Removal of Tetracycline Hydrochloride by Photocatalysis Using Electrospun PAN Nanofibrous Membranes Coated with g-C3N4/Ti3C2/Ag3PO4

In order to improve the photocatalytic performance of g-C3N4, the g-C3N4/Ti3C2/Ag3PO4 S-type heterojunction catalyst was prepared by electrostatic assembly method, and then the g-C3N4/Ti3C2/Ag3PO4/PAN composite nanofiber membrane was prepared by electrospinning technology. The morphology and chemical properties of the nanofiber membrane were characterized by SEM, FTIR, and XRD, and the photocatalytic degradation of tetracycline hydrochloride (TC) in water by the nanofiber membrane was investigated. The results showed that g-C3N4/Ti3C2/Ag3PO4 could be successfully loaded on PAN and uniformly distributed on the surface of composite nanofiber membrane by electrospinning technology. Increasing the amount of loading and catalyst, lowering the pH value and TC concentration of the system were conducive to the oxidation and degradation of TC. The nano-fiber catalytic membrane had been recycled five times and found to have excellent photocatalytic stability and reusability. The study of catalytic mechanism showed that h+, •OH and •O2− were produced and participated in the oxidation degradation reaction of TC, and •O2− plays a major role in catalysis. Therefore, this work provides a new insight into the construction of high-performance and high-stability photocatalytic system by electrospinning technology.


Introduction
Since 2020, a large number of antibiotics have been used to eliminate the global pandemic COVID-19. The following antibiotic pollution has become the main factor threatening human health and the balance of aquatic ecosystem, which seriously restricts the rapid development of China's water environment ecological compensation and livelihood project [1]. There is thereby an urgent need, but it is still a significant challenge to eliminate residual antibiotics in surface water, groundwater and even drinking water.
Photocatalytic technology has the advantages of strong oxidation capacity, simple reaction conditions, high efficiency, low cost and harmless final products, and is considered to be an effective technology for treating antibiotic wastewater and reducing its drug resistance [2]. At present, the photo-catalysts used mainly include TiO 2 [3], Ag 3 PO 4 [2,4] and ZnIn 2 S 4 [5]. Although these photo-catalysts have significant degradation effects on antibiotics, there are still many shortcomings [4]: Fast recombination rate of photo-generated carriers, low utilization rate of visible light, poor stability, poor recovery and reuse performance, etc. Therefore, it is urgent to develop a solar-driven catalyst for the efficient degradation of antibiotic pollutants in water. Metal-free organic photocatalysis merges organic synthesis with photochemistry and enables many challenging organic transformations to take place under ambient conditions while using photon as the sole energy  Figure 2 presents the XRD patterns of PAN nano-fibers and the intensity peaks at 16.33° and 22.42° correspond to typical characteristic diffraction peaks of PAN nano fibers [22]. The intensity peaks at 27.33°, 31.85° and 45.85° of g-C3N4/Ti3C2/Ag3PO4 correspond to (002), (200), (111) diffraction of g-C3N4, respectively. Some peaks also appeared at 2θ = 6.21° and 44.04°. These were detected in the X-ray diffraction pattern of Ti3C2, which were distributed to (002) and (105) diffraction of Ti3C2. The diffraction peak at 37.87° corresponds to the (211) crystal plane of Ag3PO4 in the g-C3N4/Ti3C2/Ag3PO4 catalyst. It is worth noting that the XRD spectrum of g-C3N4/Ti3C2/Ag3PO4/PAN corresponds to the obvious diffraction peak of PAN, and the diffraction peak of g-C3N4/Ti3C2/Ag3PO4 is weak. These were indicative that during electro-spinning, g-C3N4/Ti3C2/Ag3PO4 catalyst was dispersed into PAN fiber.  Figure 1. FTIR spectra of PAN, g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 and g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 /PAN. Figure 2 presents the XRD patterns of PAN nano-fibers and the intensity peaks at 16.33 • and 22.42 • correspond to typical characteristic diffraction peaks of PAN nano fibers [22]. The intensity peaks at 27.33 • , 31.85 • and 45.85 • of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 correspond to (002), (200), (111) diffraction of g-C 3 N 4 , respectively. Some peaks also appeared at 2θ = 6.21 • and 44.04 • . These were detected in the X-ray diffraction pattern of Ti 3 C 2 , which were distributed to (002) and (105) diffraction of Ti 3 C 2 . The diffraction peak at 37.87 • corresponds to the (211) crystal plane of Ag 3 PO 4 in the g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst. It is worth noting that the XRD spectrum of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 /PAN corresponds to the obvious diffraction peak of PAN, and the diffraction peak of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 is weak. These were indicative that during electro-spinning, g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst was dispersed into PAN fiber.  Figure 2 presents the XRD patterns of PAN nano-fibers and the intensity peaks at 16.33° and 22.42° correspond to typical characteristic diffraction peaks of PAN nano fibers [22]. The intensity peaks at 27.33°, 31.85° and 45.85° of g-C3N4/Ti3C2/Ag3PO4 correspond to (002), (200), (111) diffraction of g-C3N4, respectively. Some peaks also appeared at 2θ = 6.21° and 44.04°. These were detected in the X-ray diffraction pattern of Ti3C2, which were distributed to (002) and (105) diffraction of Ti3C2. The diffraction peak at 37.87° corresponds to the (211) crystal plane of Ag3PO4 in the g-C3N4/Ti3C2/Ag3PO4 catalyst. It is worth noting that the XRD spectrum of g-C3N4/Ti3C2/Ag3PO4/PAN corresponds to the obvious diffraction peak of PAN, and the diffraction peak of g-C3N4/Ti3C2/Ag3PO4 is weak. These were indicative that during electro-spinning, g-C3N4/Ti3C2/Ag3PO4 catalyst was dispersed into PAN fiber.  Figure 2. The XRD pattern of PAN, g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 and g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 /PAN.  Figure 3 shows and compares the optical absorption performance of PAN, g-C 3 N 4 /Ti 3 C 2 / Ag 3 PO 4 and g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 /PAN. DRS spectra show that PAN nano-fibers have optical absorption properties only for wavelengths less than 230 nm. In addition, it is not difficult to find that the g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst has excellent light absorption performance in the wavelength range of 200-800 nm, which provides a basic condition for preparing the g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst with excellent photocatalytic performance. It is worth noting that the DRS spectrum of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 /PAN is similar to that of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst, both of which have excellent light absorption properties [23]. This means that the prepared composite nano-fibers have excellent photocatalytic properties, and PAN nanofibers have no significant effect on the optical absorption properties of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst.

