Catalytic Activity for Dye Degradation and Characterization of Silver/Silver Oxide Nanoparticles Green Synthesized by Aqueous Leaves Extract of Phoenix Dactylifera L .

In this study, green synthesis of silver /silver oxide nanoparticles was successfully prepared from Phoenix Dactylifera L aqueous leaves extract. The effect of different volume ratios (% v/v) (Plant extract / Precursor) on the nanoparticles silver /silver oxide nanoparticles formation, optical properties, and catalytic activity for dye degradation was studied. The obtained Ag/Ag 2 O nanoparticles were characterized using various techniques, such as UV-Visible, FT-IR, XRD, SEM for this purpose. The UV-Vis spectrum shows the absorption at 430 nm associated with Ag/Ag 2 O NPs. The optical bandgap values were found to be in the range of 3.22 to 4.47 eV for the direct bandgap and 3.73 to 5.23 eV for the indirect bandgap. The functional groups present in plant extracts were studied by FTIR. XRD confirmed the crystalline nature of Ag / Ag 2 O NP, and its average particle size was between 28.66-39.40 nm. SEM showed that the green synthesized silver/silver oxide nanoparticles have a spherical shape. The purpose of this study, it highlights the high catalytic activity for dye degradation of Ag/Ag 2 O NPs green synthesized. As a result, the use of Phoenix Dactylifera L aqueous leaves extract offers a cost-effective and eco-friendly method.


Introduction
Nanotechnology is a relatively new scientific field that adopts engineering nanoparticles from different materials physically, chemically and biologically 1 . Nanotechnology is also defined as "the creation and use of structures, devices and systems that are characterized by their distinct and varied characteristics and infinitesimal size, in general to deal with particles within the range of Size <100 nanometers 2 , the world is witnessing an industrial revolution in nanoscience, which enabled it to convert negative nanostructures into positive and productive nanostructures.
Researchers and scientists have predicted the great role that nanotechnology will play in playing a great role in the development of the developing world, so that the abundance of new ways and means, and revolutionary, which contributed to advances such as energy storage and transport, medicine and pharmacy 3 .
Green synthesis is a modern area of biotechnology that is economically and environmentally beneficial as an alternative to chemical and physical methods that contain part of the hazard to the environment. Because in this method, biologically safe, non-toxic, and environmentally friendly natural reagents 4 , Mentha pulegium L 5 , Artemisia 6 , Moringa Oleifera 7 , Phoenix dactylifera L 8 , and others are used in the biosynthesis of metal oxide nanoparticles 9,10 . The researchers used widely available plant extracts in nature in the green synthesis of metal oxide nanoparticles. It is also known as "vital plants" because of its speed, environmental efficiency, and low cost. It has a high ability to absorb minerals while maintaining safety levels 11 . These methods include microbes such as fungi, bacteria, algae, and viruses as reducing agents [12][13][14][15] . It is considered environmentally friendly because during the biosynthesis of nanoparticles the resulting toxic chemicals are eliminated as they break down with the help of enzymes present in microbes.
In recent years, research has been conducted in the field of nanotechnology that uses green plant materials and extracts for the biosynthesis of metal oxide nanoparticles without adding any external chemical substances that cause environmental pollution. The synthesis of metal nanoparticles is accomplished by a green method, using different plant fractions as reducing agents and caulking agents. Among the many different metal nanoparticles that have been studied, the precious metal silver oxide hopes to be used in various fields due to its unique characteristics, so it occupies a large position in all nanomaterials research. Its characteristics are sensing, photoelectric, catalysis and drug delivery 16 .
In this study, green synthesized silver/silver oxide nanoparticles by Phoenix Dactylifera L aqueous leaf extract, was investigated. Characterizations of the obtained nanoparticles were analyzed using standard techniques such as UV-Vis, FT-IR, XRD, and SEM. In addition, the effect of different volume ratios (v /v) (plant extract / precursor) on their optical properties and catalytic activity for dye degradation were evaluated.

Materials
Silver nitrate AgNO3 obtained from (VTRS) Laboratory, Leaves of Phoenix Dactylifera L were harvested from El Oued south east of Algiers.

