g-C3N4/Ag@AgCl with Z-scheme heterojunction and Ag electron bridge for enhanced photocatalytic degradation of tetracycline wastewater

Building Z-scheme heterojunctions with an electron bridge is a favored function for increasing photocatalytic activity. A facile approach for preparing g-C3N4/Ag@AgCl ternary heterojunctions by co-precipitation and photoreduction was established in this work. First, via co-precipitation, AgCl was modified on the surface of g-C3N4 to create a broad contact area between AgCl and g-C3N4. The AgCl is then reduced to Ag via an in-situ photoreduction technique, resulting in the formation of a ternary composite. The experimental results showed that when g-C3N4 modified 25% of the Ag@AgCl, that is, g-C3N4/Ag@AgCl-25 had the best photocatalytic performance, 94.9% of TC was degraded within 240 min, and the reaction rate to TC was 0.1214 min-1, which was 4.49 times and 8.12 times higher than that of g-C3N4 and Ag/AgCl, respectively. The excellent photocatalytic performance of g-C3N4/Ag@AgCl is attributed to the LSPR effect of Ag NPs and O-doping g-C3N4, which broadens the absorbance performance of g-C3N4, the establishment of Z-type heterojunctions between AgCl NPs and g-C3N4 NSs and Ag NPs as an electron transport bridge accelerate the photogenerated electrons transfer between AgCl and g-C3N4.


Introduction
The map of antibiotic pollution in China's rivers shows an average antibiotic concentration of 303 ng/L, and major rivers from north to south are mired in antibiotic pollution (Zhang et al. 2015).Not only in China, but also in the global water environment (Fiaz et al. 2021, Wilkinson et al. 2022).However, at present, the treatment efficiency of antibiotics in sewage treatment plants is limited, resulting in large amounts of antibiotics being released directly into the environment and have been detected in various environments, especially in aquatic ecological environments.
Antibiotics in water will mainly produce selective pressure on the resistance of environmental microorganisms and selective survival of drug-resistant pathogenic bacteria, causing serious pollution to surface water and groundwater, and then drug resistance in human and aquatic organisms, resulting in "super resistant bacteria," which once again seriously threatens human health.How to scientifically and effectively remove antibiotic residues in water is the focus of domestic and foreign researchers (Anh et al. 2021, Shao et al. 2021, Zheng et al. 2021).
Photocatalytic oxidation is an excellent environmental purification technique because the process requires only inexpensive semiconductor photocatalysts and light sources (Jayaraman et al. 2022, Li et al. 2022c, Long et al. 2020).Photocatalytic oxidation has some advantages, such as simple operation, low cost, cleanliness, high degradation efficiency, good stability, and no secondary pollution (Chen et al. 2021a, Qin et al. 2021, Wei et al. 2020).However, photocatalysts are now generally faced with the problem that sunlight cannot be fully utilized and photogenerated carriers are easily recombination (Hussain et al. 2021, Yang 2021, Zhang et al. 2021).Various semiconductor nanomaterial 1 3 modification strategies are now being investigated, which includes cocatalyst modification for enhancing reaction kinetics (Wang et al. 2022, Xu et al. 2022, Zhang et al. 2022c) , loading on substrates that have excellent charge carrier mobility to achieve efficient charge extraction (Li et al. 2022d), compounding with bandgap-matched semiconductors to form heterojunctions (Liu et al. 2022), and modifying metals or oxides with surface plasmon resonance effects on semiconductor surfaces (Mei et al. 2019, Sayed et al. 2022).
Because of its simple synthesis, flexible electronic band structure, outstanding physicochemical stability, rich content, and environmental friendliness, graphitic phase carbon nitride (g-C 3 N 4 ) has attracted broad interest as an efficient non-metallic polymerized semiconductor photocatalyst (Balakrishnan & Chinthala 2022, Guo et al. 2022).However, the high recombination rate and low conductivity of photogenerated electrons-holes limit the practical application of intrinsic g-C 3 N 4 (Kong et al. 2022, Ling et al. 2022).Many methods are now being researched to improve the photocatalytic efficiency of g-C 3 N 4 .
Silver halide has outstanding photocatalytic efficiency in degrading pollutants and is considered a promising candidate material (Ghattavi & Nezamzadeh-Ejhieh 2020, Ong et al. 2016).Among them, silver chloride (AgCl) is widely used in photocatalytic degradation field since it is non-toxic and easy preparation.The band structures of AgCl (E CB = -0.05eV and E VB = 3.19 eV) and g-C 3 N 4 (E CB = -1.12eV and E VB = 1.58 eV) make AgCl and g-C 3 N 4 suitable for constructing direct Z scheme paths (Ghattavi & Nezamzadeh-Ejhieh 2020, Sun et al. 2020, Thakur et al. 2020).However, because AgCl is prone to photo-corrosion under ultraviolet irradiation, that is, AgCl is reduced to Ag 0 .Therefore, modifying Ag on the surface of AgCl can transfer photogenerated electrons in AgCl to Ag in time to achieve the purpose of inhibiting the photo-corrosion of AgCl (Sun et al. 2020).
Here, a ternary composite g-C 3 N 4 /Ag@AgCl nanomaterials have been synthesized using a simple in situ co-precipitation process and a photoreduction method in order to significantly improve the photocatalytic performance of g-C 3 N 4 .In the initial stages, a simple co-precipitation process was used to deposit AgCl NPs on the surface of g-C 3 N 4. The deposited AgCl NPs is then reduced under Xe light irradiation to form a ternary composite with Ag as the electron transport medium between g-C 3 N 4 and AgCl and light absorber.In our study, the photocatalytic performance of g-C 3 N 4 /Ag@ AgCl and relative influencing factors were investigated.The Ag@AgCl loading amounts, adsorption equilibrium time of g-C 3 N 4 /Ag@AgCl, TC concentration, and g-C 3 N 4 /Ag@ AgCl-25 dosage on TC degradation rate were also analyzed.Furthermore, the TC degradation mechanism was proposed based on the free radical capturing and LC-MS results.This work opened a novel way to fabricate Z-scheme heterojunction photocatalysts, which could contribute to visible-lightdriven photocatalytic applications.

