Emerald eco-synthesis: harnessing oleander for green silver nanoparticle production and advancing photocatalytic MB degradation with TiO2&CuO nanocomposite

The tertiary composite of TiO2/CuO @ Ag (TCA) were synthesized by the solid state method using different ratios of TiO2/CuO NCs and Ag NPs. The structural, morphological, and optical properties of nanocomposites were analyzed by scanning electron microscope, Transmission electron microscope, X-ray diffraction, Fourier transform infrared spectra, UV–Vis diffuse reflectance spectra (UV–Vis/DRS) and photoluminescence spectrophotometry. The results showed enhanced activity of TCA hybrid nano crystals in oxidizing MB in water under visible light irradiation compared to pure TiO2. The photocatalytic performance TCA samples increased with suitable Ag content. The results show that the photo degradation efficiency of the TiO2 compound improved from 13 to 85% in the presence of TiO2–CuO and to 98.87% in the presence of Ag containing TiO2–CuO, which is 7.6 times higher than that of TiO2. Optical characterization results show enhanced nanocomposite absorption in the visible region with long lifetimes between e/h+ at optimal TiO2–CuO/Ag (TCA2) ratio. Reusable experiments indicated that the prepared TCA NC photo catalysts were stable during MB photo degradation and had practical applications for environmental remediation.


Nanoparticles synthesis
Synthesis of TiO 2 nanoparticles (T) TiO 2 NPs were prepared using a sol-gel method 5 .In a typical synthesis, an appropriate amount of Ti-isopropoxide precursor was mixed with deionized water and dissolved in 87.5 mL of ethanol, stirred at room temperature for 4 h, washed several times with deionized water, and dried at 90 °C.Finally, the obtained powder was calcined in air in a muffle furnace at 500 °C for 1 h to extract TiO 2 NPs.

Synthesis of CuO nanoparticles (C)
CuO nanoparticles were prepared by a precipitation method 28 .A solution of Cu(No 3 ) 2 0.3H 2 O (0.1 M) was dissolved in 100 mL of distilled water with continuous stirring.Add 0.1 M NaOH to the above solution until the pH reaches7.The color of the solution immediately changed from light blue to black and a large amount of black precipitate was formed.The precipitate was centrifuged and washed 3-4 times with deionized water.The resulting precipitate was air dried for 24 h.This CuO powder was used to characterize the material.

Preparation of Ag NPs by green synthesis
Preparation of AgNO 3 solution.0.5 g of AgNO 3 was dissolved in distilled water (50 mL), then the solution was stirred at 60-70 °C for 15 min.until it is homogeneous.The solution is stored in a dark container to avoid oxidation.
Preparation Nerium oleander leaves extract.The oleander leaves were washed with water several times, sterilized with 50% ethanol to remove foreign substances such as dust, washed with distilled water several times, and cut into 25 cm pieces.An equal amount of gram pieces of oleander leaves was mixed with 100 ml of distilled water and boiled in a sterile flask at 80-90 °C for 5-10 min.until the water turned green color.The solution was filtered and stored in a refrigerator at 3 °C30 .

Experimental techniques
The morphology of the prepared materials was examined with a scanning electron microscope (SEM) (JEOL) and a JEOL transmission electron microscope (TEM) associated with selected area electron diffraction (SAED).The phases of the prepared samples were examined by his X-ray diffraction (XRD) using a diffractometer (Panalytical XPERT PRO MPD).CuKα radiation (λ = 1.5418Å) was used at 40 kV and 40 mA.Functional groups were identified in the wavenumber range 400-4000 cm −1 using a Fourier transform infrared (FT-IR) spectrometer model Spectrum One (Perkin Elmer, USA).Light reflectance was recorded using a UV-Vis spectrometer (Perkin Elmer Lambda 1050).Photoluminescence spectra were recorded on a Cary Eclipse fluorescence spectrophotometer.