Preparation of plant leaf extract
Leaves of Phoenix Dactylifera L were washed by distilled water, dry for 12 days in a shade place at room temperature, and then crush to obtain a fine powder. Has been put 10 grams of powdered Phoenix Dactylifera L leaves and 100 ml of distilled water into a 250 ml glass beaker to prepare the extract. The mixture was stirred stably at room temperature for one day. After that, the extract was filtered with filter paper (Whatman No: 42) and stored in a glass container at 4°C for further use 6,8,17 .

Biosynthesis of silver/silver oxide nanoparticles
The Ag / Ag2O NPs were synthesized by adding a different volume ratios (v /v) (plant extract / precursor) (1/30, 1/40 and 1/50), 1 ml of leaf extract to 50 ml of 1 mM AgNO3 aqueous solution in a 250 ml Erlenmeyer flask stirring 150 rpm at room temperature for 2 hours, the bioreduction of Ag + to Ag ° was confirmed by a color change to brown after 5 minutes.

UV-visible spectroscopy
The biosynthesis of Ag/Ag2O NPs were analyzed by using UV-vis spectrometry (shimadzu -1800), the measurement were recorded at the temperature in wavelength region of 300 to 900 nm. The stability of formation of Ag/Ag2O NPs was followed by UV-vis spectrometry using a quartz cells and distilled water as blank solution.

FTIR spectroscopy
Fourier transform infrared (FTIR) measurements of leaf extract and of green synthesis Ag/Ag2O NPs were performed by (Nicolet iS5, Thermo Fisher Scientific), to identify the functional groups in carried out in the range of 4000 to 400 cm-1.

X-Ray Diffraction (XRD)
The crystalline structure of the synthesized NPs was obtained using X-ray diffractometer (Rigaka Miniflex 600) and CuK radiation with a wavelength of 0,15406 nm in the angular range 10°< 2θ<80°.

Scanning Electron Microscopy (SEM)
The shape and morphology of the synthesis Ag/Ag2O NPs was confirmed by using (SEM-TESCAN VEGA 3) at accelerating voltage of 10KV.

Catalytic degradation of Congo red
The silver nanoparticles obtained by green method used for reducing of Congo red in the presence of sodium borohydride (NaBH4) at room temperature. Firstly, 2.5 ml of diluted Congo red was analysis ultraviolet visible, and appeared the peak occur at λmax = 488 nm. The catalytic reaction was calculated to be conducted in all experiments. To investigate the catalytic effect of the biosynthesized silver nanoparticles, NaBH4 solution (10-2 M) (considered a reducing agent) was added to the Congo red solution (10 −4 M), which was followed by the addition of biosynthesized silver nanoparticles (10 mg/l), and adjust the PH of the reaction and completed the reaction at volume of 5 ml. The degradation process was observed spectrophotometrically in a wavelength range of 250−900 nm at 10 mn to 60 mn. The decolourization process was observed as a decline in the absorbance intensity (λmax) of the solution. The Experiments showed to examine the catalytic efficacy of the biosynthesized silver nanocatalysts.

Catalytic degradation of Methylene blue
The effect of Ag/Ag2O NPs on degradation of methylene blue was estimated in the same way to the reduction of Congo red. The reaction composed 3 ml of methylene blue (2.10 -6 mol/l) and 3 ml of methylene blue (2.10 -6 mol/l). The above reaction was added to NaBH4 solution 2.25 ml (6.10 -6 mol/l), and 150 µl (80.85 mg/l Ag/Ag2O NPs on suspension). The degradation of methylene blue controlled at different time by optical absorbance at 611 nm and 663 nm.

3.1.Characterization of silver /silver oxide nanoparticles
Preliminary studies suggest that phytochemical screening of Phoenix Dactylifera L Leaves extract indicates the presence of polyphenols, flavonoids, and condensed tannins 18 .

3.1.1.UV-visible Spectroscopy
The colloidal solution of the as-prepared Ag/Ag2O and the plant extract were analyzed using UV-Vis spectroscopy ( Figure 1). Accordingly, the plant extract spectrum has exhibited two peaks at 275 and 320 nm. Meanwhile the Ag/Ag2O spectra revealed on a common peak in all the samples  Generally, the optical band gap of a semiconductor can be determined by plotting absorption coefficient verses the photon energy which could be estimated using the Tauc's formula (Equation (1)) 22 : where ℎ is the incident photon energy, is the absorption coefficient, is a constant, is the optical band gap in electron-volts (eV) and is an exponent that can take different values depending on the nature of the electronic transition, i.e.; = 2 for direct transition, and = 1 2 ⁄ for indirect transition as shown in Figure 2 and Figure 3 The results of the estimated Urbach energy values of the samples are listed in Table 1.

