Photocatalytic Degradation of Atrazine under Visible Light Using Novel Ag@Mg4Ta2O9 Nanocomposites

In this work, a novel as well as an efficient photocatalyst based Ag@Mg4Ta2O9 nanoparticles have been prepared for the photocatalytic degradation of atrazine using the hydrothermal technique. In order to measure the chemical composition as well as the phase of the novel nanoparticles, different characterization techniques were applied to confirm their structures. Furthermore, the percent of Ag in the Ag@Mg4Ta2O9 nanoparticles has been investigated on the properties of Mg4Ta2O9 (physical and chemical). The phase of new Ag@Mg4Ta2O9 was confirmed via XRD data comparing with pure Mg4Ta2O9 phase. The images of the morphologies for all samples were studied using TEM with pore size distribution around 24 nm for 2.0 wt.% Ag@Mg4Ta2O9 nanocomposite. The new Ag@Mg4Ta2O9 nanoparticles have been applied for atrazine degradation using photocatalytic method. Due to the high BET surface area and low band gap, the nanoparticles with 2.0 wt.% of Ag@Mg4Ta2O9 display the best photocatalyst efficiency for atrazine degradation. Moreover, the application and the limitation of the photodegradation process were estimated. Different conditions effect on atrazine degradation such as dosages of photocatalyst and concentration of atrazine were investigated. The ability of 2.0 wt.% Ag@Mg4Ta2O9 for atrazine degradation was reused for many times with the same efficiency.

A large number of materials of different origins have been found in ground water, surface, sewage and even drinking water as a significant class of organic pollutants in the last few decades. The biggest concern is their adverse health effects for the human and environment [1][2][3][4] . The outmost critical health issue include in most of countries in the world putting ashore of wastewater coming mainly form industrial sources 5 . Pesticides are one of the most essential categories of organic pollutants. Most of organic pollutants including pesticides, which have been displayed in water, are coming from many different resources taking in our consideration wastewater sources. As reported, almost all types of pesticides show a strong environmental impendence on the world inhabitance. In addition, all types of pesticides show forthright direct influence on the human health as well as show extremely high toxic activity 6,7 . Bactericides, herbicides, insecticides fungicides and many other derivatives are all classified as different types of pesticides 8 . Furthermore, atrazine, chlorothalonil, methamidophos, chlorpyrifos, cypermethrin, etc. have been also classified as ordinary and famous pesticides too 9,10 . The literature says, all types of underground and/or surface water including: tape and fresh water display significant pesticides contaminations [11][12][13] . It is also reported that, tape and fresh water sources with concentrations (500 ppm) display higher contamination of different types of pesticides 14 . A lot of techniques were reported for wastewater treatments as favorable techniques. These methods include membrane filtration, adsorption, oxidation & biosorption and photocatalytic degradation [15][16][17][18][19] . Photocatalytic degradation technique is considered as the most favorable method which has been utilized in wastewater remediation from variable sources. The good photocatalyst should have enhanced photocatalytic activity, facile regeneration producibility, and higher photocatalytic stability 20,21 . On the other hand, the new hot branch of science that deals with layout, preparation, and application of small elements and/or molecules in nm range is called nanotechnology [22][23][24] . Such small tiny compounds and/or elements show a lot of amazing properties and advantages which help them to be utilized for variable applications. Huge number of nanomaterials of different types have been formerly reported as excellent photocatalysts [25][26][27][28][29] . A special attention has been recently given to the ultra-small Au clusters and / or graphene supported nanocomposites for enhanced photocatalytic & photoredox catalysis behavior [30][31][32] . The most widely used nanomaterials is titanium dioxide (TiO 2 ) which has been used for the photocatalytic degradation of a lot of organic pollutants. A special attention has been reported for the photocaltalytic degradation of pesticides using TiO 2 nanoparticles [33][34][35] . This is mainly is due to its low price, commercial availability, very low toxicity, higher photocatalytic activity, higher thermal stability, as well as sensible photochemical stability [25][26][27][28] . TiO 2 is also classified as an efficient distinct photocatalyst that has been predestined for wastewater treatments. In addition it has a suitable redox behavior comparing to the rest of photocatalysts that have been utilized. Anatase, rutile and brookite represent the major crystal phases for TiO 2 that have been reported; but formally the anatase phase act as the most efficient as well as efficacious phase 29 . Furthermore, TiO 2 displaying a higher activity while interact over UV radiation coming from natural or artificial sunlight. Where from the solar spectrum (3% to 5%) has been nominated as a results of its expansive band gab (E g = 3.2 eV). For that purpose, the scientific workers doing their best to develop a new design of efficient nanomaterials to effectively minimize its band gab 18 .
