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Article

Green Synthesis of ZnO Nanostructures Using Salvadora Persica Leaf Extract: Applications for Photocatalytic Degradation of Methylene Blue Dye

by
Fahad A. Alharthi
1,*,†,
Abdulaziz Ali Alghamdi
1,†,
Asma A. Alothman
1,
Zainab M. Almarhoon
1,
Munairah F. Alsulaiman
1 and
Nabil Al-Zaqri
1,2
1
Department of Chemistry, College of Science, King Saud University, P.O. Box 2445, Riyadh 11451, Saudi Arabia
2
Department of Chemistry, College of Science, Ibb University, P.O. Box 70270, Ibb, Yemen
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Crystals 2020, 10(6), 441; https://doi.org/10.3390/cryst10060441
Submission received: 23 April 2020 / Revised: 28 May 2020 / Accepted: 28 May 2020 / Published: 30 May 2020
(This article belongs to the Special Issue Zinc Oxide Nanomaterials and Based Devices)
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Versions Notes

Abstract

:
Various ZnO nanomaterials such as nanorods, nanoparticles, and nanosheets were synthesized using Salvadora persica leaf extract via the sol–gel method. The prepared nanomaterials possess a large number of nanocavities. The synthesized nanomaterials were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), UV-visible diffuse reflectance studies (UV-DRS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HT-TEM), and these nanomaterials were used to test photocatalytic applications for the degradation of highly hazardous methylene blue dye. The degradation efficiency was higher for materials with nanorods and nanosheets with nanocavities; this was due to the presence of the nanocavities, which made the catalyst more sensitive to light absorption. This method offers a green synthesis of different nanomaterials in bulk quantity at low cost.

1. Introduction

Dye impurities in effluents from printing, textiles, and production industries play an adverse role in polluting the environment. This waste enters the aquatic ecosystem and causes environmental and health risks to the aquatic life, which, in turn, eventually affect human life. Techniques such as coagulation, adsorption, osmotic pressure, etc., have been used to remove dyes from rivers and other water bodies, but each method has unique advantages and limitations [1]. Among the various techniques employed, photocatalytic treatment for the removal of dye provides a relatively inexpensive and eco-friendly way to solve this problem [2].
Zinc oxide nanoparticles (ZnO NPs) are the essential nanoparticles that are employed in various areas of daily life due to their distinctive chemical and physical properties. They have been utilized extensively as additives in the rubber industry, textile industry, biomedical imaging, antimicrobial agents, and cosmetic products [3,4,5,6,7]. In the area of catalysis, apart from their various applications, including as oxidation catalysts for the conversion of alkanes, the functionalization of benzene, the fabrication of heavily functionalized 4H-chromenes and the N-formylation of amines [8,9], they have been extensively used as photocatalysts due to their high binding energy (60 meV) with a well-known n-type wide-bandgap metal oxide semiconductor (3.37 eV) [10,11] for the degradation of many organic pollutants [12]. Other reasons for the increased use of the ZnO NPs as photodegradation catalysts is their low cost and characteristics as the hardest material, which induce their ability to overcome physical endurance [13].
Therefore, ZnO, is the one material that can address the issue of the degradation of dye in our water system. However, the two main issues usually faced in the attempts to synthesize ZnO nanostructures are (i) a lack of synthesis techniques that are fast, economical, and environmentally benign and (ii) a difficulty in producing stable p-type doping. Keeping this in mind, a plethora of methods for the synthesis of ZnO nanocrystals has been carried out extensively using a variety of methods such as the solvothermal, hydrothermal, direct precipitation, and sol–gel methods [14,15,16,17]. However, most of these approaches require sophisticated equipment, long reaction times, difficult experimental environments, and expensive substrates.
Moreover, apart from the abovementioned protocols, the sonochemcial approach, an alternative green protocol, has been developed by Vabbina et al. for the synthesis of 1D ZnO Nanorods, core-shell nanorods, 2D nanowalls and nanoflakes on arbitrary substrates, which is a rapid, inexpensive, CMOS-compatible and environmentally benign method; however, this method cannot be implemented easily, as it requires a sonicator water bath [18,19,20]. Moreover, the other hand, other alternative routes for the synthesis of nanoparticles without the requirement of sophisticated equipment have been studied and reported extensively as green synthesis protocols [21,22,23] and can be easily extended to the synthesis of nanocomposites [24,25,26,27]. Among the various nanoparticles, green synthesis of ZnO has also been studied using various plant extracts such as Cordia myxa, Trifolium pratense flower extract, Tecoma castanifolia leaf extract, Limonia acidissima, Echinacea, Aristolochia indica, Mentha, Boswellia ovalifoliolata, etc. [28,29,30,31,32,33,34,35,36].
S. persica is an evergreen, deep-basin, hairless tree or small shrub. The branches are sagging, and the leaves are thin, oval, ovate, and mucoid. A phytochemical evaluation of S. persica leaf extract revealed that it mainly consists of aniline (28.65%), 2,6-dimethyl-N-(2-methyl-α-phenylbenzyl), homo-γ-linolenic acid (12.63%), methyl hexadecanoate (11.01%), and spiculesporic acid (13.60%), there structures are show in Scheme 1. These components act as bioagents for biological activity against potato soft and brown-rot bacteria [37], and the extract is widely used for its medicinal properties, such as antibacterial activity, and as a natural toothbrush. However, to the best of our knowledge, this plant extract has not yet been employed for the synthesis of nanoparticles.
Therefore, in this study, for the first time, a novel method of sol–gel green synthesis using S. persica leaf extract was employed as a green surfactant/capping agent for the green synthesis of ZnO nanoparticles. The obtained nanoparticles were characterized using powder X-ray diffraction, Fourier-transform infrared spectroscopy, UV-visible spectrum, scanning electron microscopy, transmission electron microscope. The prepared nanoparticles were subjected to photoluminescence studies and, later, photocatalytic evaluation for the degradation of methylene blue dye.

