English

• 0.   Introduction

  Photocatalysis is a branch of green chemistry and has important application prospects for solving energy crises, alleviating environmental problems and managing greenhouse gas emissions. TiO₂ was the first catalyst used in photocatalysis. Due to its wide band gap, TiO₂ catalysts can only use ultraviolet light with low quantum efficiencies and thus have very limited practical applications^[1-2]. In 2010, Ag₃PO₄^[3-4] was found to exhibit excellent photocatalytic oxidation abilities under visible-light irradiation; specifically, Ag₃PO₄ was found to decompose water and could be used to efficiently degrade organic dyes, providing an important basis for an in-depth study of Ag₃PO₄ catalysts. Given the poor stability of Ag₃PO₄, recent research has been focus on combining Ag₃PO₄ with other materials to improve the stability of the catalyst and to reduce costs. Currently, the most commonly used composite materials combined with Ag₃PO₄ to form composites are TiO₂^[5-7], ZnO^[8], g-C₃N₄^[9-13] and AgX^{[4, 14]}. Most Ag₃PO₄ composite catalysts have been prepared by chemical synthesis methods. The use of natural raw materials to prepare Ag₃PO₄ composite catalysts has rarely been reported.

  Oysters are one of the world′s most important aquatic products. In recent years, oyster production has surged. Large amounts of oyster shells have become a major waste and pollutant in coastal areas. Using oyster shell as a calcium source, we synthesized nanoscale hydroxyapatite (HAP) by a hydrothermal method. We then successfully loaded the HAP carrier with Ag₃PO₄ to prepare a series of Ag₃PO₄/HAP composite photocatalysts that were responsive to visible light. The surface morphology and chemical composition of the catalysts were analyzed by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectros-copy (XPS). In addition, the ability of these prepared photocatalysts with different Ag₃PO₄ contents and different catalyst amounts to degrade methylene blue (MB) under visible light was studied; the effect of the preparation temperature on the catalytic performance was also investigated. The results showed that the prepared catalysts exhibited excellent MB degradation activities. Our study not only advances the technical understanding necessary to design efficient catalysts for wastewater treatment but also provides a viable solution for reusing oyster-shell waste.

  1.   Experimental

  1.1   Materials

  Oyster shells were purchased in a seafood market; all other chemicals were acquired from commercial sources and used without further purification. Acetic acid, H₃PO₄, (NH₄)₂HPO₄, Na₃PO₄, ethyl alcohol, sodium citrate, urea, and ammonium hydroxide were purchased from Tianjin Fuyu Chemical Reagent Co., Ltd., and AgNO₃ and MB were purchased from Sinopharm Chemical Reagent Co., Ltd. Triple-distilled water was used in all experiments.

  1.2   Preparation of the catalysts

  Preparation of Ag₃PO₄: First, 6 mmol of AgNO₃ was dissolved in deionized water and 5%(w/w) aqueous ammonia solution was added dropwise until a the brown precipitate formed and then completely dissolved. Subsequently, 2 mmol of a Na₃PO₄ solution was added dropwise and the resulting mixture was stirred vigorously for 5 min to yield a Ag₃PO₄ suspension.

  Preparation of Ag₃PO₄/HAP: The oyster shells were washed and crushed. One gram of oyster shell powder was then dissolved with 10%(w/w) acetic acid. The mixture was filtered, and the filtrate was collected. 20 mL of a 0.3 mol·L^-1 (NH₄)₂HPO₄ solution was then added dropwise to achieve a Ca/P molar ratio of 1.67. The pH value of the solution was adjusted to 9~10 with aqueous ammonia, and then, the solution was transferred to an autoclave. The reaction mixture was heated in a constant temperature oven at 160 ℃ for 8 h. The product was washed with ultrapure water and anhydrous ethanol, centrifuged, and oven-dried to yield HAP. Different concentrations of the AgNO₃ solution (w_AgNO3:w_HAP=1:x, x=2, 3, 4) were dropwise added to HAP to prepare a suspension in the dark. After stirring and aging, the mixture was washed with ultrapure water and anhydrous ethanol. Ag₃PO₄/HAP composite catalysts were prepared with different drying temperatures. The resulting 1:4-Ag₃PO₄/HAP catalyst was placed in a muffle furnace and calcined at 200 or 500 ℃ for 1 h. The obtained Ag₃PO₄/HAP composite catalysts with different mass ratios were named as 1:x-Ag₃PO₄/HAP.

