Photocatalytic degradation of some dyes under solar light irradiation using ZnO nanoparticles synthesized from Rosmarinus officinalis extract

ABSTRACT ZnO nanoparticles (NPs) were synthesized using Rosmarinus officinalis leaf extract at 80 °C (ZnO-80) and 180 °C (ZnO-180). The powder was characterized and tested for the degradation of methylene blue (MB) and crystal violet (CV) under sunlight. FTIR analyzes revealed that Rosmarinus officinalis leaf extract contains phenolic compounds, and accordingly a mechanism for the formation of ZnO NPs was proposed. The characterizations by XRD, TEM and SEM and fluorescence spectroscopy showed that ZnO-80 NPs have smaller size than those of ZnO-180 which exhibited flakier agglomerated spherical structures. Photocatalytic tests showed that ZnO-80 degraded MB by 99.64% in 45 min while ZnO-180 degraded it by 98.82% in 60 min. The superior performance of ZnO-80 over ZnO-180 is due to the fact that ZnO-80 has smaller crystallites, higher specific surface area and higher pore volumes than ZnO-180. When reused, ZnO-80 only showed a slight decrease in the rate of degradation (2.95%). These results indicate that ZnO-80 can be considered as a promising photocatalyst for wastewater treatment. It was prepared by a simple and inexpensive procedure, and unlike many photocatalytic tests using an artificial light source, in the present work sunlight was used. GRAPHICAL ABSTRACT


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 compounds used in various industries, such as textiles, paper, plastics, leather plastics, food, printing and pharmaceuticals, electroplating and agriculture (6)(7)(8)(9). It should be noted that these industries use considerable quantities of water and consequently their wastewater containing dyes in significant quantities are discharged into natural waters. Some of these dyes are toxic, mutagenic and carcinogenic (10). Moreover, these dyes, by preventing the penetration of solar light into water, reduce photosynthetic activity and thus cause a disturbance of the aquatic equilibrium (11). Il should be mentioned that without appropriate treatments, these dyes can remain in the natural water for a long time (12). This is why various physical methods such as adsorption (13,14), coagulation (15,16), biodegradation (17), and various chemical methods such as chlorination, ozonation, etc. (18) have been used to reduce the environmental effects of dyes. However, physical and biological methods do not remove pollutants; they only transform them into another phase. As for chemical methods, they have the drawback of using strong oxidants such as chlorine and ozone which are themselves pollutants. The most suitable way to eliminate this waste is their degradation by photocatalysis. In fact, dyes can be degraded in the presence of photocatalyst when irradiated with visible light due to their absorption in the visible region.
In the last decades, particular interest has been focused on heterogeneous photodegradation by metal oxides because of their wide use in organic synthesis and their environmental applications (19,20). Among the different metal oxides used, it has been reported that titanium dioxide (TiO 2 ) and zinc oxide (ZnO) are the most chemically stable and are not toxic (21,22). This is what explains their applications in different fields (23,24). TiO 2 has been widely studied as a photocatalyst and found to have very good photocatalytic activity. However, its application using solar energy is strongly limited by its wide forbidden band (3.2 eV) and its low quantum efficiency (25). As for ZnO, it is a type of semiconductor which has a wide direct band gap (3.37 eV), a large excitation binding energy (60 meV) (26). In addition, ZnO not only has anti-fouling and antibacterial properties, but also good photocatalytic activity (27). In addition, ZnO has been shown to exhibit high absorption efficiency over a large fraction of the solar spectrum compared to TiO 2 (28). Taking into account these properties mentioned and the non-toxic nature, good environmental stability, strong oxidizing power, ZnO can be considered as a suitable alternative to TiO 2 photocatalyst.
Over the last decades, more attention has been paid to the synthesis of nanoparticles by the green method due to its fast achievement, its single-step realization and its use of environmentally friendly solvents (29). Several biological sources for the synthesis of metal NPs have been developed, including extracts from plants, bacteria, fungi, microorganisms and enzymes which can reduce metal ions in solution to NPs (30,31). These biological sources contain active biomolecules, such as phenols, flavonoid polyphenols, alkaloids, which act as reducing and stabilizing agents (32). Among the various biological sources used in the synthesis of NPs, it is obvious that the manipulation of the plants is more convenient compared to other biological materials. This is why many plants have been used for the synthesis of nanoparticles (33). Some studies have reported the synthesis of ZnO NPs by using various plant leaf extracts (34)(35)(36).
Sunlight is an abundantly available source of energy and its irradiation can be exploited to develop an economically process for the photodegradation of pollutants. In this study, ZnO NPs were synthesized using mediated Rosmarinus officinalis leaf extract as reducing and stabilizing agents. Rosmarinus officinalis is a plant growing wild in Saudi Arabia was selected for the synthesis of nanoparticles because it contains many phenolic compounds (37) that can be used as reducing and stabilizing agents for nanoparticles. The use of Rosmarinus officinalis plant is a valorization of local natural resources. The specific aim of this research was to demonstrate that a synthesis of ZnO nanoparticles by a simple inexpensive and environmentally friendly route produces an effective photocatalyst for the decoloration and elimination of contaminating dyes present in wastewater, such as methylene blue (MB), crystal violet (CV). The degradation of the dyes was carried out under solar irradiation with biosynthesized ZnO NPs at 80 and 180°C.