Photocatalytic activity study
The photocatalytic decomposition activity was examined using a photoreactor equipped with a 400 W halogen lamp as light source.The distance between the halogen lamp and the dye solution is 10 cm.Then, 0.025 g of photocatalyst was added to 30 ml of 20 ppm MB dye solution, and the solution was stirred in the dark for 30 min to reach adsorption-desorption equilibrium.Photolysis was started in 120 min and 5 ml of suspension was obtained in 30 min.The obtained suspension was analyzed by UV-Vis spectrophotometer at the MB solution's maximum absorption wavelength of 664 nm 32 .

Results and discussion
The SEM images of the most efficient photocatalyst of (0.05%) TiO 2 /CuO@Ag nanocomposite (TCA 2 ), TC 1 and pure TiO 2 , CuO nanoparticles are shown in Fig. 1.The SEM image of pure anatase TiO 2 grains has rounded shape and form sponge-like aggregates as presented in Fig. 1a, and the global and uniform particles indicated in images of CuO and TC 1 are coherent together as shown in Fig. 1b,c.There was also a higher tendency of agglomerations.In Fig. 1d indicates the SEM graph of net composite TiO 2 /CuO@ Ag, hence; the composite of TiO 2 with CuO appears also spherical shapes and Ag nanoparticles with even shape and spherical nature, which aggregated on the surface of the composite.This evident the composite is successfully prepared.
The TEM micrographs of the pure and composite samples are represented in Fig. 2. The figure shows that nanoparticles have almost spherical shape corroborating the images of the SEM.Other essential results can be gotten from Fig. 2 is that the particle sizes of the TC are more aggregated than that found for pure one.While the TCA composite the sliver was appeared as Irregular sphere.Moreover, a series of bright diffraction rings were observed through the corresponding ring pattern of the selected-area electron diffraction (SAED), suggesting the polycrystalline nature of the prepared material, Fig. 2e-h.The X-ray diffraction spectra was used to illustrate the purity, crystal structure and the phase formed for the as prepared nanocomposites.Figure 3 was displayed the X-ray diffraction spectra of bare samples of TiO 2, CuO, and Ag along with nanocomposites of TiO 2 /CuO and TiO 2 /CuO@Ag with different weight percentages of CuO and Ag, respectively.The diffraction peaks at 2θ of 26.69°, 36.13°,50.05° and 54.40° corresponding to lattice planes (101), (004), ( 200) and (105), respectively for TiO 2 anatase structure.Additionally, diffraction peaks characteristic to monoclinic CuO were noticed at 2θ of 35.44°, 38.81° and 48.70° representing to planes (110), (111) and (202), respectively.Upon adding CuO with various ratios of TiO 2 the resultant nanocomposits (TC) contain mixed diffraction peaks of both CuO, TiO 2 and the intensity of peaks of TiO 2 was slightly decreased without the observation of any foreign peaks, conforming the successful formation of TC composites with high purity as shown in Fig. 3a.Furthermore, the main peaks of nano silver appeared at 2θ values of 38.02°, 44.37°, 64.32°, and 77.22° corresponding to planes of (111), ( 200), ( 220) and (311), respectively.Figure 3b revealed the Bragg reflections of silver after addition of Ag to TC composites with different ratios, new main peaks were detected approximately at 2θ of 43.58°, and 64.32°, and 77.22° which indexed well with the XRD patterns of Ag nanoparticles.It is noted that the diffraction peaks have some sharpness, demonstrating the great crystallinity of the prepared photocatalysts.Figure 4 shows the FT-IR spectra of TiO 2 , CuO and TiO 2 /CuO @ Ag composite photocatalysts with mole ratios of Ag of 0%, 0.03%, 0.05%, and 0.06% for TC 1 , TCA 1 , TCA 2 and TCA 3 nanocomposites, respectively.The FT-IR spectrum of TiO 2 shows strong absorption bands in the range of 400-700 cm -1 .This band is assigned to the stretching vibration of the Ti-O-Ti bond and indicates the formation of TiO 2 at 646 cm −133 .In Fig. 4a, the FT-IR spectrum of CuO shows peaks at 718.49, 848.53 and 960.67 cm −1 revealed the formation of CuO.A broad peak noticed at 3371.57cm −1 attributed to O-H stretching of the moisture content 34 .For TC spectrum shows the presence of CuO oxide will give two characteristic low-intensity peaks of CuO