Constructed a novel of Znln2S4/S-C3N4 heterogeneous catalyst for efficient photodegradation of tetracycline

Despite S-doped C3N4 can exhibit more efficient photo-reactivity than pure C3N4, there is still some space to further improve the detaching efficiency of electron-hole and enhance the photocatalytic efficiency of S-C3N4. The construction of heterojunction is an effective method to promote the photocatalytic efficiency. ZnIn2S4, as a novel photocatalyst, its VB (1.37 V) and CB (− 1.09 V) can match with S-C3N4. Therefore, we hope to construct the ZnIn2S4/S-C3N4 heterojunction for boosting the photocatalytic activity of S-C3N4. In this paper, ZnIn2S4/S-C3N4 heterojunction was prepared through hydrothermal method using S-C3N4, ZnCl2, InCl3·4H2O, and thioacetamide as raw materials and heated at 160 °C for 16 h. The optimum 18% ZnIn2S4/S-C3N4 nanocomposites exhibit dramatically enhanced photocatalytic performance for degradation of tetracycline with 86.3% removal rate within 120 min, higher than 50% degradation efficiency of pure S-C3N4. And in the process of photodegradation for tetracycline, the largest contribution rate is the photo-excited cavity (h+), followed by ·O2− and ·OH. Herein, we have provided a good example for removing antibiotic residues by using S-C3N4-based heterojunction towards environmental remediation.