Materials
The healthy wild Rosmarinus leaves were collected from Abha region in southern of Saudi Arabia. Zinc nitrate hexahydrate and sodium hydroxide, methylene blue (MB), crystal violet from BHD. All the chemicals used without further purification.

Preparation of leaf extract
The collected fresh and healthy leaves of Rosmarinus were washed with deionized water, shadow dried for 2-3 weeks and then powdered using mixer grinder. For the preparation of aqueous leaf extract (ALE) solution, 10 g of powder dissolved in 100 mL of deionized water, boiled at 60°C for 15 min to kill the pathogens in ALE solution. After cooling, ALE solution was filtered using filter paper and stored at 4°C for further use.

Synthesis of ZnO NPs using Rosmarinus leaf extract
ZnO NPs were prepared by adding zinc nitrate hexahydrate to aqueous leaf extract solution (ALE). Two samples were prepared; the first sample was prepared by adding 18 mL of NaOH (2M) to a solution constituted of 5 g Zn(NO 3 ) 2 .6H 2 O (pH = 5.2) in 30 mL of the plant extract contained in an Erlenmeyer flask. The pH of the resulting solution The Erlenmeyer flask containing the resulting mixture was closed and then heated to 80°C with stirring for 3 hours. The catalyst thus prepared was denoted ZnO-80. The second sample was prepared in the same way but instead of being heated in the Erlenmeyer flask, it was transferred to a Teflon lined steel autoclave and heated at 180°C for 3 hours. The catalyst thus prepared was denoted ZnO-180. In both cases the obtained white precipitate was separated and washed several times with a solution of distilled water: ethanol (3:1). Then, it was left in the oven at 80°C overnight and calcined in a furnace at 500°C for 3 h. An illustrative scheme is given in Figure 1.

Photocatalyic test
1 g/L of photocatalyst and 10 mL of 10 mg/Liter of dyes was suspended in a bottle and the mixture was stirred in the dark to evaluate adsorption-desorption equilibrium. The photodegradation (Figure 2) was carried under sun light irradiation in a clear day of September (temperature 40°C) at Riyadh, Longitude 46:625 Latitude 24:727. The average sun light irradiation was 891 W/ m 2 . The resulting suspension was centrifuged at 3000 rpm for 5 minutes before measuring the absorbance using UV-Vis spectrophotometer. The photodegradation rate of methylene blue (MB) was calculated by the following Equation (1): where A 0 is the initial absorbance of MB; A t is the absorbance of the solution after sunlight irradiation at time t.

Characterization of the photocatalysts
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. The fluorescence spectra of the ZnO NPs aqueous suspension were measured using Shimadzu RF-6000 fluorescence spectrophotometer. Particle solutions have been sonicated for 30 minutes. The excitation wavelength has been 380 nm and the fluorescence spectra were recorded in the wavelength range 390-700 nm. A pass width of 10 nm was used for the measurement of the emission and excitation spectra.