vibrations in the 400-800 cm −1 region 35 .However, the spectrum of TiO 2 will appear in the same region 400-800 cm −1 , two high intensity peaks appeared for Ti-O vibrations 36 .It is easy to predict that the high-intensity characteristic peaks of TiO 2 will overlap the low-intensity characteristic peaks of CuO in this wavenumber region.That is why there is no difference in characteristic peaks in the presence or absence of CuO oxide in the photocatalysts.Figure 4b revealed the FT-IR spectra of TiO 2 -CuO/Ag nanocomposites, which show all the characteristic vibrations of TiO 2 , CuO and silver, characteristic absorption peak at 1090.72,801.81,690.40,504.43and476.31 cm −1 , hence; the peaks observed at 1090.72, and 960.67 cm −1 indicate the presence of silver nanoparticles 37 .Based on these results, a photocatalyst was prepared that did not contain any unnecessary foreign substances.The photocatalytic performance of a semiconductor significantly depends on its optical property, and thus, it is one of the important factors which should be studied.Figure 5a shows typical diffuse reflectance spectroscopy (DRS) curves for optical absorption behavior of pristine TiO 2 and CuO and their composites TC 1 , TC 2 , and TC 3 in the range of 200-800 nm.It was observed that the Pure TiO 2 photocatalyst displayed almost the strongest absorption intensity at 422 nm compared with its counter parts.Furthermore, the pristine CuO nanocomposite showed relatively small absorption band edge around 270 nm that is compatible with the small band gap of CuO 1.3 eV.As for TC 1 , TC 2 and TC 3 NCs, It can be seen the low intensity of reflectance, indicating the highly absorbance of light in visible range based on the reduction of the band gap energy.It is proposed that the combination of n-type and p-type semiconductors produces an internal electric field, resulting in the production of a p-n hetero junction.The creation of this p-n hetero junction, as well as the band alignment between CuO and TiO 2 , considerably facilitates electron-hole separation and increases catalytic activity [38][39][40] .Next, the best performing photocatalyst, TC 1 was modified with Ag nanoparticles as shown in Fig. 5b.Hence; Ag NPs modification of photocatalysts TCA 1, TCA 2 and TCA 3 endowed the TiO 2 /CuO @ Ag ternary composites the lowest reflectance, the optimum composite TCA 2 , as a result of the synergetic effect among components 41,42 .It has been demonstrated that lowering the composite's band gap energy improves the photocatalytic reaction by allowing the produced photocatalyst to absorb more photons and become more sensitive to visible light.The conjugation of two bands gap increased the stability between the e − /h + pairs.
The PL spectra demonstrate the behavior of photo generated charge carriers (e − , h + ),where a low recombination rate of photogenerated electrons and holes means a good separation between (e − , h + ), a low fluorescence intensity in PL spectra and consequently excellent photo degradation performance.Figure 6a,b displays the PL measurements of TiO 2, CuO, TC, and TCA nanocomposites 43 .The bare photocatalys, CuO, TiO 2 , and their www.nature.com/scientificreports/nanocomposites (TC 1 , TC 2 , TC 3 ) exhibte emission peaks around wavelength range of 470 to 550 cm −1 , referring the recombination of photogenerated charge carriers in visible light region as shown in Fig. 6a.Moreover, a strong PL quenching is observed for TC 1 , TC 2 , and TC 3 nanocomposites.TC 1 photocatalyst displays the smallest PL intensity, indicating that the recombination between photo-electrons generated and holes is reduced 44 .Remarkably, Fig. 6b reveals the PL emission of TCA ternary composite, TiO 2 /CuO @ Ag NCs was further quenched after the addition of Ag nanoparticles to extremely low intensity, indicating improved carriers life time and effective electron hole separation.The p-n heterojunction of CuO/TiO 2 align with the Schottky junction created with metallic Ag are responsible for facilitating charge separation and transfer through the interfaces 45 .Hence, TCA 2 nonocomposite represents a lower recombination rate provides greater dye degradation efficiency for the photocatalyst, consistent with the photocatalytic results.