Introduction
With the global scale increasing quantity of antimicrobials in the last two decades, many antimicrobial residues have been found in our surroundings.For example, some trace residual thiram and carbendazim were founded in contaminating water by Raman spectroscopy (Tran et al. 2023).And some type of isothiazolinones was also detected by gas chromatography and Raman spectroscopy on glass surface (Sohn et al. 2023).These phenomena would significantly increase the threat to human health (Murphy et al. 2012).Several traditional strategies, including physical or chemical adsorption and biological treatment, have been employed to eliminate antibiotic contamination for many years.Especially adsorption, the low cost, simple operation, and high efficiency have attracted more attention of researchers (Medjili et al. 2023).However, the absorption method can only separate antimicrobials from the polluted environment rather than thoroughly decompose them (Chen et al. 2021).And the biological treatment cannot be an efficient way to completely remove the antimicrobial residues owing to their antimicrobial properties (Guo et al. 2017a(Guo et al. , 2017b)).In contrast, photocatalysis relies on e − -h + pair activation to create the reactive oxygen species, which can effectively convert refractory pollutants into low toxicity or nontoxic compounds (Vasseghian et al. 2023).Therefore, photocatalytic degradation can be recognized as an attractive green and sustainable option for environmental protection (Wang et al. 2023).
Among the photocatalysts explored so far, g-C 3 N 4 , a non-metal semiconductor photocatalyst, has been extensively investigated due to its significant advantages like less toxic, cost-free, narrow bandgap, and stability (Kuldeep et al. 2021).Nevertheless, some inherent vices of g-C 3 N 4 , like small specific surface area, quick recombination of electric charge, and low absorption in visible region, have significantly limited its practical utilization (Chen et al. 2009).To overcome these obstacles, various efforts have been tried.Among the various previously reported strategies, heterojunction can guarantee the effective detachment of e − -h + and generate great redox potential (Cai et al. 2022).At present, the widely recognized heterojunction techniques mainly include Type-II and Z-type heterojunction.Especially Z-type heterojunction can simulate PSII and PSI of plant photosynthesis to complete carbon capture and biocatalytic oxygen production, significantly improving the ability of photocatalysis (Lim et al. 2023).Therefore, the construction of heterojunction has been ranked as an efficient method to enhance its photolysis efficiency.For instance, BiOI/C 3 N 4 (Liu et al. 2022a(Liu et al. , 2022b)), ZnS/ C 3 N 4 (Kim et al. 2017), and CdS/C 3 N 4 (Cao et al. 2022) and Bi 2 O 3 /biochar/C 3 N 4 (Huang et al. 2023) have exhibited high photocatalytic activity and drove the photodegradation efficiency of tetracycline to reach 86.7%.Another efficient method is the element doping strategy, such as Fe/ C 3 N 4 (Tonda et al. 2014), Co/C 3 N 4 (Zhang et al. 2014), and O/C 3 N 4 (Li et al. 2012), which can be regarded as a simple and feasible way to enhance catalytic efficiency.Among the doped elements, S-doping is also a significant method, which can result in charge density redistribution and improve the light absorption property of C 3 N 4 (Zhang et al. 2019).In 2010, Liu et al. synthesized S-doped C 3 N 4 using hydrothermal method, which showed high efficient photo-reactivity in the field of photocatalysis (2010).Since then, many excellent researches have been reported.For instance, Shcherban et al. (2016) discovered that the sulfur-doped C 3 N 4 by pyrolysis of melamine and sulfuric acid exhibited high photo-catalytic activity towards reduction of CO 2 with water vapor.Long et al. (2020) prepared cavernous S-C 3 N 4 nanoparticles with a narrow band gap of 2.57 eV through polymerizing urea and 2-thiobarbituric acid.The obtained photocatalyst has a superior photocatalytic H 2 evolution from water splitting.Very recently, a novel core-shell constructive catalyst S-g-C 3 N 4 @P123 has been developed, which showed good oxygen reduction reaction (ORR) persistence, dramatic tolerance to methanol, and large capability as the cathode of Zn-air cell (Ding et al. 2022).Therefore, S-C 3 N 4 can be regarded as an independent semiconductor and be hybridized with other semiconductors to obtain better photodegradation efficiency.
Znln 2 S 4 , as an important ternary chalcogenide catalyst, has drawn extensive attention due to its superior optical characteristics, excellent chemical stability, and appropriate redox potentials (Pascale et al. 1989).However, Znln 2 S 4 itself also has two major deficiencies.One is the rapid integration of photo-induced carriers, and another is the narrow response range of light absorption (Li et al. 2020).
At present, building a heterojunction between Znln 2 S 4 and other semiconductors has been considered as a promising solution.Many heterojunctions with excellent photocatalytic activity, such as WO 3 /Znln 2 S 4 (Tang et al. 2020), Znln 2 S 4 /rGO (Dang and Zhao 2021), ZnO/Znln 2 S 4 (Xu et al. 2022), and Sb 2 S 3 /Znln 2 S 4 (Xiao et al. 2022), have been prepared.Compared with the pure Znln 2 S 4 , the luminous absorption and separating capacity of photoinduced carriers can be improved significantly.According to literature survey, it is noteworthy that there is no report on the fabrication of ZnIn 2 S 4 -based heterojunction by incorporation of S-doped g-C 3 N 4 towards antibiotic removal.
Based on the above discussion, it might be a wonderful strategy to overcome the inherent defects of C 3 N 4 and Znln 2 S 4 as described above to construct a new heterojunction by using S-doped g-C 3 N 4 and Znln 2 S 4 (Jourshabani et al. 2018).Hence, a new heterojunction of Znln 2 S 4 /S-C 3 N 4 p-n photocatalysts has been fabricated by hydrothermal method, which displays remarkable photodegradation ability for tetracycline.This work may offer some ideas for constructing the other highly efficient heterojunction photocatalysts towards antibiotic residue contaminant.

Synthesis of S doped g-C 3 N 4
The S-doped g-C 3 N 4 (S-C 3 N 4 ) was synthesized through polycondensation method by using urea and 2-thiobarbituric acid as precursor (Long et al. 2020).First, 10-g urine and 0.03-g 2-thiobarbituric acid were dispersed in 10 mL 50% ethanol, stirring 20 min at room temperature.Then, under normal pressure, the solution was evaporated to obtain the white complex which was calcinated at 550 °C for 2 h under N 2 protected to obtain S-C 3 N 4 .After that, the product was collected for further using.

