ECO-FRIENDLY GREEN SYNTHESIS AND PHOTOCATALYST ACTIVITY OF Ag-ZnO NANOCOMPOSITE †

The study successfully synthesized Ag NPs, ZnO NPs, and Ag/ZnO nanocomposites using an easy, cost effect and sustainable green synthetic approach. The purpose of synthesizing Ag/ZnO nanocomposites using two different plant extracts was to study their photo-degradation activity on Methylene Blue (MB) dye. (XRD) diffraction analysis confirmed the presence of Ag crystalline size and the wurtzite hexagonal structure of ZnO. (FE-SEM) results indicated spherical, nanorods and there is Clustering of NPs with an irregular shape. The resulting metal/semiconductor oxide nanocomposites possessed unique photo degradation characteristics that were absent in the individual Ag NPs and ZnO NPs.


INTRODUCTION
In recent decades, the disposal of harmful and toxic contaminants, such as organic dyes and heavy metal ions, has led to water pollution, which has become a significant environmental issue.Industrial effluent streams are the primary sources through which heavy metals and dyes enter the ecological system, posing substantial risks to human health [1].Dyes have diverse applications across various fields in industries such as textiles, cosmetics, printing, and pharmaceuticals.Furthermore, these dyes are known to be environmentally stable and resistant to biodegradation unless a catalyst is present [2].Effluents containing dyes are typically treated using chemical, physicochemical, or biological methods [3].However, the utilization of these methods may prove to be both expensive and time-intensive.As a result, it is crucial to explore highly efficient and environmentally friendly approaches to degrade these pollutants in the environment.
Photo catalysis process is a technique that uses ultraviolet or visible light to break down organic dirt on surfaces [17][18][19][20].A typical photo-catalysis process has a high photocatalytic activity under ultraviolet or visible light for the degradation of Methylene blue.A semiconductor photo catalyst absorbs photons to generate charge separation and activate the oxidation process.Light capturing, electron-hole pair formation and separation, and catalytic reaction have all been recognized as key processes in photo catalysis in latest years [21].While numerous nanomaterials are used for wastewater treatment, many of them exhibit limitations such as low catalytic activity, limited adsorption capacity, constraints on photocatalytic efficiency, light absorption, and expensive production costs.[22,23] Researchers have dedicated significant attention to studying metal nanoparticles (MNPs) during the past few decades [24][25][26][27].Noble metal NPs have garnered greater interest compared to other metal nanoparticles because of their small size and high surface-to-volume ratio [28], the utilization of MNPs as heterogeneous catalysts offers notable advantages.[26,[29][30][31][32][33].However, one challenge that arises is the tendency of MNPs to aggregate.To address this issue and minimize the aggregation of nanoparticles (NPs), we require a support ideal material.Various inorganic compounds, including TiO2, ZnO, Fe2O3, Ta2O3, CuO, NiO, Cr2O3, RuO2, etc.), transition metal oxides with wide band gaps and stability can play an important role on the photocatalytic to remove organic [34].Between of the different heterogeneous photo catalysts, zinc oxide, silver and gold NPs appeared as a more attractive alternative for controlling the pollution-related environmental issues [35][36][37][38].
The combination of metal nanoparticles (NPs) with metal oxide semiconductor nanocomposites has garnered significant attention from researchers worldwide.This is primarily due to their remarkable optical, electrical, and chemical properties, as well as their promising potential in various applications [39][40][41].
Recently, there has been a development of biosynthetic approaches for the synthesis of MNPs.These methods make use of solvents that are comparatively less toxic, such as water, biological extracts, biological systems, and microwave technology [48][49][50][51].
Utilizing plant extracts for various applications offers several advantages, including their widespread availability, ease of handling, and possessing a diverse range of metabolites, although plant extracts contain a variety of secondary
metabolites with notable reducing potential, they can be effectively utilized as both reducing and stabilizing agents in the production of environmentally friendly nanoparticles (NPs) [52].These characteristics make plant extracts a favorable option when compared to other biosynthetic approaches [53].
The aim of the present work is synthesis Ag-ZnO Nanocomposite by eco-green approach in ambient conditions using the extract of plant which work as both reducing and stabilizing agents and study their Photo catalysis activity.

EXPERIMENTAL
2.1 Materials Silver nitrate (AgNO3), zinc nitrate hexahydrate (Zn(NO3)2•6H2O), sodium hydroxide (NaOH), and methylene blue were obtained from Carlo Erba and Himedia chemical companies in highly pure form and used without further purification.The aloe vera plant and Hibiscus sabdariffa leaves were acquired from nearby markets.All glassware used in the experiment underwent thorough cleaning, including washing twice with distilled water (DW) and ethanol, followed by drying in the oven prior to use.