Using MB dye as a model pollutant, the photocatalytic activity of the produced TiO 2 , TC and TCA nanocomposites was examined as demonstrated in Fig. 7a,b.The photocatalytic efficiency of MB decomposition was calculated from the Eq.(1): where C 0 is the initial concentration and C is the remaining concentration of MB after the reaction.Initially, to reach adsorption/desorption equilibrium, the solution was held in the dark for 30 min prior to light irradiation.On the other hand, it is obvious that the photocatalytic performance of pure TiO 2 was lower than that for their composites (TC) during the irradiation times of 0, 30, 60, 90, 120 and 150 min.This attributed to the large band gap of TiO 2, it suffer from high rate recombination between the photo induced charge carriers (e − , h + ) 46 .Form Fig. 7a revealed the combination of TiO 2 and CuO the photocatalytic activity increases to reach (85%) for TC 1 photocatalyst compared to pure TiO 2 (13%), TC 2 (79%) and TC 3 (78%).Furthermore, maximum degradation efficiency was obtained by inclusion of a suitable portion of Ag nanoparticles to TC 1 in the composite TiO 2 / www.nature.com/scientificreports/CuO @ Ag NCs with a degradation efficiency reach to (98.87%) for TCA 2 as shown in Fig. 7b.The higher photo catalytic degradation efficiency of TCA 2 to MB dye may be related to a suitable amount of Ag nanoparticles in the composite resulting in a lower band gap energy and sufficient PL characteristic.Additionally, the lower and higher amount of Ag nanoparticles in the ternary composite, TCA, was exhibited a negative effect on photodegradation performance as for TCA 1 and TCA 3 with photo degradation activity of (95.06%) and (92.07%), respectively.Where the lower Ag loading may be delay the transfer of photo generated carriers while the higher Ag loading may be hinder the light absorption.The pseudo-first-order kinetic reaction of MB dye photodecomposition under visible light irradiation was explored in Fig. 8 and Table 1.The degradation kinetics of MB by the synthesized photo catalysts was calculated according to Eq. ( 2): where C 0 is the initial concentration (mg L −1 ), C is the reaction concentration of the MB solution at time (t), and k a is the rate constant (min −1 ).
The linear relationship between the photodegradation time and dye concentration in the ln C 0 /Ct versus t time (t) plot implies that the degradation process was pseudo-first-order and, photocatalytic rate constant can obtained from it as shown in Fig. 8a.Further, the super photocatalytic performance of TCA 2 is proofed from the photocatalytic rate constant value.It could be realized from Fig. 8b and Table 1 that the photocatalytic rate constant of nano composites (TC 1 , TCA 1 , TCA 2 and TCA 3 ) was more than that of pristine bare TiO 2 taking the order of TCA 2 (0.0315 min −1 ) > TCA 1 (0.02137 min −1 ) > TCA 3 (0.01741 min −1 ) > TC 1 (0.01655 min −1 ) > T (0.00112 min −1 ) > MB (0.0001min −1 ).Additionally, adding Ag nanoparticles to TC1 photocatalyst was resulted in increased k a value.The greatest rate constant (0.0315 min −1 ) belongs to the TCA 2 photo catalyst.Which is consistent with the results of photocatalytic degradation, signifying that the catalyst has good features toward MB degrading activity under visible light.Table 2 provides a comparison of the photocatalytic efficiency of various catalysts in the degradation of a dye called Methylene Blue (MB) with respect to a synthesized nano-composite called TiO 2 /CuO @ Ag.The table provides information on the photocatalyst, dye concentration, catalyst dose, preparation method, degradation percentage, and the corresponding reference for each entry.