2. Materials and Methods

All materials were obtained from commercial sources. Sodium hydroxide pellets (NaOH) (98%) were purchased from Alfa Aesar, Karlsruhe, Germany. Hydrochloric acid (HCl) (~36%) and methylene blue were purchased from Sigma-Aldrich. All reagents were used as received without any further treatment. Deionized water was used throughout the experimental process.

2.1. Experimental

Preparation of S. persica Extract

Fresh S. persica leaves were collected from a local farm in Al Amaaria, Riyadh, Saudi Arabia. A total of 30 g of leaves was washed several times with tap water and distilled water. The leaves were dried in the shade at room temperature for two days and then powdered using a mixer. The resulting powder was extracted from the reflux extraction using 150 mL of water as a solvent.

2.2. Synthesis of ZnO Nanoparticles

In the present study, the desired ZnO nanoparticles were synthesized using two approaches, as described below and shown in Scheme 2.

2.2.1. Procedure (a)

In this procedure, to 100 mL of S. persica leaf extract was added 2 M sodium hydroxide until a pH of 12 was attained and the solution was heated to 90 °C under constant stirring. Once the desired temperature was attained, 100 mL of zinc chloride (0.1 M) was added dropwise and the resultant solution was continually stirred at 90 °C for 4 h. The mixture was then cooled down to room temperature subjected to centrifugation, washed twice with distilled water, and dried in an oven at 65 °C overnight. The ZnO nanoparticles obtained were labeled A1.

2.2.2. Procedure (b)

In this procedure, 100 mL of zinc chloride (0.1 M) solution was heated to 90 °C under constant stirring, to which 100 mL of S. persica leaf extract was added dropwise and heated for 4 h. Later sodium hydroxide (2 M) solution was added until pH 12 was reached. The mixture was cooled down to RT, subjected to centrifugation, washed twice with distilled water, and dried in an oven at 65 °C overnight. The ZnO nanoparticles obtained were labeled B1.

