Presented here is a protocol to explore a universal set of experimental procedures for comprehensive laboratory evaluation of photocatalysts in the field of environmental purification, using the example of photocatalytic removal of antibiotic organic pollutant molecules from water by phthalocyanine sensitized silver phosphate composites.
Various antibiotics such as tetracycline, aureomycin, amoxicillin, and levofloxacin are found in large quantities in groundwater and soil systems, potentially leading to the development of resistant and multi-drug resistant bacteria, posing a threat to humans, animals, and environmental systems. Photocatalytic technology has attracted keen interest due to its rapid and stable treatment and direct use of solar energy. However, most studies evaluating the performance of semiconductor catalysts for the photocatalytic degradation of organic pollutants in water are currently incomplete. In this paper, a complete experimental protocol is designed to comprehensively evaluate the photocatalytic performance of semiconductor catalysts. Herein, rhombic dodecahedral silver phosphate was prepared by a simple solvent phase synthesis method at room temperature and atmospheric pressure. BrSubphthalocyanine/Ag3PO4 heterojunction materials were prepared by the solvothermal method. The catalytic performance of as-prepared materials for the degradation of tetracycline was evaluated by studying different influencing factors such as catalyst dosage, temperature, pH, and anions at atmospheric pressure using a 300 W xenon lamp as a simulated solar light source and a light intensity of 350 mW/cm2. Compared with the first cycle, the constructed BrSubphthalocyanine/Ag3PO4 maintained 82.0% of the original photocatalytic activity after five photocatalytic cycles, while the pristine Ag3PO4 maintained only 28.6%. The stability of silver phosphate samples was further tested by a five-cycle experiment. This paper provides a complete process for evaluating the catalytic performance of semiconductor catalysts in the laboratory for the development of semiconductor catalysts with potential for practical applications.
Tetracyclines (TCs) are common antibiotics that provide effective protection against bacterial infections and are widely used in animal husbandry, aquaculture, and disease prevention1,2. They are widely distributed in water due to their overuse and improper application in the past decades, as well as the discharge of industrial wastewater3. This has caused severe environmental pollution and serious risks to human health; for example, the excessive presence of TCs in the aqueous environment can negatively affect microbial community distribution and bacterial resistance, leading to ecological imbalances, mainly due to the highly hydrophilic and bioaccumulative nature of antibiotics, as well as a certain level of bioactivity and stability4,5,6. Due to the hyper-stability of TC in the environment, it is difficult to break down naturally; therefore, many methods have been developed, including biological, physicochemical, and chemical treatments7,8,9. Biological treatments are highly efficient and low-cost10,11. However, because they are toxic to microorganisms, they do not effectively degrade and mineralize antibiotic molecules in water12. Although physicochemical methods can remove antibiotics from wastewater directly and quickly, this method only converts the antibiotic molecules from the liquid phase to the solid phase, does not completely degrade them, and is too costly13.