Analysis of UV-Vis Diffuse Reflectance Spectra (DRS)
Molecules 2023, 28, x FOR PEER REVIEW 5 of 1 Figure 3 shows and compares the optical absorption performance of PAN, g C3N4/Ti3C2/Ag3PO4 and g-C3N4/Ti3C2/Ag3PO4/PAN. DRS spectra show that PAN nano fibers have optical absorption properties only for wavelengths less than 230 nm. I addition, it is not difficult to find that the g-C3N4/Ti3C2/Ag3PO4 catalyst has excellent ligh absorption performance in the wavelength range of 200-800 nm, which provides a basi condition for preparing the g-C3N4/Ti3C2/Ag3PO4 catalyst with excellent photocatalyti performance. It is worth noting that the DRS spectrum of g-C3N4/Ti3C2/Ag3PO4/PAN i similar to that of g-C3N4/Ti3C2/Ag3PO4 catalyst, both of which have excellent ligh absorption properties [23]. This means that the prepared composite nano-fibers hav excellent photocatalytic properties, and PAN nanofibers have no significant effect on th optical absorption properties of g-C3N4/Ti3C2/Ag3PO4 catalyst.

Analysis of Zeta Potential
Malvern Zetasizer Nano ZSE analyzer (ZEN3700, MALVERNa) was used to measur the Zeta potential of g-C3N4 and Ti3C2 aqueous dispersions, and the results are shown i Figure 4. The Zeta potential of g-C3N4 and Ti3C2 dispersions is 16.8mV and −25.8mV respectively. The Zeta potential test shows that when Ti3C2 is added to the g-C3N dispersion, Ti3C2 and g-C3N4 can form the g-C3N4/Ti3C2 catalyst through electrostati attraction, and Ag + can be adsorbed on the surface of Ti3C2 through electrostatic assembly further forming the composite catalyst. This provides theoretical support for th preparation of stable composite catalysts.