  1.3   Characterization

  XRD analysis was performed using a Rigaku Ultima Ⅳ powder X-ray diffractometer equipped with a Cu Kα radiation source (λ=0.154 nm); the tube current was 40 mA, the tube voltage of 40 kV, the scan rate was 0.02°·s^-1, and the scan range was 20°≤2θ≤60°. SEM images were collected using a Nova NanoSEM 450 (FEI, USA) scanning electron micros-cope operated at an accelerating voltage of 5 kV. HRTEM images were obtained on a Jeol JEM-2100 electron microscope operated at an accelerating voltage of 200 kV. XPS experiments were performed using a Nexsa X-ray photoelectron spectrometer (Thermo Scientific, USA). The source generated Al Kα radiation, the test voltage was 12 kV, and the power was 720 W. The binding energies of the electrons in the samples were measured to analyze the composition and valence state of their elemental components.

  1.4   Photocatalytic activity

  The degradation of MB under a xenon solar simulator was used as the reaction model to evaluate the catalytic activity of the different photocatalysts. The reaction was performed in a homemade photocatalytic reactor with a 500 W xenon lamp. 0.35 g catalyst (except for the degradation experiment with different catalysts mass) was added to 250 mL of a 10 mg·L^-1 MB solution, and the resulting solution was stirred for 20 min in the dark. At 10~20 min intervals, 10 mL aliquots were collected and centrifuged. The absorbance at 664 nm was measured using a UV-Vis spectrophotometer (T6 New Century, Persee, Beijing, China) to plot the degradation curve. The degradation process was estimated by C_t/C₀ or C_t/C, where C₀ was the initial concentration of MB, C was the concentra-tion after dark adsorption and C_t was the correspond-ing concentration at the examination time.

  In situ capture experiments were carried out to identify the role of photo-induced reactive oxygen species in the degradation of MB. Three different scavengers, including isopropanol (IPA, ·OH scavenger, 2 mmol·L^-1), ascorbic acid(AA, ·O₂^- scavenger, 2 mmol·L^-1) and Na₂-EDTA (h⁺ scavenger, 2 mmol·L^-1), were added to the MB aqueous solution (250 mL, 10 mg·L^-1), along with 0.35 g of the 1:2-Ag₃PO₄/HAP photocatalyst.

  2.   Results and discussion

  2.1   Characterization of the samples

  The XRD pattern of the HAP sample hydrother-mally prepared using oyster shells as the raw materials is shown in Fig. 1A. Fig. 1A shows that the diffraction peaks of HAP prepared by this method agree with those of the standard pattern of hydroxya-patite [Ca₁₀(PO₄)₆(OH)₂, PDF No.74-0565]. The HAP diffraction peaks were sharp and no noise was detected in the pattern, indicating that the purity and crystallinity of HAP were relatively high. Fig. 1 also shows the XRD pattern of the Ag₃PO₄/HAP catalyst. Compared to the XRD pattern of HAP, the XRD pattern of Ag₃PO₄/HAP exhibited relatively intense diffraction peaks at 20.92°, 29.72°, 33.34°, 36.62°, 47.86°, 52.75°, 55.06°, and 57.28°, exactly consistent with the peaks in the standard pattern of Ag₃PO₄ (PDF No.06-0505); thus the XRD analysis shows that Ag₃PO₄ was successfully loaded onto HAP^[15-16]. The Ag₃PO₄ peaks in the diffraction pattern of 1:2-Ag₃PO₄/HAP are more intense than those in the pattren of 1:4-Ag₃PO₄/HAP, indicating that the 1:2-Ag₃PO₄/HAP catalyst had more loaded Ag₃PO₄ than the 1:4-Ag₃PO₄/HAP catalyst. In the pattern of the 1:4-Ag₃PO₄/HAP composite photo-catalysts calcined at 200 and 500 ℃, the chara-cteristic peak at 38.07° was ascribed to Ag₂O (PDF No.43-0997), indicating that Ag₂O was formed on the surface of catalyst after calcination.

  Figure 1

  

  Figure 1.  XRD patterns of HAP and Ag₃PO₄/HAP

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  Fig. 2(A, B) shows that the hydrothermally prepared HAP contained nanorod-like structures, and that the particle size and morphology were uniform. The regular shape of the rod-like structures and the existence of numerous gaps between the particles were conducive to loading Ag₃PO₄. The rings with an interplanar spacing of about 0.345 nm, corresponding to the (002) crystal planes of HAP (Fig. 2B), were consistent with the XRD patterns. The SEM images in Fig. 2C show that the morphology of the 1:2-Ag₃PO₄/HAP sample was practically unchanged compared to that of HAP. As shown in Fig. 2D, Ag₃PO₄ nanoparticles were coated on the HAP; the HRTEM image clearly displayed the lattice spacings of 0.245 and 0.268 nm, corresponding to the Ag₃PO₄ (211) and (210) planes, respectively^[17]. These results indicate that the Ag₃PO₄/HAP nanocomposite was successfully prepared.

