Preparation method and photocatalytic application of ferrophosphorus co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst

Abstract

The invention relates to a preparation method and photocatalytic application of a ferrophosphorus co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst. Compared with the prior art, the preparation method has the beneficial effects that 1) the novel S-shaped heterojunction structure iron-phosphorus co-doped composite material is prepared, the separation of photo-induced electron-hole pairs can be effectively promoted, and the preparation method is simple and low in cost; 2) the tetracycline photodegradation performance is excellent and can reach 86%, and after three times of circulation, characterization proves that the structure is stable; 3) when the color of the solution is obviously changed in the tetracycline photodegradation process, a mobile phone can be used for gray analysis to realize instant and convenient detection, and 4) a logic gate is designed to provide a new thought for the integrated application of a subsequent intelligent photocatalytic system.The method can be widely applied to the detection and degradation of tetracycline in wastewater, and has important application value and market prospect.

Description

Technical Field

[0001] This invention belongs to the field of photocatalytic degradation technology, specifically relating to a phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst and its preparation method. It can be used for convenient detection of tetracycline and has a strong degradation ability for tetracycline. The design of logic gates provides a new idea for the subsequent integrated application of portable intelligent photocatalytic systems. Background Technology

[0002] With the continuous advancement of modern industry and the acceleration of urbanization, people's awareness of the importance of water resources has significantly increased, leading to the emergence of urban wastewater treatment plants to meet societal needs. Wastewater treatment plants can be likened to key natural filters for water purification. Currently, biodegradation technologies such as the activated sludge process are widely used for water purification, effectively removing conventional dissolved organic matter (DOM) through microbial metabolism. However, these technologies have several significant limitations: on the one hand, they require long treatment times and are highly sensitive to environmental conditions; on the other hand, they exhibit limited efficiency in degrading emerging pollutants with inherent biotoxicity (such as tetracycline). After biological treatment, effluent organic matter (EfOM) inevitably accumulates in urban rivers. Especially in the Yangtze River Delta region, the concentration of tetracycline (tetracycline) in EfOM is very high. As a typical pseudo-persistent pollutant (PpP), tetracycline is not only a common detection target in the environment but also a key driver of antibiotic resistance genes (ARGs), posing a serious threat to ecosystems and human health.

[0003] Since EfOM is exposed to both sunlight and microorganisms in receiving water, and multiple studies have shown that photochemical treatment is a key factor controlling its biodegradability, photocatalysis as a pretreatment step can effectively promote the subsequent biodegradation process. Therefore, developing multifunctional composite materials that can both self-indicate the degradation process and efficiently remove recalcitrant DOM will be a key breakthrough in improving the treatment efficiency of wastewater treatment plants.

[0004] In photocatalysts, nitrided graphite carbon (g-C3N4) is widely used for the degradation of organic pollutants due to its excellent visible light response and ease of synthesis. However, the main limitations of pristine g-C3N4 include rapid recombination of photogenerated electron-hole pairs, limited solar light absorption, and inherently low specific surface area. When coupled with another semiconductor to form a heterojunction, the separation efficiency of photogenerated electron-hole pairs can be significantly improved. Therefore, many researchers have adopted this strategy to enhance the photocatalytic performance of pristine g-C3N4. Zinc oxide (ZnO) possesses high catalytic activity, a wide bandgap, and environmental sustainability, making it suitable for designing high-performance photocatalytic systems. Paul et al. prepared a GCN-ZnO type II heterojunction using a one-step thermal polymerization method, achieving a 90% photocatalytic degradation efficiency for methylene blue (MB), with a photodegradation rate constant approximately 3.2 times that of pure g-C3N4 monomer. Qamar et al. further prepared g-C3N4 / Fe@ZnO composite materials by Fe-doped ZnO. Although the composite material still exhibits the characteristics of a type II heterojunction, its degradation efficiency for MB remains above 90%. This enhancement is attributed to the modulation of the ZnO valence band position by Fe doping. By adjusting the electronic energy level structure of ZnO, the photocatalytic stability of the composite material under complex environments is significantly improved. Zhang et al. introduced n-doping during ZnO calcination using different methods, followed by electrostatic self-assembly with g-C3N4 to construct an S-scheme heterojunction. This system achieves efficient carrier separation and enhances visible light absorption. Their work provides a new strategy for photocatalytic pollutant degradation, showing superior long-term stability and wider applicability compared to traditional type II heterojunctions. Yu et al. found that s-doping into g-C3N4, followed by hybridization with ZnO, transforms the heterojunction from a type II structure to an s-structure. This shift significantly improves the interfacial charge transfer efficiency and suppresses the recombination of photogenerated carriers. Furthermore, s-doping not only broadens the visible light response range of g-C3N4 but also optimizes its band structure, resulting in a stronger built-in electric field at the ZnO / modified g-C3N4 interface. It can be observed that element incorporation can improve the photocatalytic performance of semiconductors under complex environments by adjusting the valence band position, thereby optimizing their band structure. Although many photocatalysts exhibit impressive performance, they still lag behind mainstream technologies when applied to practical wastewater treatment scenarios. Therefore, developing a novel photocatalyst that integrates economic feasibility, portability, accurate pollutant detection, and effective degradation capabilities is of great significance.

[0005] Based on this, this application was developed. Summary of the Invention

[0006] The purpose of this invention is to overcome the technical defects of existing wastewater treatment technologies and provide a phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst with strong tetracycline degradation ability. The portability and accurate pollutant detection of this catalyst have also been studied.

[0007] The present invention also provides a method for preparing the above-mentioned phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst, which can be used for convenient detection of tetracycline and has a strong degradation ability for tetracycline.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst, named CNZ-Fe X P Y It is composed of phosphorus-iron co-doped carbon nitride and zinc oxide, which are composited through an S-type heterojunction. The phosphorus-iron co-doped carbon nitride and zinc oxide exist in the carbon nitride as interstitial doping, with phosphorus (P) replacing some nitrogen atoms and iron (Fe) present as interstitial doping. Furthermore, the best effect is achieved when both Fe and P are doped at 2.5 wt%, and this is named CNZ-Fe. 2.5 P 2.5 The CNZ-Fe mentioned above 2.5 P 2.5 It has a porous nanosheet microstructure and a specific surface area of ​​30.637 m². 2 / g.