Analysis of Zeta Potential
Malvern Zetasizer Nano ZSE analyzer (ZEN3700, MALVERNa) was used to measure the Zeta potential of g-C 3 N 4 and Ti 3 C 2 aqueous dispersions, and the results are shown in Figure 4. The Zeta potential of g-C 3 N 4 and Ti 3 C 2 dispersions is 16.8 mV and −25.8 mV respectively. The Zeta potential test shows that when Ti 3 C 2 is added to the g-C 3 N 4 dispersion, Ti 3 C 2 and g-C 3 N 4 can form the g-C 3 N 4 /Ti 3 C 2 catalyst through electrostatic attraction, and Ag + can be adsorbed on the surface of Ti 3 C 2 through electrostatic assembly, further forming the composite catalyst. This provides theoretical support for the preparation of stable composite catalysts.

Morphology Analysis
The SEM studies revealed the surface morphology of PAN nano-fibers and its composite catalytic film, as shown in Figure 5. The SEM images confirm that nanofibers in the PAN before composite showed vertical, random and uniform dispersion, as shown in Figure 5(a1-a2). The transmission electron microscopy (TEM) images show that the g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst was successfully synthesized by electrostatic assembly in Figure 5c. After the composite of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst and nano-fibers, it can be observed that there are many granular substances uniformly distributed on the fiber surface ( Figure 5(b1)), which indicates that the catalyst has been successfully fixed to the fiber surface [24]. Further enlarge the multiple of the SEM diagram. From the SEM diagram, it can be seen that the diameter of the fiber increases after the catalyst is compounded,

Analysis of Zeta Potential
Malvern Zetasizer Nano ZSE analyzer (ZEN3700, MALVERNa) was used to measure the Zeta potential of g-C3N4 and Ti3C2 aqueous dispersions, and the results are shown in Figure 4. The Zeta potential of g-C3N4 and Ti3C2 dispersions is 16.8mV and −25.8mV respectively. The Zeta potential test shows that when Ti3C2 is added to the g-C3N4 dispersion, Ti3C2 and g-C3N4 can form the g-C3N4/Ti3C2 catalyst through electrostatic attraction, and Ag + can be adsorbed on the surface of Ti3C2 through electrostatic assembly, further forming the composite catalyst. This provides theoretical support for the preparation of stable composite catalysts.

Morphology Analysis
The SEM studies revealed the surface morphology of PAN nano-fibers and its composite catalytic film, as shown in Figure 5. The SEM images confirm that nanofibers in the PAN before composite showed vertical, random and uniform dispersion, as shown in Figure 5(a1-2). The transmission electron microscopy (TEM) images show that the g-C3N4/Ti3C2/Ag3PO4 catalyst was successfully synthesized by electrostatic assembly in Figure 5c. After the composite of g-C3N4/Ti3C2/Ag3PO4 catalyst and nano-fibers, it can be observed that there are many granular substances uniformly distributed on the fiber surface ( Figure 5(b1)), which indicates that the catalyst has been successfully fixed to the fiber surface [24]. Further enlarge the multiple of the SEM diagram. From the SEM diagram, it can be seen that the diameter of the fiber increases after the catalyst is compounded, which may be caused by the influence of the powder catalyst on the viscosity and other properties of the PAN spinning solution during the electro-spinning process, as shown in Figure 5(b2).

Photocatalytic Performance of Nanofiber Photocatalysis Film
Nano-fiber photocatalytic film was applied to the photocatalytic oxidation degradation of TC under different conditions, and the effect of reaction conditions on its photocatalytic performance was investigated, as shown in Figure 6. It can be seen in Figure  6 (a) that when PAN nano-fibers exist, the R% of TC at 120 min is only 17.90%, which is mainly due to the adsorption of PAN fibers on TC molecules. In the presence of nano-fiber photo-catalysis film, the R% of TC increased significantly with time, and the R% value increased with the increase of g-C3N4/Ti3C2/Ag3PO4 loading. This indicates that the increase of g-C3N4/Ti3C2/Ag3PO4 loading on the surface of nano-fibers can promote the oxidation degradation of TC. This is mainly because the increase of catalyst loading is conducive to the decomposition of water molecules or oxygen molecules to produce more highly oxidizing •OH and •O2 − under the condition of light radiation, which increases the number of active free radicals participating in the oxidation degradation reaction of TC molecules in aqueous solution, thus increasing the R% value of TC. It is worth noting that when the loading amount of nano-fibers is more than 300 mg·g −1 , the R% value of TC decreases in the same time. This means that excessive catalyst loading is not conducive to the oxidative degradation reaction. On the one hand, it is easy to agglomerate when the content of catalyst in the spinning solution is too high, reducing the specific surface area of the catalyst, and thus reducing its catalytic performance. On the other hand, in the process of electro-spinning, a large number of g-C3N4/Ti3C2/Ag3PO4 catalysts were coated