The Ag@Cu 2 O-CuO/TiO 2 catalyst was prepared using the sol-gel technique.It was tested against MB dye at a concentration of 20 ppm, with a catalyst dose of 0.5 g/L.The degradation percentage achieved was 83.00% 42 .Additionally, CuO-TiO 2 catalyst was synthesized using a green synthesis method involving C. benghalensis plant extracts.The MB dye concentration used was 20 ppm, and the catalyst dose was 1 g/L.The degradation achieved was 33.00% 47 .Also, the catalyst of Ag@CuO that prepared using an apolyol-mediated refluxing method, was   42 .Furthermore, the hydrothermal method was used to synthesize of Ag/TiO 2 /CuFe 2 O 4 photocatalyst and then tested against MB dye at a concentration of 5 ppm, with a catalyst dose of 1 g/L.The catalyst exhibited degradation efficiency of 85.00% 48 .The degradation efficiency of 91% was achieved via TiO 2 @CoFe 3 O 4 nanocomposite against MB dye at a concentration of 100 ppm, with a catalyst dose of 1.0 g/L.TiO 2 @CoFe 3 O 4 catalyst was synthesized using the co-precipitation method 5 .The sol-gel method was further used to prepare Pt/ZnO-MWCNT catalyst and tested against MB dye at a concentration of 100 ppm, with a catalyst dose of 0.4 g/L, achieving 74% 47 .Finally, Ag@CuO/TiO 2 nano-composite that synthesized for the current work.The MB dye concentration used was 20 ppm, and the catalyst dose was 0.83 g/L.It was prepared using a green synthesis method involving nerium oleander leaves extract.The degradation achieved by this composite was 98.87%.In summary, the table compares the photocatalytic efficiency of relevant catalysts, including the TiO 2 /CuO @ Ag nano-composite synthesized in the current work.It demonstrates that the TiO 2 /CuO @ Ag composite showed the highest degradation percentage (98.87%)among all the tested catalysts for the degradation of MB dye at a concentration of 20 ppm and a catalyst dose of 0.5 g/L.Reusability experiments were conducted using FT-IR after 120 min of photocatalysis, as can be seen in Fig. 9a.There was no change in the peak position, and the peaks exactly matched those of the FT-IR catalysis from before the photocatalytic degradation reaction.These findings explain how the produced catalyst can keep its stability and catalytic effectiveness even after numerous repeat usage.As demonstrated in Fig. 9b, the optimized photocatalyst TCA 2 for MB degrades at a rate that can be recycled, and after six cycles of usage, its photo degradation activity was found to slightly decline, showing strong stability.The interaction of TiO 2 @CuO and Ag, which can immobilize silver nanoparticle active sites in photocatalysis, is responsible for stability feature of TCA NCs.
The photocatalytic dye degradation process includes light absorption, generation of electron-hole pairs, and redox reactions with the dye adsorbed on the surface of the photocatalyst, upon exposing the photocatalysts to adequate light as shown in Fig. 10. which during the degradation process, the charge species such as OH -, h + , and e -act as oxidizing and reducing agents, and OH ., O 2 ., and HO 2 .radicals and holes (h + ) are intermediates that react spontaneously on the neighboring ions, and degrading the dye compounds to environmental friendly CO 2 and H 2 O as shown at the following Equations.
(3)    The developed photocatalyst, TiO 2 /CuO@Ag has a p-n heterojunction formed as a result of the combination between p-type (CuO) and n-type (TiO 2 ) semiconductors.Furthermore, the conduction band (CB) level of TiO 2 is higher than the CB level of CuO.Leading to the diffusion of holes from p-type to n-type region and electrons from n-type to p-type via the heterojunctions or to the attached Ag nanoparticles.As a result, it makes the separation of photogenerated electrons and holes easier along with the influence of Ag nanoparticles' surface plasma resonance (SPR) that enhances the collection of visible light harvesting.Resulting in enhanced photocatalytic efficiency 39,[49][50][51] .

Conclusion
In conclusion, this study successfully synthesized a green-synthesized TiO 2 /CuO@Ag nanocomposite using Nerium oleander leaves extract.The TCA nanocomposite exhibited superior photocatalytic activity for the degradation of MB under visible light.The presence of Ag in the nanocomposite significantly enhanced its photocatalytic efficiency, achieving a degradation percentage of 98.87% for MB.The TCA nanocomposite demonstrated stability and reusability, making it a promising material for environmental remediation applications.This research contributes to the development of sustainable and efficient photocatalytic materials for water treatment and pollution control.Further investigations can explore optimization and scalability of the TCA nanocomposite for broader environmental applications.

Table 2 .
Comparison of photocatalytic efficiency of relevant catalysis with respect to our synthesized TiO 2 / CuO @ Ag nano-composite.