Highly effectual photocatalytic degradation of tartrazine by using Ag nanoparticles decorated on Zn-Cu-Cr layered double hydroxide@ 2D graphitic carbon nitride (C3N5)

Pollution of water resources is one of the main concerns of many countries. This issue originates from the entry of diverse pollutants, including dye compounds, into water sources. In this work, ternary Zn-Cu-Cr layered double hydroxides (LDH) supported on graphitic carbon nitride (g-C3N5) decorated by silver nanoparticles (C3N5-LDH-Ag) was first prepared. Application of various characterization techniques such as SEM, XRD, and FT-IR revealed that the synthesized nanocomposite was composed of Zn-Cu-Cr LDH nanoparticles, g-C3N5 nanosheets, and Ag nanoparticles. The prepared nanomaterials were employed for the photodegradation of tartrazine in aqueous solutions. It was found that the C3N5-LDH-Ag catalyst outperformed their pure g-C3N5, Zn-Cu-Cr LDH, and C3N5-LDH composite in photocatalytic degradation of tartrazine under visible light irradiation. Tartrazine (20 mg/L) can be entirely removed by 0.25 g/L C3N5-LDH-Ag photocatalyst under 1 h visible light irradiation (200 W) at pH 6 with a rapid degradation rate constant (k) that is 4.4, 3.9, and 2.6 times higher than that of pure C3N5, Zn-Cu-Cr LDH, and C3N5-LDH component, respectively. The formation of hydroxyl radicals on the surface of C3N5-LDH-Ag as the main active species was approved by the capturing experiment. The finding results approved the stability and reusability of C3N5-LDH-Ag in four photocatalytic degradation cycles. In general, our findings revealed that the synthesized nanocomposite could be employed as an efficient photocatalyst in environmental remediation.