In contrast to conventional methods, semiconductor photocatalysis has been widely used for the degradation of pollutants in the past decades due to its efficient catalytic degradation properties14. For example, the noble metal-free magnetic FexMny catalyst of Li et al. achieved efficient photocatalytic oxidation of a variety of antibiotic molecules in water without the use of any oxidant15. Yan et al. reported the in situ synthesis of lily-like NiCo2O4 nanosheets on waste biomass-derived carbon to achieve efficient photocatalytic removal of phenolic pollutants from water16. The technology relies on a semiconductor catalyst excited by light to generate photogenerated electrons (e–) and holes (h+)17. The photogenerated e– and h+ will be converted into superoxide anion radicals (O2–) or hydroxyl radicals (OH–) by reacting with absorbed O2 and H2O, and these oxidatively active species oxidize and decompose organic pollutant molecules in water into CO2 and H2O and other smaller organic molecules18,19,20. However, there is no unified field standard for photocatalyst performance evaluation. The evaluation of a material's photocatalytic performance should be investigated in terms of the catalyst preparation process, environmental conditions for optimal catalytic performance, catalyst recycling performance, etc. Ag3PO4, with its prominent photocatalytic ability, has triggered substantial concern in environmental remediation. This new photocatalyst achieves quantum efficiencies of up to 90 % at wavelengths greater than 420 nm, which is significantly higher than previously reported values21. However, the severe photo corrosion and unsatisfactory electron-hole separation rate of Ag3PO4 limit its wide application22. Therefore, various attempts have been made to overcome these drawbacks, such as shape optimization23, ion doping24, and heterostructure building25,26,27. In this paper, Ag3PO4 was modified using morphology control as well as heterojunction engineering. First, rhombic dodecahedral Ag3PO4 crystals with high surface energy were prepared by solvent phase synthesis at room temperature under ambient pressure. Then, organic supramolecular BrSubphthalocyanine (BrSubPc), which can act as both electron acceptor and electron donor, was self-assembled on the silver phosphate surface by the solvothermal method28,29,30,31,32,33,34,35. The photocatalytic performance of the prepared materials was evaluated by investigating the effect of different environmental factors on the photocatalytic performance of the prepared samples to degrade trace amounts of tetracycline in water. This paper provides a reference for the systematic evaluation of the photocatalytic performance of the materials, which is of significance for the future development of photocatalytic materials for practical applications in environmental remediation.
1. Preparation of the BrSubPc
NOTE: The BrSubPc sample was prepared according to a previously published work36. The reaction is carried out in a double-row tube vacuum line system, and the reaction process is strictly controlled under water-free and oxygen-free conditions.
2. Preparation of the Rhombic dodecahedral Ag3PO4
NOTE: Rhombic dodecahedral Ag3PO4 was prepared according to the previously reported literature35.
3. Preparation of BrSubPc/Ag 3PO4
NOTE: Four different composite ratios of BrSubPc to Ag3PO4 were prepared according to the mass ratios of 1:25, 1:50, 1:75, and 1:100.
4. Characterization of the samples
5. Photocatalytic activity test
NOTE: The light source is a 300 W xenon lamp, and a 400 nm filter is used to remove ultraviolet light from the light source. The xenon lamp was mounted 15 cm above the solution, and the light intensity was determined to be 350 mW/cm2.
The rhombic dodecahedron Ag3PO4 was successfully synthesized using this solvent phase synthesis method. This is confirmed by the SEM images shown in Figure 1A,B. According to the SEM analysis, the average diameter of the rhombic dodecahedral structure was found to be between 2-3 µm. The pristine BrSubPc microcrystals show a large irregular flake structure (Figure 1C). In the composite sample, the titanium dioxide still kept the original nanosphere structure, but no phthalocyanine sheet structure was found, which means that the phthalocyanine molecules were uniformly self-assembled on the titanium dioxide surface (Figure 1D). As shown in Figure 2A, all the samples show a characteristic peak located at 20.9°, 29.7°, 33.3°, 36.6°, 42.5°, 47.8°, 52.7°, 55.0°, 57.3°, 61.6°, 65.8°, 69.9°, 71.9°, and 73.8° which were attributed to the (110), (200), (210), (211), (220), (310), (222), (320), (321), (400), (330), (420), (421) and (332) facets of the body-centered cubic structure of Ag3PO4 (JCPDS No. 06-0505)21. On the other hand, BrSubPc/Ag3PO4 samples did not show additional characteristic peaks of BrSubPc, mainly because of the amount of BrSubPc loaded on the surface of Ag3PO4 was low and the intensity of the main diffraction peak of Ag3PO4 decreased as the amount of BrSubPc increased. The FT-IR spectra of the as-prepared samples are analyzed as shown in Figure 2B. For BrSubPc, the more abundant characteristic peaks in the FT-IR spectrum are peaks at 743 cm-1, 868 cm-1, 943 cm-1, and 1452 cm-1; this feature is the stretching and bending vibration of the C-C and C-N bonds of the benzene ring backbone. The weak peak at 624 cm-1 is the characteristic peak of the stretching of the B-Br bond. The symmetric and asymmetric stretching vibrations of P-O-P caused the same FT-IR peaks at 546 cm-1 and 931 cm-1 for pristine Ag3PO4 and BrSubPc/Ag3PO4, respectively. The pristine Ag3PO4 can absorb light at wavelengths less than 530 nm, and BrSubPc has two characteristic peaks at 310 nm and 570 nm, respectively (Figure 2C). Compared with pure Ag3PO4, the BrSubPc/Ag3PO4 composite sample shows significantly increased absorption in the visible region, confirming that the Ag3PO4 particles are successfully covered by BrSubPc microcrystals. This can prove that the BrSubPc/Ag3PO4 composite is a very promising visible-light-induced photocatalyst.