[0009] A method for preparing the above-mentioned phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst includes the following steps: (a) A carbon nitride precursor (g-C3N4) was prepared by melamine. (b) A zinc oxide sol precursor was prepared by zinc acetate dihydrate, 2-methoxyethanol and ethanolamine; (c) The carbon nitride precursor obtained in step (a) is ground and mixed with ferric nitrate and ammonium phosphate, then uniformly dispersed in water, and then calcined under an inert gas atmosphere to obtain ferric nitride co-doped with phosphorus. (d) Dissolve the product obtained in step (c) in the zinc oxide sol precursor obtained in step (b) and calcine it at 550±50℃ for 1-2 h in an inert gas atmosphere to obtain the product.

[0010] Further, step (a) specifically involves: placing melamine into a crucible, sealing the crucible with a lid, transferring it to a tube furnace, heating it to 550±50℃ and holding it at this temperature for 3-5 hours, and then cooling it to room temperature to obtain the final product.

[0011] Further, step (b) specifically involves dissolving 11-15g of zinc acetate dihydrate in 40-60mL of a mixed solvent composed of 2-methoxyethanol and ethanolamine, stirring at room temperature for 1-5h, and allowing it to stand for 48±12h to obtain the zinc oxide sol precursor; the volume ratio of 2-methoxyethanol to ethanolamine is 20-28:1.

[0012] Further, in step (c), the carbon nitride is calcined in a tube furnace at 550±50℃ for 1-3 h under a nitrogen atmosphere to obtain co-doped carbon nitride with iron and phosphorus; the doping amounts of iron nitrate and ammonium phosphate are calculated as iron and phosphorus, respectively, and the mass percentages of iron nitrate and ammonium phosphate doping amounts (compared to the carbon nitride precursor g-C3N4) are 2-3% and 2-3%, respectively.

[0013] Furthermore, in step (d), 0.1-0.3 g of the product obtained in step (c) is dissolved in 1-3 mL of the zinc oxide sol precursor obtained in step (b).

[0014] This invention provides a phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst prepared by the above method.

[0015] This invention also provides the application of the above-mentioned phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst in the photocatalytic degradation of tetracycline antibiotics.

[0016] Furthermore, in the above applications, the photocatalytic reaction is performed at 400 W / cm². 2 The process was carried out under xenon lamp irradiation, with a tetracycline degradation concentration of 60 mg / L and a pH value of 5.

[0017] Furthermore, the above-mentioned applications demonstrate the stable color change that occurs during photocatalytic degradation, which can be used for real-time portable detection of tetracycline in water and the design of logic gate sensing systems.

[0018] The S-type heterojunction photocatalyst described in this invention relates to a novel semiconductor material composed of Fe and P doped with g-C3N4 and then composited with ZnO. It achieves a degradation rate of over 85% in high-concentration tetracycline wastewater and exhibits stable performance across three degradation cycles. During the degradation process, a clear color change was observed upon initial exposure to light, with the color gradually fading over time; this color change trend was roughly consistent across eight experiments. Scientific extraction of grayscale values ​​using the mobile app "Color Grab" revealed a correlation between grayscale values ​​and degradation capability, indicating its potential for outdoor use. Users can determine the presence of tetracycline in wastewater via mobile phone and assess the degradation progress through water color. This visual color response is similar to the quantifiable detection principle of smart devices, and by implementing logic gates, it could provide a feasible reference for the subsequent design of novel photodegradation sensors.

[0019] The photocatalyst (CNZ-Fe) of this invention X P Y This invention relates to a novel composite material, which involves first doping g-C3N4 with elements and then combining it with ZnO to form an S-type heterojunction. The photocatalyst of this invention is synthesized through a multiple calcination method. Compared with existing photocatalytic composite materials, this invention has the following advantages and beneficial effects: 1) This invention uses a calcination method, which is safe and environmentally friendly in experimental operation; moreover, the raw materials are inexpensive and suitable for mass production; 2) This invention uses FTIR, XRD, SEM and TEM to accurately determine the specific structure of the composite material; 3) This invention uses UV-vis and Mott-Schottky to determine that the heterojunction changed from type II to type S; 4) This invention systematically verifies the superiority of the composite material from both macroscopic performance and microscopic mechanism perspectives by testing its photocatalytic performance and combining BET, EIS and transient photocurrent response. 5) This experiment determined the degradation pathway using mass spectrometry and liquid chromatography, and predicted the toxicity of degradation intermediates using toxicity assessment software, confirming that the metabolic pathway can significantly reduce the ecotoxicity of tetracycline. 6) This study used the mobile application "Color Grab" to analyze the grayscale values ​​of the solution color during the experiment. The mass of the composite material and the illumination time were set as input signals a and b, respectively, with threshold values ​​set for each. A significant grayscale change occurred when both conditions were simultaneously met. This change, quantified by grayscale, was converted into a binary "1" signal; otherwise, it was a "0" signal. This method transforms the visual colorimetric response into a clear digital logic signal, enabling quantifiable and automated detection compatible with smart devices. Attached Figure Description

