Synthesis of ZnO Nanoparticles by using Rosmarinus officinalis Extract and their Application for Methylene bleu and Crystal violet Dyes Degradation under Sunlight irradiation

Zinc oxide (ZnO) nanoparticles (NPs) were synthesized using Rosmarinus officinalis leaf extract at 80 ° C (ZnO-80) and 180 ° C (ZnO-180). The biosynthesized ZnO NPs were characterized and their photocatalytic activity was evaluated for the degradation of methylene blue (MB) and crystal violet (CV) under sunlight irradiation. The results of the characterizations by XRD, TEM and SEM showed that the size of the NPs of ZnO-80 was smaller than that of ZnO-180 which exhibited flakier agglomerated spherical structures. Photocatalytic tests showed ZnO-80 which was prepared by a cheap and easy procedure compared to ZnO-180 effectively degrades MB and CV dyes under sunlight. The superior performance of ZnO-80 over ZnO-180 can be explained by the differences in their textural properties. This is because ZnO-80 has a smaller crystallite size, a specific surface area and a higher pore volume than ZnO-180. Fourier-transform infrared spectroscopy (FTIR) analyzes revealed that both samples contained an adsorbed carboxylate group (COO), and accordingly a mechanism was proposed for the formation of ZnO NPs that include the carboxyl group.


Introduction
The removal of organic contaminants from wastewater, and especially those resulting from dyes, remains a major concern for several countries. Indeed, environmental contamination caused by dyes leads to health problems due to their toxicity [1][2][3][4][5]. Dyes are organic where 0 is the initial absorbance of MB; is the absorbance of the solution after sunlight irradiation at time t.

Characterization of the catalysts
Infrared spectra were recorded with an infrared spectrometer GENESIS II-FTIR (4000-400 cm -1 ) using the KBr Pellet technique. X-ray diffraction (XRD) measurements were carried out employing an Ultima IV, X-ray Rigaku difractometer using Cu-Kα radiation. UV-Vis characterization was achieved by means of double beam UV-Vis spectrophotometer (Philips 8800). Catalysts Surface morphology was analyzed by using a JSM-7600F (JEOL Ltd, Japan) Transmission electron microscopy (TEM). Thermogravimetric Analysis (TGA) were performed using a Netzsch Thermogravimetric analyzer TGA model SAT 449 F3.
The specific surface area (B.E.T), pore volume and average pore diameter of the fresh and used catalyst was measured in Micromeritics Tristar II 3020 surface area and porosity analyzer.  ZnO-180 showed the bands at 420 and 492 cm -1 and 1383 cm -1 . The pair of bands observed at 416 and 490 cm -1 and at 420 and 492 cm -1 are assigned to the tensile bond of ZnO and the oxygen vacancies in ZnO respectively. These results are in agreement with those reported in the literature [38][39][40]. As for the peak at 1369 cm -1 and 1383cm-1 observed in the spectrum of ZnO-80 spectrum and ZnO-180 spectrum respectively, it is due to the symmetrical stretching of the zinc carboxylate. As the size of the NPs increases, the content of the carboxylate (COO − ) group in the samples decreased.

XRD analysis
The XRD patterns of bio-synthesized ZnO NPs from leaf extract of Rosmarinus officinalis for Zno-80 and ZnO-180 are shown in Figure 3. The sharp and narrow diffraction peaks indicate that the materials are well crystallized.
where D is the crystallite size, β is the full width at half maximum, θ is the diffraction angle and λ is the wavelength of X-rays. The average crystallite size obtained for ZnO-80 and ZnO-180 was found to be 14.7 and 15.5 nm respectively.

TEM
In order to see the effect of temperature on the texture of ZnO NPs synthesized using aqueous extract of Rosmarinus officinalis leaves, characterizations by TEM were carried out. The results of the analyzes illustrated in Figure 4 show that the particles synthesized at The results showed that the synthesis temperature affects the shape and size of ZnO NPs.
Spherical shaped particles were observed at 100 o C. With the increase in the reaction temperature, spherical and rod-shaped shapes of ZnO NPs were observed due to the increased growth rate. A further increase in temperature, almost all the ZnO NPs appeared in nanorod shaped clusters due to the fusion of smaller NPs and the formation of larger particles, that is, say clusters. Hassan Basri et al. [42] analyzed the structure of ZnO NPs synthesized at 28 o C and 60 o C by FESEM. NPs prepared at 28 °C clearly exhibited a mixture of spherical and rod-shaped particles, but better separated and less agglomerated compared to NPs prepared at 60 °C, which agglomerated in the shape of a flower rod. In turn, these flower-shaped particles tend to stick together to form large clusters. Dutta et al. [43] They studied the effect of temperature on the aggregation of Au NPs in the temperature range of 20 °C to 60 °C and found that the rate of aggregation increases with temperature.
TEM measurements showed the formation of aggregate of Au NPs with different morphologies. The acceleration of the aggregation of NPs at higher temperatures can be explained by the decrease in the electrostatic repulsion force between NPs with increasing temperature [44][45][46][47]. It has been reported in other research work [47,48], that the agglomerations could be due to a high surface energy of ZnO-NPs resulting from a narrow space between the NPs which generally has been observed for synthesis carried out in aqueous medium. The increased rate of aggregation with temperature is similar to ordinary chemical reactions generally observed. At higher temperatures, the NPs come together to form nanoclusters, i.e. the average grain size increases with the annealing temperature and this was confirmed by XRD.