Synthesis of Znln 2 S 4 /S-C 3 N 4 catalysts
A series of Znln 2 S 4 /S-C 3 N 4 (donated as ZSC) catalysts with various mass proportions were synthesized according to the route shown in Scheme 1.As 36 wt.% Znln 2 S 4 /S-C 3 N 4 (36% ZSC) for example, firstly, 0.3 g S-C 3 N 4 was inputted into 40-mL non-ion water and sonicated 30 min.Second, ZnCl 2 (0.16g), InCl 3 •4H 2 O (0.35g), and thioacetamide (0.18g) were resolved in a beaker with 100-mL non-ion water, adjusting the pH to 2.5 by hydrochloric acid.Then, the S-C 3 N 4 suspension was drop-wisely instilled into the later mixture and vigorously agitated about 1 h.Subsequently, the composition was heated to 160 °C for 16 h to obtain 36% ZSC, which was rinsed three times by EtOH and H 2 O, and finally desiccated in an oven at 60 °C for about 6 h.According to the same steps, different amount of ZnIn 2 S 4 (3~18wt.%)was loaded with S-C 3 N 4 to obtain 3% ZSC, 9% ZSC, and 18% ZSC.The raw Znln 2 S 4 was prepared under the same process without adding S-C 3 N 4 .

Photolysis performance
The photolysis performances of ZSC photocatalyst for tetracycline (TC) were evaluated in a quartz photo-reactor under a high-pressure Xe lamp (300 W) at room temperature, using filter to cut off the light below 400 nm.Firstly, 30-mg ZSC was dispersed in 50-mL TC aqueous solution (20 mg/L).Pre-irradiation, the suspension was mixed for 30 min by magnetic stirrer to reach absorption-desorption equilibrium.Then, at each 20-min interval, 3-mL aliquots were extracted and monitored the absorption at 357 nm on a UV spectrophotometer to detect the degradation rates of TC.

Active species capturing
For better understanding the photocatalytic degradation mechanism of ZSC, tetracycline was used as an example to conduct the active radical trapping experiments.Scavenger-like EDTA (1 mmol/L) (disodium ethylenediaminetetraacetate), IPA (isopropanol), and BQ (1,4-benzoquinone) were used severally to catch light-induced h + , •OH, and •O 2 − (He et al. 2019a(He et al. , 2019b)).In addition, the electron spin resonance (ESR) was applied to further assay the existence of h + and •O 2 − radicals in the photodegradation system.One-milligram photocatalyst was dispersed to 0.1 mol/L DMPO solvent to determine the existence of superoxide radicals (DMPO-•O 2 − ) and free h + respectively.