Synthesis of g-C 3 N 4
All the reagents were of analytical grade and no further purification.g-C 3 N 4 were prepared via thermal polymerization (Li et al. 2022b).Forty grams of urea was placed in a porcelain crucible with a cover.The crucible was heated in a Ksl-1100x muffle furnace at a rate of 10°C per minute from room temperature to 500°C, held for 4 h, and subsequently allowed to cool naturally to room temperature.To create g-C 3 N 4 powder, lumpy yellowish g-C 3 N 4 was placed in an agate mortar and then milled continuously for 15 min.

Synthesis of g-C 3 N 4 /Ag@AgCl
In situ co-precipitation method and photoreduction method were used to construction g-C 3 N 4 /Ag@AgCl in Scheme 1. Depending on the loading levels, 141.2 mg and 266.7 mg of AgNO 3 were dissolve in distilled water with 30 mL, respectively.Subsequently, 0.8 g of g-C 3 N 4 was fully dispersed in the above AgNO 3 aqueous solution and sonicated in Scheme 1 Manufacturing procedure of g-C 3 N 4 /Ag@AgCl ultrasonic cleaners for 10 min.Excess 10% sodium chloride aqueous solution needs to be added dropwise and vigorously stirred for 10 h in dark conditions before the products are washed six times with distilled water and ethanol.The products are then collected and dried in an oven at 50 °C for a total of 10 h to obtain g-C 3 N 4 /AgCl-X, where X represents g-C 3 N 4 was modified by AgCl with 15wt% and 25wt% respectively.For the formation of g-C 3 N 4 /Ag@AgCl-X, the prior obtained g-C 3 N 4 /AgCl-X with 30 mg was dispersed in 100 mL of distilled water, sonicated for a total of 10 min, followed by being placed under a xenon lamp (>400 nm, 300 W) for 30 min.The end product was then collected and dried in an oven at 50°C for a total of 10 h to obtain g-C 3 N 4 / Ag@AgCl-X, where X represents g-C 3 N 4 was modified by Ag@AgCl with 15wt% and 25wt% respectively.The sample prepared without the g-C 3 N 4 using the same preparation method is Ag@AgCl.To generate AgCl, an excess of 10% sodium chloride solution was slowly added to a silver nitrate solution dropwise while stirring vigorously.The resulting mixture was then stirred in dark conditions at ambient temperature for 10 h.Ag@AgCl was obtained by dispersing the synthesized AgCl in 100 mL of purified water, sonicating it for 10 min, and then exposing it to xenon radiation (>400 nm, 300 W) for 30 min.The end product was then collected and dried in an oven at 50°C for 10 h.