UV-visible analysis
UV-Vis spectroscopy was carried out to confirm the formation of the NPs of ZnO and to estimate the band gap value (Eg). The band gap energies of the samples were estimated using the Tauc equation : where the terms h, ν, α, and Eg represent Planck's constant, frequency, absorption coefficient, and band gap energy, respectively. A is a proportionality constant, and n denotes the type of electron transition (for directly allowed transitions, n = 1/2). As can be seen from  [51,52]. The bandgap of ZnO-80 material is slightly higher than that of ZnO-180. This could be due to the difference in the size of their NPs. It is well known that the variation in band gap can be due to a structural parameter and to the size of the grains.
In fact, a strong correlation between absorption peak and particle size has been observed [53]. Therefore, this result indicates that the crystal particle size of ZnO-80 is smaller than that of ZnO-180, which is in agreement with those of XRD and TEM analyzes.   [54]. A third weight loss of 5.9 % and 1.9% respectively appeared while temperature increases at 260-900°C which, probably was due to the thermal degradation of less volatile aromatic compounds [55]. It should be noted that beyond 220 o C the weight loss for ZnO-180 becomes greater than that observed for ZnO-80. This is consistent with the results of SEM which shows that ZnO-180 exhibits more agglomerations which may be organic compounds coating the ZnO NPs.

Mechanism for the formation of ZnO
To establish a possible mechanism for the formation of ZnO NPs involving one of the substances present in the plant extract, it is first necessary to verify the formation of the NPs after addition of the extract. To do this, the ZnO-80 particles formed after the action of the substance but before calcination were analyzed by UV-Visible spectroscopy. By way of comparison, the characterization of the ZnO-80 particles after calcination was also carried out. The analysis was performed under aqueous conditions in the wavelength range from 250 nm to 550 nm and the results are presented in Figure 8. The absorption peaks observed at 371 and 382 nm for Zno-80 before and after calcination respectively confirms the formation of ZnO NPs. Similar results of absorption band were also observed by various researchers [56][57][58]. The absorption peaks observed at 371 and 382 nm are due to the intrinsic band gap of Zn-O absorption. As for the substance probably involved in the synthesis of ZnO-80 NPs, the results of FTIR (Figure 2), showed that carboxylate species remained adsorbed on the NPs even after calcination. It is therefore probable that these species are involved in the formation of NPs. It should be noted that various leaf extracts contain substances which can act as reducing and capping agent, thus preventing aggregation of NPs. Indeed, it has been observed that the action of certain biological compounds leads to the reduction of Zn 2+ to ZnO NPs [59]. It has also been observed that proteins and functional groups (carboxylates) are involved in the reduction of gold NPs [60].
On the other hand, it has been shown that when a zinc oxide powder is dispersed in distilled water in the pH range 7-9, it is in equilibrium with various species in aqueous solution and that is the species Zn 2+ and Zn(OH) + which are the predominant [61]. It is therefore probable that the formation of ZnO NPs goes through the formation of Zn(OH) + . Based on the above results, a mechanism ( Figure 9) has been proposed.

Photocatalytic activity
The photocatalytic activities of the synthesized ZnO NPs were evaluated via the photodegradation of methylene blue (MB) and CV under sunlight irradiation. Prior to illumination, 10mg photocatalyst was added to the dye aqueous solution (10 mL, 10ppm).
The solution was stirred in the dark for 20 minutes in order to achieve absorption-desorption equilibrium, then the photocatalytic reaction was started. The photocatalyst will then be exposed to the sunlight for the desired time at 40 o C. Figure 10 shows the UV-Vis absorption spectra of MB and CV absorbance with respect to time for ZnO-80 and ZnO-180. The aqueous solution of the MB molecules exhibits two peaks, one at 664 and the other at 615 nm, which correspond respectively to monomers and dimers. [62]. Upon irradiation, the peak at 664nm has a progressively blue shift to shorter wavelength (Figure 10 (a)) because of hypsochromic effect [63,64]. In the presence of ZnO-80 the absorbance of MB decreased sharply after 30 min. Initially, the absorption peak at 664 nm was much larger than the absorption peak at 615 nm which gives a big difference between their intensities. After 30 min, this difference is attenuated, thus indicating that the rate of degradation of the monomers is much higher than that of the dimers [65]. In addition to the decrease in the intensities of the two peaks, a slight shift towards the blue of the bands located at 664 nm also observed. This is caused by the N-demethylated degradation concomitant with the degradation of phenothiazine [66].
The influence of the irradiation time on the discoloration of the CV (Figure 10 (b)) was followed by the characteristic peak at 590 nm, corresponding to the conjugated triphenylmethane chromophore. The decrease in absorbance at 590 nm with irradiation is due to the degradation of the chromophore responsible for the characteristic color of the CV.
The hypochromic shift of the peak at 590 nm of the chromophore at about 575 nm indicates an N-dimethylation reaction [67] leading to NO 3 − ions [68,69].   (Table 1). It can be seen that ZnO-80 presents lower crystallite size, higher surface area and higher pore volume than ZnO-180.

Reusability
In order to examine the reusability of the biosynthesized ZnO-80 and ZnO-180 photocatalysts, we tested them after use to see the stability of their photocatalytic activity.
Both solids were tested under the same conditions as those in which they gave complete degradation of MB and CV, ie 100%. The results obtained ( to 95.1%. A slight decrease in activity therefore occurred. This is expected because a decrease in photocatalytic activity after reuse of the catalyst has been observed by many researchers [70,71]. This slight decrease in degradation is acceptable and allows reuse of these catalysts.