2.3. Photocatalytic Activity

The photo-reactor “Heber” was purchased from Chennai, India. The photocatalytic reactor consisted of a medium-pressure mercury vapor lamp (λ max = 365 nm, 250 W) in a jacketed quartz crystal tube; the heat caused by the lamp was removed by the continuous circulation of cold water throughout the jacket. The 37-cm-long tube with an inner diameter of 2.3 cm and an outer diameter of 2.7 cm had a capacity of 100 mL.
The gap between the mercury lamp and the quartz tube containing pollutant was 6 cm. The uniform distribution of catalytic particles throughout the solution was achieved using an air pump [38]. The solution was continuously boiled in the dark for 30 min to ensure the organization of an adsorption–desorption equilibrium between the MB and photocatalyst prior to irradiation by a light source. Over 30 min intervals, 2 mL of the suspension was withdrawn from the reactor, and the solution was centrifuged to remove the ZnO nanoparticles from the mixture. The dye concentration of leftover aqueous solution was measured using a UV-Vis spectrophotometer at 664 nm. The degradation of the MB was calculated using the following equation:
%   o f   d e g r a d a t i o n = C i − C f C i × 100
where Ci and Cf are the initial and final concentrations of dye in ppm, respectively. The photocatalytic experiment was repeated by changing various parameters, such as catalytic load, concentration of dye, pH, variation, catalyst recycling, etc.

2.4. Characterization

The synthesized nanoparticles were characterized using powder X-ray diffraction (XRD; Rigaku Smart Labs). Fourier-transform infrared spectroscopy (FT-IR; Thermo Scientific Nicole iS10) was used to analyze the metal oxide bond-stretching frequencies. The UV-visible spectrum of the samples was measured using a UV-visible (UV-Vis) spectrophotometer (Lab India-Diffuse reflectance spectra and Cary 60 Agilent technologies-absorbance spectra). The morphologies of the prepared nanoparticles were scanned using scanning electron microscopy (SEM; JEOL JSM-6380 LA). The shape and size of the ZnO crystallites were determined using a transmission electron microscope (TEM; JEOL JEM-1011). Photoluminescence studies were recorded using the Cary Eclipse fluorescence spectrophotometer (Agilent).

3. Results and Discussion

3.1. ZnO Nanostruture Characterization

3.1.1. Microscopic Analysis

1)
Scanning Electron Microscopy Analysis
Figure 1 demonstrates the SEM images of compounds A1 (Figure 1a,b) and B1 (Figure 1c,d). From an evaluation of the surface morphology, it is evident that the change in procedure resulted in different surface morphologies of the ZnO nanoparticles. Procedure (a), wherein the Zinc precursor was added to the solution of S. persica leaf extract at pH 12, yielded a rugged surface morphology, whereas procedure (b), wherein the Zinc precursor was added to the solution of S. persica leaf extract and later the pH of the solution was changed to 12, yielded a rugged surface morphology with a rod-like structure.
2)
Transmission Electron Microscopy
Further studies using transmission electron microscopy shed further light on the effect of different procedures on the size and shape of the ZnO nanoparticles obtained using S. persica leaf extract by the sol–gel method. The microscopic images obtained are illustrated in Figure 2 As seen in Figure 2a,b, the ZnO nanoparticles prepared employing procedure (a) were found to be of a spherical in shape, with a size of about 30 to 50 nm, with some instances of agglomeration leading to the formation of honeycomb-like structures. However, with the change in the synthetic pathway (i.e., procedure (b)), some of the nanoparticles obtained were found to be shaped like nanorods in the nanoparticle mixture obtained. The lengths of the nanorods were in the range of 230 to 400 nm, and their width varied from 25 to 35 nm. Furthermore, sheet-like structures with nanocavities caused due to the plant extract were clearly observed (Figure 2). These structures absorbed the light more efficiently, which cause light sensitivity [39].
From the microscopic analysis, it can be concluded that the different protocols resulted in different morphologies of ZnO nanoparticles obtained. The protocol wherein the S. persica leaf extract was heated with 2 M NaOH at 90 °C, followed by the addition of the ZnCl2, produced the ZnO nanoparticles in Scheme 2a. However, the protocol wherein the S. persica leaf extract was heated with ZnCl2, followed by the addition of NaOH, yielded a mixture of ZnO nanorods and nanoparticles. From the results obtained, it can be concluded that the S. persica leaf extract, plays a very efficient role as a surfactant and capping agent, wherein it controls the morphology of the nanoparticles obtained based on the approach adopted. The probable hypothetical mechanism for the role of the plant extract is depicted in Scheme 3.
Figure 3 shows the TEM, HR-TEM, and selected area electron diffraction (SAED) patterns of B1. As seen in Figure 3a,b, sheet-like structures were visible, along with nanorods and nanoparticles, which could be the plant extract that acts as a capping agent, retaining the rod-like morphology of the ZnO nanorods formed. Figure 3c shows an HR-TEM image of material B1 and reveals a d-spacing value of 0.145 nm belonging to plane (103). The SAED pattern confirmed that B1 was crystalline in nature (Figure 3d); these reflection fringes that emerged from the particular planes were well matched with the d-spacing values obtained by HR-TEM and were later confirmed by XRD.