Structural characterization
Firstly, the infrared spectrum of S-C 3 N 4 , ZnIn 2 S 4 , and ZSC is described in Fig. 1a.The main characteristic absorption bonds in aromatic rings (Wang et al. 2009).However, due to that ZnIn 2 S 4 is an inorganic compound with no significant infrared characteristic peaks, herein, the absorption bands of ZSC cannot reveal the existence of ZnIn 2 S 4 .Therefore, further analysis and detection are needed.002) peak, which attributes to the interlayer stacked aromatic rings (He et al. 2019a(He et al. , 2019b)).And the pure ZnIn 2 S 4 (purple trace) shows three major characteristic diffraction apexes at 2θ = 21.3°,27.4°, and 47.1° assigning to (006), (102), and (110) diffraction planes (JCPDS No. 65-2023), respectively (Ye et al. 2014).Note that after coupling S-C 3 N 4 and ZnIn 2 S 4 (ZSC, black, red, blue, and yellow trace), the characteristic peaks were completely overlapped with the pure ZnIn 2 S 4 and S-C 3 N 4 , suggesting that hybridization of these two semiconductors does not affect the crystal structure of each other.In addition, as the mass percentage of ZnIn 2 S 4 in hybrid is increased, the strength of peak at 27.4° was also decreased continuously, which is due to the overlap of the (002) crystal plane of C 3 N 4 with the (102) crystal plane of ZnIn 2 S 4 (Battula et al. 2019).
The elemental composition and valence conditions of 18% ZSC were disclosed by X-ray photoelectron spectroscopy (XPS).As shown in the XPS survey spectrum (Fig. 2a), the main elements are C, N, S, In, and Zn, which explains the hybrid of ZnIn 2 S 4 with S-C 3 N 4 was successful.For C 1s in Fig. 2b, two main signals centered at 281.0 and 284.1 eV were related to =C of graphite and C=N units in triazine skeleton, respectively (Jiang et al. 2015).The N 1s in Fig. 2c (394.8,399.9, and 403.0 eV) was fitted with N-C, N-H x , and N=C, respectively (Zhang et al. 2016).Correspondingly, 166.0 eV in Fig. 2d (S 2p) was ascribed to S 2p 1/2 .A total of 449.7 and 457.2 eV in Fig. 2e (In 3d) were associated with In 3d 5/2 and In 3d 3/2 .For Zn 2p spectra, main peaks located at 1027.2 and 1050.3 eV (Fig. 2f) correspond to Zn 2p 3/2 and 2p 1/2 .Noteworthy, the bonding power of each element in ZnIn 2 S 4 was consistent with those reported in the references, which demonstrated ZnIn 2 S 4 is hybridized with S-C 3 N 4 successfully (Shi et al. 2015).
The SEM images were applied to assess the morphological and microscopic characteristics of S-C 3 N 4 , ZnIn 2 S 4 , and 18% ZSC. Figure 3 a shows the s-C 3 N 4 image, appearing as a 3D hollow tubular structure (Long et al. 2020).The ZnIn 2 S 4 (Fig. 3b) consisted of large amount of microspheres with 2-μm mean diameter (Liu et al. 2022a(Liu et al. , 2022b)).The single microsphere appears as a flower-like spherical constructional assembled from nanosheets.For ZnIn 2 S 4 /S-C 3 N 4 , it is obvious from Fig. 3c and d that the ZnIn 2 S 4 has been effectively coated on S-C 3 N 4 nanotube.Unfortunately, the microsphere of ZnIn 2 S 4 collapsed, which may be relevant to the high temperature in preparation.
For better analyzing the microstructure of ZSC sample, the TEM and HRTEM were employed in Fig. 4 a and b.As shown in Fig. 4a, the darker nanosheets can be attributed to ZnIn 2 S 4 , while the lighter part correlated to S-C 3 N 4 (Guo et al. 2017a(Guo et al. , 2017b)).The HRTEM in Fig. 4b revealed the crystal-line with a planar spacing of 0.32 and 0.40 nm, corresponding to the ( 006) and ( 102) peak of ZnIn 2 S 4 (Guo elements is homogeneous, and the position of each unit conforms to the composite.All these phenomena illustrated that the combination of ZnIn 2 S 4 and S-C 3 N 4 was successful, and the structure of ZnIn 2 S 4 was well preserved, which played a crucial role for improving the photocatalytic performance of ZSC.

Photocatalytic performance
The photocatalytic efficiency of different ratio of ZSC heterojunctions was evaluated by using tetracycline hydrochloride (TC, 20 mg/L, 30 mL) as pharmaceutical residue and irradiating with visible light (Fig. 5a).Firstly, the blank control trial showed that TC solvent is stable under visible light.Without photocatalyst ZSC, only 12.6% self-degradation rate can be neglected.Secondly, compared with pure S-C 3 N 4 samples, the ZSC composites show remarkably increased photocatalytic activities to degrading the tetracycline, indicating the formation of ZSC heterojunction is an efficient solution to upgrade the photocatalysis level of S-C 3 N 4 .The highest photocatalytic efficiency belongs to the 18% ZSC, which can remove 86.3% TC within 120 min, higher than 50% degradation efficiency of pure S-C 3 N 4 .Furthermore, the ln(C/C 0 ) curve exhibits a good linear relationship (Fig. 5b), illustrating that the photo-induced degradation of TC solution catalyzed by ZSC conforms to the first-order dynamic model.In addition, the time variation of tetracycline concentration (Fig. 5c) can directly prove that TC molecules were decomposed by irradiating by visible light in the presence of 18% ZSC.With the extension of light irradiation, the absorption intensity of tetracycline hydrochloride at 370 nm drastically decreased and blue shift occurred.For further surveying the application prospect of ZSC, the stability of this heterojunction under the photolysis of TC was also evaluated.As illustrated in Fig. 5d, after three successive runs, the degradation efficiency of TC with 18% ZSC can still be retained above 80%.Even recycling for five times, the degradability of TC still