Characterization
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were measured on a JEOL-2100F microscope equipped with a 200 kV accelerating voltage to investigate both the microstructure and the morphology of the nanomaterials as-prepared.The DX2700B X-ray diffractometer, which has a scanning speed of 4°/min, a step size of 0.06, and a scanning range of 10°-80°, was used to capture the crystal structure of each component in the sample.The sample's surface content, chemical state, and elemental composition were all measured using an X-ray photoelectron spectrometer made by Thermo Scientific, model number ESCALAB 250Xi.Test conditions: The X-ray source is Al target Kα 1486.6 eV, and the binding energy of each element in the sample is calibrated with contaminated carbon C 1s 284.6 eV.XPS Peak 4.1 was used for fitting and peak splitting.The absorption range of light from the samples were tested by a UV-Vis absorption spectrum (Thermo Scientific Evolution 220 spectrophotometer).Test conditions: wavelength range 200-1000 nm, with barium sulfate (BaSO 4 ) as blank.The identification of functional groups present on the surfaces of the materials was carried out using a Fourier Transform infrared spectrometer (Nicolet iS5) capable of scanning at a range of 400-4000 cm -1 .The Brunauer-Emmett-Teller (BET) was measured at Micromeritics ASAP 2020 apparatus.Total organic carbon (TOC) was conducted by TOC-Vcph at TC/IC model.

Photocatalytic degradation experiments
Under visible light irradiation, the samples' photocatalytic degradation activity in TC solution has been evaluated.The photocatalyst (20 mg) was used in the photocatalytic process, suspended in a 100 mL TC aqueous solution (30 mg/L).Moreover, to sustain the room temperature of the photocatalytic reaction system, a cooling water circulation device was installed within the reactor.After 30 min of stirring in a dark environment with the constructed nano-materials dispersed in TC solution, the adsorption-desorption equilibrium between the material and TC was achieved.Then, at a height of 13 cm from the surface of the liquid level to the lamp, a 300 W xenon lamp (CEL-HXF300-T3, Beijing Zhongjiao-Jinyuan Technology Co., Ltd.) with a filter (>400 nm) is illuminated from the reactor's top.The resulting suspension (3 mL) was taken out from the reactor at regular intervals and filtered using a 0.45 m aqueous filter head.
The absorbance of the filtrate was measured with a UV-1200 ultraviolet (UV) spectrophotometer at the maximum absorption wavelength of TC at 356 nm, and the degradation rate (η) was obtained using Equations ( 1) and ( 2), which are: where C 0 and A 0 are the initial concentration and absorbance of TC, while C and A are the concentration and absorbance of TC at the reaction time t (min), respectively.K (min -1 ) is the rate constant, and t (min) is the time of photocatalytic degradation.The numbers in the results and discussion section show the standard deviation of the average values from the three degradation tests.
In addition, the Ultimate 3000 UHPLC-Q Exactive (Thermo Scientific, US) equipment was used to examine the intermediates of TC degradation.

Photoelectrochemical measurement
A CHI660E electrochemical workstation (Shanghai Chen-Hua Instrument Co., Ltd.) equipped with a 300 W Xenon lamp (CEL-HXF200-T3, Beijing ZhongjiaoJinyuan Technology Co., Ltd.) is used to measure the photoelectrochemical characteristics (PEC) of nanomaterials.Standard three-electrode electrochemical analysis apparatus is used.The reference and counter electrodes utilized were Ag/AgCl and Pt wires, respectively.The working electrode is constructed with a series of nano-material modified conductive glass modified (FTO glass).In the electrochemical impedance test (EIS), the high-frequency region of the Nyquist diagram is controlled by the electrode reaction kinetics, so the impedance of nanomaterials can be tested by comparing the size of the semicircle radius in the high-frequency region.
The frequency range of the EIS test is from 0.005 to 10 6 Hz with amplitude of 10 mV.The three electrodes was soaked in 30 mL PBS solution (pH 7.4) and 5 mM [Fe(CN) 6 ] 3-/4-.The photocurrent response can directly reflect the separation efficiency of photogenerated carriers in nanomaterials.In the identical three-electrode setup, photocurrents carried out.The three electrodes were measured in 30 mL PBS solution (pH 7.4), irradiated with a 300 W xenon lamp.

