Preparation of thiourea derivative incorporated Ag3PO4 core shell for enhancement of photocatalytic degradation performance of organic dye under visible radiation light

Photocatalysis is a promising technique to reduce hazardous organic pollutants using semiconductors under visible light. However, previous studies have been concerned with the behavior of silver phosphate (Ag3PO4) as n-type semiconductors, and the problem of their instability is still under investigation. Herein, 4,4′-(((oxalylbis(azanediyl)) bis(carbonothioyl)) bis(azanediyl)) dibenzoic acid is synthesized by green method and used to enhance the photocatalytic behavior for Ag3PO4. The incorporated Ag3PO4 core–shell is prepared and characterized via XRD, FT-IR, Raman, TEM and BET. Besides, the thermal stability of the prepared core shell was investigated via TGA and DSC measurements. The optical properties and the energy band gap are determined using photoluminescence and DRS measurements. The photodegradation of methylene blue in the presence of the synthesized Ag3PO4 core–shell under visible light is examined using UV/Vis measurements. The effect of initial dye concentration and contact time are studied. In addition, the kinetic behavior of the selected dye during the photodegradation process shows a pseudo-first order reaction with rate constant of 0.015 min−1 for ZAg. The reusability of the Ag3PO4 core shell is evaluated, and the efficiency changed from 96.76 to 94.02% after three cycles, indicating efficient photocatalytic behavior with excellent stability.

Green synthesis methods are used for the preparation without the addition of any solvents [35][36][37] .The co-precipitation was applied to synthesize the Ag 3 PO 4 nanoparticles, and the procedure was as follows: a suitable quantity of AgNO 3 salt was completely dissolved in 50 ml of deionized water while continuously stirring.Also, an equivalent volume of crystalline NH 4 H 2 PO 4 was dissolved in 50 ml of deionized water with constant magnetic stirring.The second solution, comprising NH 4 H 2 PO 4 , was then gradually added to the first solution, containing AgNO 3 , while being continuously stirred until forming a yellow precipitate.The precipitate was then crushed and calcined at 400 °C for one hour to produce the Ag 3 PO 4 nanocrystalline sample, which was then ready for usage.

Synthesis of core-shell
A calculated amount of the Z as shell was combined with the silver phosphate core, followed by a 10-min period of rigorous grinding to produce a homogeneous coating of the Z over the Ag 3 PO 4 core.To produce prepared core-shell nanoparticles, the resultant mixture was heated at a 3°/min for 1 h in a muffle furnace at 400 °C.Scheme 2 declares the structure of the synthesized compound.

Characterization techniques
Using the Philips X'Pert X-ray diffraction (XRD) system with Cu-Ka radiation (= 1.54056 Å), the crystallographic pattern of the produced nanocomposite was examined.A typical 2 scan had a scan speed of 4o/min and ranged from 10° to 80°.The functional groups were identified using Nicolet iS10 FT-IR spectroscopy, which uses Fourier-transform infrared spectroscopy.A JEOL-JEM 3200 electron microscope operating at 300 kV was used to capture TEM pictures.To examine the optical characteristics of the produced materials, a Shimadzu IRS-2200 diffuse reflection (DRS) system in combination with an ultraviolet-visible spectrometer (Jasco V-507) was utilized.A Perkin Elmer fluorometer (model LS-55) was used to analyze solid-state photoluminescence (PL) at ambient temperature.

Photocatalytic activity study
500 Watt halogen lamp used as a radiation source to evaluate the photocatalytic activity of ZAg for the degradation of the molecules of MB dye as a model of organic pollutants (as shown in Fig. 1S).In each test, 50 mL of the dye solution with varying initial concentrations of 10, 20, 35 and 50 ppm were taken in a glass beaker, and the effect of the prepared catalyst with 1.0 g/L were added.For 30 min, the mixture was agitated in the dark to achieve adsorption/desorption equilibrium between the catalyst and the MB dye molecule and then continued while being exposed to visible light while being constantly stirred.Then, 3 mL of suspensions were withdrawn regularly from the reactor at predetermined intervals (30, 60, 90, 120,150,180 and 210 min).ZAg was separated using a centrifuge, and the supernatant was measured using UV/Vis measurements at maximum absorption wavelength of 660 nm.Before the addition of ZAg, and after doing a blank experiment, it was discovered that dye degradation was constrained.