Introduction
The widespread issue of water pollution is endangering human health so that every year the human death due to polluted waters are more than war. Toxic substances from agriculture, urban, and industry readily enter and cause water pollution. Almost every industry utilizes dyestuffs to dye their products and its estimated 10-15% of the dye is lost into the effluent during the process (Gupta et al. 2011). There is number of coloring agents that are commonly used in pharmaceutical, texture, and food industries, among which tartrazine is most widely used. Tartrazine is a yellow to orange colored synthetic azo-dye, which is also known as FD&C yellow 5 and/or E102. Tartrazine is a water-soluble colorant used in many food products like cotton candy, corn flakes, soft drinks, foodstuffs, pickles, chewing gum, and drugs (Mehedi et al. 2009). Tartrazine has been known to show side effects like hyperactivity, asthma, migraines, eczema, thyroid cancer (Skinner et al. 2020), and also lupus chromosomal aberration in somatic cells of rats Responsible Editor: George Z. Kyzas (Elhkim et al. 2007). Despite such toxic effects observed on tartrazine exposure, this azo-dye is still cheaply available worldwide and has been observed to be used. Accordingly, it is necessary to develop efficient and sustainable methods to remove tartrazine from wastewater. Consequently, eliminating organic and inorganic contaminants from contaminated water is of crucial significance. Several techniques, including adsorption (Gautam et al. 2017;Goscianska and Ciesielczyk 2019;Grover et al. 2022;Kumar Biswal et al. 2022), photocatalytic degradation (Balu et al. 2019;Dalponte et al. 2019;Alcantara-Cobos et al. 2020;Cubas et al. 2020;Dalponte Dallabona et al. 2021;Abd-Ellatif et al. 2022), electrochemical and sonochemical oxidation (Donoso et al. 2021), and electro-Fenton reaction (Zhang et al. 2019), have been employed in the treatment of tartrazine.
Graphitic carbon nitrides (CNs) are exciting materials with unique properties such as high hardness, semi-conductivity, and adsorption properties, which are mainly controlled by the structure, composition, and crystallinity of the CN framework. According to the C/N ratio, carbon nitride materials ranging from 0.4 to 3 have been synthesized and denominated as C 3 N 2 , C 3 N 4 , C 3 N 5 , C 3 N 6 , C 3 N 7 , and so on. Graphitic carbon nitrides have attracted growing attention as a semiconductor polymeric photocatalyst because of their facile synthesis procedure, attractive electronic band structure, easy functionalization, light absorption in the visible spectrum, and high physicochemical stability (Vadivel et al. 2020). The catalytic properties of a material are often related to its electronic structures. Chemical doping on CNs is an effective method to regulate the electronic structures, ionic conductivity, and surface properties. It has been found that the addition of extra nitrogen-rich moieties in the CNs framework to increase the C/N ratio can reduce the band gap significantly due to a more extended conjugated network. For example, C 3 N 5 in comparison with C 3 N 4 displayed improved photosensitization properties at longer wavelengths and solar cell devices fabricated using low band gap C 3 N 5 demonstrated improved power conversion efficiency and open-circuit voltage (Kumar et al. 2019). Vadivel et al. (Vadivel et al. 2020) studied the catalytic activity of AgClloaded carbon nitride (AgCl/g-C3N5) composite in rhodamin B photo-degradation where the effective formation of a heterojunction and excellent visible light absorption of AgCl by g-C3N5 incorporation increased the photocatalytic of g-C3N5.
Layered double hydroxides (LDHs) constitute a class of stacked inorganic sheets with simple abbreviation formula M II M III -X where M II and M III represent divalent and trivalent cations, respectively, within the layer of hydroxides. Their layered structure, high adsorption capacity, comprehensive chemical composition, variable layer charge density, ion-exchange properties, reactive interlayer space, tunable acidity-basicity surface, and environment-friendly property make it a promising material for various applications (Forano et al. 2013). Besides, LDHs are prospective candidate catalysts for water treatment owing to their excellent structural and physicochemical properties interacting with pollutants in aqueous solutions (Karim et al. 2022). The catalytic activity and further application of LDHs are limited owing to the lack of functional groups and structural components in pure LDHs. To overcome these drawbacks, modifications of LDHs have been performed by introducing functional groups or structural components. Modifications of LDHs have been caused to design and yield novel functional LDHs-based catalysts. Meanwhile, various modification strategies of LDHs, such as polyoxometalates, metal nanoparticles, metal oxide, carbon-based nanomaterials, have been proposed (Chen et al. 2022;Qiu et al. 2022;Salem et al. 2022  ) and used as phothocatalyst in degradation of phenol. The enhanced photocatalytic activity of the photocatalyst has been attributed to (1) effective holes transfer from BiPO4 to CoAl-LDH which hinders the recombination of photogenerated charge carriers and (2) the combination of BiPO4 and CoAl-LDH avoids the agglomeration of BiPO4 and improves the stability of BiPO4. Li et al. ) designed a new ternary layered double hydroxides (LDHs) containing Ni, Co, and Mn for removal arsenate. They optimized the ratio of metals in the ternary LDHs, favoring for arsenate removal, at 1:2:1. They also confirmed the existence of ligand covalent bonds between arsenate and LDHs.
Herein, we report a novel Zn-Cu-CrLDH@C 3 N 5 /AgNPs (C 3 N 5 -LDH-Ag) composite for the first time, which was synthesized step by step. Zn-Cu-Cr ternary layered double hydroxide was synthesized by the co-precipitation method (Liu et al. 2018). Graphitic carbon nitride g-C 3 N 5 was synthesized by hydrothermal method from melamine (Liu et al. 2020). Solvothermal method was used to prepare Zn-Cu-CrLDH@C 3 N 5 , and at the last step, silver nanoparticles were decorated onto the Zn-Cu-CrLDH@C 3 N 5 nanocomposite. The photocatalytic activity of (C 3 N 5 -LDH-Ag) nano-composite in the degradation of tartrazine azo dye was investigated. Tartrazine can completely be degraded by 2% (C 3 N 5 -LDH-Ag) in 60 min and pH 6.0. Compared with pure Zn-Cu-CrLDH, g-C 3 N 5 , and C 3 N 5 -LDH, the as-prepared C 3 N 5 -LDH-Ag photocatalyst has improved photocatalytic activity for the removal of tartrazine in such a way the degradation rate constants k are 4, 3, and 2 times higher than that of pure C 3 N 5 , Zn-Cu-CrLDH, and C 3 N 5 -LDH, respectively.
The structure of C 3 N 5 -LDH-Ag and the applicability of the improved photocatalytic performance were systematically investigated.