[0020] Figure 1 This describes the synthetic pathway for composite materials. Figure 2 In the middle, (a)g-C3N4, ZnO, CNZ and CNZ-Fe X P Y XRD patterns of the samples. (b) g-C3N4, ZnO, CNZ and CNZ-Fe 2.5 P 2.5 FT-IR spectra of the sample; Figure 3 Among them, (a) ZnO, (b) g-C3N4, (c) CNZ, (d) CNZ-Fe 2.5 P 2.5 SEM image of the sample, (e)CNZ-Fe 2.5 P 2.5EDX images of N, C, Zn, O, Fe and P in the sample; Figure 4 Among them, (a)g-C3N4, (b)CNZ, (c)CNZ-Fe 2.5 P 2.5 TEM images of the samples, and (d)CNZ and CNZ-Fe. 2.5 P 2.5 HRTEM image of the sample; Figure 5 The middle is g-C3N4, ZnO, CN-Fe 2.5 P 2.5 CNZ and CNZ-Fe 2.5 P 2.5 XPS spectra of the samples: (a) Measured spectra; (be) High-resolution XPS spectra: O 1s, N 1s, Zn 2p and C 1s; Figure 6 In the middle, (a) g-C3N4, ZnO, CNZ and CNZ-Fe X P Y (b) UV-Vis diffuse reflectance spectrum of the sample. (c) ZnO, (d) g-C3N4, (e) CN-Fe 2.5 P 2.5 Mott-Schottky diagram, (f) band structure of CNZ, (g) CNZ-Fe 2.5 P 2.5 The band structure of CNZ, (h) is a schematic diagram of carrier transfer, and (i) is a CNZ-Fe 2.5 P 2.5 A schematic diagram of carrier transfer; Figure 7 In the middle, (a)g-C3N4, ZnO, CNZ and CNZ-Fe X P Y Comparison of photocatalytic performance, (b) CNZ-Fe 2.5 P 2.5 Optical photographs of the photodegradation process. Different trapping agents: (c) CNZ, (d) CNZ-Fe. 2.5 P 2.5 The performance of the sample in degrading tetracycline. ESR spectra of •OH radicals generated by the sample: (e) CNZ, (f) CNZ-Fe. 2.5 P 2.5 ESR spectra of •O-2 radicals generated by the sample (g) CNZ, (h) CNZ-Fe 2.5 P 2.5 (i) CNZ-Fe under dark and light irradiation 2.5 P 2.5Time dependence of H2O2 evolution in the presence and absence of tetracycline photocatalysis; Figure 8 In the image, (a) shows nitrogen adsorption-desorption in the sample, and (b) shows the pore size distribution of the isotherms. Figure 9 For (a) CNZ-Fe 2.5 P 2.5 Recyclability of degraded tetracycline. (b) FTIR spectrum, (c) XRD pattern, (d) CNZ-Fe before and after three consecutive visible light irradiations. 2.5 P 2.5 (e) A calibration curve showing the relationship between absorbance (y) and PO3-4 concentration (x) for quantifying PO3-4 leaching after cycling experiments. (f) After cycling experiments, for quantifying Fe... 3+ The absorbance (y) of the leached sample and Fe 3+ Calibration curve of concentration (x) relationship; Figure 10 In the middle, (a) g-C3N4, ZnO, CNZ and CNZ-Fe X P Y (a) Transient photocurrent response and (b) electrochemical impedance spectroscopy of the sample; Figure 11 In, (a) CNZ-Fe 2.5 P 2.5 Potential pathways for catalytic degradation of tetracycline, (b) blackhead fish LC50 (96 h), (c) Tetrahymena pyriformis LC50 (48 h), (d) Daphnia magna LC50 (48 h), (e) degradation product heatmaps predicting the toxicity values ​​of each substance and toxicity endpoint. Figure 12 In the diagram, (a) the colorimetric sensing behavior exhibits an "on-off" response under different concentrations of tetracycline and (b) different illumination durations. (c) Logic gates. (d) Gray intensity analysis corresponding to illumination time. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] In the following embodiments, unless otherwise specified, all raw materials used are ordinary commercially available products that can be directly purchased or can be prepared by conventional methods in the art.

[0023] Room temperature refers to 25±5°C.

[0024] Example 1: A phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst, named CNZ-Fe X P Y It is composed of phosphorus-iron co-doped carbon nitride and zinc oxide, wherein the phosphorus-iron co-doped carbon nitride and zinc oxide are composited through an S-type heterojunction, and the P exists in the carbon nitride in the form of interstitial doping, replacing some of the N atoms in the carbon nitride.

[0025] The CNZ-Fe X P Y The preparation method of [the substance] specifically includes the following steps: (a) A carbon nitride precursor (g-C3N4) was prepared from melamine using an easy-burning method. Specifically, 5 g of melamine was placed in a 50 mL crucible. The crucible was then sealed with a lid and transferred to a tube furnace. The furnace was heated to 550 °C at a heating rate of 5 °C / min and held at this temperature for 4 h. After cooling to room temperature, a yellow powdery g-C3N4 sample was obtained.

[0026] (b) A zinc oxide sol precursor was prepared by means of zinc acetate dihydrate, 2-methoxyethanol, and ethanolamine. Specifically, 13.17 g of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) was dissolved in a mixed solvent system consisting of 48 mL of 2-methoxyethanol and 2 mL of ethanolamine. The solution was stirred continuously at room temperature for 2 hours, and then allowed to stand for 48 hours to obtain the ZnO sol precursor.

[0027] (c) 0.5 g of carbon nitride (g-C3N4), 0.09 g of ferric nitrate nonahydrate, and 0.06 g of ammonium phosphate were placed in an agate mortar and manually ground to obtain a homogeneous mixture. The resulting powder was then transferred to a beaker containing 25 ml of deionized water, ultrasonically dispersed for 10 minutes, and magnetically stirred for 12 hours. The solution was then transferred to a crucible and placed in a tube furnace. Initially, the furnace was purged with high-purity nitrogen to establish an inert atmosphere. After the internal environment stabilized, the temperature was gradually increased to 550 °C at a rate of 5 °C / min and calcined at this temperature for 2 hours to obtain iron-phosphorus co-doped carbon nitride nanosheets, named “CN-Fe”. X P Y ",in" X "This indicates the weight percentage of ferric nitrate (as iron)." Y "Indicates the weight percentage of ammonium phosphate (calculated as phosphorus). In this step, the doping amounts of ferric nitrate and ammonium phosphate are calculated as iron and phosphorus, respectively. A co-doped g-C3N4 nanosheet sample with both ferric nitrate and ammonium phosphate doping amounts of 2.5 wt% is labeled as "CN-Fe". 2.5 P 2.5".

[0028] Furthermore, the doping amounts of ferric nitrate and ammonium phosphate were calculated as iron and phosphorus, respectively. Iron-doped g-C3N4 nanosheets with 5 wt% ferric nitrate and phosphorus-doped g-C3N4 nanosheets with 5 wt% ammonium phosphate were labeled "CN-Fe5" and "CN-P5," respectively. Iron-phosphorus co-doped g-C3N4 nanosheets with 1.0 wt% ferric nitrate and 4.0 wt% ammonium phosphate were labeled "CN-Fe1P4"; and iron-phosphorus co-doped g-C3N4 nanosheets with 4.0 wt% ferric nitrate and 1.0 wt% ammonium phosphate were labeled "CN-Fe4P1."