3.1.2.Fourier Transform Infrared (FTIR) spectroscopy
The FTIR analysis was carried out to identify the potential presence of reducing and stabilizing biomolecules in the Phoenix Dactylifera L extract. The resultant FTIR spectrum ( Figure 5) has exhibited several absorption bands that correspond to the functional groups of the biomolecules existing in the plant extract. Five main absorption peaks were observed, the broad peak centered at 3340 cm -1 is assigned to O-H stretching vibrations 6 , the intense peak at 1650 cm -1 is due to C=O stretching and N-H bending vibrations of primary amides group which is commonly found in the protein 26 . The two peaks located at 1364 cm -1 and 1204 cm -1 are assigned to the nitro banding N-O vibrations and C-O-C stretching vibrations of the aromatic ring respectively 27 . In addition, the peak located at 650 cm -1 corresponds to C-H bending vibrations out of plane 28 .

Figure 5 FTIR spectra of Phoenix Dactylifera L extract and the biosynthesized Ag/Ag2O.
The FTIR results shows that Phoenix Dactylifera L extract contain many different functional groups such as carboxyl, carbonyls, amides and phenols, which could serve as bio-reducing and capping agents for Ag/Ag2O synthesis 7 .

1..1 3.1.3.X-ray diffraction (XRD)
The Where D is the crystalline size (nm), β is the full width at half maximum of the diffraction peak (FWHM) of the most intense diffraction peak, λ the X-ray wavelength (1.5406 Å) and θ is the Bragg angle of diffraction 32 .
The effect of the different volume ratios on crystallite size of Ag/Ag2O are shown in Table 2. The decrease in crystallite size by increasing the amount of surfactant (plant extract) ratio has been also reported by many previous studies 33,34 .

3.1.4.Scanning Electron Microscopy (SEM)
SEM was used to study the formation of silver/silver oxide NP and its morphological size. Figure   7(a-c) shows the SEM images of the synthesized silver/silver oxide. It was observed that almost of them are spherical in nature shaped. Note that for the mixture of Ag and Ag2O nanoparticles; in addition, these particles converge into clusters to form foam (Figure 7 a and b). A similar phenomenon has been reported in previous studies.