On the other hand, the form Mg 4 Ta 2 O 9 is considered as one of the most important reported three forms of a binary MgO-Ta 2 O 5 system. From the crystal structure point of view of Mg 4 Ta 2 O 9 , it is easily to detect that, Mg 4 Ta 2 O 9 display a corundum structure as a common crystallization form which can be derived from a hexagonal close packing of oxygen atoms with two-thirds of the octahedral sites occupied by Mg and Ta atoms; rather than, the other forms display a trirutile structure [36][37][38][39][40] . The study of Mg 4 Ta 2 O 9 nanomaterials shows a considerable interest due to its fantastic and powerful properties which leads to its use over a wide range of industrial applications; moreover, Mg 4 Ta 2 O 9 nano-powder was successfully prepared as reported in the literature by different working groups [41][42][43] . Therefore, it is important to develop a new design of modern nanomaterials to be applied in the field of photocatalysts degradation. Hence, the current manuscript is aimed to synthesize novel Ag@Mg4Ta2O9 composite materials known through hydrothermal technique. Cyclohexylamine has been consumed for synthetization of Mg 4 Ta 2 O 9 as a matrix during the operated process. Moreover, the role of Ag loading on the total performance of Mg 4 Ta 2 O 9 including its physical as well as chemical has been studied through its related nanocomposites formation with a general formula in the form of Ag@Mg 4 Ta 2 O 9 . Furthermore, the photocatalytic degradation of these new fabricated products will be tested against atrazine as an important application of such materials. All the optimization procedures which have a direct effect on degradation of atrazine will be also studied discussed and evaluated in details. Such procedures include: atrazine concentration, photocatalytic performance, amount of used photocatalyst and all other factors. www.nature.com/scientificreports www.nature.com/scientificreports/ experimental Materials. Tantalum isopropoxide, magnesium isopropoxide, cyclohexylamine, silver nitrate and methanol were purchased from Sigma-Aldrich and used as purchased without any extra purification process.
Ag doped Mg 4 ta 2 o 9 nanoparticles preparation. A typical method to synthesize the required Mg 4 Ta 2 O 9 nanoparticles is summarized in the following few lines. Two separated solutions (1 and 2) were prepared as follows. Solution 1: cyclohexylamine (0.02 g) was dissolved in a mixture of ethanol (40 mL) and double distilled water about (60 mL); after that tantalum isopropoxide (8 mmol) was added. The mixture was stirred for 60 min. However, solution 2: magnesium isopropoxide (16 mmol) was directly mixed with the same mixture of ethanol and double distilled water (40 mL and 30 mL respectively). Then, solution 2 was added to solution 1 and continues stirring for 60 min. The total mixture transferred to a Teflon-lined autoclave at 180 °C for 24 hrs. Then, the whole reaction mixture was gradually cooled to room temperature and the final products were collected. Ethanol, double distilled water, and acetone were used to wash the materials, and then eventually dried at 100 °C for 24 h.
Deposition process used to synthesize Ag@Mg 4 Ta 2 O 9 samples. An ideal method to prepare the Ag@Mg 4 Ta 2 O 9 is as follows. Under sonication irradiation, Mg 4 Ta 2 O 9 nanoparticles were dispersed in 100 mL distilled water. Before irradiation using strong UV lamp, the silver nitrate solution was slowly added to the Mg 4 Ta 2 O 9 suspension for 24 hrs. Finally, Ag@Mg 4 Ta 2 O 9 nanoparticles were obtained after drying of the materials for 24 h at 80 °C. The silver contents that loading on Mg 4 Ta 2 O 9 were determined as 0.5, 1.0, 1.5, 2.0 and 2.5 wt., and denoted as (x wt.%) Ag@Mg 4 Ta 2 O 9 and x is wt.% of silver.