Characterization of the synthesized materials
The crystalinity of ZAg are confirmed with the sharp characteristic peaks illustrated in Fig. 1.Characteristic diffraction peaks are located at different 2θ (degrees) that are assigned to the indexed planes of silver phosphate that match with the previous research and the standard XRD data for Ag 3 PO 4 JCPDS file NO. 00-006-0505 (see Fig. 2S).The peaks at 2θ and their related crystal plane is summarized as: 20  38,39 .The average particle size of ZAg calculated using Debye-Scherrer equation 40,41 and found to be 36.55nm.
Figure 2 shows FT-IR spectra of Z and ZAg core-shell.Concerning the spectra of Z, the absorption band that present at 333.14 cm −1 corresponds to the stretching vibration of NH and OH groups 42,43 .Symmetric and asymmetric stretching vibration COO groups are located at 1400 cm −1 and 1620 cm −1 , respectively 44,45 .The bands present at 1091.92 cm −1 and 1629.12cm −1 are due to the stretching vibration of COH and C=O groups 46,47 .In addition, the absorption peak of S=O is located at 1184.09 cm −1 , while that present at 1311.12 cm −1 is related to C-O and C-N groups.The peak found at 3030.64 cm −1 is due to the stretching vibration of the C-H group of the benzene ring 48,49 .The FT-IR spectra of ZAg show the characteristic peaks related to Z with low intensity confirming their interaction.Moreover, the peaks at 555.66 cm −1 and 1007 cm −1 are assigned to the bending vibration of O=P-O and asymmetric stretching vibration of O-P-O, respectively 50,51 .Besides, new peak present at 495 cm −1 in ZAg is attributed to the formation of Ag-N bond 40 .The previous data confirm not only the formation of Z (due to the presence of all the prepared functional groups) but also the formation of ZAg (see Fig. 3).
Raman spectroscopy is applied as a complement analysis to FT-IR is measured for the ZAg and displayed in Fig. 3.The symmetrical vibration of the AgO bending bond is located at 113 cm −152 .The peak located at 500 cm −1 are attributed to the symmetrical str.vibration of P-O-P bonds 53 .In addition, the peak present at 919.13 cm −1 is due to the vibration of the terminal oxygen of the phosphate group 51,54 .The aromatic ring and C=S are strong peaks in the range 995-1050 cm −155, 56 .
The stability of the photocatalysts is an important parameter that determines their efficiencies over higher temperatures 57 .Figure 4a declares the thermal stability of ZAg using Thermal gravimetric analysis.The decomposition of any impurities and the adsorbed water molecules on the surface of the investigated photocatalytic substances (ZAg) is present in the 100 to 200 °C range.Then, the decomposition of organic material started Besides, the DSC of ZAg is determined and displayed in Fig. 4b.A weight loss of ≈10% accompanies the sharp endo peak near 400 °C (see Fig. 4), confirming the starting decomposition of the organic molecules (Z).
The surface area of the investigated photocatalyst was determined via BET measurements and found to be 0.395 m 2 /g, indicating moderate surface area.Concerning Fig. 5, The synthesized ZAg shows Type IV isotherm   www.nature.com/scientificreports/with H3 hysteresis determined from Nitrogen adsorption-desorption isotherm.The previous results suggest that ZAg is a mesoporous material 59 .The morphology of the synthesized ZAg was obtained from TEM images (see Fig. 6) Irregular spherical particles of ZAg are distributed indicating the coating of the Ag 3 PO 4 with Synthesis of 4,4′-(((oxalylbis(azanediyl)) bis(carbonothioyl))bis(azanediyl)) dibenzoic acid.Besides, the image shows aggregation of the core shell Ag 3 PO 4 that occur due to the intermolecular interaction between the coated Z. the previous data is matched with FT-IR and confirming the formation of heterojunction photocatalyst.
Using UV/Vis-DRS optical spectroscopy in the 200-800 nm region, the diffuse reflection spectra of the Z and ZAg core-shell were examined (see Fig. 7a).It has been found that the ZAg core-shell absorbs visible light at a substantially faster rate than the Z.
The production of electron-hole pairs, light absorption, charge carrier transfer, and charge carrier utilization are a few of the mechanisms that control photocatalytic activity 60 .The photocatalytic material's energy band gap (E g ), which affects the efficiency of the product and the transfer of the e/h pairs, is crucial for maximizing the photocatalytic activity.The graph relationship of the Kubelka-Munk equation was used to determine the E g value 61,62 : where n: the proportionality constant (n = 1), v: the frequency of light, and α: the absorption coefficient.
The diffuse reflectance spectra and the Z and ZAg band gab are illustrated (see Fig. 8a and b).As shown in Fig. 7b, the optical band gaps were computed using plots of (αhʋ) 2 versus photon energy (hʋ).The band gap narrowed due to the conjugation of two bands, increasing the stability of the e/h couples (quantum confinement effect).The ZAg core shell has a band gap of 2.24 eV which is lower value than the value of band gap for pure Ag 3 PO 4 (2.39 eV) 63 indicating that the formed core shell silver phosphate (ZAg) move towards the redshift.
Mulliken electrochemical equation are used to determine the energy of valence band (E VB ) and the energy of the conduction band (E CB ) 64 : (2)  www.nature.com/scientificreports/ where, E e : the energy of free electron (4.5 eV), E g : the energy of band gap, and x : the electronegativity of Ag 3 PO 4 = 5.96 eV 63 .The estimated E CB and E VB of Ag 3 PO 4 are found to be − 0.265 eV and 2.655 eV, respectively.The PL spectra of pure Z and ZAg at a 370 nm excitation wavelength are shown in Fig. 8.The PL spectra show the rate of charge separation and recombination within the photocatalysts.The rate of charge separation and recombination within the photocatalysts is shown by the PL spectra.As a result of the effective charge transfer at the heterostructure interface, the PL spectrum of pure Z has a prominent emission band at 498 nm.At the same time, the PL intensity of the ZAg core-shell has significantly reduced, indicating that the recombination between photo-electrons generated and holes has decreased.Additionally, pure Z and ZAg as a heterostructure have a synergistic effect of reducing PL intensity, which increases the lifetime of electron stability 65,66 .