Catalytic activity towards CR dye degradation
Congo red consists of two phenyl ring bonded to two naphthalene terminal flats containing amino and sulfonic groups. Congo red is a toxic and carcinogenic metabolite dye used in industries such as the textile, paper, and rubber industries and causes bladder cancer in humans. Therefore, its reduction is an important issue due to its high environmental toxicity. The catalytic degradation of CR was monitored by biosynthetic silver nanoparticles under various experimental conditions. The Congo red aqueous solution shows two peaks at 340 nm and 490 nm in the UV-vis region, which binds to the azo ( N N ) bond. During the CR reduction process, azo bonds in the dye molecule decompose and produce various aromatic amine products (sodium 4-amino-1naphthalene sulfonate and 1,1'-Biphenyl ). The CR dye molecules cannot be reduced in the aqueous medium in the presence of sodium borohydride (NaBH4) as the reducing agent because this reaction is thermodynamically achievable but is not kinetically possible. Therefore, the use of silver / silver oxide nanoparticles as nanocatalysts provides the support and pathway via the transfer of electrons between the receiver (CR) and the donor (borohydride ion (BH4 −1 )). In addition, the silver nanoparticles provide a suitable surface for binding CR particles and borohydride ions (BH4 −1 ) to interact with each other to form decomposition products. These nano stimuli make the CR dye degradation process kinetically feasible, and reduction is complete in a very short period. A plausible decomposition mechanism for CR on the surface of bio-silver/ silver oxide nanoparticles is presented in Figure 8 35 . Figure 8 The Proposed mechanism for the silver/silver oxide nanoparticle-catalyzed reduction of CR using NaBH4 35 .
Upon application of optimal experimental conditions, achieve an 80% degradation rate within 60 minutes was observed Figure 9 and Figure 10. The degradation efficiency is calculated by the following formula: Degradation ratio (%) = 0 − × 100 Where 0 is the initial concentration of CR and is the immediate concentration in the sample. Figure 9UV-vis spectra versus congo red dye under optimal reaction conditions. Figure 10 The graph of the decrease in absorbance and height percent of Congo red dye degradation versus time.
In order to determine the dye degradation kinetics of CR, the relationship between ln(C0/Ct)and irradiation time was plotted (as shown in Figure 11). It is found that under the catalysis of the silver/silver oxide nanocatalyst, the degradation reaction of CR basically obeys the first-order reaction kinetics 35 . Figure 11 shows a graph of ln (C0/Ct) versus time, which helps to understand the catalytic performance of biosynthetic silver nanoparticles. By plotting the relationship between ln (C0/Ct) and time, the kinetic parameters of CR dye degradation under optimal reaction conditions were studied. As shown in Figure 10, a linear relationship between ln (C0 / Ct) and time is observed, and the reaction follows a pseudo-first order kinetics. Therefore, the reaction rate is determined by where C0 and Ct are the concentration or absorbance of CR dye before and after degradation, and Kapp is the apparent rate (min -1 ). The value of the apparent rate constant (kapp) is calculated from the slope of the straight line using the above formula kapp=0.01151 min -1 36 . The results further prove that the silver/silver oxide nano-catalyst exhibits good photoreactivity, which confirms the corresponding degradation efficiency 37 . Figure 11 Plot of ln (C0/Ct) versus time for the silver /silver oxide nanoparticle-catalysed degradation of Congo red.

Catalytic Degradation of MB
The catalytic hydrolysis of the dye in the presence of NaBH4 was examined, which is another typical reaction to confirm the catalytic activity of Ag/Ag2O NPs and MB. The stimulation is monitored by UV-Vis spectroscopy. By adding Ag/Ag2O NPs as compounds to the reaction mixture, the catalytic reduction of the dye takes place immediately. The strong blue color of the MB solution fades and becomes colorless after 8 minutes during the degradation process Figure   13. Calculate the degradation percentage ( Figure 14) as a quantitative expression of degraded dyes.
The presence of amide groups of Ag/Ag2O NPs in the transfer of electrons from BH4anions to MB cations, which increases with increasing time, which was also similar to the previously reported micro Ag/Ag2O NPs [38][39][40][41][42][43][44][45] . The initial absorption peak at 663 nm gradually decreased over time, confirming the catalytic activity of the composite Ag/Ag2O NPs ( Figure 14). Figure 12 schematically illustrates the mechanism of the catalytic degradation process. The addition of biosynthetic silver nanoparticles improves the reduction process (the dye degrades up to 84.60% in 50 minutes). Analysis of the degradation reaction kinetics data showed pseudo first-order reaction kinetics. The reaction rate is determined by ln (C t / C 0 ) = -K app .t, where C 0 and Ct are the concentration or absorbance of MB dye before and after degradation. The slope of the curve determines the kapp (min-1) value. The linear graph of ln (Ct / C0) versus time ( Figure 15) supports the kinetic theory, where the k value is 0.137 min-1 46 .

Conclusion
In this study, the green synthesis of silver/silver oxide nanoparticles was successfully performed using Phoenix Dactylifera L aqueous leaf extract. The process is relatively easy, fast, cheap, environmentally friendly, and does not require any organic solvents or other toxic reagents.
Therefore, this synthesis method is more beneficial than conventional methods for the synthesis of Ag/ Ag2O NP. The shape of the prepared Ag / Ag2O NPs is close to spherical, the crystal in nature, and the average diameter is 28.66-39.40 nm. In addition, this study shows that the prepared Ag/Ag2O NPs have good catalytic activity for dye degradation of MB and CR stains under environmental conditions. The compound Ag/Ag2O NP is proved to be useful in the treatment of wastewater (dye degradation) in medicines, cosmetics, paints, plastics, and textiles.

CONSENT FOR PUBLICATION
Not applicable.

FUNDING
None.