Green Synthesis of ZnO Nanoparticles: Effect of Synthesis Conditions on Their Size and Photocatalytic Activity

Abstract

Green technologies are actively being used to produce nanosized zinc oxide, which is in demand for water purification processes to remove pollutants. Despite the success of the green synthesis of ZnO nanoparticles, no scientific approach exists for selecting plant extracts to produce nanoparticles with the desired properties. This study shows that the antioxidant activity of the plant extracts used is a key parameter influencing the properties of the resulting ZnO nanoparticles. This conclusion is based on the results of nanoparticle synthesis with the use of various plant extracts. The antioxidant activity of the extracts increases in the following order: plum–gooseberry–black currant–strawberry–sea buckthorn. The synthesized ZnO nanoparticles were characterized by UV–visible spectroscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The catalytic properties of ZnO nanoparticles were tested under the degradation of a synthetic methylene blue dye after exposure to UV light. We found that with an increase in the AOA of plant extracts, the size of the nanoparticles decreases, while their photocatalytic activity increases. The smallest (d = 13 nm), most uniform in size (polydispersity index 0.1), and most catalytically active ZnO nanoparticles with a small band gap (2.85 eV) were obtained using the sea buckthorn extract with the highest AOA, pH 10 of the reaction mixture and 0.1 M Zn(CH₃COO)₂∙2H₂O as a precursor salt. ZnO nanoparticles synthesized in the sea buckthorn extract demonstrated the highest dye photodegradation efficiency (96.4%) compared with other nanoparticles. The established patterns demonstrate the “antioxidant activity–size–catalytic activity” triad can be considered as a practical guide for obtaining ZnO nanoparticles of a given size and with given properties for environmental remediation applications.

1. Introduction

The textile industry, which uses synthetic dyes on a large scale, annually disposes of large volumes of waste water containing pollutants. Not all harmful substances can be removed from water. They continue to enter the environment and pollute ecosystems [1]. Synthetic dyes are relatively stable under normal conditions, but over time, they can form hazardous chemical reactions that pollute ecosystems. Methylene blue (MB) is one of the most widely used synthetic dyes in the textile, paper, leather, and medical industries [2]. It is bright blue, readily soluble, and stable in water. It has reproductive effects and is mutagenic [3,4].

A variety of traditional methods, such as adsorption, coagulation, ultrafiltration, and reverse osmosis [5,6,7,8], are used to remove dyes from water. These methods are quite simple and do not require complex equipment or highly qualified personnel for their maintenance; however, the organic reagents they employ may cause secondary water pollution, requiring repeated purification. This prolongs the dye removal process and significantly increases its cost [9,10]. Photocatalytic pollutant removal has attracted the attention of researchers due to the wide range of materials that can be used in this process [11,12], along with the rapidity of the purification process, which occurs under mild conditions (room temperature and standard atmospheric pressure) and does not require additional oxidizing reagents, since reactive radicals are generated on the surface of a semiconductor under light radiation. Moreover, the ability to reuse the catalyst over multiple cycles, along with its availability and low cost, makes photocatalytic pollutant removal technology economically efficient and environmentally sustainable, as compared to other methods.

Metal oxide semiconductors (Co₃O₄, NiO, ZnO, etc.) have long been used as photocatalysts for dye degradation [13,14]. Titanium oxide (TiO₂) is considered the “gold standard” photocatalyst. Due to its wide band gap (3–3.6 eV), it can only absorb light in the ultraviolet region (λ ≤ 400 nm) [15]. ZnO, with a band gap of 3.37 eV, is the closest to TiO₂ in terms of semiconductor properties. It has proven to be an excellent pollutant adsorbent and photocatalyst. ZnO offers several advantages over TiO₂, including higher quantum efficiency, which facilitates faster pollutant degradation; strong UV absorption due to a shifted absorption edge, which provides better activity under UV radiation; and more economical and easier synthesis with greater morphological diversity [16,17,18].