Chemicals and reagents
All of the materials were used in this study, such as melamine, hydrazine, Zn(NO 3 ) 2 .6H 2 O, Cu(NO 3 ) 2 .3H 2 O, and Cr(NO 3 ) 3 .6H 2 O Ag(NO 3 ), were purchased from Merck (Darmstadt, Germany). All chemicals were analytical grade reagents and were used as received without further purification. Double distilled water was used for the preparation of solutions. A stock solution of 1000 ppm tartrazine was prepared and other working solutions were obtained by successive dilution of stock solutions.

Synthesis of melem
To synthesize melem (2,5,8 -triamino-s-heptazine), 6 g of melamine was added in an alumina crucible covered with a lid and heated at an oven with a heating rate 20 °C/min and holding at a final temperature of 425 °C for 12. The obtained yellowish tinge powder was crushed and suspended in 100 ml DI water in an ultrasonic bath for 10 min. The obtained suspension was refluxed for 5 h to remove unreacted melamine and other impurities. The resulting white product was collected by centrifugation at 7000 rpm for 12 min and dried at room temperature.

Synthesis of melem hydrazine
A hydrothermal reaction between melem and hydrazine was used to synthesize the monomeric unit of melem hydrazine or 2,5,8-trihydrazino-s-heptazine. In brief, 1.6 g of melem was dispersed in 15 mL of hydrazine hydrate solution (55%) by an ultrasonic probe and sealed in a 25 mL stainless steel reactor with Teflon liner and heated at 140 °C for 24 h. After cooling the reactor to room temperature, the obtained yellowish solution suspension was transferred to a beaker and pH was adjusted 1-2 by adding 10% HCl solution. Then the solution was filtered to remove unreacted solid, and the filtrate was precipitated by adding 10% NaOH solution to maintain pH in the range of 7.5-8.5. The obtained solid was filtered and dissolved in HCl, and filtered and reprecipitated in NaOH. This procedure was repeated three times.
Finally, the obtained white solid was washed three times with DI water and ethanol, centrifuged at 5000 rpm for 5 min, and dried at room temperature.

Synthesis of C 3 N 5 polymer
The as-prepared melem hydrazine was heated at 450 °C for 2 h (heating rate of 2 °C) to synthesize C 3 N 5 polymer. The obtained orange powder was used for subsequent experiments to modify by LDH and Ag nanoparticles.

Synthesis of Zn-Cu-Cr-LDH
The co-precipitation method, which is a common technique for direct synthesis of double-layer hydroxides, was used in this study. For the synthesis of Zn-Cu-Cr-LDH at first, 12 mmol Zn(NO 3 ) 2 .6H 2 O, 4 mmol Cu(NO 3 ) 2 .3H 2 O, and 8 mmol Cr(NO 3 ) 3 .6H 2 O were dissolved in 200 ml of ultrapure water. Then 50 ml of an alkaline solution containing NaOH (2 M) and Na 2 CO 3 (1 M) was prepared. The alkaline solution was then slowly added dropwise to the solution of metal cations under a magnetic stirrer until the pH of the solution reached 10. The resulting suspension is stored at 700 °C for 24 h. After cooling, the resulting precipitate was filtered and washed with ultra-pure water until the pH of the filtrate was neutralized. Finally, the product was dried in an oven at 650 °C for 24 h. To compare, Zn-Cr-LDH, Zn-Cu-LDH, and Cu-Cr-LDH double layer hydroxides were synthesized similarly. It is worth to mention, the chloride salts of metals were used for the two-component synthesis of LDH.

Synthesis of Zn-Cu-Cr-LDH@g-C 3 N 5 composite
Zn-Cu-Cr-LDH@g-C 3 N 5 (C 3 N 5 -LDH) composite was prepared by solvo-thermal method. For this purpose, 0.4 g of graphite nitride carbonate (g-C 3 N 5 ) and 2 g of sodium hydroxide were added to 35 ml of ethylene glycol and stirred. A total of 12 mmol Zn(NO 3 ) 2 .6H 2 O, 4 mmol Cu(NO 3 ) 2 .3H 2 O, and 8 mmol Cr(NO 3 ) 3 .6H 2 O (molar ratio 3: 1: 2) were solved into another 35 mL of ethylene glycol. The two ethylene glycol solutions were then mixed and stirred for 1 h. The suspension was transferred to a stainless steel reactor with Teflon liner and heated at 120 °C for 24 h. The obtained solid was collected by centrifugation, washed twice with ethanol and three times with ultra-pure water, and dried in an oven at 80 °C for 24 h.

