPA, the degradation of MB was slightly weakened, but the decrease was minor. However, when TEMPOL and EDTA-2Na were added, the reaction was mostly inhibited, which strongly affected the photodegradation of MB. This indicates that ·O2 and h+ are the main types of free radicals involved in the reaction, and only a small amount of ·OH is involved in the photocatalytic degradation process. To verify the above results, we also conducted ESR experiments, and the test results are shown in Figure 8b,c. Figure 8b,c show obvious characteristic peaks of ·O2 and h+, indicating that the composite material can indeed generate these two free radicals under light [49].
The valence band (VB) spectra of Ag3PO4 and CNH are shown in Figure 9a,b. As seen from the figure, the maximum VB values for Ag3PO4 and CNH are 2.64 eV and 2.21 eV, respectively [50,51]. According to the formula EVB = ECB + Eg, the conduction band edge potentials (ECB) of Ag3PO4 and CNH are 0.29 eV and −0.35 eV. A schematic diagram of the energy band structure of the two materials is shown in Figure 9c.
Based on the above analysis, a possible Z-scheme photocatalytic degradation mechanism of MB is proposed in Figure 10. Under light irradiation, the electrons in the VB of Ag3PO4 and CNH are excited and transition to the conduction band (CB), producing photogenerated electrons and holes. However, the electrons in the CB of Ag3PO4 are more likely to recombine with the holes on CNH, so more holes and electrons accumulate on the VB of Ag3PO4 and the CB of CNH, respectively. Since the CB of CNH is −0.35 eV, which is more negative than the standard redox potential of ·O2/O2 (−0.33 eV) [52], the electrons clustered in the CB of CNH can reduce O2 in the environment, forming superoxide radicals (·O2), which promotes the degradation of methylene blue. The holes in the VB of Ag3PO4 are another important reaction species involved in the reaction and can also degrade MB. Therefore, this Z-scheme mechanism promotes the separation of photogenerated electrons and holes in the composites, which makes the composites exhibit good photocatalytic degradation ability.

3. Materials and Methods

3.1. Raw Material

Oyster shells were obtained from the seafood market in Weihai, Shandong Province, China. Phosphoric acid (AR), glacial acetic acid (AR), hydrogen diamine phosphate (AR), ammonia (AR), and anhydrous ethanol (AR) was obtained from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). Silver nitrate (AR) and methylene blue (AR) were obtained from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, China). Urea (AR) was obtained from Tianjin Beichen Founder Reagent Factory (Tianjin, China). The water used in this experiment was homemade ultrapure water.

3.2. Preparation of Materials

The preparation methods of HAP and Ag3PO4 were the same as those in previous studies [53]. The preparation methods of the other three materials are shown in Figure 11.
Preparation of g-C3N4: Urea was transferred to a crucible after full grinding. The crucible was placed in a muffle furnace and roasted at 550 °C for 5 h. After cooling, the material was removed and ground to obtain a light yellow g-C3N4 solid, denoted as CN.
Preparation of g-C3N4/HAP: The initial steps were the same as those for HAP; then, ultrasonicated g-C3N4 was added to the solution after adjusting the solution pH value. After even mixing, the solution was transferred to a hydrothermal reaction kettle for reaction, and the subsequent treatment steps were the same as those for HAP. The obtained material was g-C3N4/HAP, denoted as CNH or CNH(X), where X indicates the mass of g-C3N4 added per 10 g of oyster shells.
Preparation of Ag3PO4/g-C3N4/HAP: First, 0.5 g CNH(X) was dispersed in 50 mL water and stirred. Then, an appropriate amount of AgNO3 was dissolved in water, added dropwise to the above solution and stirred for another 6 h before standing overnight. The solids were separated and rinsed three times each with water and anhydrous ethanol. The remaining solid was dried at 100 °C to obtain the Ag3PO4/g-C3N4/HAP composite material, which was denoted as Y:1-Ag/CNH(X), where the mass ratio of silver nitrate to CNH(X) is denoted as Y:1.