[0029] (d) Take 0.2 g of the CN-Fe obtained in step (c) X P Y The sample was dissolved in 1 mL of the ZnO sol precursor obtained in step (b) and calcined in a tube furnace at 550 °C for 1 h under a nitrogen atmosphere. The resulting yellowish-brown powder composite sample was named "CNZ-Fe". X P Y The meanings of "X" and "Y" are the same as above. The composite sample without P and Fe doping is named "CNZ". For the specific synthetic route, please refer to [link to synthetic route]. Figure 1 .

[0030] After obtaining CNZ-Fe X P Y After a series of samples, the composition and crystal structure of the synthesized composite material were characterized using powder X-ray diffraction (XRD). Figure 2 As shown in (a), the ZnO nanoparticles exhibit an XRD pattern similar to that of commercial ZnO, conforming to the standard reference data for ZnO (JCPDS card number 00-036-1451). The symbols “◇” and “△” represent the characteristic diffraction peaks of ZnO and g-C3N4 crystals, respectively. Except for the g-C3N4 sample, the characteristic peaks at 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8°, 67.9°, and 69.2° represent the (100), (002), (101), (102), (110), (103), (112), and (201) planes of ZnO, respectively. Notably, no additional impurity peaks were observed after Fe and P were doped into the composite system, indicating that they were successfully incorporated into the lattice or formed an amorphous phase. Furthermore, the characteristic peaks of g-C3N4 disappeared in the XRD peaks of CNZ-Fe5, CNZ-P5, and CNZ-Fe4P1 materials, indicating that the introduction of Fe and P effectively disrupted the layered stacking structure of g-C3N4. The Fourier transform infrared (FTIR) images of the prepared samples are shown below. Figure 2 As shown in (b). At 810 cm-1 Absorption peaks corresponding to the 3-S-triazine moiety can be clearly observed at 1240 ~ 1700 cm⁻¹. Additionally, absorption peaks are observed at 1240 ~ 1700 cm⁻¹. -1 The absorption peaks in the wavenumber range are caused by the vibrations of aromatic heterocycles, 2950 ~ 3650 cm⁻¹. -1 The absorption peaks in the wavenumber range are caused by the NH stretching vibration. In the ZnO sample, the peaks are at 465, 1385, and 1633 cm⁻¹. -1 The absorption peaks at these locations correspond to the vibrations of Zn-O, CO, and OH bonds, respectively. In CNZ-Fe... 2.5 P 2.5 In the sample, at 2100 cm -1 A new absorption peak appeared on both sides, which is due to the formation of Fe-C coordination bonds between the hybrid iron and the electron-deficient carbon atoms in g-C3N4. These XRD and FTIR tests confirmed the successful synthesis of the Fe-P co-doped CNZ complex.

[0031] The morphological evolution and structure of the synthesized composite material were characterized using scanning electron microscopy (SEM). Figure 3 As shown in (a, b), the original ZnO exhibits aggregated nanoparticles with a particle size of approximately 100 nm, while g-C3N4 possesses both layered and bulk structures. Figure 3 (c) It can be clearly seen that the ZnO nanospheres in the CNZ sample are uniformly distributed on the g-C3N4 nanosheets. CNZ-Fe 2.5 P 2.5 High-resolution images of the sample, such as Figure 3 As shown in (d). Compared to the intact g-C3N4 nanosheet structure in the CNZ sample, CNZ-Fe 2.5 P 2.5 The doping of Fe and P in the sample disrupted the g-C3N4 nanosheet structure in the composite material. A rough, porous morphology formed by the aggregation of numerous quasi-spherical particles was observed. These particles were densely packed, with abundant protrusions on the surface and clearly defined pore features. This morphological transformation not only increased the specific surface area of ​​the material but also promoted the exposure of additional active sites, thus providing favorable structural properties for improving performance in catalytic and adsorption applications. CNZ-Fe 2.5 P 2.5 The elemental energy dispersive X-ray energy spectrum (EDS) of the sample is shown below. Figure 3 As shown in (e), the composition of C, N, O, Zn, Fe and P is highlighted. EDS measurements confirm that only the essential elements are present in a uniform distribution in the sample.

[0032] The prepared samples were studied in more detail using TEM measurements, such as... Figure 4As shown in (ac), g-C3N4 has a thin-layer structure composed of nanosheets. These nanosheets are 50 nm thick, with smooth surfaces and clear edges. Furthermore, in the CNZ sample, ZnO nanoparticles are clearly visible on the surface of the g-C3N4 nanosheets, and the ZnO nanosheets are 30 nm thick. Then, as... Figure 4 As shown in (d), CNZ and CNZ-Fe 2.5 P 2.5 High-resolution images of the samples show that the lattice fringes with spacing of 0.218 nm and 0.243 nm belong to the (101) plane of wurtzite ZnO and the (100) plane of g-C3N4, respectively.

[0033] Example 2: XPS was used to study the elemental composition and surface electronic flow state of the composite material. Figure 5 (a) is g-C3N4, ZnO, CN-Fe 2.5 P 2.5 CNZ and CNZ-Fe 2.5 P 2.5 Full spectrum of the sample. CNZ-Fe 2.5 P 2.5 The measured spectra confirmed the presence of C, N, O, Zn, Fe, and P, consistent with the EDS analysis results. Notably, iron and phosphorus signals were not clearly observed, likely due to their relatively low concentrations in the composite material. Figure 5 (b) shows g-C3N4, ZnO, CN-Fe 2.5 P 2.5 CNZ and CNZ-Fe 2.5 P 2.5The O1s orbital binding states in the samples. In the ZnO sample, the peaks with binding energies (BE) of 529.818, 531.037, and 532.810 eV belong to lattice oxygen (OL), VO defects, and surface adsorbed oxygen (Oads), respectively. In contrast, the g-C3N4 sample has two characteristic peaks at 531.881 and 533.804 eV, corresponding to COC bonds and OAds, respectively. When ZnO combines with g-C3N4 to form CNZ, its O-1s spectrum exhibits four sub-peaks. The BE values ​​are located at 530.006 (OL), 531.003 (COC), 532.084 (VO defects), and 533.288 eV (Oads), respectively. Notably, compared to single ZnO, the OL peak position of the ZnO component in CNZ shifts positively from 529.818 eV to 530.006 eV. The increase in binding energy clearly indicates a directional electron transfer from ZnO to g-C3N4, resulting in an embedded electric field at the heterojunction interface pointing from ZnO to g-C3N4. This electric field is a key driving force for the effective separation of photogenerated electron-hole pairs. Furthermore, co-doping Fe and P into CNZ composites synthesizes CNZ-Fe. 2.5 P 2.5 After sample processing, the OL peak position shifted significantly to 529.678 eV. This spectral evolution originates from the local electronic structure modulation induced by Fe-P co-doping, which enhances the interfacial charge transfer dynamics by strengthening the orbital hybridization between the transition metal d orbitals and the carbon π* state, thereby optimizing the built-in electric field magnitude and interfacial charge migration efficiency.