  Figure 2

  

  Figure 2.  SEM and HETEM images of HAP (A, B) and Ag₃PO₄/HAP (C, D)

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  The XPS spectra of the 1:2-Ag₃PO₄/HAP catalyst in Fig. 3 shows the characteristic peaks of Ag, P, and O. As shown in Fig. 3A, there were two distinct peaks at 367.87 and 373.87 eV in the Ag3d region, which are generated by Ag⁺3d_5/2 and Ag⁺3d_3/2, respectively^[18-20]. In Fig. 3B, the peak at 132.95 eV was attributed to P2p_3/2 of the P element in Ag₃PO₄^[21]. In Fig. 3C, the peaks at 530.7 and 531.9 eV were attributed to the lattice oxygen in Ag₃PO₄ and to the hydroxyl groups on the surface of the Ag₃PO₄/HAP sample, respective-ly^[22]. The XPS spectra again indicate that Ag₃PO₄ was successfully loaded onto HAP and are consistent with the XRD and TEM results.

  Figure 3

  

  Figure 3.  XPS spectra of 1:2-Ag₃PO₄/HAP catalysts

  (A) Ag3d; (B) P2p; (C) O1s

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  2.2   Photocatalytic activity

  The photocatalytic activity of the different catalysts was evaluated by studying the degradation of MB under simulated solar radiation. Specifically, the changes in the time required for the four catalysts to degrade MB at a room temperature (35 ℃) were investigated. The four catalysts were carrier-free Ag₃PO₄, 1:4-Ag₃PO₄/HAP, 1:4-Ag₃PO₄/HAP calcined at 200 ℃ (1:4-Ag₃PO₄/HAP (200 ℃)), and 1:4-Ag₃PO₄/HAP calcined at 500 ℃ (1:4-Ag₃PO₄/HAP (500 ℃)). Carrier-free Ag₃PO₄ exhibited a degradation rate of only 9.31% after 120 min of light irradiation, whereas Ag₃PO₄/HAP showed dramatically increased photodegradation activities, indicating that HAP prepared using oyster shells as the raw materials was a viable carrier for preparing photocatalysts. Combining Ag₃PO₄ and HAP could improve the stability of Ag₃PO₄ and lead to better photocatalytic performance. However, after the 1:4-Ag₃PO₄/HAP catalyst was calcined, the degradation of MB worsened. At 120 min, Ag₃PO₄/HAP, 1:4-Ag₃PO₄/HAP (200 ℃) and 1:4-Ag₃PO₄/HAP (500 ℃) exhibited degradation rates of 71.08%, 60.55% and 33.40%, respectively. Calcination did not improve the photocatalytic activity toward MB degradation. Fig. 1B shows that after the photocatalyst was roasted in air, a small amount of silver phosphate in the catalyst was decomposed into silver oxide, resulting in a decrease in the degradation activity of the catalyst. Therefore, calcination was not used in subsequent catalyst preparations.

  Interestingly, as the room temperature was slowly decreased, the catalytic activity of the catalyst changed substantially. That is, the photocatalytic activity of the catalyst was very sensitive to changes in the room temperature. Fig. 4B shows that the catalytic activity significantly increased with decreasing room temperature. At 120 min, the MB degradation rate of 1:4-Ag₃PO₄/HAP increased from 71.08% at 35 ℃ to 91.56% at 23 ℃. Therefore, a low room temperature of 23~35 ℃ was more conducive to the photocatalytic degradation of MB. Therefore, subsequent experiments were performed at a room temperature of approximately 23 ℃.

  Figure 4

  

  Figure 4.  Effect of preparation temperatures on degradation activity of the catalyst: calcination temperature (A), room temperature (B) and drying temperature (C)

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  We subsequently investigated the effects of the catalyst drying temperature on the activity, as shown in Fig. 4C. After drying at 70 ℃ for 12 h, the 1:4-Ag₃PO₄/HAP catalyst exhibited very poor photo-catalytic activity and an MB degradation rate of only 13.95% at 120 min. The catalysts dried at 100 and 130 ℃ did not exhibit substantially different catalytic activities, indicating that 100 ℃ was the ideal drying temperature for the 1:4-Ag₃PO₄/HAP catalyst. There-fore, in subsequent experiments, all of the catalysts were dried at 100 ℃.