Photocatalytic Performance of Nanofiber Photocatalysis Film
Nano-fiber photocatalytic film was applied to the photocatalytic oxidation degradation of TC under different conditions, and the effect of reaction conditions on its photocatalytic performance was investigated, as shown in Figure 6. It can be seen in Figure 6a that when PAN nano-fibers exist, the R% of TC at 120 min is only 17.90%, which is mainly due to the adsorption of PAN fibers on TC molecules. In the presence of nano-fiber photo-catalysis film, the R% of TC increased significantly with time, and the R% value increased with the increase of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 loading. This indicates that the increase of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 loading on the surface of nano-fibers can promote the oxidation degradation of TC. This is mainly because the increase of catalyst loading is conducive to the decomposition of water molecules or oxygen molecules to produce more highly oxidizing •OH and •O 2 − under the condition of light radiation, which increases the number of active free radicals participating in the oxidation degradation reaction of TC molecules in aqueous solution, thus increasing the R% value of TC. It is worth noting that when the loading amount of nano-fibers is more than 300 mg·g −1 , the R% value of TC decreases in the same time. This means that excessive catalyst loading is not conducive to the oxidative degradation reaction. On the one hand, it is easy to agglomerate when the content of catalyst in the spinning solution is too high, reducing the specific surface area of the catalyst, and thus reducing its catalytic performance. On the other hand, in the process of electro-spinning, a large number of g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalysts were coated inside the nano-fibers, which could not be effectively contacted during the oxidation degradation of TC.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 14 inside the nano-fibers, which could not be effectively contacted during the oxidation degradation of TC.  Figure 6b shows the effect of pH value on the photocatalytic degradation performance of nano-fiber photo-catalysis film. It can be seen from the figure that in the range of pH = 3-9, the R% value of TC increases significantly with time, and the R% value at 120 min is higher than 70%. This means that the nano-fiber photocatalytic film has wide pH adaptability. It is not difficult to find that the R% value of TC gradually decreases with the increase of pH value at the same time. This indicates that raising the pH value is not conducive to the oxidative degradation reaction. This is mainly because under acidic conditions, TC mainly exists in the form of cationic TCH3 + [25], and the surface of nanofibers has negative ionic groups. The electrostatic attraction enables the nano-fibers to rapidly absorb TC, thus accelerating the reaction. When the pH value increases, TC exists in the form of zwitterion (TCH2 0 or TCH1 − ) [25], which leads to the reduction of electrostatic attraction and adsorption between nanofibers and TC molecules. Under the condition of light radiation, the nano-fiber photocatalytic film was applied to the oxidation degradation reaction of different initial concentrations of TC to study the relationship between the concentration of pollutants and the catalytic performance of the system (Figure 6c). It can be seen from the figure that with the increase of TC concentration, its R% value gradually decreases. This means that the increase of TC concentration in aqueous solution in the presence of nano-fiber photocatalytic film is not conducive to its oxidative degradation reaction. On the one hand, the increase of the initial concentration of TC makes many TC molecules gather to form a multi-molecule complex, resulting in the increase of steric hindrance, which increases the resistance for the photocatalytic oxidation degradation of TC molecules. On the other hand, the concentration of active free radicals (•OH and •O2 − ) produced by the decomposition of water molecules or dissolved oxygen molecules by g-C3N4/Ti3C2/Ag3PO4 catalyst remains unchanged, and the number of active digits of the reactants adsorbed on the surface of the catalyst is limited. The increase of TC concentration leads to the reduction of the rate of its oxidative degradation reaction with active free radicals. In addition, Figure 6d shows the effect of the amount of nano-fiber photocatalytic film on the R% value. The results  Figure 6b shows the effect of pH value on the photocatalytic degradation performance of nano-fiber photo-catalysis film. It can be seen from the figure that in the range of pH = 3-9, the R% value of TC increases significantly with time, and the R% value at 120 min is higher than 70%. This means that the nano-fiber photocatalytic film has wide pH adaptability. It is not difficult to find that the R% value of TC gradually decreases with the increase of pH value at the same time. This indicates that raising the pH value is not conducive to the oxidative degradation reaction. This is mainly because under acidic conditions, TC mainly exists in the form of cationic TCH 3 + [25], and the surface of nano-fibers has negative ionic groups. The electrostatic attraction enables the nano-fibers to rapidly absorb TC, thus accelerating the reaction. When the pH value increases, TC exists in the form of zwitterion (TCH 2 0 or TCH 1 − ) [25], which leads to the reduction of electrostatic attraction and adsorption between nanofibers and TC molecules.
Under the condition of light radiation, the nano-fiber photocatalytic film was applied to the oxidation degradation reaction of different initial concentrations of TC to study the relationship between the concentration of pollutants and the catalytic performance of the system (Figure 6c). It can be seen from the figure that with the increase of TC concentration, its R% value gradually decreases. This means that the increase of TC concentration in aqueous solution in the presence of nano-fiber photocatalytic film is not conducive to its oxidative degradation reaction. On the one hand, the increase of the initial concentration of TC makes many TC molecules gather to form a multi-molecule complex, resulting in the increase of steric hindrance, which increases the resistance for the photocatalytic oxidation degradation of TC molecules. On the other hand, the concentration of active free radicals (•OH and •O 2 − ) produced by the decomposition of water molecules or dissolved oxygen molecules by g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst remains unchanged, and the number of active digits of the reactants adsorbed on the surface of the catalyst is limited. The increase of TC concentration leads to the reduction of the rate of its oxidative degradation reaction with active free radicals. In addition, Figure 6d shows the effect of the amount of nano-fiber photocatalytic film on the R% value. The results showed that the R% value of TC gradually increased with the increase of the amount of fiber membrane. It is noteworthy that when the fiber dosage is greater than 0.2 g, the R% value of TC will not increase significantly. This is because in the oxidation degradation reaction of TC, the light radiation intensity does not increase, that is, the number of photons exposed to the nano-fiber photo-catalysis film is certain, and the concentration of active free radicals in the solution cannot be increased by continuing to increase the amount of fiber.
To study the decomposition and destruction of aromatic ring structure and unsaturated bond in the process of catalytic oxidation and degradation of TC by nano-fiber photo-catalysis film, Lambda 950 UV-Vis-NIR spectrophotometer was used to analyze the oxidation and degradation of TC in the wavelength range of 220-500 nm, as shown in Figure 7. During the oxidation degradation reaction of TC, the characteristic absorption peak of carbonyl at λ = 357 nm and the structure of TC aromatic ring at λ = 276 nm decreased with the prolongation of reaction time, indicating that most of TC was degraded during the reaction (Figure 7). With the short reaction time, the characteristic absorption peak at λ = 276 nm shifted to 272 nm and 259 nm. These results mean that the conjugated structure of TC is destroyed and decomposed into small molecular fragments during the oxidative degradation reaction.
Molecules 2023, 28, x FOR PEER REVIEW showed that the R% value of TC gradually increased with the increase of the am fiber membrane. It is noteworthy that when the fiber dosage is greater than 0.2 g, value of TC will not increase significantly. This is because in the oxidation degr reaction of TC, the light radiation intensity does not increase, that is, the num photons exposed to the nano-fiber photo-catalysis film is certain, and the concentr active free radicals in the solution cannot be increased by continuing to incre amount of fiber.
To study the decomposition and destruction of aromatic ring structu unsaturated bond in the process of catalytic oxidation and degradation of TC b fiber photo-catalysis film, Lambda 950 UV-Vis-NIR spectrophotometer was analyze the oxidation and degradation of TC in the wavelength range of 220-500 shown in Figure 7. During the oxidation degradation reaction of TC, the chara absorption peak of carbonyl at λ = 357 nm and the structure of TC aromatic ring at nm decreased with the prolongation of reaction time, indicating that most of degraded during the reaction (Figure 7). With the short reaction time, the chara absorption peak at λ = 276 nm shifted to 272 nm and 259 nm. These results mean conjugated structure of TC is destroyed and decomposed into small molecular fra during the oxidative degradation reaction. It is of great significance for practical application to study the reusability o fiber photocatalytic film as heterogeneous photocatalytic reaction catalyst. Un same conditions, five oxidation degradation experiments of TC were carried out o fiber photocatalytic film. As shown in Figure 8, the R% value of TC did not d significantly with the increase of the number of times of reuse of the nano-fiber catalysis film. At the fifth time of reuse, R% value at 120 min was still as high as which showed that the catalytic oxidation activity of the nano-fiber photo-cataly on TC was still high after five repetitions, and there was almost no deactivatio means that the nano-fiber photocatalytic film can be used as a long-term heterogeneous catalyst for the oxidative degradation of TC in water. It is of great significance for practical application to study the reusability of nano-fiber photocatalytic film as heterogeneous photocatalytic reaction catalyst. Under the same conditions, five oxidation degradation experiments of TC were carried out on nano-fiber photocatalytic film. As shown in Figure 8, the R% value of TC did not decrease significantly with the increase of the number of times of reuse of the nano-fiber photo-catalysis film. At the fifth time of reuse, R% value at 120 min was still as high as 78.43%, which showed that the catalytic oxidation activity of the nano-fiber photo-catalysis film on TC was still high after five repetitions, and there was almost no deactivation. This means that the nano-fiber photocatalytic film can be used as a long-term stable heterogeneous catalyst for the oxidative degradation of TC in water.