3.1.2. Spectral Analysis

1)
XRD Analysis
Figure 4 shows the XRD patterns of materials A1 and B1. The materials were synthesized in different ways using S. persica leaf extract. The XRD results indicated that ZnO nanoparticles were highly crystalline and had a hexagonal wurtzite structure, resembling the standard JCPDC card number 36–1451 in the space group P63mc. No extra peaks were found in the pattern, which indicated the purity of the samples.
2)
FT-IR Analysis
The FT-IR spectra of the ZnO nanomaterials were recorded in the range from 4000 to 400 cm−1, as shown in Figure 5. The board peaks at 3425 cm−1 demonstrate the O–H bonds of phenols and the peaks at 1600 cm−1 and 1417 cm−1 correspond to the C=O stretch in polyphenols and C–N stretch in primary amines, respectively. Obtainable weak peaks at 1087 and 850 confirm the presence of C–O stretching in amino acid and C–H bending, respectively. The peak observed at 569 cm−1 corresponds to the vibrational mode of ZnO bonding. The above inference justifies the fact that the presence of phenols, polyphenols and primary amines in the plant extract could be implicated for the capping and stabilization of ZnO nanomaterials [40].
3)
UV-Visible diffuse reflectance studies
Figure 6 shows the UV-visible spectra of ZnO nanoparticles synthesized by the green sol–gel method using S. persica leaf extract. The spectra demonstrated an absorption band at around 378 nm. Because of the electron transfer from the valence band to the conduction band, by using the Kubelka–Munk relation, the calculated Eg values were found to be 3.3 and 3.26 eV for A1 and B1, respectively [41] (Figure 6b). The higher band gap indicates the formation of smaller NPs; however, it also means that there is a greater restriction of movement on electrons. From this, it can be concluded that sample B1 has a better conductance, which may be the reason for its better photocalytic performance, as described below.

3.2. Photocatalytic Activity of ZnO Nanostructres

3.2.1. Degradation of Methylene Blue

Under UV irradiation, a semiconductor absorbs photon energy equal to or greater than the bandgap of the semiconductors, which results in the generation of electrons and holes on the surface of the photocatalyst. If the charge carriers do not recombine, they migrate to the surface, wherein in the presence of O2 and H2O, ends up forming four major reactive oxygen species (ROSs), which are hydroxyl radical (•OH), superoxide anion radical (•O2−), singlet oxygen (1O2) and hydrogen peroxide (H2O2) [42].
This OH• (hydroxyl radical) formed species is highly reactive and unstable. It ultimately acts on organic compounds and oxidizes carcinogenic dyes to CO2, water, and inorganic acids. The photocatalytic action of the catalyst on the dye was reportedly enhanced by various factors, such as the particle size of the catalyst, phase composition, shape, crystallinity, size distribution, surface area, surface OH group density and bandgap [43]. A photocatalytic evaluation of the as-synthesized ZnO nanorods/nanoparticles was carried out, employing them as catalysts for the degradation of methylene blue under UV rays, in a UV-Vis reactor. The progress of the reaction was carried out by periodic collection, i.e., every 30 min, with 2 mL aliquots of the reaction mixture, which were centrifuged to remove the suspended ZnO nanorods/nanoparticles. The absorption spectrum of the reaction mixture was then recorded using a UV-Vis spectrophotometer. Upon evaluation of the results obtained, it was found that the B1 displayed a degradation efficiency of 95% within 150 min of exposure time, whereas the sample A1 showed a degradation efficiency of about 75%. A graphical representation of the results obtained are given in Figure 7. The enhanced photocatalytic activity of B1 can be attributed to the presence of nanocavities and the nanorods of ZnO (lengthy, spindle-like structures) on the surface of the ZnO nanoparticles.