3.3. Testing and Characterization

Sample characterization: X-ray diffraction (XRD) analysis of the samples was performed by a Rigaku UltimaIV X-ray powder diffractometer (Japanese Science Company, Tokyo, Japan). Scanning electron microscopy (SEM, Nova nanoSEM450, FEI, Hillsboro, OR, USA) and high-resolution transmission electron microscopy (HRTEM, JEM-F200, JEOL, Akishima, Tokyo, Japan) were used to analyze the microstructure of the samples. An ASAP 2460 instrument (Micromeritics, Norcross, GA, USA) was used to measure the specific surface area of the materials. X-ray photoelectron spectroscopy (XPS) analysis of the samples was performed using a Nexsa X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The light absorption properties of the materials were analyzed by using a UV-3600 instrument (Shimadzu, Tokyo, Japan). The fluorescence spectra of the samples were measured with an Edinburgh FLS1000. Photoelectric measurements of the samples were taken by a CHI660E electrochemical workstation with a standard three-electrode configuration. Electron spin resonance (ESR) analysis of the samples was performed by an EMXPLUS ESR spectrometer (Bruker).
Photocatalytic performance test: A Xe lamp was used to simulate sunlight to reduce methylene blue (MB) solution as the reaction model, and the catalytic activity of different photocatalysts was evaluated. The reaction took place in a self-assembled photocatalytic reactor. The light source was a 500 W long-arc xenon lamp (λ = 300~800 nm). Catalyst (0.35 g) was added to 250 mL 10 mg/L methylene blue solution and stirred in the dark for 40 min. Samples were taken at an interval of 10–20 min. The liquid was filtered with a filter membrane and placed in a UV–VIS spectrophotometer (Hitachi, Tokyo, Japan). The absorbance was tested at 664 nm, and the degradation curve was obtained.
Free radical capture experiment: Similar to the above photocatalytic performance tests, 1 mmol isopropanol (IPA, ·OH trapping agent), 2,2,6,6-tetramethylpiperidinooxy (TEMPOL, ·O2 trapping agent) and EDTA-2Na (h+ trapping agent) were added to a 250 mL MB (10 mg/L) solution containing catalyst, and the effect on the degradation of MB was investigated.

4. Conclusions

In this work, a series of Ag3PO4/g-C3N4/HAP composites were successfully prepared by a hydrothermal method. The materials showed excellent performance in the sunlight catalytic degradation of MB. Among the materials, the degradation effect of 3:1-Ag/CNH(III) was the best (94% within 60 min). Through characterization, it was found that the addition of g-C3N4 did not change the morphology of the material, but greatly improved its band gap structure and improved its response range to visible light. In addition, the composite has good photogenerated electron and hole separation efficiency, and ·O2 and h+ are the main active species involved in the reaction.

Author Contributions

Conceptualization and Writing: C.S. (Cui Song); Data curation and software: C.S. (Changyu Shang); formal analysis: S.L.; methodology: W.W.; investigation: M.Q.; supervision: J.C.; resources: H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (4217030124), Natural Science Foundation of Shandong Province (ZR2021MB052, ZR2020MB140), and Shandong Unversity-Weihai Research Institute of Industry Technology Research Projects (0004202107020002).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