Characterization techniques. The normal and popular characterization procedures were adopted to identify the synthesized nanoparticles as follows: X-ray diffraction (XRD) analysis that applied to detect the crystalline phase (using Bruker axis D8 with Cu Kα radiation instrument at RT). Nanostructure and surface morphologies were checked out applying JEOL-JEM-1230 transmission electron microscopy (TEM). To perform this measurement, the specimens were suspended in ethanol and subjected to ultrasonicator for nearly 30 min, after that; a tiny dose of the suspended and sonicated solution was left to dry on a copper grid coated with carbon and loaded into the TEM instrument. A Thermo Scientific spectrometer (K-ALPHA-type) was adopted to perform the X-ray photoelectron spectroscopy (XPS) measurements. Surface characterization was accomplished applying Nitrogen-adsorption assessments on the specimens with a Chromatech instrument (Nova 2000 series). On the other hand, a fluorescence spectrophotometer (Shimadzu RF-5301) was adopted to display the Photoluminescence emission spectra (PL). Whereas, UV-Vis-NIR spectrophotometer (V-570, Jasco, www.nature.com/scientificreports www.nature.com/scientificreports/ Japan) was supported in order to estimate the band gap performance from the UV-Vis diffuse reflectance spectra (UV-Vis-DRS); the experiment was performed in normal condition (air) at ambient temperature to identify absorption from 200 up to 800 nm. photocatalytic activity. Visible light radiation was utilized as an important tool to determine the catalytic activity of the produced materials and to measure the atrazine degradation as well. 300-W Xenon lamp was utilized as a main origin of irradiation during the running experiments. Together with an optical cut-off filter which is important to cancel other extra illumination below the wavelength of 420 nm. Prior the illumination process, a solution of atrazine (100 ppm) was stored in a murky place for half an hour to insure that a complete equilibrium (adsorption-desorption) was achieved. The changes in the atrazine concentrations during the experiments were obviously determined using A Shimadzu LC 20 A High-pressure liquid chromatography with a C18 column UV detector. Moreover, atrazine photoxidation produces a different tiny particles was clearly examined throughout measuring its concentrations by using DX-300 ion chromatography in the presence of a CDM-II conductivity detector as well as an AS4A-SC column. These particles include: carbon dioxide, NO 3 and Cl ions. The end material produced by the atrazine photoxidation process include carbon dioxide gas which is confirmed by passing the evolved flow gases meanwhile a sodium hydroxide solution (0.2 M). Barium nitrate solution precipitate a white material as an end material; filtered off, washed, dried and analyzed by common predictable techniques.

Results and Discussion
In the current research, novel materials of Ag@Mg 4 Ta 2 O 9 nanocomposites have been synthesized using hydrothermal technique. In addition study the performance of the new synthesized Mg 4 Ta 2 O 9 nanocomposites in the presence of doped Ag of variable loading including its physical and chemical properties as well. The new nanoparticles were characterized using different techniques such as TEM, BET, XPS, UV-Vis spectra, XRD, PL and N 2 adsorption-desorption isotherms.
Photocatalysts identification and characterizations. The XRD diffractograms of pure Mg 4 Ta 2 O 9 and new Ag@Mg 4 Ta 2 O 9 nanoparticles are displayed in Fig. 1. XRD patterns approve that the formation of Ag@  According to the existence of different binding energies for Ta 4f 7/2 at 26.2 eV and Ta 4f 7/2 at 29.0 eV, the Ta 5+ ions are highly present in the XPS spectrum of Ta species in Fig. 2A. Figure 2B shows that the three binding energy peaks for oxygen at 529.6, 530.0 and 531.1 eV that confirm the presence of three form of oxygen, such as Mg-O, Ta-O and Ta-OH, respectively. However, the XPS spectrum of Mg particles in high-resolution is shown in Fig. 2C. The observed data also prove the existence of Mg 2+ ions which is mainly attributed to the existence of binding energy value at 1304.5 eV which is due to Mg-O. Also, Fig. 2D shows the XPS spectrum of Ag particles with high-resolution. The results signalize that the two binding energies peaks at 368.2 eV and 374.1 eV, are mainly attribute to Ag3d 5/2 and Ag3d 3/2 respectively. This observation approves the presence of silver metal as expected.
As revealed by the TEM images in Fig. 3, the morphological features for Mg 4 Ta 2 O 9 and the new Ag@Mg 4 Ta 2 O 9 nanoparticles showed that the silver was doped on the surface of Mg 4 Ta 2 O 9 as spots. It is established that the size of Mg 4 Ta 2 O 9 effect of silver content which come to an agreement with XRD results.
