Green synthesis of Ag NPs on magnetic polyallylamine decorated g-C3N4 by Heracleum persicum extract: efficient catalyst for reduction of dyes.

Silver nanoparticles were immobilized on magnetic polyallylamine (PAA) decorated g-C3N4 by using Heracleum persicum extract as a biological reducing and stabilizing agent. The resulting nanocomposite, Fe3O4-g-C3N4-TCT-PAA-Ag, was then characterized using BET, VSM, XRD, TGA, FTIR, TEM, EDS and ICP. The catalytic performance of the synthesized nanocatalyst was considered in the reduction of rhodamine B, and methyl orange in the presence of sodium borohydride in the aqueous medium at room temperature. The results showed that Fe3O4-g-C3N4-TCT-PAA-Ag nanocomposite could promote both reduction reactions efficiently in very short reaction times (70-100 s). In addition, Fe3O4-g-C3N4-TCT-PAA-Ag could be magnetically recovered and recycled for several cycles with no significant decrease in its catalytic performance. Using the experimental results, the rate constant, enthalpy, and entropy of the reduction reactions of both dyes were estimated.

health, medicine, animal husbandry, agriculture, household, packaging, and electronics 13 . One of the main problems in the application of nanoparticles in reactions is relevant to their agglomeration. Immobilization of NPs on suitable supports overcomes the problems regarding their separation, stability, and recovery. In this context, several supports such as graphene oxide, TiO 2 , zeolite, and Fe 3 O 4 have been applied for the immobilization of nanoparticles [14][15][16][17] . Among various supports, g-C 3 N 4 displays good chemical stability [18][19][20] . g-C 3 N 4 provides more applications than carbon materials due to the incorporation of nitrogen atoms in the carbon structure that can have great versatility and simplicity in designing and enhancement of its chemical, catalytically, electrical, and optical properties [21][22][23][24] .
In continuation of our research on the heterogeneous catalyst [25][26][27][28] , recently we disclosed the catalytic utility of the g-C 3 N 4 hybrids and composites [29][30][31][32][33] . Inspired by the promising results of these hybrids/composites, in the present study, silver nanoparticles were synthesized using Heracleum persicum extract as the reducing and stabilizing agent. These nanoparticles were immobilized on the magnetic polyallylamine decorated g-C 3 N 4 substrate (Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag) (Fig. 1). The catalytic activity of the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocomposite is investigated in the reduction of rhodamine B, and methyl orange dyes and the kinetic and thermodynamic terms of these reactions were also discussed. Moreover, the recyclability of the nanocatalyst for both reduction reactions is investigated.