Results and discussion
Catalyst characterizations

FTIR analysis Analysis by FTIR of ZnO nanoparticles synthesized at 80°C
(ZnO-80) and at 180°C (ZnO-180) as well as the Rosmarinus leaf extract used for their synthesis was performed to find out the compounds involved in the formation of ZnO nanoparticles. The FTIR spectra obtained are shown in Figure 3. The FTIR spectrum of the leaf extract showed the bands at 605, 758, 1075, 1242, 1425, 1612, 2825, 2931, and 3389 cm −1 in the region of 400 cm −1 -4000 cm −1 . The broad band at 3389 cm −1 can be attributed to the hydroxyl groups of phenolic compounds and N-H stretching vibration of group NH 2 (38,39). The peak 1612 can be due to the C = O stretching of amide I or to C = C groups of aromatic rings (38). The peak at 2931 cm −1 is assigned to C-H Figure 1. Schematic presentation of ZnO-NPs synthesis using Rosmarinus officinalis leaf extract. stretching vibrations to CH 3 and CH 2 (40). The presence of the bands at 2825 cm −1 and 2931 cm −1 due to aliphatic stretching vibrations of CH, CH 2 and CH 3 suggests that the bands at 1383 cm −1 and 1369 cm −1 in the FTIR spectrum of ZnO NPs can be attributed to aliphatic compounds. The peak at1425 cm −1 is assigned to C-O stretching vibration of amide and C-C stretching from phenyl groups and carboxylate COO − symmetric stretchig (41). The peak at 1075 is assigned to C-O stretching vibration (42,43). The spectrum of ZnO-80 sample showed the bands at 416, 490 cm -1 and 1369 cm −1 . Similarly, the spectrum of 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 (44)(45)(46).

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 4. The sharp and narrow diffraction peaks indicate that the materials are well crystallized. The peaks found for ZnO-180 correspond to Bragg reflections with 2θ values of 31.84°, 34.48°,     (1 0 0) The average crystal size (D) of the particles was determined using Debye Scherer's formula, given below: 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 analysis
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 5 show that the particles synthesized at 80°C (ZnO-80) are mainly spherical with sizes varying from 18 to 40 nm. However, some grains of larger sizes are also formed. On the other hand, NPs synthesized at 180°C (ZnO-180), they mainly have an elongated structure with sizes varying from 20 to 123 nm. Some spherical particles of small sizes are also observed. The agglomerates were obtained by coalescing spherical NPs. These results are in agreement with those reported in the literature (47)(48)(49) (47) analyzed the structure of ZnO NPs synthesized at 28°C and 60°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. (48) 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 (49)(50)(51)(52). It has been reported in other research work (30,53), 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. Figure 6 shows the SEM morphology of ZnO-80 and ZnO-180 synthesized at 80 and 180°C respectively. For the ZnO-80 sample, the micrograph shows individual particles with irregular morphology and an agglomerated spherical particle. On the other hand, for the ZnO-180 sample, SEM imaging shows the flaky agglomerated spherical structures. It should be noted that for the two samples ZnO-80 and ZnO-180, the agglomerated particles exhibited pores, probably resulting from the escape of volatile substances or gases formed during calcination. Similar observations were reported in the literature (54,55).

SEM analysis
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 (αhv) 2 versus energy plots (Figure 7), ZnO-80 and ZnO-180 have a bandgap value of 3.13 eV and 3.10 eV respectively. The values of these band gaps are in the range of reported values of ZnO NPs (56,57). 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 (58). Therefore, this result confirms 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. It should be noted that studies reported in the literature have mentioned that the band gap narrowing could be due to the improvement of the crystallinity with the increase of the annealing temperature (59), to the loading of a dopant on ZnO (58), to O and zinc vacancies in ZnO (60,61).