Results and discussion
Morphology and structure characterization of g-C 3 N 4 /Ag@AgCl-X The morphological features of g-C 3 N 4 and g-C 3 N 4 /Ag@ AgCl-X are observed in Fig. 1.The TEM of g-C 3 N 4 and g-C 3 N 4 /Ag@AgCl-X indicated that g-C 3 N 4 has a layered shape and an amorphous framework, as illustrated in Fig. 1a-b.In the TEM of g-C 3 N 4 /Ag@AgCl-X (Fig. 1c and Fig. S2), with an average particle size of 4.76 nm, the Ag@AgCl NPs were evenly modified on the external layer of g-C 3 N 4 .Figure 1d shows Ag@AgCl NPs anchored at the edge of g-C 3 N 4 .Therefore, a heterojunction is formed between g-C 3 N 4 and Ag@AgCl NPs.HRTEM reveals visible lattice distance at 0.277 nm and 0.236 nm (Fig. 1d), which are nearly identical to the (200) plane of AgCl and the (111) plane of Ag (Yao et al. 2014).
The FT-IR spectra of these photocatalyst were examined to elucidate its functionalities.Figure 3a displays the obtained spectra.The chemical bond of these samples exhibits similarity in the FTIR spectra at 813 cm -1 , which corresponds to the tris-triazine system's breathing mode.Additionally, the deformation mode of N-H can be found at 890 cm -1 .The stretching vibration modes of C-N and C = N in the range of 1200-1700 cm -1 , as well as the stretching vibration modes of O-H and N-H in the range of 3000-3500 cm -1 , were also observed (Zhang et al. 2022b).Notably, there were two obvious changes.Firstly, the intensity of the bands at 813 cm -1 and 890 cm -1 shows a clear drop in the g-C 3 N 4 /Ag@AgCl composite.Secondly, the characteristic peaks at 3000-3500 cm -1 in the g-C 3 N 4 @AgCl and g-C 3 N 4 / Ag@AgCl become a clearer broad band.These changes indicated the interaction between g-C 3 N 4 and Ag@AgCl (Fan et al. 2021, Yang et al. 2017).
The N 2 adsorption-desorption isotherm was used to assess the photocatalysts' BET specific surface area and pore structure.The g-C 3 N 4 and g-C 3 N 4 /Ag@AgCl isotherms in Fig. 3b show typical type-IV isotherms with H 3 hysteresis loops, indicating that both of them are narrow slit-shaped pores (Fan et al. 2021).The specific surface area and pore volume of g-C 3 N 4 /Ag@AgCl were lower than g-C 3 N 4 (Table S1), showing that Ag@AgCl nanoparticles were successfully anchored on g-C 3 N 4 .
XPS spectroscopy is applied to investigate the surface chemical state and the interactions between g-C 3 N 4 /Ag@ AgCl-X and g-C 3 N 4 materials, as shown in Fig. 4. From the XPS full spectrum (Fig. 4a), g-C 3 N 4 /Ag@AgCl-X contains peaks of Ag and Cl in addition to C, N, and O peaks compared with g-C 3 N 4 , indicating that the g-C 3 N 4 surface is successfully loaded Ag@AgCl, which corresponds with the XRD results.In Fig. 4b, after depositing Ag and AgCl on the surface of g-C 3 N 4 by ordinary chemical precipitation, the O content in g-C 3 N 4 /Ag@AgCl increases significantly, because the process of forming Ag and AgCl is carried out in an aqueous solution environment, so that O is doped into g-C 3 N 4 in the form of O-H, C-O, C=O, and O-C=O (Yang et al. 2021).Three peaks for g-C 3 N 4 at 397.7 eV, 399.6 eV, and 400.6 eV in the N1s spectra (Fig. 4c) correspond to C-N=C, N(C) 3 , and NH X in the triazine ring structure, respectively.In contrast to C 3 N 4 , g-C 3 N 4 /Ag@AgCl exhibits a novel N-C=O functional group, demonstrating that part of the N in the compound has been replaced by O.This is in line with the rise in O content depicted in Fig. 4b.In addition, after the modification of Ag and AgCl, the peaks of C-N=C, N(C) 3 and NH x transfer to higher binding energies.This is because nitrogen groups in g-C 3 N 4 with high local electron densities can donate solitary pairs of electrons to the empty orbitals of Ag atoms, stabilizing Ag via metal support interactions and resulting in significant dispersion of Ag and AgCl (Zhang et al. 2023).In addition, the C1s spectra of g-C 3 N 4 /Ag@AgCl in Fig. 4d contain three peaks of 284.8 eV, 286.1 eV, and 288.3 eV that are attributable to N=C-N, C-O, and graphitic carbon in the triazine framework (Wu et al. 2018, Zhang et al. 2022b).C1s of g-C 3 N 4 /Ag@AgCl have a new C-O peak compared to C1s of g-C 3 N 4 , consistent with O1s results.Four peaks can be seen in the high-resolution XPS spectra of Ag3d, which is depicted in Fig. 4e.The peaks at 367.3 eV and 373.3 eV are associated with Ag 3d5/2 (Ag + ) and Ag 3d3/2 (Ag + ), respectively.The two remaining peaks, assigned to Ag 3d5/2 (Ag 0 ) and Ag 3d3/2 (Ag 0 ), are situated at 374.1 eV and 368.1 eV, respectively.The Cl2p XPS peak can also be split into two characteristic peaks that are both associated with AgCl, at 197.6 eV and 199.3 eV, respectively (Yu et al. 2021).The coexistence of Ag@AgCl and g-C 3 N 4 in g-C 3 N 4 /Ag@AgCl composites is confirmed by all of these investigations.