Result and Discussion
characterization of the as-prepared nanocomposite. FTIR analyses were carried out to get reliable information about the functional group in Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocatalyst. In Fig. 2a, FTIR spectrum of Fe 3 O 4 -g-C 3 N 4 , the strong absorption peak at 588 cm −1 was related to the stretching vibration the Fe-O bond. The strong absorption peak that appeared at 804 cm −1 corresponded to the bending vibration of the s-triazine ring. The absorption bands at 1200-1400 cm −1 were due to types of C-N stretching vibration mode. The peak present at the region 1621 cm −1 was due to C=N stretching vibration mode. In addition, the peaks at 3200-3500 cm −1 were due to NH, and OH stretching vibration mode. It, therefore, confirmed the presence of Fe 3 O 4 and g-C 3 N 4 structures 34 . Figure 2b showed the FTIR spectrum of the Fe 3 O 4 -g-C 3 N 4 -polyallylamine nanostructure. When the polymer was connected to Fe 3 O 4 -g-C 3 N 4 , the peak intensity was increased in the NH region and on the other hand, a weak peak appeared in the 2929 cm −1 region, which may be related to the stretching vibration of the C-H bond of the polymer chain.
The FTIR spectrum of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag was shown in Fig. 2c. It could be seen from the FTIR spectrum of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag that after the deposition with Ag NPs on Fe 3 O 4 -g-C 3 N 4 -Polyallylamine, there was no change in the FTIR spectrum of the prepared nanocomposite, indicating that the Fe 3 O 4 -g-C 3 N 4 -Polyallylamine remained stable during the synthesis of the silver nanoparticles.   www.nature.com/scientificreports www.nature.com/scientificreports/ and (4 4 0), are observed in the XRD pattern of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag. These peaks can be indexed to a face-centered cubic structure with the Fd3m space group of Fe 3 O 4 correspondent to JCPDS card no. 19-0629. Moreover, it can be observed that the XRD pattern of the catalyst exhibited the characteristic band of g-C 3 N 4, i.e., a strong peak in the diffractogram at 2θ = 28° (JCPDS card no. 87-1526). XRD pattern of crystalline planes of cubic Ag gave four characteristic crystalline peaks at 2θ = 38.0°, 44.3°, 64.4°, and 77.5° that could be attributed to the reflections of the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) (JCPDS card no. 65-2871).
To evaluate the elemental composition and distribution of nanoparticles on the synthesized nanocomposite, EDX and elemental mapping analysis were performed, Fig. 4. The study results showed that besides the C and N elements, the Ag, Fe, and O elements are identified. The mapping analysis, Fig. 4 demonstrated the successful loading of Ag and Fe 3 O 4 nanoparticles onto the g-C 3 N 4 surface. This analysis also illustrated the uniform dispersion of these nanoparticles on the g-C 3 N 4 substrate.
To obtain information about the morphology, TEM analysis was performed. The TEM images of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag are demonstrated in Fig. 5. The g-C 3 N 4 thin film is observed, which is decorated by the Ag and Fe 3 O 4 nanoparticles (black spots). As shown, Fe 3 O 4 and Ag nanoparticles were almost uniformly dispersed on g-C 3 N 4 .
The evaluation of the magnetic properties of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocomposite was performed using the VSM technique at room temperature. The VSM plot of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocomposite is shown in Fig. 6. The appraised value of Saturation magnetization (Ms) was 20.22 emu/g, indicated that the catalyst has paramagnetic behavior. This result showed the synthesized nanocomposite could be simply separated with the help of an external magnet (Fig. 6). This confirmed that the magnetic property of Fe 3 O 4 nanoparticles was partially maintained even after its composition with Ag nanoparticle, g-C 3 N 4 , and polyallylamine.
Thermal properties of the synthesized nanocomposite before and after the formation of polymer were assessed by the TGA technique, as exhibited in Fig. 7. The degradation of nanocomposite occurred in several stages. A slight weight loss was observed at a temperature below 200 °C due to the loss of the adsorbed water. The subsequent mass loss of about 75.61% in the curve a (Fe 3 O 4 -g-C 3 N 4 -TCT), was related to the loss of organic www.nature.com/scientificreports www.nature.com/scientificreports/ compounds in this structure. In the curve b (Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag), apart from the weight loss due to the loss of water, an additional weight loss of 79.77% was observed. Comparing the two cures, it can be inferred that the polyallylamine content was ~3 wt%. evaluation of the catalytic performance. MO and RhB as common contaminants were chosen to investigate the catalytic efficiency of the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag with sodium borohydride as a reducing agent. In the lack of the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag, the reduction with NaBH 4 was deficient, and after 3 h, no notable variations in the concentration of dyes were apperceived (Conversion = 2%). Furthermore, in the presence of   www.nature.com/scientificreports www.nature.com/scientificreports/ Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag without NaBH 4 , no considerable change in the concentration of dyes was observed (Conversion ~ 0%). These observations showed that neither nanocatalyst nor NaBH 4 was able to reduce these dyes individually. Therefore, both Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocomposite and sodium borohydride were required in the catalytic reduction of MO and RhB. As presented in Fig. 8, when the reduction reactions of MO and RhB with a given amounts of the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocomposite was initiated, the absorption peak at 460 nm for MO and 556 nm for RhB gradually decreased in intensity.
The effect of the amount of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag (1, 2, 3, 4, and 5 mg) on the catalytic performance in the reduction of dyes was investigated. As expected, the catalytic activity of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag was enhanced by increasing the nanocatalyst amount. As exhibited in Table S1, the low amount of nanocatalyst in a short time was required for the reduction of two dyes. The best results were obtained with 2.0 mg of the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag for MO and 4.0 mg of this nanocomposite for RhB in the presence of fresh NaBH 4 solution (0.1 M) at ambient temperature.
The variation of ln(A t /A 0 ) vs. reaction time was linear; indicating that the reaction follows Langmuir-Hinshelwood model with pseudo-first-order kinetics, where A 0 and A t were the absorbance of these dyes after time 0 and t, respectively. As ln(C t /C 0 ) = ln (A t /A 0 ) = −k.t, the k (apparent rate constant) was obtained from the slope of this lines. These constants for MO and RhB are reported in Fig. S1 and Table 1 at four different temperatures. thermodynamic study. The activation energies of RhB and MO reduction reactions at four various temperatures were determined by the Arrhenius equation (ln k = ln A-(E a /RT)). In this equation, A is the Arrhenius factor, E a is the activation energy, R is the ideal gas constant (8.314 JK −1 mol −1 ), and T is temperature. From drawing the graphs of lnk vs 1/T, the activation energy values were obtained as 36.72 and 44.55 KJ mol −1 for MO and RhB, respectively, as exhibited in Fig. S2 and Table 1.  Table 1. − molecules as H − donor are adsorbed on the surface of the nanocatalyst via the electrostatic interaction, hydrogen bonding, and π-π interaction. In other words, these molecules transfer from the solution to the nanocatalyst surface. Then, electrons transfer from BH 4 − to the organic dyes, MO or RhB, and this process leads to the decolorization and reduction of dyes (Fig. 9). The formed product desorbed from the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag surface and diffused from the surface to the solution region. Therefore, it can be accepted that the catalytic system is a requirement for performing the MO or RhB reduction because it overcomes the kinetic barrier and catalyzes the reduction reaction by simplifying the electron transition between electron donor i.e. BH 4 − and electron acceptor i.e. dye, as a result, the reduction and decolorization of dye occurred. The cycle again continues after the discharge of the active sites by desorption of the products.