Fluorescence measurements
In order to determine the relationship between ZnO NPs size and emission intensity, a quantitative investigation for both ZnO-80 and ZnO-80 samples was conducted.
The fluorescence spectrum of the aqueous suspension of ZnO NPs at room temperature with an excitation wavelength of 380 nm is shown in Figure 8. This spectrum  contains three important emission peaks positioned at 412, 437., 470, and a very low yellow emission at 572 nm. However, the intensity of the violet emission localized around 437 nm is predominant in the spectrum. It is well known that crystal size as well as surface and volume defects content influence the fluorescence spectrum of ZnO nanocrystallines (62,63). According to the results reported in the literature, the emission peak around 412 nm is attributable to the formation of interstitial Zn defects, the strong emission at 437 nm and the weak emission at 485 nm are attributable to the formation of oxygen vacancies (46). The emission around 572 nm may be due to anti-site defects. Indeed, it has been reported that visible luminescence is mainly due to defects related to emissions at deep levels, such as oxygen vacancies and Zn interstitials (64). It should be mentioned that the physical properties of semiconductor materials undergo changes when their dimensions are reduced to the nanoscale, called 'quantum size effects.' The results of this work showing the high intensity of the peaks of ZnO-80 compared to those of ZnO-180 means that the crystallites of ZnO-80 are smaller than those of ZnO-180. This result is corroborated by those obtained by XRD and TEM and it is in agreement with those reported in the literature where it has been shown that the fluorescence intensity increases with the decrease in the size of the nanoparticles (65).

Thermogravimetric analysis
TGA performed in 25°C-1000°C at a constant heating rate of 10°C/minute under air atmosphere. The thermograms of both metal oxide NPs are shown in Figure 9. As can be seen, the thermogravimetric (TG) curve for both samples shows three stages of weight loss. The first stage indicates a weight loss of 1.4% and 1.3% for ZnO-80 and ZnO-180 respectively was observed at around 140°C due to water evaporation. A second weight loss of 1.7% and 1.2% for ZnO-80 and ZnO-180 respectively appeared while temperature increases at 140-260°C that could be attributed to the decomposition of the bio entities from the extract capped on the synthesized zinc oxide NPs. This thermal decomposition can be yet another confirmation for the presence of phytoconstituents that are functionalized on the NPs. Similar results of thermal decomposition of phytochemicals coated on the NPs have been reported (66). 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 (67). It should be noted that beyond 220°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 10. The absorption peaks observed at 371 and 382 nm for ZnO-80 before and after calcination respectively confirms the formation of ZnO NPs. The absorption peaks observed at 371 and 382 nm are due to the intrinsic band gap of Zn-O absorption. Similar results of absorption band were also observed by various researchers (68)(69)(70). Similar results of absorption band were observed by various researchers [56][57][58]. Similar results were also reported. In fact, in a study by UV-vis analysis (71), the UV-vis spectra of a colloidal solution of ZnO, show a broad peak at 298.8 nm indicating that the blue shift is observed with a decrease in the size of the particles. compared to bulk ZnO (λ = 376 nm). This result was attributed to the confinement effects. In another study (72), analysis of the optical properties of ZnO showed that ZnO synthesized at a higher calcination temperature increases light absorption. These results are in agreement with ours. Indeed, after calcination the SEM images showed the agglomeration of the particles. As for the substance probably involved in the synthesis of ZnO-80 NPs, the results of FTIR (Figure 3), showed the presence of phenolic compounds in the leaf extract. It should be noted that various leaf extracts contain substances which can act as stabilizing and capping agent, thus preventing aggregation of NPs. Based on the above results, a mechanism has been proposed. In a first step the zinc nitrate is dissolved in the aqueous solution giving rise to the Zn 2+ ions (Equation (4)), then the Zn 2+ ions are precipitated in the form of hydroxide by NaOH (Equation (5)) and finally in the last step, the hydroxide under the action of temperature leads to the formation of ZnO nanoparticles (Equation (6)).
The nanoparticles formed are then more or less agglomerated, depending on several factors. Analogously to chemical synthesis, where a capping and stabilizing agent is added to reduce the aggregation, in the biosynthesis, a capping and stabilizing agent is provided by the leaf extract. It should be noted that a qualitative and quantitative analysis carried out by the HPLC-DAD method of the extract of Rosmarinus officinalis collected in Saudi Arabia (37) on selected phenolic compounds a very large quantity of rosmarinic acid and other polyphenols including caffeic acid, caftaric acid, 3,4 dihydroxyphenylacetic acid, ferulic acid, gentistic acid, vanilic acid, rutoside were present in the extract. It is therefore probable that these substances played the role of capping to reduce the aggregation of ZnO nanoparticles. Figure 11 illustrates the possible mechanism for the formation of ZnO nanoparticles.