[0034] Figure 5 (c) gives g-C3N4, CNZ, CN-Fe 2.5 P 2.5 and CNZ-Fe 2.5 P 2.5 The N 1s orbital binding state of the samples. Three fitting peaks were observed in the g-C3N4 sample at 398.289 eV, 399.977 eV, and 400.973 eV, corresponding to the CN=C bond, N-(C)3 bond, and N-(C)3 bond, respectively. For the CNZ sample, the three fitting peaks were located at 398.231 eV, 399.677 eV, and 400.647 eV. CN-Fe 2.5 P 2.5 The three fitting peaks of the sample are located at 398.273 eV, 399.791 eV, and 400.499 eV, respectively, while CNZ-Fe 2.5 P 2.5 The three fitting peaks of the sample are located at 398.304 eV, 399.816 eV and 400.898 eV, respectively. Figure 5(d) is ZnO, CNZ and CNZ-Fe 2.5 P 2.5 The orbital binding states of Zn 2p in the sample. In the ZnO sample, the characteristic peaks at 1021.139 eV and 1044.213 eV correspond to Zn 2p orbitals, respectively. 3 / 2 and Zn 2p 1 / 2 The spin-orbit splitting peaks were observed. The binding energy difference between the two peaks was 23.074 eV, confirming that Zn exhibits spin-orbit splitting. 2+ The Zn 2p binding energy peaks in the CNZ sample are 1021.226 eV and 1044.387 eV, respectively. Compared with the ZnO sample, the Zn 2p binding energy peak in the CNZ sample shifts towards higher binding energies. This phenomenon indicates that in the CNZ composite material, electrons transfer from ZnO to g-C3N4, thereby forming a built-in electric field from ZnO to g-C3N4. In CNZ-Fe... 2.5 P 2.5 In the sample, the Zn 2p binding energy peak shifts towards lower binding energies, meaning the electron migration direction is opposite to that in the CNZ sample. Therefore, the direction of the internal electric field is opposite, from g-C3N4 to ZnO. Figure 5 (e) g-C3N4, CN-Fe 2.5 P 2.5 CNZ and CNZ-Fe 2.5 P 2.5 In the C 1s XPS of the sample, the carbon orbital binding states are deconvolved through peak fitting to form two characteristic peaks, corresponding to sp... 2 Hybridized NC=N bonds and amorphous CC bonds. The BE values ​​of the characteristic peaks of each sample are as follows: the two characteristic peaks of g-C3N4 are located at 287.760 eV and 284.376 eV, respectively; CN-Fe 2.5 P 2.5 The values ​​for CNZ were 287.750 eV and 284.555 eV, respectively; the values ​​for CNZ and Fe were 287.726 eV and 284.404 eV, respectively; CNZ-Fe 2.5 P 2.5 The values ​​were 288.130 eV and 284.484 eV, respectively. Compared to g-C3N4, the characteristic peak corresponding to the NC=N bond in the CNZ sample shifted towards lower binding energies. And compared to CN-Fe... 2.5 P 2.5 In comparison, CNZ-Fe 2.5 P 2.5 The peak corresponding to the NC=N bond in the sample shifts to a higher binding energy. This shift mode confirms the presence of CNZ and CNZ-Fe. 2.5 P 2.5 The direction of the embedded electric field in the composite material sample is consistent with the analysis results of the Zn 2p orbital binding state.

[0035] Various photocatalysts were measured using ultraviolet-visible diffuse reflectance spectroscopy to evaluate the solar energy harvesting capacity of the prepared samples. For example... Figure 6 As shown in (a), the spectral absorption edges of pristine ZnO and g-C3N4 are located at approximately 400 nm and 460 nm, respectively, indicating their strong visible light trapping ability. These CN-Fe X P Y The samples show that Fe-P co-doping leads to a blue shift in the intrinsic emission of g-C3N4, which is due to the hybridization between the electron orbitals of C, Fe, P, and N, and the corresponding band gap (E). g The values ​​were 2.51 eV (g-C3N4) and 2.87 eV (CN-Fe), respectively. 2.5 P 2.5 ) and 3.20 eV (ZnO), such as Figure 6 (b) shows the determination of g-C3N4 and CN-Fe by Mott-Schottky diagram. 2.5 P 2.5 And the flat band potential of the ZnO sample, such as Figure 6 As shown in (ce). The positive slope of the linear region confirms the n-type semiconductor behavior of the material. Considering that the planar band potential is generally approximated as the conduction band bottom (CBB) of an n-type semiconductor, the CBB potential relative to the saturated calomel electrode (SCE) is denoted as E. CBB Furthermore, the Mott-Schottky diagram of the synthesized photocatalyst shows that ZnO, g-C3N4, CN-Fe 2.5 P 2.5 The flat-band potentials are -0.33, -0.38, and -1.29 V, respectively. (Using the relationship...) At that time, their potentials relative to the standard hydrogen electrode (NHE) were -0.09, -0.24, and -1.05 V, respectively. Based on equation... ZnO, g-C3N4, CN-Fe 2.5 P 2.5 The valence band potentials of the samples were 3.11, 2.37, and 1.82 eV, respectively.