Effect of the intial dye concentration
According to Fig. 9, which reveals that the best effectiveness was seen at an MB starting concentration of 50 ppm, the influence of MB initial concentration was examined depending on UV/Vis measurements in the range of 10-50 ppm.
It is clear that the removal efficiency increase significantly with the initial concentration of dye from 1-20 ppm.In the range from 20-40 ppm and 40-50 ppm the MB degradation increase slowly (around 2%) with increasing the initial dye concentration due to the trapping of photon to the solution.

Effect of contact time
Figure 8a depicts the photocatalytic degradation of MB dye under irradiation of visible light in the presence of Z and ZAg core shells with time irradiation of 0, 30, 60, 90, 120, 150, 180, and 210 min.It is evident that the ZAg is more photocatalytically active than Z for the MB degradation (see Fig. 11a).The photocatalyst's MB solution was left in the dark for 30 min before initiating the photocatalytic process to achieve the adsorption-desorption equilibrium (see Fig. 3S).The relation between the dye concentration in the absence and in the presence of ZAg is illustrated in Fig. 4S.
The following relation provided the efficiency of the deterioration of MB under visible light:  where C 0 and C: the initial and the remaining concentration of the investigated dye.As shown in Fig. 10a, the efficiency of Z and ZAg as photocatalytic martials was found to be 87.82% and 96.76%, respectively after 210 min.The next pseudo-first-order equation represents the kinetics of the photocatalytic degradation reaction of MB in accordance with the L-H kinetics model.
where k a : the rate constant (min −1 ), the concentration in mg L −1 and t: time in min.Concerning Fig. 10b, K a is the slope of the linear relation between ln (C 0 /C) and t and found to be 1.240 × 10 -4 , 0.010, and 0.015 min −1 for MB, Z, and ZAg, respectively.The rate constants are increased in the following order: ZAg > Z > MB (see Fig. 5S), indicating excellent catalytic efficiency of the ZAg core-shell.From all the previous data, ZAg core-shell can work as an effective photocatalyst for organic compound degradation with good stability.The photocatalytic degradation of similar materials listed in Table 1.