  Fig. 5 shows the photocatalytic degradation curves obtained for different catalyst amounts and different Ag₃PO₄ contents. These two experiments included data obtained before and after treatment under dark conditions and can therefore be used to compare the MB adsorption performances of the catalysts. Fig. 5A shows that, for the 1:4-Ag₃PO₄/HAP catalyst, the degra-dation activity increased with increasing catalyst amount. In particular, 1.25 g of the 1:4-Ag₃PO₄/HAP catalyst resulted in the best catalytic performance, and could completely degrade MB in approximately 40 min. Given the reaction time and cost, 0.35 g of catalyst was used in subsequent catalyst performance experiments.

  Figure 5

  

  Figure 5.  Photocatalytic degradation curves for different catalyst amounts (A) and different Ag₃PO₄ contents (B)

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  Fig. 5B shows the degradation activity of the three catalysts with w_Ag3PO4:w_HAP=1:4, 1:3, and 1:2. Increasing the Ag₃PO₄ content caused the catalysts to exhibit stronger MB adsorption and better photocatalytic activity for degrading MB. Specifically, the 1:3-Ag₃PO₄/HAP could completely degrade MB in 60 min; 1:2-Ag₃PO₄/HAP achieved approximately 50% in 10 min and completely degradation MB in 40 min, exhibiting excellent photocatalytic activity.

  To reveal the main reactive species in the degradation of MB, radical-trapping experiments were conducted using several scavengers, IPA (·OH radical scavenger), AA (·O₂^- radical scavenger) and Na₂-EDTA (hole scavenger)^[23-24]. As presented in Fig. 6A, the add-itation of Na₂-EDTA and AA resluted in a decrease in the MB degradation rate from 0.076 to 0.003 1 and 0.000 2 min^-1, indicating that ·O₂^- and h⁺ species were the main oxidative centers in the Ag₃PO₄/HAP. Fig. 6B and C show the ESR signals of the as-prepared composite with DMPO (5, 5-dimethyl-1-pyrroline N-oxide) and TEMPO (2, 2, 6, 6-tetramethylpiperidinooxy) as scavengers. No signals were detectable in the absence of light irradiation. Under visible-light irra-diation of the composite for 10 min, six characteristic peaks of ·O₂^- were observed. Therefore, the bireactive species (·O₂^- and h⁺) mainly improve the catalytic performance. The HAP might act as a charge transmission bridge to form the Ag₃PO₄/HAP Z-scheme system. As a result, the connection between Ag₃PO₄ and HAP made the electrons on the CB of Ag₃PO₄ transfer to HAP because of the electro-conductivity that resulted from the alteraion of the electron state of the surface PO₄^3- groups under light irradiation^[24].

  Figure 6

  

  Figure 6.  Free radicals capture experiments using 1:2-Ag₃PO₄/HAP (A), and EPR signals of 1:2-Ag₃PO₄/HAP: ·O₂^- (B) and h⁺ (C)

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  3.   Conclusions

  Natural oyster shell waste was used to prepare a HAP material used for photocatalytic degradation. A series of photocatalysts used to degrade an organic dye was prepared by loading Ag₃PO₄ onto the prepared HAP. The effect of factors such as the calcination temperature, room temperature, drying temperature, catalyst amount and Ag₃PO₄ content on the MB degradation rate was investigated. The 1:2-Ag₃PO₄/HAP catalyst prepared with optimal preparation conditions and an ideal Ag₃PO₄ content was selected for further study. The morphologies, compositions and valence states of the elements were analyzed. The selected 1:2-Ag₃PO₄/HAP catalyst achieved a 50% degradation rate in approximately 10 min and completely degradation of MB in 40 min, demonstrating excellent photocatalytic performance. This type of photocatalyst solves problems encountered when using traditional photocatalysts, such as ineffective use of solar radiation, and alleviates the environmental pollution of waste materials. Therefore, this type of photocatalyst has broad application prospects in the photocatalytic treatment of pollutants.

  
  
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  Figure 1  XRD patterns of HAP and Ag₃PO₄/HAP

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  Figure 2  SEM and HETEM images of HAP (A, B) and Ag₃PO₄/HAP (C, D)

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  Figure 3  XPS spectra of 1:2-Ag₃PO₄/HAP catalysts

  (A) Ag3d; (B) P2p; (C) O1s

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  Figure 4  Effect of preparation temperatures on degradation activity of the catalyst: calcination temperature (A), room temperature (B) and drying temperature (C)

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  Figure 5  Photocatalytic degradation curves for different catalyst amounts (A) and different Ag₃PO₄ contents (B)

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  Figure 6  Free radicals capture experiments using 1:2-Ag₃PO₄/HAP (A), and EPR signals of 1:2-Ag₃PO₄/HAP: ·O₂^- (B) and h⁺ (C)

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