[0036] Figure 6(fi) illustrates the photocatalytic mechanism of tetracycline degradation in the proposed CNZ composite material under visible light irradiation. XPS analysis of the CNZ sample confirmed the spontaneous formation of an intrinsic electric field, pointing from ZnO to g-C3N4. Upon sunlight irradiation, electrons in the VBs of both ZnO and g-C3N4 are excited into their respective CBs, while an equal number of holes are generated in the VBs. Subsequently, under the influence of the intrinsic electric field, conduction band electrons from g-C3N4 migrate to the conduction band of ZnO, while valence band holes from ZnO migrate to the valence band of g-C3N4, establishing a type II carrier transfer mechanism. In CNZ-Fe 2.5 P 2.5 In the sample, a spontaneously generated internal electric field points from g-C3N4 to ZnO. When exposed to sunlight, electrons in the conduction band of ZnO transfer to the valence band of g-C3N4 under the influence of the internal electric field and recombine with photogenerated holes. Therefore, electrons accumulate in the conduction band of g-C3N4, while holes remain in the valence band of ZnO. Subsequently, these spatially separated electrons and holes can directly participate in the photocatalytic degradation of tetracycline. This charge transfer behavior is observed in CNZ-Fe 2.5 P 2.5 An S-type heterojunction mechanism was established in the sample. Furthermore, CN-Fe 2.5 P 2.5 Photoelectrons in the CB are transferred to dissolved oxygen (DO) in the reduced tetracycline solution, generating •O₂. These reactive oxygen species (ROS) act as key oxidation intermediates, promoting the degradation of tetracycline through a redox coupling mechanism.

[0037] Example 3: Take 10 mg of newly synthesized CNZ-Fe X P Y The composite material was added to 80 mL of a 60 mg / L tetracycline solution. The reaction was carried out in a double-layered beaker with a 25°C circulating water jacket: the mixture was first stirred in the dark for 30 min to achieve adsorption-desorption equilibrium, and then exposed to visible light for 2 h. During the experiment, 2 mL samples were taken every 30 min, and after standing and centrifugation, the absorbance of the solution was measured using a UV-Vis spectrophotometer to monitor the degradation process in real time.

[0038] The photocatalytic performance of g-C3N4, ZnO, and CNZ-FeXPY was evaluated by comparing their degradation efficiencies of tetracycline under the same conditions. Figure 7As shown in (a), all samples reached adsorption equilibrium after 30 min of dark adsorption. After 120 min of visible light irradiation, pure g-C3N4 only degraded about 25% of tetracycline, while the CNZ heterojunction composite material showed significantly enhanced photocatalytic activity, with a degradation rate of up to 70% within the same time period. Notably, co-doping with Fe and P in the g-C3N4 component significantly improved the photocatalytic activity of the composite photocatalyst. CNZ-Fe 2.5 P 2.5 The tetracycline degradation rate of the sample was 0.1052 min. -1 The degradation performance of CNZ-Fe was 18.13 times that of the original g-C3N4 and 5.62 times that of the original ZnO, achieving the highest degradation performance. The Fe and P atoms co-doped in g-C3N4 not only provided additional electrons but also induced the level of impurities acting as acceptors. This is beneficial for solar light absorption and the separation of photoexcited charge carriers, thereby improving the degradation performance of CNZ-Fe. X P Y The photocatalytic activity of composite materials in degrading organic pollutants was investigated. An interesting phenomenon was observed in the experiment: specifically, a significant color change was observed in the tetracycline solution before and after the photocatalytic reaction. For example... Figure 7 As shown in (b), before the reaction begins, the solution without catalyst is colorless and transparent. During the dark adsorption stage (corresponding to the third image from the left), the solution color deepens but remains transparent. After illumination is initiated (corresponding to the fourth image from the right), the solution rapidly turns brownish-yellow and gradually fades to light yellow as the reaction time increases. This color evolution is directly related to the concentration changes of tetracycline and its degradation intermediates in the reaction system. In eight repeated cycles of experiments, this phenomenon was consistently reproduced, and the depth of solution color showed a strong positive correlation with its absorbance at a characteristic wavelength. This indicates that, in actual detection or processing, the degree of tetracycline degradation can be rapidly and intuitively assessed by visually observing the solution color change, providing experimental evidence for the application of this material in the visual detection or process monitoring of trace tetracycline.

[0039] In the photocatalytic process, a specific scavenger was applied to the tetracycline degradation system to identify the main reactants. Figure 7 In (c, d), EDTA-2Na, AgNO3, isopropanol (IPA), and p-benzoquinone (BQ) were used as scavengers of holes, electrons, hydroxyl radicals (•OH), and superoxide radicals (•O- 2), respectively. (The text abruptly ends here, likely due to an incomplete sentence or missing information.) 2.5 P 2.5 Compared to the CNZ sample, there was no significant effect before and after using BQ, indicating that no •O- 2 free radicals were generated during photodegradation. Figure 7 (a) shows that the degradation performance of CNZ-Fe 2.5 P2.5 The significantly higher photocatalytic activity of the two materials compared to CNZ confirms that the •O- 2 free radical plays a crucial role in the degradation process. As it is the main active substance in the pores and plays a significant role in tetracycline degradation, it may be the main factor leading to the difference in photocatalytic performance between the two materials. Figure 7 As shown in (eh), electron paramagnetic resonance (ESR) tests revealed that no •O- 2 radicals were detected for CNZ, but a large number of •OH radicals were observed under illumination. Conversely, for CNZ-Fe... 2.5 P 2.5 Under light irradiation, both •OH and •O- radicals were observed. These ESR results are consistent with radical capture experiments, confirming the presence of CNZ and CNZ-Fe. 2.5 P 2.5 Determination of carrier transport mechanism in composite samples. Compared with CNZ, CNZ-Fe... 2.5 P 2.5 The degradation performance of CNZ-Fe remains significantly different. Therefore, the study... 2.5 P 2.5 The presence of a heterogeneous Fenton photocatalytic synergistic degradation and antibacterial system in the sample is a key research objective. The standard curve of absorbance versus H₂O₂ concentration is shown in Figure S4. Figure 7 As shown in (i), TC represents the reaction occurring in a tetracycline-containing environment, while DW represents the reaction occurring in a deionized water system. Under 120 minutes of illumination, the deionized water system produced 18 μM H₂O₂, while the tetracycline solution produced only 9.8 μM H₂O₂. To eliminate interference from other factors, the experiment was repeated under dark conditions. These results indicate that the presence of tetracycline activates more of the generated H₂O₂ into •OH radicals, thereby mineralizing the organic molecules in the tetracycline and consuming them in the process.