Absorbance and carrier separation characteristics
As we all know, metal modification and heterostructure between semiconductors are significant strategies for controlling the optical and electrical properties, which in turn impacts their photocatalytic efficacy.In Fig. 5, the absorbance properties of g-C 3 N 4 , AgCl, Ag@AgCl, g-C 3 N 4 /AgCl, and g-C 3 N 4 /Ag@AgCl were evaluated by UV-vis diffuse reflectance spectra.Due to its absorption edge at about 432 nm, g-C 3 N 4 exhibits a 2.87 eV band gap.Due to its absorption edge at 419 nm, AgCl exhibits a 2.96 eV band gap.The surface plasmon resonance (SPR) effect of the Ag NPs on the surface of AgCl NPs by in situ photoreduction (Bao & Chen 2016) and O-doped g-C 3 N 4 (Jing et al. 2023) is responsible for the slight redshift of the absorption edge observed in g-C 3 N 4 /Ag@AgCl comparison with g-C 3 N 4 and AgCl.Furthermore, the SPR effect of Ag nanoparticles may be used to explain the considerable improvement in the visible area in the absorption strength of g-C 3 N 4 /Ag@AgCl composites (Mei et al. 2019).For the purpose of to achieve high photocatalytic performance, the integration of g-C 3 N 4 with Ag@AgCl NPs may significantly enhance the visible light response and the utilization efficiency of solar energy.
EIS and photocurrent responses were used to examine the charge separation and transfer capabilities of g-C 3 N 4 and g-C 3 N 4 /Ag@AgCl.Figure 6a shows that g-C 3 N 4 /Ag@ AgCl-25 shows significantly enhanced photocurrent density compared to g-C 3 N 4 and g-C 3 N 4 /Ag@AgCl-15, further verifying that g-C 3 N 4 /Ag@AgCl-25 exhibits higher charge separation and transfer efficiency.The sample's arc radius rose in the following order: g-C 3 N 4 ; g-C 3 N 4 /Ag@AgCl-15; and g-C 3 N 4 /Ag@AgCl-25 in Fig. 6b, revealing that this material has the lowest electrochemical impedance during the photocatalytic reaction.The results above imply that heterojunction formation between AgCl and g-C 3 N 4 , Ag SPR, Ag as an electron bridge, and O-doping in g-C 3 N 4 are responsible for the improvement in carrier separation and transfer efficiency.