The photocatalytic activity of the as-prepared materials was assessed following the degradation of the antibiotic TC in pure water under simulated visible light irradiation (λ > 400 nm). As shown in Figure 3A, the photocatalytic performance of pristine Ag3PO4 showed only 72.86% degradation of TC after 0.5 h of visible light irradiation. It can be observed that all the composite photocatalysts showed enhanced degradation of TC when BrSubPc supramolecular nanocrystals were loaded on the surface of Ag3PO4. In particular, BrSubPc/Ag3PO4 (1:50) achieved 94.54% degradation of TC after 0.5 h of visible light illumination, respectively. A pseudo-first-order reaction model (l−ln (C/C0) = kt)28, where k is the apparent rate constant, was used to fit the kinetics of photodegradation of TC by different samples. As shown in Figure 3B, the apparent rate constant of TC degradation by BrSubPc/Ag3PO4 (1:50) composites was 1.69 times higher than that of the pristine Ag3PO4. The above results indicate that the photocatalytic performance of Ag3PO4 is significantly improved when Ag3PO4 is combined with BrSubPc supramolecular nanocrystals.
The photostability and reusability of photocatalysts are important factors affecting their practical applications, and recycling degradation experiments were conducted on the as-prepared pristine Ag3PO4 and BrSubPc/Ag3PO4 (1:50) composites. Figure 3C shows that after five cycles of the prepared catalysts, the composite still showed a high TC removal rate of 77.5%. However, the TC removal by pristine Ag3PO4 decreased from 72.86% to 20.84%. In addition, XRD analysis of the cycled composite BrSubPc/Ag3PO4 (1:50) samples showed that the XRD peaks of the cycled samples did not change compared with the XRD of the original samples (Figure 4), which proved the good stability of the composite samples in the photocatalytic reaction. The ICP-OES test results of the reaction solution after five cycles showed that the concentration of elemental silver in the solution after the reaction of pristine Ag3PO4 was 1.3 mg/L, while the concentration of elemental silver in the solution after the reaction of the composite sample of BrSubPc/Ag3PO4 (1:50) was 0.1 mg/L (Table 1). This indicates that the composite sample photocatalytic reaction has better stability compared to that of pristine Ag3PO4.