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, 10 mg photocatalyst was added to the dye aqueous solution (10 mL, 10 ppm). 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°C. Figure 12 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 (29). Upon irradiation, the peak at 664 nm has a progressively blue shift to shorter wavelength (Figure 12 (a)) because of hypsochromic effect (73,74). In the presence of ZnO-80 the absorbance of MB decreased sharply after 30 min. Initialy, 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 (75). 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 (76).
The influence of the irradiation time on the discoloration of the CV (Figure 11(b)) was followed by the evolution of the characteristic peak positioned at 590 nm, corresponding to the conjugated triphenylmethane chromophore. The decrease in the intensity of the absorbance at 590 nm with irradiation is due to the degradation of the chromophore emitting the characteristic color of the CV. The hypochromic shift of the peak at 590 nm of the chromophore at about 575 nm has been shown to indicate the degradation reaction of Ndimethylation (77) leading to NO − 3 ions (78,79). Figure 13 shows the decreasing concentration of MB and CV with respect to time for ZnO-80 and ZnO-180.
As shown in Figure 13(a) MB was degraded completely after irradiating 45 min and 60 min by ZnO-80 and ZnO-180, respectively. Whereas, the complete degradation of CV (Figure 13(b)) takes 60 and 75 min for ZnO-80 and ZnO-180 respectively. The higher MB and CV degradation rate over ZnO-80 proves its better performance compared to ZnO-180. These results are consistent with the texture properties of the prepared ZnO-80 and ZnO-180 photocatalysts (Table 1). It can be seen that ZnO-80 presents lower crystallite size, higher surface area and higher pore volume than ZnO-180.
A comparison of the performance of the ZnO photocatalyst of the present work with other recent work is presented in Table 2. The results clearly show that the ZnO nanoparticles of the present work exhibit superiority in terms of degradation time and efficiency. In fact, the photocatalyst degrades almost 100% of MB in a shorter period of time than the ZnO nanoparticles synthesized by using various plant leaf extracts.

Reusability
In order to examine the reusability of the biosynthesized ZnO-80 and ZnO-180 photocatalysts, they were tested after use to see the stability of their photocatalytic activity. After used, the NPs were separated from the solution by centrifugation, washed three times with an ethanol-acetone solution (1:1 in volume) and then put in an oven at 80°C overnight to be dried. After drying, the NPs was characterized by FTIR and tested for the degradation reaction. Both solids were tested under  the same conditions as those in which they gave almost complete degradation of MB and CV. The results obtained (Table 3) show that ZnO-80 degrades MB by 96.7% while ZnO-180 degrades it by 93.8%. As for the discoloration of the CV, the results obtained show that the two samples, ZnO-80 and ZnO-180, discolored it 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 (80,81). This slight decrease in degradation is acceptable and allows reuse of these catalysts.

Conclusion
In the present study, the degradation of methylene blue and crystal violet dyes by ZnO NPs under sunlight irradiation was investigated. ZnO NPs were synthesized using Rosmarinus officinalis leaf extract at 80°C (ZnO-80) and 180°C (ZnO-180).
The FTIR spectra of the Rosmarinus officinalis leaf extract showed the presence of polyphenolic compounds. It is therefore probable that these species are involved in the formation of NPs. Based on these results, a mechanism has been proposed.
The XRD, TEM and SEM results showed that the synthesis temperature affects the shape and size of ZnO NPs. A lower temperature leads to the production of smaller, spherical shaped and less agglomerated particles.
Photocatalytic tests revealed that ZnO-80 NPs were more efficient in photo-degradation of MB and CV dyes compared to ZnO-180. The high activity of ZnO-80 was due to better physicochemical properties compared to ZnO-180.
ZnO-80, prepared by a cheap and easy procedure effectively degrades MB and CV dyes under sunlight.
Photo-degradation of dyes under sunlight as an abundantly available energy source by ZnO, prepared by a simple method, can be exploited to develop an environmentally friendly and economical process.

Disclosure statement
No potential conflict of interest was reported by the author(s).