3.2.2. Mechanism of Photodegradation

OH• ions are an unstable and reactive chemical species that play an important role in the degradation of organic dyes. In order to identify whether the OH• free radical is being generated by the photocatalyst B1, coumarin was used as a probe molecule, which is a simple and sensitive approach for OH• detection. In the presence of an OH• ion generated by the photocatalyst, coumarin converts to 7-hydroxyl coumarin, a luminous material that exhibits a photoluminescent peak at 455 nm wavelength. In this study, 100 mg of the catalyst (B1) was added to 50 mL of 1 mM coumarin solution irradiated by a 120-W UV source. At an interval of 10 min, 2 mL of the aliquot sample was withdrawn and measured using a photoluminescence spectrometer, which revealed the production of OH• free radicals, a vital species for the degradation of organic dyes [44]. Moreover, it was observed that, as the irradiation time increases, the intensity of the peak/band increases at 455 nm, which indicates that, as the exposure time increases, there is a steady production of OH• radicals, as well as an increase in the rate of production in the system. (Figure 8) Hence, it can be confirmed that the degradation of methylene blue takes place via a free radical mechanism in the presence of a catalyst (B1).

3.3. Optimization Studies

Since the ZnO nano rods and nanoparticles (B1) yielded the best photocatalytic activity, they were selected for further optimization studies in order to find out the optimum dye concentration, catalyst loading and pH. The prepared nanocatalyst was also subjected to recycling studies to evaluate its stability.

3.3.1. Effect of Dye Concentration

The photocatalytic activity of ZnO nanoparticles was also affected by the concentration of the dye. To determine the optimum concentration of dye required for efficient oxidation, an experiment was performed with the catalyst (B1) load (10 mg) kept constant, while the dye concentration varied from 5 ppm to 25 ppm, as shown in Figure 9. Under the UV light, it was observed that, upon increasing the dye concentration from 5 ppm to 20 ppm, the photocatalytic degradation of catalyst B1 reduced from 100% to 82%. The 5-ppm dye concentration showed the highest photocatalytic degradation activity (i.e., 100%) within 2.5 h of UV exposure. Generally, as the concentration of the dye increases, the penetration of light through the dye solution decreases; therefore, more dye molecules are adsorbed on the surface of the catalyst. Because of this effect, the production of OH• and O22− in the solution mixture decreased, which lowered the degradation efficiency [45].

3.3.2. Effect of Catalyst Load

In general, increasing the photocatalyst load in the reaction system would lead to a higher production of OH• free radicals, due to the increased light absorption, which, in turn, causes increased catalytic activity. In this study, the oxidative degradation of methylene blue was carried out by varying the catalyst (B1) load from 5 to 25 mg, while the dye concentration was kept constant (5 ppm). In 150 min, the degradation of the dye increased from 88% to 100%, while the catalyst load increased from 5 to 10 mg; however, upon a further increase in the catalyst load up to 25 mg, the degradation efficiency of the dye was found to decrease to <90%. This could be attributed to the light scattering and screening effects, which arise due to the increase in the catalyst load in the reaction system, which results in a reduction in the catalytic performance [46,47]. In addition, catalyst aggregation might also reduce the catalytic activity [47]. Consequently, in this work, the optimal amount of B1 catalyst to reach the best degradation rate was found to be 10 mg. Figure 10 shows the degradation kinetics of methylene blue by varying the catalyst load (5 to 25 mg B1) at a constant dye concentration (5 ppm).