Synthesis of Zn-Cu-Cr-LDH@g-C 3 N 5 /AgNPs composite
In order to synthesize Zn-Cu-Cr-LDH@g-C 3 N 5 /AgNPs (C 3 N 5 -LDH-Ag) nanocomposite, 50 mg of the as-synthesized C 3 N 5 -LDH was dispersed in 50 ml of DI water in an ultrasonic bath for 1 h. Then 15 mL of silver nitrate with a concentration of 0.25 M was added drop-wise and stirred for 1 h. Then 15 mL of sodium hydroxide (2 g/L) was added to the suspension, which changed the color from brown to black. The suspension was stirred for 6 h and then centrifuged at 6000 rpm for 10 min. The precipitate was first washed with water and then with ethanol and dried for 12 h at 60 °C.

Material characterization
The morphology and elemental analysis of the synthesized nano-composite were assessed by obtaining SEM images, recorded using a Czech Republic MIRA3, TES-CAN scanning electron microscope equipped with an EDS accessory. FT-IR spectra of the synthesized compounds were measured with FT-IR spectroscopy (Rayleigh WQF-510A, Beijing, China). The microcrystalline structures of the compounds were analyzed by X-ray diffraction (D8 Advanced Bruker diffractometer) with Cu Kα source radiation within an angle range (2θ) 4-80 operated at 40 kV and 40 mA. The surface area and pore structure of the materials were analyzed by N 2 adsorption-desorption on a Quanta Chrome Autosorb-Iq-Mp analyzer (Quanta Chrome Instruments, USA).
The pH of the point of zero charge (pH ZPC ) of C 3 N 5 -LDH-Ag (0.1 g/L) was found in NaCl solution (40 ml) with different pH values in range of 2-11 adjusted with HNO 3 or NaOH. The solutions were immersed into conical flasks which placed in a shaker-incubator for continuous shaking for 24 h at 20 °C. Final pH of each solution was measured and the difference between initial and final pH was calculated as ΔpH for each sample. From the plot of ΔpH vs initial pH, the PZC was noted as ΔpH zero. All pH was measured by 780 pH meter (Metrohm, Switzerland).

Photocatalytic degradation
To investigate the photocatalytic degradation of tartrazine in the presence of synthesized nanocomposite, a 500 mL photocatalytic reactor was used. A total of 300 mL of 20 mg/L solution of tartrazine was poured into a photocatalytic reactor. A total of 5 mL of 0.25 g/L well-dispersed synthesized nanomaterials was added to the photo-reactor. The mixture was stirred for 1 h in the absence of light to balance the possible adsorption of tartrazine on the photocatalyst (it was considered time 0 in all graphs). After 1 h, the reactor lamp was turned on and the photocatalytic degradation of tartrazine was followed under the radiation of visible light. All experiments were conducted at room temperature (22 ± 2 °C). During each experimental process, 2.0 mL samples was withdrawn from the photoreactor at 10-min intervals and centrifuged to remove the photocatalyst. UV-Vis spectrum of each samples was recorded in the range of 700-200 nm to evaluate the concentration of tartrazine, and the percentage of degradation was obtained using Eq. 1.
where C 0 and C t are initial and instantaneous concentration of tartrazine at time t.