In the photocatalytic process, the amount of photocatalyst dosage also has an important influence on the photocatalytic effect, too little dosage may lead to lower light utilization efficiency and poor photocatalytic effect, and too much photocatalyst dosage may lead to higher cost and uneconomical. Too little amount of photocatalyst may lead to lower light utilization efficiency and poor photocatalytic effect, while too much amount of photocatalyst may lead to higher cost and uneconomical treatment of wastewater. Therefore, it is important to determine the optimal photocatalyst dosage. As can be seen from Figure 5A, after 30 min of dark reaction, the adsorption and removal of tetracycline increased as the concentration of the photocatalyst in the reaction solution increased (the dosage increased) because the concentration of tetracycline as the adsorbent in the solution remained the same, while the concentration of the photocatalyst as the adsorbent increased, which means that the active point on the surface of the adsorbent in the solution also increased, and the probability of collision adsorption with the adsorbent increased. This means that the probability of collisional adsorption with the adsorbate increases, resulting in a decrease in the concentration of adsorbate in the solution. The degradation rate of TC by photocatalysts at 0.6 g/L, 0.8 g/L, 1 g/L, 1.2 g/L, and 1.4 g/L was 71.6%, 75.0%, 94.5%, 95.7%, and 95.7% after 30 min of light reaction, respectively. When the concentration of the catalyst exceeded 1.0 g/L, the degradation rate of TC could reach more than 90% in 30 min of photoreaction. From the above analysis, it can be seen that when the concentration of photocatalyst is 1.4 g/L, the best removal effect of tetracycline is achieved, and the photocatalytic effect was not greatly improved compared with the catalyst concentration of 1.0 g/L, while the catalyst dosage was 40% higher. The analysis of the degradation kinetic data in Figure 5B also shows that 1.4 g/L and 1.2 g/L are not significantly different compared to 1.0 g/L. From the economic point of view, the optimal dosage of composite material is 1.0 g/L.
As can be seen in Figure 5C, the effect of pH on the photocatalytic degradation of the composite material for the removal of TC is relatively large. The TC aqueous solution pH was detected to be 6, showing the best degradation efficiency. The photocatalytic performance of the composites was slightly reduced in acidic solutions, while the TC degradation efficiency was more attenuated in neutral and alkaline solutions. The maximum kinetic data for degradation TC can also be seen in Figure 5D at solution pH = 6. In alkaline solutions with high pH, tetracycline will be present in the solution in the form of TC–, which will have electrostatic repulsion with the catalyst, resulting in poor degradation of tetracycline. In acidic solutions with low pH, tetracycline is mainly present in the solution as TC+, and H+ will compete with TC+ in the solution to be absorbed by the photocatalyst, inhibiting the TC+ contact with the photocatalyst, thus reducing the photocatalytic activity in the system.
In reality, antibiotic wastewater often also contains some anions (Cl–, SO42-, NO3–, CO32-, etc.), and these common anions may also affect the photocatalytic process. As can be seen in Figure 5E, the addition of SO42- inhibited the adsorption of TC molecules on the catalyst surface during the dark reaction phase. This may be since SO42-, as a negatively charged anion, competes with the tetracycline molecules for the active site on the photocatalyst surface, resulting in a reduction in the number of tetracycline molecules that can undergo catalytic oxidation or the formation of a highly polar environment close to the photocatalyst surface, preventing the expansion of tetracycline to the active site of the photocatalyst37. When the light reaction was carried out for 30 min, the TC degradation rate in the system without the anion was 94.5%, while in the system with the Cl–, SO42-, NO3–, and CO32- anion, the TC degradation rate was 79.2%, 77.3%, 85%, and 80.3%, respectively. TC degradation kinetic data also reflects the inhibition of TC degradation by the addition of all anions (Figure 5F). The addition of all anions had an inhibitory effect on the photocatalytic degradation of TC, but the TC degradation rate was not overly affected.
The results of the effect of temperature on the photocatalytic degradation of TC are shown in Figure 5G. The degradation rates were 35.3%, 70.6%, 94.5%, 96.5%, and 98.0% for 30 min of photoreaction at 10°C, 20°C, 30°C, 40°C, and 50 °C, respectively. The degradation rate of tetracycline gradually increased with the increase in temperature. The degradation kinetic data for TC from Figure 5H also shows that temperature has a large effect on the degradation efficiency. Tetracycline molecules migrate more quickly as a result of the rising temperature of the solution, making them easier to adsorb when in contact with the catalyst surface. Additionally, at higher temperatures, photogenerated electron-hole pairs more actively, allowing electrons to bind to adsorbed oxygen more quickly and holes to produce hydroxyl radicals with –OH in water more quickly, which speeds up the destruction of tetracycline38.