[0040] The specific surface area and pore properties of the synthesized samples were characterized using N2 adsorption-desorption isotherms, such as... Figure 8 As shown in (a, b), all samples exhibited type IV adsorption isotherms, with hysteresis loops in the relative pressure (P / P0) range of 0.750–0.980, exhibiting characteristics of type H3. CNZ-Fe 2.5 P 2.5 The pore volume is significantly higher than other materials, exhibiting a well-developed pore structure including micropores, mesopores, and even macropores. This material possesses a wide pore size distribution and extremely high specific porosity. (ZnO, CN-Fe) 2.5 P 2.5 CNZ, CNZ-Fe 2.5 P 2.5 The specific surface areas are 6.9753 m². 2 / g, 27.959 m 2 / g, 14.385 m 2 / g, 30.637 m 2 / g. Compared with the specific surface area of ​​the CNZ sample, the obtained CNZ-Fe 2.5 P 2.5 The specific surface area of ​​the sample increased significantly; this increase is attributed to the disruption of the nanosheet structure of g-C3N4 by Fe-P co-doping. CNZ-Fe 2.5 P 2.5 The excellent porous structure of the composite material enables the efficient transport of pollutant molecules and active free radicals, which is beneficial to CNZ-Fe 2.5 P 2.5 Enhanced photocatalytic performance.

[0041] Structural stability and reusability are key parameters for evaluating the performance of photocatalytic composite materials. Therefore, in Figure 9 (a) Under the same conditions, CNZ-Fe 2.5 P 2.5 The composite material was repeatedly used for three consecutive cycles to degrade tetracycline. Furthermore, to minimize experimental error, the experiment was repeated three times in this study. The results showed that the material maintained a high degradation efficiency even after three cycles.

[0042] To gain a deeper understanding of the functional groups in the sample, we further characterized it using FTIR, XRD, and BET, with the results shown below. Figure 9 As shown in (bd). After cyclic experiments, CNZ-Fe 2.5 P 2.5 The FTIR and XRD spectra of the catalyst showed no significant changes, indicating that its crystal structure was stable. Although the specific surface area decreased slightly during the reaction due to possible blockage or collapse of some micropores, the overall microstructural integrity of the catalyst was well maintained. Furthermore, the stability of the catalyst was evaluated by determining the leaching amounts of Fe and P. The P and Fe elements in the solution were determined using the molybdenum blue method and the o-phenanthroline method, respectively, and corresponding standard curves were plotted, as shown below. Figure 9 As shown in (e, f), after three reaction cycles, the leaching concentrations remained very low, with Fe and P as low as 1.4 wt% and 0.8 wt%, respectively, indicating that CNZ-Fe 2.5 P 2.5 The catalyst exhibits good structural stability and recyclability.

[0043] Figure 10 g-C3N4, ZnO, CNZ and CNZ-Fe were tested X P Y Electrical properties of the sample. Figure 10(a) shows that the pristine g-C3N4 and ZnO samples exhibit low photocurrent responses, confirming their poor charge separation efficiency. CNZ-Fe is formed through Fe-P co-doping and recombination with ZnO. 2.5 P 2.5 The photocurrent response of the sample was significantly enhanced compared to the original g-C3N4 (by 6.53 times), indicating its superior charge separation efficiency. Figure 10 As shown in (b), the impedance plots of g-C3N4 and ZnO exhibit larger arc radii than other composites, indicating the lowest interfacial migration resistance. In contrast, CNZ-Fe 2.5 P 2.5 The sample exhibits the smallest arc radius, corresponding to the lowest interfacial charge migration resistance, theoretically validating its optimal photocatalytic performance. The trend in photocurrent response is consistent with the impedance measurement results. Notably, compared to the CNZ sample, CNZ-Fe... 2.5 P 2.5 The samples exhibited faster photocurrent response dynamics, further confirming that Fe-P co-doping enhances carrier mobility and improves material properties.

[0044] Example 4: The intermediates and degradation pathways of tetracycline were investigated using mass spectrometry-high performance liquid chromatography (MS-HPLC). For example... Figure 11 As shown in (a), the effects of tetracycline on CNZ-Fe under visible light irradiation were systematically studied. 2.5 P 2.5 The degradation behavior of the composite material surface was investigated, identifying four distinct degradation pathways. Pathway 1: Tetracycline degradation begins with adsorption at the active site, followed by charge-induced hydroxylation, leading to the intermediate P2 (m / z = 475). The electron-rich hydroxyl groups on P2 are then subjected to h... +Alternatively, O₂ is oxidized to a ketone, forming a more conjugated P₃ (m / z = 459). The electron-withdrawing effect of ketones increases the ring strain in P₃, triggering ring-opening cleavage via CC / CN bond breaking, ultimately generating low-molecular-weight nitrogen-containing intermediates P₄ (m / z = 127) and P₅ (m / z = 80). Pathways II / III: As a key intermediate, P₆ (m / z = 443) can be generated directly from P₁ via dehydrogenation, and also from P₂ via a concerted pathway of small side chain cleavage and amino rearrangement to a tertiary amine. Subsequently, the P₆ side chain undergoes cleavage to generate P₇ (m / z = 338). P₇ further undergoes aromatic ring-opening degradation, releasing the characteristic degradation intermediate P₈ (m / z = 149) of tetracycline antibiotics, a typical fingerprint product of tetracycline antibiotic degradation. Pathway IV: Another branch of P₁ is characterized by the preferential cleavage of its hydroxyl side chain, producing intermediate P₈ (m / z = 403). Due to the absence of this side chain, the electron cloud distribution of the parent nucleus in P9 is reshaped, leading to the cleavage of its macrocyclic structure. This process generates a polyhydroxy carbonyl-substituted aromatic derivative, P10 (m / z=319). Subsequently, P10 undergoes a series of ring-opening decomposition reactions, ultimately releasing the small molecule ketone product P11 (m / z=98).