Photocatalytic mechanism analysis
To deeply understand the activity of composite photocatalyst 18% ZSC under visible light, some experiments like UV-Vis/ DRS, PL emission, photo-electrochemical, and ESR properties are carried out to assess the charge carrier dissociation and transfer rate of the composites S-C 3 N 4 , ZnIn 2 S 4 , and 18% ZSC.
In Fig. 6a, it is obvious that the absorbed boundary of S-C 3 N 4 is about 475 nm, slightly higher than 460 nm of pure C N 4 , because of the doping effect of S (Long et al. 2020).Compared with S-C 3 N 4 , the ZnIn 2 S 4 as a semiconductor with UV-visible light response offers a wide absorption (300-500 nm), with an absorption boundary around 570 nm, although the intensity of absorption peak is not as high as S-C 3 N 4 in the ultraviolet region.Notably, when incorporating of ZnIn 2 S 4 within S-C 3 N 4 , the obtained absorption spectra exhibit all the characteristics of ZnIn 2 S 4 and S-C 3 N 4 .The optical absorption almost covers the entire spectral range.And although the intensity of absorption in 450-800 nm is not stronger than ZnIn 2 S 4 and S-C 3 N 4 , the absorbed boundary of composite displays a significant redshift to 650 nm, which is beneficial to improve the utilization of visible light for photocatalysts.
Generally, the fluorescence intensity is related to the re-compounding of charge carriers.Therefore, photoluminescence (PL) spectrum of ZnIn 2 S 4 , S-C 3 N 4 and 18% ZSC is investigated and displayed in Fig. 6b.Obviously, ZnIn 2 S 4 has a strong absorption band in a broad spectral range from 400 to 600 nm.In the case of S-C 3 N 4 , a weaker emission peak centered at 475 nm is observed, which is consistent with the previous literature reported (Shi et al. 2014).For 18% ZSC, the PL peak is much weaker than ZnIn 2 S 4 and S-C 3 N 4 , which alludes that the synergetic effect occurred between the two semiconductors.It suggests that the recombination rate of photogenerated carriers has been inhibited significantly, which is a reasonable explanation for the improvement of photocatalytic efficiency of 18% ZSC.In addition, this result is further validated by photocurrent time curves in Fig. 6c.In comparison to ZnIn 2 S 4 and S-C 3 N 4 , the 18% ZSC heterojunctions show a remarkable increased photocurrent, illustrating the 18% ZSC heterojunctions can act as effective mediators for promoting photo-induced electron-hole pair separation and diffusion.Simultaneously, the electrochemical impedance spectroscopy (EIS) of 18% ZSC, ZnIn 2 S 4 , and S-C 3 N 4 is delineated in Fig. 6d.The much smaller arc size of 18% ZSC discloses that the resistance of photo-induced electronhole pair migration within the 18% ZSC electrode is much lower than ZnIn 2 S 4 and S-C 3 N 4 electrodes.All these above To further expose the photocatalytic mechanism of 18% ZSC, radical trapping experiments are conducted to reveal the primary active components in photodegradation.In Fig. 7a, it can be illustrated that the degradability of TC is reduced obviously to 10.4% after adding EDTA-2Na, demonstrating that the photo-excited cavity (h + ) plays a vital effect in the degradation of TC.In addition, adding BQ directly results in the elimination of 50.9%, revealing that •O 2 − also contributes greatly to photodegradation.After adding IPA, the eliminated efficiency of TC slightly decreases to 77.2%, indicating that •OH species has slight affected on the photolysis of TC.In short, the order of contribution to degradation efficiency is h + > ⋅O 2 − > •OH.To further verify the contribution rate of •OH to degradation, we designed the fluorescence experiment to detect •OH, according to the principle that p-phthalic acid can react on •OH to generate some strong fluorescent substances.In Fig. 7b, we find a weak fluorescence peak at 430 nm in absence of photocatalyst, illustrating that the content of •OH was very small before photodegradation.After adding catalyst, as shown in Fig. 7c, although strong fluorescence absorption peaks appeared at 340 nm and 370 nm, there was no characteristic fluorescence absorption at 430 nm, possibly due to the appearance of another strong fluorescent substance in the reaction system.This is in agreement with the result of trapping test, suggesting that •OH is not the primary active components for TC photolysis catalyzed by 18% ZSC.
Moreover, the ESR trapping experiment was used to evaluate the efficiency of superoxide anion (•O 2 − ) and photo-excited cavity (h + ) in the TC degradation.In Fig. 7d, the characteristic absorption of − components appeared under visible light cannot be detected in the dark.By contrast, as in Fig. 7e, the peaks of h + with same intensity can be seen either under visible light or in dark.These results further confirm that photo-excited cavity is the crucial active component in the TC photocatalysis, which is in concordance with the experiment data obtained from the radical capture experiments above.
To further estimate the photocatalytic degradation mechanism of ZSC, the band gaps and valence band (VB)-XPS of catalysts were tested and exposed at Fig. 8.As shown in Fig. 8 a and b, the bandgap width of the ZnIn 2 S 4 and S-C 3 N 4 was measured individually to be 2.46 and 2.68 eV.In Fig. 8 c and d, the valence band potential of S-C 3 N 4 and ZnIn 2 S 4 was 1.85 and 1.37 V. Based on the band gap and valence band potential obtained above, the conduction band potential of ZnIn 2 S 4 and S-C 3 N 4 can be calculated as − 1.09 and − 0.83 V, respectively.
According to the data required before, the possible photocatalysis principle of ZSC composites is proposed in Fig. 9.With the compact hetero-interface and well-matched band structure, a local field electric field would be established between S-C 3 N 4 and ZnIn 2 S 4 , which leads to remarkably accelerating the segregation of charge (e − )-cavity (h + ) pairs.When irradiated through visible spectrum, the electronics on the valence band of ZnIn 2 S 4 are motivated to its conduction band, and then transferred to the conduction band of S-C 3 N 4 .In the meantime, h + is transferred from the valence band of S-C 3 N 4 to ZnIn 2 S 4 .This process conforms to the charge transfer mechanism of type II heterojunction.Such rapid segregation of charge-cavity pairs would lead to the accumulation of h + in VB of ZnIn 2 S 4 and form the oxidation centers.Meanwhile, the electronics would be stored in conduction band of S-C 3 N 4 to become reduction sites.All these would significantly decrease the reassociation of electronic charge-cavity pairs and upgrade the photocatalysis effect.Because the conduction band potential of S-C 3 N 4 is more negative than O 2 /•O 2 − couple (− 0.33 V), the electrons of S-C 3 N 4 can better react with O 2 to form •O 2 − , which can be partially converted to •OH (He et al. 2022), then degrade tetracycline.In addition, the oxidation voltage of •OH/ OH − (1.99 V) and •OH/H 2 O (2.68 V) is more positive than the VB of S-C 3 N 4 (1.87 V) and Znln 2 S 4 (1.37 V).So the •OH radicals cannot be produced directly either from OH − or H 2 O by oxidation reaction.Considering the continuous accumulation of h + at the heterointerface, which can directly oxidize the pollutants, as well as partially consume •O 2 − , it can be sure that the forming amount of these radicals in the photodegradation of tetracycline is h + > •O 2 − > •OH.This was unanimous with the results of actives species detection experiments.