3.3.3. Effect of pH

To optimize the pH for the efficient photodegradation of methylene blue, experiments were performed at different pH levels (2 to 12), while keeping the catalyst load (B1) and dye concentration constant. The results are shown in Figure 11. These experiments revealed that the degradation of methylene blue was effective in a basic medium [44,48,49], and the highest degradation rate was observed at pH 10. Above this pH 10, the degradation decreased and could be explained based on the zero-potential charge (ZPC). For ZnO, the ZPC was 9.0 ± 0.3. If the ZPC is larger than this value, the surface net negative charge is high, due to the increase in the OH- ions adsorbed on the surface. Due to the presence of many OH- ions on the surface of the catalyst, OH• free radical generation is lowered, a radical species that is the primary oxidant responsible for the degradation of methylene blue [49].

3.3.4. Catalyst Recycling

The recyclability experiment of catalyst B1 was carried out using 10 mg of catalyst, in a dye concentration of 5 ppm at pH 7 and the catalyst was reused for about five cycles. After UV light irradiation for 150 min, each of the photodegradation mixtures was centrifuged and filtered. The catalyst B1 residue was washed several times with distilled water. Recovered B1 was then reused for a new photodegradation batch, without any further treatment. From the outcomes of this study, it can be concluded that the catalytic efficiency of the catalyst B1 decreases by 8% after five cycles of reuse, with an efficiency reduction of 1.5% after each cycle of reuse. The regeneration stability of nanorods and nanoparticles in the green synthesized ZnO nanomaterials using S. persica leaf extract can be attributed to the prevention of the oxidative decomposition of the catalyst by the S. persica leaf extract, which acts as a capping agent. The results obtained are illustrated in Figure 12.

4. Conclusions

In this study, ZnO nanorods and nanoparticles were synthesized with a simple green sol–gel method using S. persica extract as a surfactant and capping agent. The XRD pattern showed that the ZnO nanorods/nanoparticles obtained were crystalline in nature and possessed a hexagonal lattice structure with an average crystallite size of 21.5 nm. The SEM and TEM analysis revealed that the morphology of ZnO nanoparticles depended exclusively on the synthetic pathway chosen. The TEM image showed that the particles were virtually agglomerated rod-like structures with a few agglomerated nanoparticles around them. The prepared ZnO nanorods/nanoparticles exhibited excellent photocatalytic activity against the photodegradation of methylene blue, indicating that a 10-mg catalyst load and a pH of 10 are the optimum conditions for the ZnO nanoparticles to display the best catalytic performance.

Author Contributions

F.A.A. and N.A.-Z. played an important role in the project design and execution, while M.F.A. carried out the experimental work. F.A.A., A.A.A. (Asma A. Alothman) and Z.M.A. played a crucial role in the characterization and elucidation of the results. F.A.A. and A.A.A. (Abdulaziz Ali Alghamdi) compiled the data and prepared the manuscript. All authors read and approved the final manuscript.

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through Research Group No. RGP-1438-040.

Conflicts of Interest

The authors declare that they have no conflict of interests.