Photocatalytic Mechanism
The adsorption of TC onto the photocatalysts had an important effect photodegradation of TC. So, the specific surface areas (BET) of blank P C3N4/Ti3C2/Ag3PO4 and g-C3N4/Ti3C2/Ag3PO4/PAN have been determined adsorption using Micromeritics ASAP 2460 equipment, which are 10.0041 m 2 •g −1 m 2 •g −1 and 20.0023 m 2 •g −1 respectively. As displayed in Figure 9a, when the bla exists and without catalyst, the concentration of TC hardly decreased with the in of time. The TC decreased by 4% after 120 min. This suggested that the adsorp photo-degradability performance of blank PAN and light to TC was weak in solution. Under the same conditions, the powder catalysts g-C3N4 C3N4/Ti3C2/Ag3PO4 were used for oxidative degradation of TC in Figure 9a. It sho the R% values of g-C3N4, g-C3N4/Ti3C2/Ag3PO4 and their nano-fiber membranes t 45.7%, 96.2% and 82.8% at 120 min, respectively, which means that the performance of g-C3N4 can be significantly improved through the construction o junction. Moreover, the catalytic performance of the nano-fiber photocatalytic fi not significantly affected. To further clarify the process and mechanism photocatalytic reaction, the effect of free radical trapping agent on the catalytic ac the nano-fiber photocatalytic film was studied, and the main active substances oxidative degradation of TC were determined. As shown in Figure 9b, EDTA-2 and BQ are the trapping agents of hole (h + ), hydroxyl radical (•OH) and sup radical (•O2 − ) respectively [5,26], and their dosage is 1 mM•L −1 . After adding ED IPA and BQ into the reaction system, the R% value of TC decreased from 82.8% t 62.2% and 14.6%, respectively within 120 min of light radiation time. This shows •OH and •O2 − are produced and participated in the photocatalytic degradation r and •O2 − plays a major role in the oxidative degradation of TC.