Recyclability.
Encouraged by high efficiency of the catalyst for dye reduction, the stability and reusability of the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocatalyst were investigated. Recycling was repeated for eight times for reduction of MO and RhB dyes. For each individual cycle, the nanocatalyst was separated with the help of an external magnet from the reaction solution. The separated nanocatalyst was then rinsed with water and ethanol several times, dried, and applied for the subsequent run. It was observed that the catalytic activity of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag remained almost constant up to 8 cycles for dye reduction reactions. In more detail, MO conversion decreased from 99 to 96 and the loss of the catalytic activity of the catalyst for RhB reduction was 5% after eight reaction runs (Fig. 10). To study Ag leaching, ICP technique was applied. The amount of Ag loaded on the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag catalyst after 8 cycles was obtained as 0.32 wt%, which was slightly lower compared to the fresh catalyst (0.34wt %). Figure S4 illustrated the FTIR spectrum of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanostructures recycled after catalytic cycles along with that of fresh RhB. The similarity of two spectra confirmed that Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag preserved its structure and recycling did not destruct it. experimental Materials. Silver nitrate (AgNO 3 , 99%), iron(III) chloride hexahydrate (FeCl 3 .6H 2 O, 97%), iron(II) chloride tetrahydrate (FeCl 2 .4H 2 O, 99%), sodium borohydride (NaBH 4 , 96%), melamine (C 3 H 6 N 6 , 99%), ammonium hydroxide solution (NH 4 OH, 25%), cyanuric chloride (2,4,6-trichloro-1,3,5-triazine, C 3 Cl 3 N 3 , TCT, 99%), Heracleum persicum extract preparation. Heracleum persicum herb was collected and entirely washed with distilled water. After drying, in 100 mL of distilled water, 10 g of the grass was added, and using Soxhlet, the extraction process was performed. After the completion process, the extract was dried to get Heracleum persicum extract. The extract was stored at 4 °C.

Synthesis of Fe 3 O 4 -g-C 3 N 4 .
Typically, 1.5 g of g-C 3 N 4 was dispersed in 140 mL of distilled water, and then 1.37 g of FeCl 3 .6H 2 O and 0.5 g of FeCl 2 .4H 2 O were added to it. This mixture was heated at 60 °C. Next, 11 mL of NH 4 OH solution (25%) was added to the above mixture. The reaction mixture was stirred for another 60 min, and then it was cooled to ambient temperature. After separating of the magnetic nanostructures with an external magnetic, the product was washed several times with water and dried at room temperature.

Synthesis of Fe 3 O 4 -g-C 3 N 4 -PAA.
Fe 3 O 4 -g-C 3 N 4 was dispersed in dried tetrahydrofuran (THF) (25 mL) and stirred for 20 min at 0-5 °C. Then a solution of cyanuric chloride (2 mmol in 25 mL THF) was transferred to the abovementioned mixture. The suspension was stirred for 4 h. At the completion of this reaction, the product was separated magnetically and rinsed with THF frequently. In the next step, the resulting product was dispersed in THF (50 mL), and 3 mmol allylamine was added. The mixture was stirred and heated at 80 °C for 5 h. After the end of this reaction, the precipitate was isolated from the reaction mixture using a magnet and rinsed with ethanol. To form PAA polymer, the resulting product was dispersed in EtOH (30 mL) and 10 mmol allylamine and AIBN as an initiator of radical polymerization were added. The mixture was stirred and refluxed overnight. The product was isolated using a magnet, rinsed with ethanol, and dried under ambient condition.

Synthesis of Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag
. 300 mg of Fe 3 O 4 -g-C 3 N 4 -PAA nanocomposite was dispersed to 50 mL of AgNO 3 (1.5 mM) solution, then, 10 mL of Heracleum persicum extract (2%) was added to the abovementioned solution under stirring at 60 °C for 2 h. After cooling the reaction mixture, the Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag was separated with an external magnet, washed several times with ethanol/water, and dried. Using ICP analysis, the content of Ag was estimated as 0.34wt % Reduction of dyes using fe 3 o 4 -g-c 3 n 4 -tct-pAA-Ag catalyst. The evaluation of the catalytic performance of the synthesized nanocomposite was performed in a quartz cell. MO aqueous solution (10 ppm) and freshly prepared NaBH 4 solution (0.1 M) were taken in quartz cell. Then, Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag nanocatalyst (2.0 mg) was added into the above mentioned solution. The progress of the catalytic reduction reaction was monitored by recording the time-dependent spectra with a UV-Vis spectrophotometer. In order to investigate catalyst recycling, the nanocatalyst was separated by an external magnet, washed, and reused for consequent reactions with fresh dye solutions. Also, the same process was performed for RhB except that the amount of nanocatalyst was 4.0 mg.

conclusion
In this work, eco-friendly and green synthesis of Ag nanoparticle was reported using Heracleum persicum extract as a reducing and stabilizing agent. The nanoparticles were supported on the magnetic polyallylamine decorated g-C 3 N 4 substrate to furnish magnetic heterogeneous Fe 3 O 4 -g-C 3 N 4 -TCT-PAA-Ag. It was found that