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Crystals 10 00441 sch001 550
Scheme 1. Structure of (a) salvadorine, (b) salvadoside, and (c) salvadoricine.
Scheme 1. Structure of (a) salvadorine, (b) salvadoside, and (c) salvadoricine.
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Figure 1. (a,b) SEM images of ZnO nanoparticles prepared.by procedure (a); (c,d) SEM images of ZnO nanoparticles prepared by procedure (b) using S. persica leaf extract.
Figure 1. (a,b) SEM images of ZnO nanoparticles prepared.by procedure (a); (c,d) SEM images of ZnO nanoparticles prepared by procedure (b) using S. persica leaf extract.
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Figure 2. (a,b) Transmission electron microscopy (TEM) images of ZnO nanoparticles (A1) prepared.by procedure (a); (c,d) TEM images of ZnO nanoparticles (B1) prepared by procedure (b) using S. persica leaf extract.
Figure 2. (a,b) Transmission electron microscopy (TEM) images of ZnO nanoparticles (A1) prepared.by procedure (a); (c,d) TEM images of ZnO nanoparticles (B1) prepared by procedure (b) using S. persica leaf extract.
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Scheme 2. Flow chart of the two different synthetic protocols applied for the synthesis of ZnO nanoparticles.
Scheme 2. Flow chart of the two different synthetic protocols applied for the synthesis of ZnO nanoparticles.
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Scheme 3. Schematic representation of the role of phytomolecules of S. persica leaf extract in the synthesis process of ZnO nanorods/nanoparticles.
Scheme 3. Schematic representation of the role of phytomolecules of S. persica leaf extract in the synthesis process of ZnO nanorods/nanoparticles.
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Figure 3. (a), (b) TEM images. (c) HR-TEM images. (d) SAED pattern of material of S. persica leaf extract.
Figure 3. (a), (b) TEM images. (c) HR-TEM images. (d) SAED pattern of material of S. persica leaf extract.
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Figure 4. XRD pattern of A1 and B1 synthesized using S. persica leaf extract.
Figure 4. XRD pattern of A1 and B1 synthesized using S. persica leaf extract.
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Figure 5. FT-IR spectra of A1 and B1 synthesized using S. persica leaf extract.
Figure 5. FT-IR spectra of A1 and B1 synthesized using S. persica leaf extract.
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Figure 6. (a) UV-visible diffuse reflectance spectrum. (b) Bandgap calculation graph.
Figure 6. (a) UV-visible diffuse reflectance spectrum. (b) Bandgap calculation graph.
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Figure 7. Degradation efficiency of catalysts A1 and B1.
Figure 7. Degradation efficiency of catalysts A1 and B1.
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Figure 8. Photoluminescence spectrum indicating generation of OH• ion by B1 at different time intervals.
Figure 8. Photoluminescence spectrum indicating generation of OH• ion by B1 at different time intervals.
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Figure 9. Degradation of MB with varying dye concentration and a constant catalyst load (B1).
Figure 9. Degradation of MB with varying dye concentration and a constant catalyst load (B1).
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Figure 10. Degradation of MB with varying catalyst loads (B1) and constant dye concentration.
Figure 10. Degradation of MB with varying catalyst loads (B1) and constant dye concentration.
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Figure 11. Degradation of MB with varying pH in the solution, constant catalyst load (B1) and dye.
Figure 11. Degradation of MB with varying pH in the solution, constant catalyst load (B1) and dye.
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Figure 12. Degradation of MB carried out using recycling of the catalyst load (B1).
Figure 12. Degradation of MB carried out using recycling of the catalyst load (B1).
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MDPI and ACS Style

Alharthi, F.A.; Alghamdi, A.A.; Alothman, A.A.; Almarhoon, Z.M.; Alsulaiman, M.F.; Al-Zaqri, N. Green Synthesis of ZnO Nanostructures Using Salvadora Persica Leaf Extract: Applications for Photocatalytic Degradation of Methylene Blue Dye. Crystals 2020, 10, 441. https://doi.org/10.3390/cryst10060441

AMA Style

Alharthi FA, Alghamdi AA, Alothman AA, Almarhoon ZM, Alsulaiman MF, Al-Zaqri N. Green Synthesis of ZnO Nanostructures Using Salvadora Persica Leaf Extract: Applications for Photocatalytic Degradation of Methylene Blue Dye. Crystals. 2020; 10(6):441. https://doi.org/10.3390/cryst10060441

Chicago/Turabian Style

Alharthi, Fahad A., Abdulaziz Ali Alghamdi, Asma A. Alothman, Zainab M. Almarhoon, Munairah F. Alsulaiman, and Nabil Al-Zaqri. 2020. "Green Synthesis of ZnO Nanostructures Using Salvadora Persica Leaf Extract: Applications for Photocatalytic Degradation of Methylene Blue Dye" Crystals 10, no. 6: 441. https://doi.org/10.3390/cryst10060441

APA Style

Alharthi, F. A., Alghamdi, A. A., Alothman, A. A., Almarhoon, Z. M., Alsulaiman, M. F., & Al-Zaqri, N. (2020). Green Synthesis of ZnO Nanostructures Using Salvadora Persica Leaf Extract: Applications for Photocatalytic Degradation of Methylene Blue Dye. Crystals, 10(6), 441. https://doi.org/10.3390/cryst10060441

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