Photocatalytic Mechanism
The adsorption of TC onto the photocatalysts had an important effect on the photodegradation of TC. So, the specific surface areas (BET) of blank PAN, g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 and g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 /PAN have been determined by N 2 adsorption using Micromeritics ASAP 2460 equipment, which are 10.0041 m 2 ·g −1 , 81.3913 m 2 ·g −1 and 20.0023 m 2 ·g −1 respectively. As displayed in Figure 9a, when the blank PAN exists and without catalyst, the concentration of TC hardly decreased with the increasing of time. The TC decreased by 4% after 120 min. This suggested that the adsorption and photo-degradability performance of blank PAN and light to TC was weak in aqueous solution. Under the same conditions, the powder catalysts g-C 3 N 4 and g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 were used for oxidative degradation of TC in Figure 9a. It shows that the R% values of g-C 3 N 4 , g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 and their nano-fiber membranes to TC are 45.7%, 96.2% and 82.8% at 120 min, respectively, which means that the catalytic performance of g-C 3 N 4 can be significantly improved through the construction of hetero-junction. Moreover, the catalytic performance of the nano-fiber photocatalytic film was not significantly affected. To further clarify the process and mechanism of the photocatalytic reaction, the effect of free radical trapping agent on the catalytic activity of the nano-fiber photocatalytic film was studied, and the main active substances for the oxidative degradation of TC were determined. As shown in Figure 9b, EDTA-2Na, IPA and BQ are the trapping agents of hole (h + ), hydroxyl radical (•OH) and superoxide radical (•O 2 − ) respectively [5,26], and their dosage is 1 mM·L −1 . After adding EDTA-2Na, IPA and BQ into the reaction system, the R% value of TC decreased from 82.8% to 59.3%, 62.2% and 14.6%, respectively within 120 min of light radiation time. This shows that h + , •OH and •O 2 − are produced and participated in the photocatalytic degradation reaction, and •O 2 − plays a major role in the oxidative degradation of TC. With BaSO 4 as the reference, the optical properties of g-C 3 N 4 and Ag 3 PO 4 were tested by UV-Vis-NIR spectrophotometer, and according to the formula (αhv) 1/2 = A(hv − E g ) is calculated [27] in Figure 10a. The band gap widths of the two are 2.66 eV and 2.41 eV respectively. According to the literature calculation, the valence band (VB) and conduction band (CB) positions of [28] g-C 3 N 4 are 1.55 eV and −1.11 eV, respectively, and the valence band and conduction band positions of Ag 3 PO 4 are 2.675 eV and 0.265 eV, respectively. Ti 3 C 2 as an electron transport body can build a S-type heterostructure between the three. Under the condition of light radiation, the electrons in the valence band of g-C 3 N 4 and Ag 3 PO 4 migrate to their conduction bands respectively, and then through the rapid transport of Ti 3 C 2 , the holes in the valence band of g-C 3 N 4 and the electrons in the conduction band of Ag 3 PO 4 recombine, thus inhibiting the recombination of photo-generated electrons and holes in g-C 3 N 4 and Ag 3 PO 4 . Since the potential of the g-C 3 N 4 conduction band is higher than the standard reduction potential of O 2 /•O 2 − (−0.33 eV) [26], the photo-generated electrons retained in the g-C 3 N 4 conduction band can reduce O 2 to •O 2 − . At the same time, because the potential of Ag 3 PO 4 valence band is lower than OH − /•OH (2.38 eV) [26], the photogenerated holes in its valence band can oxidize OH − to •OH. In addition, some photo-generated holes in the valence band of Ag 3 PO 4 also participate in the oxidative degradation reaction. The resulting h + , •OH and •O 2 − subsequently participate in the photocatalytic oxidation degradation of TC. In summary, the main reaction steps of this mechanism under the condition of light radiation are as follows (1)- (5): With BaSO4 as the reference, the optical properties of g-C3N4 and Ag3PO4 were tested by UV-Vis-NIR spectrophotometer, and according to the formula (αhv) 1/2 = A(hv − Eg) is calculated [27] in Figure 10a. The band gap widths of the two are 2.66 eV and 2.41 eV respectively. According to the literature calculation, the valence band (VB) and conduction band (CB) positions of [28] g-C3N4 are 1.55 eV and −1.11 eV, respectively, and the valence band and conduction band positions of Ag3PO4 are 2.675 eV and 0.265 eV, respectively. Ti3C2 as an electron transport body can build a S-type heterostructure between the three. Under the condition of light radiation, the electrons in the valence band of g-C3N4 and Ag3PO4 migrate to their conduction bands respectively, and then through the rapid transport of Ti3C2, the holes in the valence band of g-C3N4 and the electrons in the conduction band of Ag3PO4 recombine, thus inhibiting the recombination of photogenerated electrons and holes in g-C3N4 and Ag3PO4. Since the potential of the g-C3N4 conduction band is higher than the standard reduction potential of O2/•O2 − (−0.33 eV) [26], the photo-generated electrons retained in the g-C3N4 conduction band can reduce O2 to •O2 − . At the same time, because the potential of Ag3PO4 valence band is lower than OH − /•OH (2.38 eV) [26], the photogenerated holes in its valence band can oxidize OH − to •OH. In addition, some photo-generated holes in the valence band of Ag3PO4 also participate in the oxidative degradation reaction. The resulting h + , •OH and •O2 −