As is well known, catalyst properties directly depend on the size and morphology of the material and are determined by the catalyst’s synthesis method. Currently, many different methods can be used to synthesize zinc oxide nanoparticles (ZnO NPs), for example, the sol–gel method, precipitation, the hydrothermal method, and spray pyrolysis [19,20,21,22]. In recent years, the “green” synthesis of oxide nanomaterials has become increasingly popular. It offers certain environmental and economic advantages over chemical and physicochemical methods of nanoparticle synthesis. In the green synthesis of ZnO NPs, bacteria [23,24], fungi [25,26], yeasts [27], and algae [28] are used, but most commonly, leaves, fruits, roots, and other parts of plants are used to obtain extracts. The extracts’ phytochemicals act as stabilizing and reducing agents [29,30,31,32,33,34].

A large number of studies have confirmed the binding of functional groups of phytocompounds in onion, cabbage, carrot, and tomato extracts [35], from species, including Pelargonium odoratissimum (L.) [36], Rhamnus prinoides [37], Artemisia absinthium [38,39], Punica granatum [40], Green Algae [28], Phlomis [41], and Southern African medicinal plants [42], to the ZnO NPs surface. FTIR spectroscopy has revealed that plant extracts contain various phenolic compounds, including flavonoids, iridoids, diterpenoids, phenylpropanoids, triterpenoids, as well as tannins, saponins, alcohols, amines, carboxylic acids, and other bioactive substances, featuring functional groups, such as –OH, –C=O, C–O, N–H, CN, C–O–C, –COOH, and –NH₂. In the FTIR spectra of the nanoparticles, characteristic peaks were observed in the region from 700 cm⁻¹ to 400 cm⁻¹, corresponding to metal–oxygen vibrations, which confirmed the formation of ZnO NPs. Additional peaks of phytochemical compounds, including a broad peak at 1500–2000 cm⁻¹ corresponding to the stretching vibrations of N–H and O–H [35], absorption bands at 1440 cm⁻¹, 1075 cm⁻¹, 1371 cm⁻¹ corresponding to the stretching vibrations of the C=O, C–O bond and the bending of the O–H bond, respectively [37], a peak at 1571 cm⁻¹ corresponding to the C=C stretching vibrations [39]. In the spectra of green ZnO NPs, a significant decrease in the intensity of the coupled peaks was observed near 3400 cm⁻¹ [38] and at 1588 cm⁻¹ for N–H primary amines [40], as well as a shift in the peak frequencies [43] compared to the spectrum of the plant extract. The results of the FTIR analysis indicate the interaction between the different functional groups of the plant extracts’ phytocompounds and the green ZnO NPs. It has been suggested that the OH-groups of flavonoids, as well as free carbonyl and NH₂– groups of proteins and amino acid residues, are able to bind to the surface of ZnO NPs and form a protective layer [28,36,39,40]. Thus, the biomolecules of the plant extracts act as effective capping and stabilizing agents, providing structural and chemical modification of green nanoparticles, as compared to pure ZnO NPs.

There are several major governing factors that affect the morphology and size of the synthesized ZnO NPs: the antioxidant activity (AOA) of the plant extract, the pH of the medium, and the concentration of the precursor salt. Joelis Vera et al. showed that to obtain ZnO nanopowders with a small size and narrow particle distribution, plant extracts with greater AOA are required [44]. Hajira Arif et al., who synthesized ZnO NPs using Clinopodium vulgare L. extract at pH 5 and 9, found that at an alkaline pH, the size of the synthesized nanoparticles decreases and their shape becomes more spherical [45]. Huzaifa Umar et al. obtained more monodisperse ZnO NPs by increasing the precursor concentration [46]. In their study, Jagdamni et al. found that small ZnO NPs were formed at low concentrations of zinc acetate, while excessive precursor concentrations resulted in the formation of polydisperse particles with a wide range of sizes [30].