The photocatalytic mechanism
Reactive oxidizing species were produced concurrently with the photocatalytic process to accomplish the photocatalytic destruction of organic contaminants.Two different processes produce reactive oxidizing species: first, the adsorbed water is oxidized with the help of the photocreated holes (h + ), and second, the photostimulated electrons reduce the oxygen that has been adsorbed on the surface.The excited electrons (e − ) in the conduction band will interact with the surface-adsorbed oxygen to produce reactive superoxide anion radicals ( − O 2 • ) when the adsorbed OH on the photocatalyst interacts with the reactive holes in the valence band, resulting in the formation of hydroxyl radicals (OH • ).The two most well-known and influential oxidizing species are OH • and − O 2 •65 .In this instance, the two reactive species were involved in the photocatalytic degradation of MB.Organic molecules-derived carbon, sulfur and nitrogen decrease the Ag 3 PO 4 band gap by generating energy levels above the Ag 3 PO 4 valence band (VB) and oxygen vacancies.When visible light excites the electrons in the conduction band (CB) of Ag 3 PO 4 band, these photostimulated electrons move from the (CB) to the lower CB.As a result, the transferred electrons' reduction potential will decrease.The potent reactive oxidizing species (OH • and − O 2 • ) produced by these excited photocarriers will cause the photodegradation of MB and the conversion into H 2 O and CO 2 .The photodegradation mechanism is represented in Fig. 11.

The reusability and stability of ZAg after photocatalytic degradation reaction
The effectiveness of the photocatalytic degradation of MB using ZAg for three cycles under visible light illumination was 94.02% (See Fig. 12).The FT-IR measurements (see Fig. 13) utilized to verify the stability of the synthesized materials after three cycles further show that ZAg has been applied as easily manufactured with low environmental impact and considered efficient and economical photocatalysis for organic pollutants under visible light.

Conclusion
Photodegradation enhancement of organic pollutants in aqueous media under visible light using photocatalysis consider serious challenge.Therefore, Ag 3 PO 4 is prepared and enhanced by incorporating with 4,4′-(((oxalylbis(azanediyl))bis(carbonothioyl))bis(azanediyl)) dibenzoic acid forming ZAg.The performance of ZAg as photocatalytic material on MB under visible light was evaluated using UV-Vis measurements and  www.nature.com/scientificreports/found to be 96.76%.Excellent stability of the investigated photocatalytic materials up to 800 °C was confirmed via DCS and TGA measurements.The effect of initial concentration of the investigating dye and the contact time was determined.The photodegradation reaction of MD under visible light in the presence of ZAg is pseudo-first order.The data show that ZAg is highly efficient and stable after three cycles.

Figure 6 .
Figure 6.TEM image of ZAg with different scale.

Figure 8 .
Figure 8. Photoluminescence spectra of pure Z and ZAg.

Figure 9 .
Figure 9.The effect of MB degradation using ZAg after 120 min of irradiation.

Figure 10 .
Figure 10.(a) The photocatalysis degradation curve of Z and ZAg under irradiated light (after 30 min dark), (b) Kinetic behavior of MB degradation using Z and ZAg under irradiation of visible light.

Figure 11 .
Figure 11.Schematic representation of the photo-degradation process.

Figure 12 .Figure 13 .
Figure 12.The degradation percentage of MB after three cycle using ZAg.

Table 1 .
Photocatalytic activities of various photo-catalyst and organic pollutants.