[0045] The toxicity of degradation intermediates was assessed using toxicity assessment software (TEST) based on quantitative structure-activity relationship (QSAR). Furthermore, a toxicity classification table was derived with reference to Van Leeuwen & Vermeire (2007). Figure 11 As shown in (bd), the experimental data provide predicted 96-hour and 48-hour median lethal concentrations (LD50) for blackhead fish, large fleas, and *Tetrahymena piriformis*. Analysis of the four metabolic pathways revealed that the toxicity of the final products was significantly lower than that of tetracycline. Specifically, the Log(LC50) or Log(IGC50) values ​​of all final products were within the "harmless" range (corresponding to the lowest risk category in the standard toxicity classification). Conversely, the toxicity distribution of intermediate metabolites was closer to "harmful" or even "toxic." Furthermore, compared to the developmental toxicity baseline of tetracycline, most intermediate metabolites showed reduced toxicity to aquatic organisms. These results collectively indicate that the metabolic byproducts of tetracycline pose a significantly lower risk to aquatic organisms than the parent compound itself. Figure 11 As shown in (e), the heatmap systematically illustrates the toxicity characteristics of tetracycline-derived intermediate metabolites to three aquatic organisms in four degradation pathways. The color gradient from dark blue to light blue visually represents the transition from "very toxic" to "harmless" effects. These findings are reinforced in the heatmap, confirming that metabolic pathways can significantly reduce the ecotoxicity of tetracycline. This breakthrough provides a scientific basis for optimizing wastewater treatment processes aimed at removing tetracycline.

[0046] Example 5: To achieve this integration, a logic gate system was designed. This system converts visual colorimetric responses into logic-based signals, facilitating a quantifiable detection process compatible with smart devices. Figure 12 As shown in (a, b), the system uses the photocatalyst CNZ-Fe 2.5 P 2.5 The mass (0-1 mg) is represented by the input signal "A", and the irradiation time (0-60 min) is represented by the input signal "B". Experiments show that when the catalyst mass is less than 0.1 mg (A = 0) or the irradiation time is 0 (B = 0), the photocatalytic degradation reaction does not occur, the solution remains colorless, and the output Q = 0. Conversely, when both A and B simultaneously meet the threshold conditions, the solution becomes... Figure 7 (b) The brownish-yellow color and output Q = 1 indicate the presence of tetracycline and that the degradation reaction is proceeding normally. Furthermore, the A / A0 value was determined by UV-Vis absorption spectroscopy, and a threshold was set to digitize the output Q (0 or 1). Figure 12 The truth table in (c) summarizes the input combinations and corresponding outputs, intuitively illustrating the "on-off" switching process: even in the presence of a catalyst (A>0), if irradiation is insufficient (B≤0) or tetracycline has been completely degraded, the output remains Q = 0. This system achieves integrated tetracycline detection and degradation through the coordinated control of catalyst quality and irradiation time. Based on CNZ-Fe... 2.5 P 2.5 The logic gate design provides a new approach for integrating intelligent logic operations. Furthermore, to achieve real-time monitoring and rapid on-site evaluation of the photocatalytic degradation process, this application develops a simple detection method based on dynamic changes in solution color. With increasing illumination time, CNZ-Fe... 2.5 P 2.5 The solution undergoing catalytic degradation of tetracycline gradually lightened in color, exhibiting good stability and visual clarity. Based on this, CNZ-Fe... 2.5 P 2.5 The material was introduced into water samples for testing. Samples were collected periodically, and the RGB values ​​of the solutions were measured using the smartphone app ColorGrab. The grayscale value was then calculated using the formula "grayscale = 0.299R + 0.587G + 0.114B" to quantitatively reflect the degradation process.

[0047] Finally, it should be noted that the above embodiments are only for illustration and not for limiting the technical solutions of the present invention. Any equivalent substitutions and modifications or partial substitutions made to the present invention that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst, characterized in that, It is composed of phosphorus-iron co-doped carbon nitride and zinc oxide, wherein the phosphorus-iron co-doped carbon nitride and zinc oxide are composited through an S-type heterojunction, and the P exists in the carbon nitride in the form of interstitial doping, replacing some of the N atoms in the carbon nitride.

2. The preparation method of the phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst according to claim 1, characterized in that, Includes the following steps: (a) A carbon nitride precursor was prepared by melamine; (b) A zinc oxide sol precursor was prepared by zinc acetate dihydrate, 2-methoxyethanol and ethanolamine; (c) The carbon nitride precursor obtained in step (a) is ground and mixed with ferric nitrate and ammonium phosphate, then uniformly dispersed in water, and then calcined under an inert gas atmosphere to obtain ferric nitride co-doped with phosphorus. (d) Dissolve the product obtained in step (c) in the zinc oxide sol precursor obtained in step (b) and calcine it at 550±50℃ for 1-2 h in an inert gas atmosphere to obtain the product.

3. The preparation method according to claim 2, characterized in that, Step (a) specifically involves placing melamine into a crucible, sealing the crucible with a lid, transferring it to a tube furnace, heating it to 550±50℃ and holding it at this temperature for 3-5 hours, and then cooling it to room temperature to obtain the final product.

4. The preparation method according to claim 2, characterized in that, Step (b) specifically involves dissolving 11-15g of zinc acetate dihydrate in 40-60mL of a mixed solvent composed of 2-methoxyethanol and ethanolamine, stirring at room temperature, and allowing it to stand to obtain the zinc oxide sol precursor; the volume ratio of 2-methoxyethanol to ethanolamine is 20-28:

1.

5. The preparation method according to claim 2, characterized in that, In step (c), carbon nitride co-doped with iron phosphate is obtained by calcining in a tube furnace at 550±50℃ for 1-3 h under a nitrogen atmosphere. The doping amounts of iron nitrate and ammonium phosphate are calculated as iron and phosphorus, respectively, and the mass percentages of iron nitrate and ammonium phosphate doping amounts are 2-3% and 2-3%, respectively.

6. The preparation method according to claim 2, characterized in that, In step (d), 0.1-0.3 g of the product obtained in step (c) is dissolved in 1-3 mL of the zinc oxide sol precursor obtained in step (b).

7. A phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst prepared by any one of the methods described in claims 2 to 6.

8. The application of the phosphorus-iron co-doped carbon nitride / zinc oxide S-type heterojunction photocatalyst according to claim 1 or 7 in the photocatalytic degradation of tetracycline antibiotics.

9. The application according to claim 8, characterized in that, Photocatalytic reaction at 400 W / cm 2 The process was carried out under xenon lamp irradiation, with a tetracycline degradation concentration of 60 mg / L and a pH value of 5.

10. The application according to claim 8, characterized in that, A stable color change occurs during photocatalytic degradation, which can be used for the instantaneous detection of tetracycline in water.

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