Characterization of nano-composite
The morphology of the prepared nano-photocatalyst was evaluated by FE-SEM technique. The FE-SEM images of C 3 N 5 , Zn-Cu-Cr-LDH, C 3 N 5 -LDH, and C 3 N 5 -LDH-Ag have been shown in Fig. 1. The rough and tumbled carbon nitride (C 3 N 5 ) layered structure toward irregular thickness has been clearly remarked in Fig. 1a. The FE-SEM images of Zn-Cu-Cr-LDH are exhibited in Fig. 1b. A hexagonal layer structure was observed in Zn-Cu-Cr-LDH. Moreover, in Fig. 1b, the layers collapse due to thermal treatment upon calcination and material accumulated can be viewed. In the case of C 3 N 5 -LDH, good interaction between double layered hydroxide and C 3 N 5 is apparent from the growth of particle-shape LDH configuration grown on the C 3 N 5 sheets of C 3 N 5 -LDH composite. As can be seen in Fig. 1d, after Ag nanoparticles doping, the surface of C 3 N 5 -LDH is uniformly deposited by metallic Ag NPs as well as shown.
Energy dispersive spectroscopy (EDS) analysis was performed to determine the elemental composition of the synthesized compounds ( Fig. 2a and b). The EDS spectrum demonstrated the presence of C, O, N, Cr, Cu, and Zn in the structure of C 3 N 5 -LDH composite. C and N elements come from g-C 3 N 5 and Zn, Cr, Cu, and O attributed to LDH nanoparticles decorated on g-C 3 N 5 sheets. In addition to the elements observed in Fig. 2a, silver is also identified in EDS spectrum of C 3 N 5 -LDH-Ag, which approves decoration of the silver layer on the surface of the synthetic nanocomposite. Elemental dot mapping images of C 3 N 5 -LDH-Ag have been shown in Fig. 2. Zn, Cr, and cu come from LDH and carbon and nitrogen are belong to C 3 N 5 . Homogeneous distribution of Ag element without any agglomeration can be seen in dot mapping images of the nano-composite. This confirms the uniform hybridizing of Ag nanoparticle and Zn-Cu-Cr-LDH layers with the g-C 3 N 5 .
Fourier transform infrared (FTIR) spectroscopy is used to identify structure vibration of the pure C 3 N 5 , pure Zn-Cu-Cr-LDH, C 3 N 5 -LDH, and Ag-doped C 3 N 5 -LDH material that has been shown in Fig. 3. The FT-IR spectrum of pure g-C 3 N 5 has a specific broad peak at 3416 cm −1 which corresponds to stretching vibration -OH, -NH and the stretching modes of C = N and C-N bonds are located in the range of 1216-1622 cm −1 . A featured sharp band at 808 cm −1 related to breathing bending modes of triazine units (Fig. 3) (1) %Removal = C 0 − C t C 0 × 100 (Kumar et al. 2019) is observed which shifts to 810 cm −1 for C 3 N 5 -LDH suggests a strong chemical interaction between g-C 3 N 5 and Zn-Cu-Cr-LDH in the nano-composite (Xia et al. 2021). For LDH structure, a broad absorbance peak at approximately 3416 cm −1 is due to stretching vibration of hydrogen bonding between LDH layers due to hydroxyl groups and water interlayers (Zhou et al. 2022). The emergence of CO 3 2− ions in the LDH interlayers proved a sharp band at 1358 cm −1 . A peak around 1624 cm −1 can be correspond to water molecules bending vibration. The presence of metal-oxygen and metal-hydroxyl vibrational peaks is obviously confirmed in the 400-1000 cm −1 range (Prasad et al. 2019). C 3 N 5 -LDH exhibits the predominant characteristic peak for Zn-Cu-Cr-LDH and C 3 N 5 , indicating that both components are present in the composite as prepared. As can be seen, after doping Ag NPs in Ag-doped C 3 N 5 -LDH compound, the characteristic peaks exhibited slight shifts toward lower wavenumbers. Furthermore, after the Ag nanoparticles were merged into the composite, the intensity was significantly decreased.
The specific surface areas and porosity of the as-prepared materials were measured by using nitrogen adsorption-desorption technology. Figure 5 displays the N 2 adsorption-desorption plots of the C 3 N 5 , Zn-Cu-Cr-LDH, C 3 N 5 -LDH, and C 3 N 5 -LDH-Ag and the insets are their corresponding BJH pore size distribution. According to IUPAC classification, the isotherm type IV was detected for all samples, with hysteresis loop of H3 type, which are indicative of mesoporous materials (Sadeghi Rad et al. 2022). The specific surface area of the photocatalyst is a key factor that plays a decisive role in the photocatalytic performance, because it provides active catalytic sites for the absorption of pollutants and thus affects the catalytic activity. The BET surface areas of C 3 N 5 , Zn-Cu-Cr-LDH, C 3 N 5 -LDH, and C 3 N 5 -LDH-Ag are 61.87, 75.32, 84.65, and 87.79 m 2 g −1 , respectively. The results show that the hybridization of CN and LDH increases the surface area of the catalyst and thus the catalytic activity. Although the deposition of silver nanoparticles on the surface of the catalyst does not significantly increase the surface area of C 3 N 5 -LDH-Ag, but the photocatalytic performance of the composite is significantly improved due to the effectiveness of noble metal nanoparticles for charge transfer and the limiting electron-hole recombination (Liu et al. 2021).  Fig. 2 EDS pattern of C 3 N 5 -LDH, C 3 N 5 -LDH-Ag, and elemental mapping of C 3 N 5 -LDH-Ag