XRD and SEM are supported by Physical-Chemical Materials Analytical & Testing Center of Shandong University at Weihai. And the authors would like to thank Shiyanjia Lab (www.shiyanjia.com, 14 December 2021) for the support of XPS, HRTEM, PL and ESR analysis.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. XRD patterns of (a) CN, HAP, CNH and (b) Ag/CNH.
Figure 1. XRD patterns of (a) CN, HAP, CNH and (b) Ag/CNH.
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Figure 2. SEM images of (a) CN, (b) HAP, (c) CNH(II), (d) CNH(III), (e) CNH(IV), (f) 3:1-Ag/CNH(II), (g) 3:1-Ag/CNH(III), (h) 3:1-Ag/CNH(IV), and (i) EDS spectrum of Ag/CNH.
Figure 2. SEM images of (a) CN, (b) HAP, (c) CNH(II), (d) CNH(III), (e) CNH(IV), (f) 3:1-Ag/CNH(II), (g) 3:1-Ag/CNH(III), (h) 3:1-Ag/CNH(IV), and (i) EDS spectrum of Ag/CNH.
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Figure 3. (a,b) HRTEM images of CNH(III), (c) rutile spots of CNH(III), HRTEM images (d,e), elemental mapping images (f) of 3:1-Ag/CNH(III), image of Ag (f-1), Ca (f-2), O (f-3), P (f-4) and N (f-5) elements.
Figure 3. (a,b) HRTEM images of CNH(III), (c) rutile spots of CNH(III), HRTEM images (d,e), elemental mapping images (f) of 3:1-Ag/CNH(III), image of Ag (f-1), Ca (f-2), O (f-3), P (f-4) and N (f-5) elements.
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Figure 4. (a) XPS survey spectra of different samples; (b) C 1s peaks and (c) N 1s peaks over CN; (d) Ag 3d peaks and (e) O 1s peaks over Ag/CNH(III) samples.
Figure 4. (a) XPS survey spectra of different samples; (b) C 1s peaks and (c) N 1s peaks over CN; (d) Ag 3d peaks and (e) O 1s peaks over Ag/CNH(III) samples.
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Figure 5. (a) UV–VIS absorption spectra, (b) Tauc plots, and (c) N2 adsorption-desorption isotherms of different samples.
Figure 5. (a) UV–VIS absorption spectra, (b) Tauc plots, and (c) N2 adsorption-desorption isotherms of different samples.
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Figure 6. (a) Steady-state PL emission spectra, (b) transient photocurrent curves, and (c) EIS spectra of CNH(III), Ag3PO4 and Ag/CNH(III).
Figure 6. (a) Steady-state PL emission spectra, (b) transient photocurrent curves, and (c) EIS spectra of CNH(III), Ag3PO4 and Ag/CNH(III).
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Figure 7. (a,b) Comparison of degradation MB curves over as-prepared photocatalysts; (c) photocatalytic kinetics of different materials; (d) UV-VIS absorption spectrum of MB solution; (e) three cycling runs for degradation of MB over 3:1-Ag/CNH(III); (f) XRD patterns of the composite before and after reaction.
Figure 7. (a,b) Comparison of degradation MB curves over as-prepared photocatalysts; (c) photocatalytic kinetics of different materials; (d) UV-VIS absorption spectrum of MB solution; (e) three cycling runs for degradation of MB over 3:1-Ag/CNH(III); (f) XRD patterns of the composite before and after reaction.
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Figure 8. (a) Effects of active species scavengers on the degradation of MB for 3:1-Ag/CNH(III); ESR spectra of 3:1-Ag/CNH(III) for detecting (b) DMPO-·O2 and (c) TEMPO-h+.
Figure 8. (a) Effects of active species scavengers on the degradation of MB for 3:1-Ag/CNH(III); ESR spectra of 3:1-Ag/CNH(III) for detecting (b) DMPO-·O2 and (c) TEMPO-h+.
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Figure 9. (a,b) VB-XPS spectra and (c) energy diagrams of Ag3PO4 and CNH(III).
Figure 9. (a,b) VB-XPS spectra and (c) energy diagrams of Ag3PO4 and CNH(III).
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Figure 10. Possible photocatalytic mechanism for the degradation of MB over Ag/CNH(III).
Figure 10. Possible photocatalytic mechanism for the degradation of MB over Ag/CNH(III).
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Figure 11. Synthesis process of Ag/CNH composite.
Figure 11. Synthesis process of Ag/CNH composite.
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Song, C.; Shang, C.; Li, S.; Wang, W.; Qi, M.; Chen, J.; Liu, H. Efficient Visible-Light-Responsive Ag3PO4/g-C3N4/Hydroxyapatite Photocatalyst (from Oyster Shells) for the Degradation of Methylene Blue: Preparation, Properties and Mechanism. Catalysts 2022, 12, 115. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12020115

AMA Style

Song C, Shang C, Li S, Wang W, Qi M, Chen J, Liu H. Efficient Visible-Light-Responsive Ag3PO4/g-C3N4/Hydroxyapatite Photocatalyst (from Oyster Shells) for the Degradation of Methylene Blue: Preparation, Properties and Mechanism. Catalysts. 2022; 12(2):115. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12020115

Chicago/Turabian Style

Song, Cui, Changyu Shang, Shuqian Li, Wenhao Wang, Mingying Qi, Jingdi Chen, and Haijun Liu. 2022. "Efficient Visible-Light-Responsive Ag3PO4/g-C3N4/Hydroxyapatite Photocatalyst (from Oyster Shells) for the Degradation of Methylene Blue: Preparation, Properties and Mechanism" Catalysts 12, no. 2: 115. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12020115

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