Figure 1: SEM images. (A,B) Ag3PO4. The left side shows a low-resolution image, and the right side provides a magnified image. (C) BrSubPc and (D) BrSubPc/Ag3PO4. All samples were measured in the powder state. Please click here to view a larger version of this figure.
Figure 2: XRD, FT-IR, and UV-Vis spectra of the samples. (A) XRD patterns. For XRD analysis, the scanning range was 10°-80°, and the scanning speed was 8°/min. The numbers placed vertically at the bottom indicate the corresponding crystal plane. (B) FT-IR spectrum. All samples were tested in the dried powder state. (C) UV-vis spectra of the samples. Solid powders were used for the measurement at a range of 200-800 nm. Please click here to view a larger version of this figure.
Figure 3: TC photocatalytic degradation. (A) TC photocatalytic degradation, the vertical coordinate C0 indicates the initial absorbance of TC (0.664) measured using a UV-vis spectrophotometer, and C indicates the absorbance of TC at each sampling point. (B) The apparent rate constants k for TC photodegradation of Ag3PO4 and BrSubPc/Ag3PO4, calculated from the pseudo-first-order reaction model (l-ln(C/C0) = kt). (C) Cycle experiment of BrSubPc/Ag3PO4 (1:50) for TC photocatalytic degradation reaction, the latter reactions are all based on the samples collected after the previous step. Please click here to view a larger version of this figure.
Figure 4: XRD patterns of BrSubPc/Ag3PO4. XRD patterns of BrSubPc/Ag3PO4 (1:50) before and after the photocatalytic reaction at a scanning range of 10°-80° and a scanning speed of 8°/min. Please click here to view a larger version of this figure.
Figure 5: Exploring TC photocatalytic degradation under the influence of different factors. (A) different catalyst dosages, (C) different pH, (E) different anions and (G) different temperatures. The apparent rate constants k for TC photodegradation using (B) different catalyst dosages, (D) different pH, (F) different anions, and (H) different temperatures. Please click here to view a larger version of this figure.
Sample | Test elements | Sample elemental content (mg/L) |
Ag3PO4 | Ag | 1.3 |
BrSubPc:Ag3PO4 (1:50) | Ag | 0.1 |
Table 1: ICP-OES data. Ag elemental concentration data in the reaction solution after five cycles of testing using ICP-OES.
In this paper, we present a complete methodology for evaluating the catalytic performance of photocatalytic materials, including the preparation of catalysts, the investigation of factors affecting photocatalysis, and the performance of catalyst recycling. This evaluation method is universal and applicable to all photocatalytic material performance evaluations.
In terms of material preparation methods, many schemes have been reported for the preparation of rhombic dodecahedral Ag3PO4 using different precursors21,22. The method we have used is relatively homogeneous in terms of the shape of the Ag3PO4 synthesized, the synthesis process is simple, large quantities can be synthesized, and there are fewer factors affecting the experimental process. It should be noted that ammonium nitrate, a raw material for the synthesis of Ag3PO4, is an oxidizing agent and is subject to explosive decomposition by violent impact or heat, so it should be stored and used to avoid violent impact. In the synthesis of the composites, BrSubPc was firstly dissolved in sufficient amount of ethanol solution to destroy the weak forces between BrSubPc molecules (hydrogen bonding, π−π interaction), then Ag3PO4 was added in an appropriate amount, and the ethanol was evaporated by heating, during which the BrSubPc molecules reassemble themselves on the Ag3PO4 surface through intermolecular hydrogen bonding and π−π interaction.
The effect of different catalyst amounts, solution pH, anions in solution, and reaction temperature on the photocatalytic performance of the prepared materials was investigated. The airflow rate, the intensity of the light source, and the distance of the light source from the reactor should be controlled when doing photocatalytic reactions with different influencing factors. When filtering samples using 0.22 µm nylon membrane, it should be noted that not all degradation contaminants are suitable for use with 0.22 µm nylon membrane as some contaminants are inherently blocked by 0.22 µm nylon membrane, in which case centrifugation should be used to separate the catalyst from the reaction solution. Therefore, a 0.22 µm nylon membrane should be used to filter a simple solution of contaminants without a catalyst to exclude the possibility that the contaminants themselves may be blocked by the 0.22 µm nylon membrane.