The photocalalytic degradation of tartrazine
Photocatalytic degradation of tartrazine in the presence of pure g-C 3 N 5 , pure Zn-Cu-Cr-LDH, C 3 N 5 -LDH, and C 3 N 5 -LDH-Ag was investigated, and the corresponding results were illustrated in Fig. 6. The degradation reaction was also followed by using no photocatalyst but under the same reaction conditions. However, no decolorization was detected in the absence of the photocatalysts. Figure 6a, b, c, and d show the UV-Vis spectrum of tartrazine for all samples at different irradiation times. Before starting the catalyzed degradation reactions, the tartrazine solutions were stirred for 1 h in the dark to establish an adsorption − desorption equilibrium between the photocatalyst particles and the tartrazine molecules. No significant adsorption of tartrazine onto the photocatalysts was observed. As shown in Fig. 6e, g-C3N5, Zn-Cu-Cr LDH, and C 3 N 5 -LDH can respectively provide 63, 65, and 88% tartrazine degradation during 1 h, whereas the C 3 N 5 -LDH-Ag exhibits a greater photocatalytic activity over the same irradiation period. In other words, a combination of the two individual materials (g-C 3 N 5 and Zn-Cu-Cr LDH) leads to the improved photocatalytic performance of the resultant composites and the presence of Ag nanoparticles made the C 3 N 5 -LDH-Ag the best photocatalyst. Ag loading enabled the C 3 N 5 -LDH to absorb visible light effectively and improved charge separation ability during photocatalysis process. The C 3 N 5 -LDH-Ag composite exhibited enhanced performance toward tartrazine photodegradation, which could be ascribed to the fast separation and transfer of photogenerated electron-hole pairs and the modified band structure (Liu et al. 2021). In order to confirm the effect of the catalyst on the photocatalytic activity, an experiment was conducted in the absence of C 3 N 5 -LDH-Ag. Without the photocatalyst, the concentration of tartrazine did not decrease under visible light irradiation. Accordingly, it was concluded that both light and the catalyst are important for the degradation of tartrazine. Kinetics of the photocatalytic degradation of tartrazine was explored using different catalysts and 2500 W visible light. A linear relationship between the natural logarithm of the changes in the tartrazine concentration, i.e., − ln(Ct/C0), and irradiation time (t) was observed. The observed linear relationship suggests that the degradation process can be described as a pseudo-first-order reaction. Its associated rate constant (k) can be calculated using − ln(Ct/ C0) = kt. Accordingly, the rate constants of the photocatalysis processes of the dye degradation were obtained as 0.018, 0.020, 0.031, and 0.078 min −1 , for g-C 3 N 5 , Zn-Cu-Cr-LDH, C 3 N 5 -LDH, and C 3 N 5 -LDH-Ag, respectively (Fig. 6f). It means that the photocatalytic degradation reaction in the presence of C 3 N 5 -LDH-Ag is 4.4, 3.9, and 2.6 times faster than the photocatalytic reaction in the presence of g-C 3 N 5 , Zn-Cu-Cr-LDH, and C 3 N 5 -LDH, respectively.