Conclusion
In conclusion, a new binary heterojunction ZSC was designed and constructed via hydrothermal method towards removal of TC.The optimized 18% ZSC Fig. 9 The proposed photocatalytic mechanism for 18% ZSC composites display the most efficient photodegradation activity for the elimination of TC, which was 1.7 times that of net S-C 3 N 4 (50%).The architectural construction of heterojunctions inside of 18% ZSC and the formation of type II charge transmitting system between S-C 3 N 4 and ZnIn 2 S 4 play a key role for promoting photogenerated charge carrier separation and migration.Most importantly, the 18% ZSC heterojunction retains high stability and has 73.4% photocatalytic degradation efficiency after being reused for five times in view of its practical application in environmental remediation.In addition, due to the high temperature during preparation, the morphology of ZnIn 2 S 4 microspheres collapsed, which would cause the decrease of photocatalytic efficiency.Therefore, further investigations are still warranted to explore new strategy for constructing highly efficient S-C 3 N 4 -based photocatalyst at low temperature.

Fig. 7 a
Fig. 7 a Degradation efficiency of tetracycline over 18% ZSC with different sacrificial agents.b Fluorescence spectra for •OH without 18% ZSC.c Fluorescence spectra for •OH in the presence of 18% ZSC.d ESR spectra for DMPO-•O 2 − .e ESR spectra for DMPO-h +