Synthesis of Catalyst
(1) g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 composite photo-catalyst: Put the specified amount of urea in a crucible, heat it up to 100 • C in a tubular furnace, and keep it for 20 min, with the heating rate of 5 • C/min; Then raise the temperature to 550 • C, the heating rate is 10 • C/min, and the temperature drops naturally to room temperature after holding for 3 h. After taking out and grinding, add 4.0 g of g-C 3 N 4 powder into 200 mL of 0.5 mol/L nitric acid aqueous solution. After ultrasonic and vigorous stirring, centrifuge and wash it to pH = 7.0. Then, Ti 3 C 2 with 7% mass fraction of g-C 3 N 4 was added into the g-C 3 N 4 dispersion under stirring conditions. Then add 30 mg of silver nitrate into the solution, stir it in dark for 2 h, and then add sodium dihydrogen phosphate. After the electrostatic assembly is completed, centrifuge the precipitate, dry it in vacuum for 12 h at 60 • C, and grind it for use.
(2) Nanofiber photo-catalyst film: 300 mg of composite photo-catalyst into the spinning solution dissolved with 1.0 g of PAN, stirring for 1 h, and ultrasonic treatment for 2 h to obtain a uniformly dispersed functional spinning solution. Subsequently, the g-C 3 N 4 /Ti 3 C 2/ Ag 3 PO 4 nanofiber membrane was prepared by electrospinning technology. The spinning voltage was 20 kV, the receiving distance was 15 cm, and the propulsion speed was 1 mL/h.