Effect of initial solution pH
The solution pH is a key parameter that affects the removal efficiency of tartrazine in the photocatalytic degradation process in the presence of C 3 N 5 -LDH-Ag. The effect of pH in the range 3-10 on tartrazine (20 mg/l) removal after  Fig. 7a. As can be seen, increasing the initial solution pH from 3.0 to 6.0 increased the removal efficiency of tartrazine from 82% in pH 3.0 to more than 99% in pH 6.0 after 60 min reaction time. But the increasing in solution pH from 6.0 to 10 decreased the removal efficiency to 76%. In a similar pattern when the solution pH was raised from 3.0 to 6.0, increase in k obs was seen in which the rate constant was increased from 0.028 min −1 to 0.078 min −1 , but the rate constant dropped with increasing pH from 6.0 to 10.0, (0.027 min −1 ) Fig. 7d.
To interpret the effects of pH on the photo-degradation process, the zero point charge pH (pHpzc) of the photocatalyst was evaluated, and a pHpzc of 7.3 was obtained for the C 3 N 5 -LDH-Ag photocatalyst. Therefore, the catalyst surface has a positive charge at pH < 7.3 and a negative charge at pH > 7.3. Tartrazine is an anionic dye because the sulfonate and carboxylate groups in tartrazine (D-SO3Na, D-COONa) are dissociated and converted to anionic dye ions (D-SO 3 − , D-COO − ). Therefore, at pH 6, tartrazine is easily adsorbed on the positively charged C 3 N 5 -LDH-Ag surface and increases the photocatalytic activity. On the other hand, by increasing the pH of the solution, the negative charge on the surface of the photocatalyst causes electrostatic repulsion and as a result reduces the amount of pollutant absorption on the surface of the catalyst and photocatalytic destruction. The same optimum pH has been reported by other authors (Abdul Rahman et al. 2018).

Effect of catalyst dosage
The effect of C 3 N 5 -LDH-Ag dosage (0.1-0.5 g/L) on the degradation of tartrazine (20 mg/L) was investigated in the photo-reactor. As demonstrated in Fig. 7c, the efficiency of tartrazine degradation increased from 83% to more than 99% with increase in catalyst concentration from 0.1 to 0.25 g/L, but a decreasing trend was observed by increasing the photocatalyst dose beyond 0.25 to 0.5 g/L in such a way that tartrazine degradation dropped to 89% in catalyst dose 0.5 g/L. The increasing catalyst dose increased the amount of oxidizing radicals produced since, the active surface sites increased, more light was absorbed and caused the formation of more active species (Chekir et al. 2017). Therefore, the more tartrazine degraded by increasing photocatalyst from 0.1 to 0.25 g/L. When an excess amount of C 3 N 5 -LDH-Ag was added (more than 0.25 g/L), the percentage of degradation decreased because the photocatalyst suspension could notably inhibit the penetration of visible light into

Effect of initial dye concentration
The initial concentration of the dye solution influenced the photocatalytic degradation of tartrazine as shown in Fig. 7b. Firstly, decolorization efficiency increased when the initial tartrazine concentration was increased from 10 to 20 mg/L. Then, increasing the concentration of tartrazine beyond 20 mg/L resulted in a decrease in the decolorization efficiency. The rate and amount of the degradation reaction is dependent on the rate of formation of the active species during photodegradation process as well as the probability of the radicals reacting with the tartrazine molecules. As the concentration of dye is increased, there will be more molecules of dye which could react with the oxidizing radicals, consequently resulting in an enhancement of the rate of the degradation of tartrazine (Bouarroudj et al. 2021). However, as the concentration of dye is increased beyond 20 mg/L, the decolorization efficiency decreases, showing that the decrease in the formation of active species on the surface of the photocatalyst has been taking place. The reason could be that the active sites on the catalyst surface are occupied by layers of dye molecules (Faramarzpour et al. 2009). On the other side, at a high concentration of dye, a significant amount of the visible radiation could be absorbed by the dye molecules rather than the photocatalyst particles and this could reduce the efficiency of the decolorization process (Shanthi and Kuzhalosai 2012). A comparison of the photo-catalytic degradation of tartrazine by different photocatalysts gathered from the literatures has been summarized in Table 1. It is observed that the 100% removal of tartrazine was achieved at 60 min by C 3 N 5 -LDH-Ag which is faster than the most of the reported photocatalysts. In the some reports, the optimum pH around 3 (relatively high acidic) or 11 (highly basic) have been reported which could be a secondary contaminant source in wastewater treatment while in this work pH 6 (near to neutral) is the best one. Generally, the data show that the fabricated nano-composite provides a promising new photocatalyst for the degradation of tartrazine.