A catalyst can only be considered to be a promising photocatalyst if it shows good catalytic performance under this evaluation system and not if only a single influencing factor is studied without taking into account environmental factors. In addition, to promote the healthy development of the field of photocatalytic environmental purification, we believe that the same evaluation criteria should be set for the same pollutant, for example, a uniform TC concentration of 20 mg/L, a catalyst dosage of 1 g/L, a light intensity of 350 mW/cm2, an airflow rate of 100 mL/min and a temperature of 30 °C should be used for TC degradation, so that the best catalyst for degrading the same pollutant can be selected by comparing different literature reports.
The photocatalytic performance of the photocatalyst is more comprehensive than that reported in some papers39,40,41, especially in the laboratory photocatalytic experiments to ensure a stable oxygen content in the water and to take into account the thermal effect. The limitation of this scheme is that it does not consider the effect of reactor optical thickness and catalyst optical properties on photocatalytic performance, both of which are important when performing scale-up labs42,43,44. This scheme provides a reference for evaluating the removal of antibiotic-like molecules from water by photocatalysts in the laboratory and compensates for the lack of uniform criteria for evaluating the photocatalytic water purification ability of photocatalysts in the field. This research protocol can be extended to other photocatalytic fields, such as photocatalytic hydrogen production and photocatalytic carbon dioxide reduction45,46. It is recommended that each field should have a set of strict research protocol criteria for evaluating the catalytic performance of catalysts, which will help to select the best photocatalysts for early experimental industrial applications.
The authors have nothing to disclose.
This work was supported by the National Natural Science Foundation of China (21606180), and the Natural Science Basic Research Program of Shaanxi (Program No. 2019JM-589).
300 W xenon lamp | CeauLight | CEL-HXF300 | |
AgNO3 | Aladdin Reagent (Shanghai) Co., Ltd. | 7783-99-5 | |
Air Pump | Samson Group Co. | ACO-001 | |
BBr3 | Bailingwei Technology Co., Ltd. | 10294-33-4 | |
Constant temperature circulating water bath | Beijing Changliu Scientific Instruments Co. | HX-105 | |
Dichloromethane | Tianjin Kemiou Chemical Reagent Co., Ltd. | 75-09-2 | |
Ethanol | Tianjin Fuyu Fine Chemical Co., Ltd. | 64-17-5 | |
Fourier-transform infrared | Bruker | Vector002 | |
Hexane | Tianjin Kemiou Chemical Reagent Co., Ltd. | 110-54-3 | |
HNO3 | Aladdin Reagent (Shanghai) Co., Ltd. | 7697-37-2 | |
ICP-OES | Aglient | 5110 | |
K2HPO4 | Aladdin Reagent (Shanghai) Co., Ltd. | 16788-57-1 | |
Magnesium Sulfate | Tianjin Kemiou Chemical Reagent Co., Ltd. | 10034-99-8 | |
Methanol | Tianjin Kemiou Chemical Reagent Co., Ltd. | 67-56-1 | |
NaOH | Aladdin Reagent (Shanghai) Co., Ltd. | 1310-73-2 | |
NH4NO3 | Sinopharm Group Chemical Reagent Co., Ltd. | 6484-52-2 | |
o-dichlorobenzene | Tianjin Fuyu Fine Chemical Co., Ltd. | 95-50-1 | |
o-dicyanobenzene | Sinopharm Group Chemical Reagent Co., Ltd. | 91-15-6 | |
Scanning electron microscopy | JEOL | JSM-6390 | |
Trichloromethane | Tianjin Kemiou Chemical Reagent Co., Ltd. | 67-66-3 | |
Ultraviolet-visible Spectrophotometer | Shimadzu | UV-3600 | |
X-ray diffractometer | Rigaku | D/max-IIIA |