Preparation of Ag NPs, ZnO NPs and Ag-ZnO nanocomposite via Aloe vera plant extract
Initially, we produced the aloe vera extract by washing and cutting the leaves, extracting the gel, and blending it with a mixer.We took 100 grams of the gel and mixed it with 300 milliliters of distilled water, before placing it on a magnetic stirrer for 30 minutes.Subsequently, once it had cooled down we centrifuged the solution at 4000 rotations per minute for approximately 10 minutes.Then to get the Ag NPs, we prepared 0.1M of AgNO3 solution, the solution was stirred for 30 min at 60 C° on the magnetic stirrer and then we added 50 ml of the aloe vera extract to 100 ml of AgNO3.On the other hand, we prepared other solution from mixing 20 ml of aloe vera gel which was diluted with 150 ml of deionized water .After filtered and purified, 2 g of zinc nitrate hexahydrate (Zn(NO 3 ) 2 • 6H 2 O) was dissolved in 150 ml of the plant extract with stirring using the magnetic stirrer for 1 h at an 80 °C.Leave the zinc solution overnight.Separately, 4 g of Sodium hydroxide (NaOH) is dissolved in 35 ml of deionized water.Then 12 ml of NaOH solution was added to the precursor solution with the extract.Following the formation of a white precipitate, the Ag/ZnO nanocomposite is produced by gradually adding 100 milliliters of Ag nanoparticles to 100 milliliters of ZnO nanoparticles at 60 degrees Celsius, while continuously stirring to ensure thorough mixing.Final indicate was Ag/ZnO nanocomposite.Then all solutions were deposit on slides for further work.

Preparation of Ag NPs, ZnO NPs and Ag-ZnO nanocomposite via Hibiscus sabdariffa plant extract
Similar method was adapted to prepare Ag/ZnO nano-campsite via Hibiscus sabdariffa.First, we take the Hibiscus flowers then washed thoroughly with DW.Then they were sun dried and grinded into small pieces we mixed 2 gr with 100 ml of DW then placed on a magnetic stirrer at 60 C° for 30 min.The red extract was filtered twice with cotton wool balls.After the completion of the extraction process, we take 50 ml of aqueous extract and repeat the same operation to prepare the Ag NPs.Then to prepare ZnO NPs, 4g of zinc nitrate hexahydrate (Zn(NO3)2 6H 2 O) was dissolved in 150 ml of the plant extract with stirring using the magnetic stirrer for 1 h at an 80 °C.Leave the solution overnight.Separately, 4 g of Sodium hydroxide (NaOH) is dissolved in 35 ml of deionized water.Then 20 ml of NaOH solution was added to the precursor solution with the extract.After the formation of a white precipitate, the Ag/ZnO nanocomposite is also produced by gradually adding 100 milliliters of Ag nanoparticles to 100 milliliters of ZnO nanoparticles at 60 degrees Celsius, while continuously stirring to ensure thorough mixing.Then we deposit all the solutions on slides for further experiments.

Photocatalytic activity
The procedure was conducted in according to previous work [54,55].The efficiency of the synthesized photocatalyst samples were assessed by monitoring the degradation of MB dye as a model pollutant.To control the photo-catalytic degradation process, the optical absorption peak of MB dye at 664.5 nm was determined.We prepared solution of MB dye by dissolving 10 mg in 1000 ml of distilled water and stirring it on a magnetic stirrer for two hours.The solution was left overnight to ensure complete dissolution of the MB dye before using it.Approximately 30 mg of nanomaterial (Samples) was adding, 50 ml of MB (10 mg/ml), the solution was stirred in a dark room for one hour to ensure adsorption equilibrium between Methylene Blue (MB) and the catalyst surfaces.Subsequently, the solution was exposed to UV light from a 6W lamp with a wavelength of 254 nm serving as the irradiation source.At equal intervals of time (15 minutes), samples were collected, filtered, and monitored for MB degradation using UV-visible spectroscopy (Shimadzu 1900i-Japan).Using the following eqution [56] to calculate the degradation efficiency.
C 0 , C, A 0 , and A refer to the concentration and absorbance of methylene blue (MB) before and after exposure to UV light, respectively.diffraction peaks with 2θ = 35°, 46.4°, 65°, and 72.2° for Hibiscus sabdariffa respectively.Which both can be corresponded to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) Braggs planes.Also, the X-ray diffraction powder pattern in the two extract also had small peaks.This could be formed because of the organic ingredients of the leaves and flower extracts and because of the small residual amounts of AgNO3 that it is did not diminished.It is notable that the peak was associated with the (111) plane is the highest and most intense of the peaks.The Ag NPs produced in the present process are naturally crystalline with an FCC structure and the determined patterns of XRD matched closely with the standard (JCPDS: 01-1164) [57].