Effect of scavengers
The radical scavenger experiment was conducted to highlight the roles of oxidizing radicals during the tartrazine photodegradation process. Typically, tri-ethanol amine (TEOA), isopropanol (IP), and benzoquinone (BQ) were selected as the scavenger of holes, •OH and •O 2 − , respectively (Liu et al. 2018). Figure 8a shows the removal efficiency of tartrazine decrease from > 99 to 59.7, 82.1, and 33.2% in the Fig. 7 Evaluation of different parameters in photocatalytic degradation of 20 mg/L tartrazine in the presence of 0.25 g/L of C 3 N 5 -LDH-Ag, a the different initial pH (the inset is point of zero charge (pHpzc) of C 3 N 5 -LDH-Ag), b the different initial concentration of tartrazine, c different dosages of C 3 N 5 -LDH-Ag, and d kinetic constant at different pH  presence of TEOA, IP, and BQ, respectively. We could find that the addition of BQ made an impediment on photocatalytic efficiency, suggesting the key roles of •O 2 − played in tartrazine photodegrdation. On other word, the radical scavenging experiment revealed that •O 2 − was the main oxidizing radicals to degrade tartrazine in the presence of C 3 N 5 -LDH-Ag as photocatalyst.
The photocatalysis reaction begins with the photogeneration of active radicals. Absorption of radiation can lead to the transfer of the holes from the valence band of the C 3 N 5 -LDH-Ag to the conductance band in its surface, oxidation of the adsorbed hydroxide ions, and water molecules to the •OH and •O 2 − radicals took place, and finally, degradation of dye molecules occurred. The capturing experiment showed •O 2 − radicals was the main active species to degrade dye molecules, and after, that holes and •OH were responsible for degradation of tartrazine. The corresponding reactions follow Eqs. 2-5.

Catalyst reusability
To evaluate the practical applicability of the developed photocatalyst, the reusability and stability of the photocatalyst, C 3 N 5 -LDH-Ag, has been assessed by performing four consecutive catalytic cycles. The photocatalyst was recovered by centrifugation after each reaction cycle, washed with methanol, and dried at 100 °C. Figure 8b depicts (2) the tartrazine degradation efficiency over four sequential photocatalytic experiments. The photocatalytic activity decreased to some extent upon four successive cycles, from the 99% removal achieved in the first cycle to 80% at the 4th cycle. The observed loss of photocatalytic activity can be attributed to the loss of active particles during the washing and separation processes. However, the photocatalyst is able to conserve its activity after 4 catalytic runs and could be classified as a stable and durable catalyst.

Conclusion
In this study, g-C 3 N 5 nanosheets, ternary Zn-Cu-Cr LDH nanoparticles, Zn-Cu-Cr LDH-C 3 N 5 composite, and their combination Zn-Cu-Cr LDH-C 3 N 5 -Ag nanocomposites with high purity were designed for the first time. Furthermore, the architected nanocomposites were applied for photocatalytic removal of tartrazine from aqueous solutions under visible radiation. Among the synthesized materials, Zn-Cu-CrLDH-C 3 N 5 -Ag (C 3 N 5 -LDH-Ag) exhibited the highest photocatalytic efficiency. Although the increase in the surface area due to the hybridization of g-C 3 N 5 and LDH plays an important role in improving the photocatalytic activity of the C 3 N 5 -LDH in the degradation of the catalyst, the presence of silver nanoparticles also improves the photocatalytic properties of the compound due the effectiveness of noble metal nanoparticles for charge transfer and limiting electron-hole recombination. The C 3 N 5 -LDH-Ag nano-photocatalyst was found to be highly efficient and recyclable and therefore reusable. The excellent photocatalytic properties of C 3 N 5 -LDH-Ag for degradation of tartrazine have been found in 1 h light time. Therefore, this synthesized nanocomposite has potential and can apply as an efficient catalyst for the degradation of tartrazine in environment.