Tetracycline photocatalytic degradation properties
The dosing quantity and desorption adsorption equilibrium time of g-C 3 N 4 /Ag@AgCl were tuned to increase the photocatalytic effectiveness, as demonstrated in the supporting information (Figure S4-S5).
A 300 W xenon lamp (wavelength greater than 400 nm) was used to photocatalyze the degradation of TC (30 mg L -1 ) under visible light for 60 min using 20 mg of each of the following catalysts: AgCl, g-C 3 N 4 , Ag@AgCl, g-C 3 N 4 @AgCl, and g-C 3 N 4 /Ag@AgCl.The results are shown in Figs.7a  and b.TC virtually does not self-degrade when exposed to visible light in Figure S3.The order of each nanomaterial's photocatalytic performance was g-C 3 N 4 /Ag@AgCl > g-C 3 N 4 @AgCl > g-C 3 N 4 > Ag@AgCl = AgCl.The adsorption performance of each material to TC is poor, so TC degradation is mainly a photocatalytic degradation process.It can be demonstrated from the significantly improved photocatalytic performance of g-C 3 N 4 @AgCl compared to g-C 3 N 4 and AgCl that the creation of heterostructures between g-C 3 N 4 and AgCl improved the separation efficiency of photogenerated electrons and holes.Ag@AgCl had similar degradation TC properties to AgCl, indicating that AgCl played a major role in Ag@AgCl and AgCl, and the cocatalyst and SPR effect of Ag were negligible.g-C 3 N 4 / Ag@AgCl has the best photocatalytic performance, indicating that the heterogeneous structure between g-C 3 N 4 and AgCl, the Ag as electron transport, and the SPR of Ag synergistic enhance its photocatalytic properties.
Further research was done on the impact of Ag@AgCl loading on the photocatalytic performance of g-C 3 N 4 /Ag@ AgCl. Figure 7c demonstrates that as the amount of Ag@ AgCl alteration increases, so does the photocatalytic performance.g-C 3 N 4 /Ag@AgCl-25 demonstrated improved photocatalytic performance, and the degradation efficiency of TC reached 85.2% in 60 min, which was 4.49 times and 8.12 times higher than that of g-C 3 N 4 and Ag/AgCl, respectively.g-C 3 N 4 /Ag@AgCl-25 had the best photocatalytic performance, 94.9% of TC was degraded within 240 min (Figure S8), and the reaction rate to TC was 0.1214 min -1 .Furthermore, Figure S6 shows visible light illumination g-C 3 N 4 /Ag@AgCl acts on the mineralization curve of TC solution at different times.The removal of total organic carbon (TOC) was 26.1% and 29.5% after 15 min and 30 min, respectively, indicating that some intermediates in the photocatalytic process were more stable than TC, and the rate of mineralization tended to increase slowly over time, indicating that TC would take longer to fully mineralize.When combined with the structure and photoelectric characterization of g-C 3 N 4 /Ag@AgCl, the results show that the SPR effect of Ag NPs and O-doping broadens the absorbance performance of g-C 3 N 4 and Ag NPs acts as an electron transport bridge to promote the transfer of photogenerated electrons between AgCl and g-C 3 N 4 .
The pH of the TC solution was adjusted using HNO 3 and NaOH solutions, respectively, and the influence of pH on degradation rate was explored.The results are presented in Fig. 7d under the conditions of dark reaction for 20 min, catalyst dosage of 20 mg, and initial TC content of 20 mg L -1 .The pH of the TC degradation rate from high to low was 7.0, 3.0, and 11.0 after 30 min of reaction time.The degradation rate of TC under acidic and neutral circumstances for 30 min was nearly identical, reaching 80.78% and 80.91%, respectively, but it was only 73.34% under alkaline conditions.This result indicates that TC can be well degraded under both acidic and neutral conditions.In addition, the initial concentration of TC is examined in Figure S7, which indicating g-C 3 N 4 /Ag@AgCl-25 efficiently degrades TC solutions from 10 to 30 mg L -1 .