Field emission scanning electron microscopy (FE-SEM)
The FE-SEM images in demonstrate the formation of agglomerate ZnO NPs with hexagonal nanorod shape, with particle sizes ranging from (23.04 -32.58) nm and (37.99 -79.59) nm, the metallic silver nanospheres are evenly dispersed on the surface of ZnO NPs, loose agglomerations of NPs have grown larger and formed clusters, resulting in the formation of nanorod-like Ag/ZnO NCPs with particle sizes ranging from (22.39 -40.05) nm and (59.73 -87.05) nm as in Fig. 2 (e & f).It is evident that the agglomerations of Ag/ZnO NCPs were significantly larger in size compared to pure Ag NPs and ZnO NPs.In this process, silver nanoparticles form an outer layer coating on the surface of ZnO particles.In one possible method, that the silver nanoparticles introduced to a solution containing ZnO particles and allowed to bond with the ZnO surface due to attractive electrostatic forces or chemical interactions.The silver nanoparticles can then continue to accumulate on the surface of the ZnO particles, forming a layer.This can be facilitated by agitation or heating of the solution.Our results agree with the results of other work [59][60][61].

Dye degradation activity
The photocatalytic effectiveness of Ag nanoparticles, ZnO nanoparticles, and Ag/ZnO nanocomposite powder, the degradation rate of Methylene Blue dye in a solution was measured under UV light.Samples of the degraded dye solutions were collected at different time intervals, and their absorbance was determined using a UV-Visible Spectrophotometer to quantify the dye degradation rate using the equation (1) above.Figure 3 showed UV-visible absorption spectra of the 6 samples, where Fig. 3 (a & b) represent the Ag NPs efficiency from Aloe vera, Hibiscus sabdariffa respectively.Fig. 3 (c & d) represented the ZnO NPs efficiency from Aloe vera, Hibiscus sabdariffa respectively.And Fig. 3 (e & f) represented the Ag/ZnO nanocomposite efficiency from Aloe vera, Hibiscus sabdariffa respectively.The above results indicate that the Ag/ZnO nano composite had the highest photo degradation activity as shown in Figure 4, with a degradation rate of 97% for Aloe vera and 87% for Hibiscus sabdariffa.The ZnO NPs also showed a high level of photo degradation activity, with degradation rates of 95% and 75% for Aloe vera and Hibiscus sabdariffa, respectively, while the Ag NPs had lower levels of activity, with degradation rates of 86% and 21% for Aloe vera and Hibiscus sabdariffa, respectively.These results suggest that the Ag/ZnO nano composite could be a promising material for photocatalytic applications.
To determine the rate of photocatalytic reactions, the pseudo-first order rate constant (k min −1 ) can be calculated by estimating the slope of the lines in Figure (5).When exposed to UV light, electrons in the Ag/ZnO NCs can absorb photon energy and move to the conduction band (CB), generating an equal number of holes in the valence band (VB).Since the conduction band CB energy level of ZnO is higher than the Fermi level of Ag-ZnO, the electrons can transfer from ZnO to Ag.Consequently, the photogenerated electrons can be trapped by Ag, preventing their recombination with holes.This leads to an accumulation of more electrons and holes at the Ag/ZnO interface, ultimately enhancing the photocatalytic activity of the Ag/ZnO NCs.
The conduction band (CB) electrons may subsequently generate superoxide anion radicals (O2−), while the valence band (VB) holes may react with H2O to create hydroxyl radicals (•OH).These hydroxyl radicals can facilitate the degradation of MB.The reactions can be summarized as follows [62 -64].

Figure 4 .
Figure 4.The efficiency of Ag NPs, ZnO NPs and Ag/ZnO NCs against MB dye.

Figure 5 .
Figure 5. First-order kinetics of MB dye degradation against Ag NPs, ZnO NPs, Ag/ZnO NCs using different plant extracts with irradiation time.These lines are plotted between ln (A0/A) and irradiation time, and the results indicate an increase in the value of k for samples prepared from Aloe vera extract.Table(1) presents the efficiency values and k for each sample.Figure(6) demonstrates a possible mechanism of the Ag/ZnO NCs photocatalyst.

Figure ( 6 )
demonstrates a possible mechanism of the Ag/ZnO NCs photocatalyst.