The porosity of 2.0 wt.% Ag@Mg 4 Ta 2 O 9 was determined using N 2 adsorption desorption isotherms and pore size distribution curve (Fig. 4A,B) respectively. Appearance of narrow hysteresis and pore size distribution of 2.0 wt.% Ag@Mg 4 Ta 2 O 9 indicates the occurrence of mesoporous in the networks. The pore size of 2.0 wt.% Ag@ Mg 4 Ta 2 O 9 is calculated using the BJH method and it has about 24 nm as revealed in Fig. 4B. Table 1   samples, respectively. Thus, while the percent composition of silver increased from 0 to 2.0 wt%, the band gap of energy decreased for Mg 4 Ta 2 O 9 from 3.70 to 2.59 eV, respectively. The data shows that above 2.00 wt% of Ag on Mg 4 Ta 2 O 9 has no significant effect on the band gap energy of Mg 4 Ta 2 O 9 . As a result, the best wt% of Ag is 2.00 wt%. Figure 5 photocatalytic activities. Figure 6(A) displays the photocatalytic activity of Ag@Mg 4 Ta 2 O 9 nanoparticles for atrazine degradation. The result shows the addition of silver weight has significant effect on the photocatalytic activity of Mg 4 Ta 2 O 9 . While the loading of silver percent increased from 0 to 2.0 wt%, the photocatalytic activity enhanced from 1 to 100%. However, above 2.0 wt% of silver shows no effect on photocatalytic activity of Mg 4 Ta 2 O 9 . The data confirms that the composition of silver acts a significant factor for control band gap of Mg 4 Ta 2 O 9 . Therefore, the most Ag@Mg 4 Ta 2 O 9 nanocomposite displays as effective photocatalyst for atrazine degradation is 2.0 wt% Ag@Mg 4 Ta 2 O 9 nanoparticles. www.nature.com/scientificreports www.nature.com/scientificreports/ The nature of gases that begin through the photocatalyst chemical reaction is investigated. The photocatalytic reaction pumped an outflow throughout a sodium hydroxide solution (0.2 M). A white precipitate obtained, after adding barium nitrate solution. Figure 6(B) shows the XRD pattern for the white precipitate that promise the formation of barium carbonate which consider as the related carbonate salt with a correct card number (05-0378). The data mentions that the most important gas evolved during the atrazine photocatalytic oxidation is carbon dioxide. In the complete photocatalytic oxidation of atrazine, nitrate, H 2 O, carbon dioxide and chloride ions are confirmed as final products during this process using 2.0 wt% Ag@Mg 4 Ta 2 O 9 nanoparticles.
As shown in Fig. 7(A), the effect of 2.0 wt% Ag@Mg 4 Ta 2 O 9 nanocomposite as a photocatalyst in the degradation of atrazine illustrates the activity of such photocatalyst improved from 80 to 100% by increasing the dosage of 2.0 wt% Ag@Mg 4 Ta 2 O 9 catalyst within the range from 0.5 to 1.0 g/L, respectively. In order to complete the atrazine decomposition, time required is found to be from 60 to 40 min by increasing the dose of 2.0 wt% Ag@Mg 4 Ta 2 O 9 nanocomposite within the range from 1.0 to 2.0 g/L, respectively. While the dose of 2.0 wt% Ag@Mg 4 Ta 2 O 9 increases, the corresponding photocatalytic oxidation of atrazine is regularly increased through increasing the total number of active sites. However and while using an over dose (above 2.0 g/L) from the same photocatalyst 2.0 wt% Ag@Mg 4 Ta 2 O 9 , the activity of photocatalyst is obviously reduced. As a result, 2.0 g/L dose shows the best dose using for photocatalytic oxidation reaction because of after using over 2.0 g/L dose, the light scattering to the photocatalyst surface will be highly forbidden and hindered.
The reusing and recycling of 2.0 wt% Ag@Mg 4 Ta 2 O 9 nanocomposite as photocatalyst for degradation of atrazine was studied as illustrated in Fig. 7(B). The data shows the efficiency and stability of this photocatalyst against atrazine decomposition is planned for five times. www.nature.com/scientificreports www.nature.com/scientificreports/

Conclusions
A new set of Ag@Mg 4 Ta 2 O 9 nanocomposites via hydrothermal method technique has been prepared using different composition of silver. Furthermore, the physical and chemical properties of Mg 4 Ta 2 O 9 have been affected of Ag content in Ag@Mg 4 Ta 2 O 9 nanocomposites. According to lower loading of Ag, XRD data demonstrates no bands are detected for Ag or Ag 2 O in the Ag@Mg 4 Ta 2 O 9 nanoparticles. The occurrence of O 2− , Ta 5+ , Mg 2+ ions and silver metal were clearly confirmed using XPS spectra for 2.0 wt.% Ag@ Mg 4 Ta 2 O 9 nanocomposite. Moreover, the TEM images show the size of the Mg 4 Ta 2 O 9 effect with adding of Ag as also provides from XRD data. However, the photocatalytic activity increases from 1 to 100% with increasing the silver content from 0 to 2.0 wt%. Carbon dioxide, NO 3 − , H 2 O, and Cl − were produced after completing degradation of atrazine using the presence of this novel 2.0 wt% Ag@Mg 4 Ta 2 O 9 nanoparticle.