Photocatalytic degradation pathway
To study the TC degradation route, LC-MS was used to identify the intermediates created during the photocatalytic process over the g-C 3 N 4 /Ag@AgCl heterojunction, and the m/z, formulae and proposed molecular structures of the degradation products are provided in Table S2.Based on the aforementioned findings (Barhoumi et al. 2017, Jia et al. 2021, Li et al. 2022a)

Photocatalytic degradation mechanism
Photogenerated electrons (e -) and holes (h + ) on the photocatalyst surface engage in the degradation reaction under visible light circumstances, producing various photocatalytic degradation pathways (Fig. 9).Ascorbic acid (AA), EDTA-2Na, isopropanol (IPA), and AgNO 3 were used to capture superoxide radicals (•O 2 -), h + , hydroxyl radicals (•OH), and electrons (e -) in order to explore the photocatalytic mechanism of the g-C 3 N 4 /Ag@AgCl-25 photocatalyst on TC under visible light conditions (Chen et al. 2021b, Wang et al. 2017).The photocatalytic degradation rates of g-C 3 N 4 /Ag@AgCl-25, as shown in Fig. 10, were 18.35% and 13.41%, respectively, with the addition of AA and EDTA-2Na, revealing that •O 2 -and h + were the main active species in the degradation process and the h + effect was stronger than that of •O 2 -.Interestingly, when AgNO 3 is used as an electron trapper, the degradation efficiency of TC is slightly improved, because after AgNO 3 captures electrons, it facilitates the separation of electrons and holes, so that more holes are used to degrade TC.In addition, the addition of IPA has almost no effect on g-C 3 N 4 /Ag@ AgCl-25 degradation TC.In summary, •O 2 -and h + are the main active species in the degradation of TC by g-C 3 N 4 / Ag@AgCl-25 photocatalysts under visible light conditions.

Conclusion
In this paper, a novel Z-type g-C 3 N 4 /Ag@AgCl heterojunction photocatalyst was successfully obtained, and the photocatalytic performance of the prepared photocatalyst was systematically investigated by oxidation of tetracycline under visible light irradiation.In these samples, the Ag@ AgCl loading was 25 wt%, that is, g-C 3 N 4 /Ag@AgCl-25 showed the highest photocatalytic performance, and the TC degradation efficiency reached 94.9%.The improvement of photocatalytic performance is attributed to the LSPR effect of Ag NPs and O-doping, which broadens the absorbance performance of g-C 3 N 4 , and the formation of Z-type heterojunctions between AgCl and g-C 3 N 4 and Ag as an electron transport bridge promote the transfer of photogenerated electrons between AgCl and g-C 3 N 4 .In addition, based on the results of the capture experiment, the g-C 3 N 4 /Ag@AgCl heterostructure follows the typical Z-scheme charge transfer mechanism instead of the traditional type II heterojunction charge transfer mechanism.

Fig. 5
Fig. 5 UV-vis absorption spectra (a), Tauc plots (b) of the prepared nano-materials as well as earlier research, the hypothesized TC degrading routes are depicted in Fig. 8.Because of the high electron density of the double bond, TC (m/z = 445) might originally be transformed to P1 (m/z = 461) by a hydroxylation operation.Following then, •O 2 -continued to attack P1's double bond, resulting in the creation of P2 (m/z = 477).Then, P3 (m/z = 459) might be produced by the detachment of a H 2 O molecule from P2, followed by a sequence of N-demethylation, deamination, dehydroxylation, and deamidation reactions of P3 to create P4 (m/z = 343).These intermediates subsequently break down into low molecular weight organics like P5-P7 (m/z = 288, 167, and 114) as the degradation reaction advances due to the ringopening process and the loss of certain functional groups caused by the active substance's redox activity.Finally, under the influence of the active ingredient, all intermediate products are mineralized to CO 2 and H 2 O (Fig. 8).

Fig. 7
Fig. 7 TC photocatalytic performance (a, c, d) and ratios (b) of the prepared nano-materials

Fig. 8
Fig. 8 Speculation of the pathway of photocatalytic degradation of TC