Characterization
Scanning electron microscopy (SEM) images were examined with a Hitachi S4800 field emission scanning electron microscope under 20 kV accelerating voltage. The morphology was determined by transmission electron microscopy (TEM, FEI Tecnai G20). The sample structure was determined by X-ray powder diffraction (XRD, Bruker D8). Fourier transform infrared (FTIR) spectroscopy was examined by a Shimadzu IR Prestige-21 spectrometer to collect the vibration modes of functional groups within 4000-400 cm −1 .

Photocatalytic Activity
Use distilled water to prepare 50 mL of tetracycline hydrochloride (TC) aqueous solution with a concentration of 20 mg L −1 (except for special instructions) for use. Adjust the pH value of TC solution by adding H 2 SO 4 or NaOH. Then cut 0.2 g (except for special instructions) of nanofiber photocatalytic film and put it into the above TC aqueous solution and put it in the photo-reactor to keep it at room temperature and dark state for 2 h. After adsorption equilibrium, turn on the light source to carry out the photocatalytic oxidation degradation reaction of TC. The light source is a 300 W xenon lamp and a filter is used (λ ≥ 420 nm). After the reaction starts, at the maximum absorption wavelength of TC at certain intervals (λ = 357 nm), calculate the degradation rate (R%) of TC in the reaction process according to Formula (6): where R% is the degradation rate, C 0 and C t are the initial concentration and time t concentration of TC solution.

Conclusions
The g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 hetero-junction catalyst was prepared by electrostatic assembly method, and then the nano-fiber composite photo-catalyst film was prepared by electrostatic spinning technology. The effects of load, pH value, TC concentration and nanofiber dosage on its catalytic performance were investigated, and the catalytic mechanism was researched. SEM, FTIR and XRD studies showed that g-C 3 N 4 /Ti 3 C 2 /Ag 3 PO 4 catalyst has been successfully loaded onto the surface of nano-fibers. Increasing the amount of loading and catalyst, decreasing the pH value and TC concentration of the system are conducive to the oxidation and degradation of TC. The nano-fiber catalytic membrane has been tested for five time cycles and found to have excellent photocatalytic stability and reusability. The study of catalytic mechanism shows that h + , •OH and •O 2 − are produced and participated in the oxidation degradation reaction of TC, and •O 2 − plays a major role in catalysis.