It was shown that an increase in the concentration of Assam green tea extract initially enhanced the intensity of the characteristic UV–vis absorption peak in the range of 390–410 nm, which corresponds to the surface plasmon resonance of ZnO NPs, and then by its decrease and a red shift at concentrations above 50% (v/v), which was explained by the agglomeration and growth of ZnO NPs [47]. A similar red shift was observed for the ZnO NPs synthesized using Solanum nigrum [48] and Pomegranate peel [49] extracts. The crucial issue in studying the effect of the plant extract on the size characteristics of ZnO NPs is the lack of quantitative characteristics of the plant extract. The volume concentration (v/v) used by the authors does not provide reliable information about the content/concentration of those substances that are directly involved in the formation of ZnO NPs and affect their size. Using extracts prepared via the same technology and the same plant, but grown in different geographic zones under different climatic conditions, does not guarantee the production of ZnO NPs with identical properties. Our expertise in the phytosynthesis of gold and silver nanoparticles shows that an important quantitative characteristic of plant extracts is their AOA, which refers to the total active concentration of extract compounds, primarily antioxidant, possessing reducing properties and being involved in nanoparticles synthesis. The correlation established during the phytosynthesis of gold and silver nanoparticles demonstrates that increasing the AOA of plant extracts leads to the formation of smaller, more aggregation-stable, and electrochemically active phytonanoparticles capable of reducing charge transfer resistance [50,51]. It has been shown that at a certain AOA value, regardless of plant type, metallic nanoparticles of the same size are formed, which might be essential for practical applications. So far, only a single study is known to have used the AOA as an indicator of the efficiency of plant extract-mediated synthesis of ZnO NPs [44]. The authors of this study demonstrated that obtaining ZnO nanopowders with a small particle size and narrow particle distribution requires plant extracts with greater AOA. Therefore, more work is needed to research how the AOA of different plant extracts affects the properties of oxide nanoparticles and how these patterns can be applied in the synthesis of new green nanomaterials.

The photocatalytic properties of phytosynthesized ZnO NPs are being actively studied through the degradation of the hazardous synthetic methylene blue (MB) dye. Alprol et al. obtained ZnO NPs with an average size of 20 nm from Padina pavonica extract and used them to remove 98% of the MB dye under sunlight exposure [52]. Cervantes-Gaxiola et al. synthesized ZnO NPs using Sorghum bicolor seed extract. These NPs were characterized by high crystallinity, an average size of 12 nm, and the ability to rapidly decompose the MB pollutant in water under visible radiation [53]. It is worth noting that comprehensive studies of the impact of a number of phytosynthesis parameters on the ZnO NPs properties are quite common. For example, Mebrahtu Hagos Kahsay made an attempt to optimize the synthesis parameters (precursor salt concentration, pH, extract concentration) by using Design Expert 11 software. However, the resulting ZnO NPs had low catalytic activity and were able to decompose 70% of MB within 200 min exposure to UV radiation [54]. When synthesizing ZnO, Bozkurt and Şahin varied several parameters (extract volume, pH of the medium, microwave power, and precursor salt concentration) using Rosa canina L. seed extract. As a result, they obtained relatively large (d = 66 nm) and size nonuniform (polydispersity index = 0.24) ZnO NPs without their subsequent practical application [55].

The literature review revealed insufficient information on the effect of the plant extracts’ AOA on the size characteristics of phytosynthesized ZnO NPs. It also shows a gap in our knowledge regarding the impact of the AOA on the nanoparticles’ photocatalytic properties. The purpose of this study is to investigate the impact of various factors, namely the AOA of the extracts of several plants widespread in temperate zones worldwide, pH of the reaction mixture, and precursor salt concentration, on the “green” synthesis of ZnO NPs, and, most importantly, on their size, morphology, polydispersity, and photocatalytic properties, thus establishing a correlation within the triad “antioxidant activity (AOA)—particle size—catalytic activity”. Using the photodegradation reaction of the synthetic MB dye as an example, the photocatalytic activity of ZnO NPs will be assessed and the “green” synthesis governing parameters that allow one to obtain the most effective nanocatalyst will be identified.

2. Materials and Methods

2.1. Reactive and Materials

Plant material (leaves of plum, gooseberry, black currant, strawberry and sea buckthorn) was collected in early summer on a household plot near Yekaterinburg (Russia). Then, the leaves were washed and dried in the air in a well-ventilated room. The dried raw materials were stored in a dry dark room.

The following chemically pure (≥99%) reagents were used in the research: potassium hexacyanoferrate (III), potassium hexacyanoferrate (II), potassium chloride, dimethyl sulfoxide (JSC ChemReactiveSnab, Ufa, Russia), sodium hydroxide (JSC Ekros, St. Petersburg, Russia), methylene blue, zinc acetate dehydrate, silver nitrate, ammonium oxalate (JSC Lenreactive, St. Petersburg, Russia), isopropyl alcohol (JSC Ekos-1, Moscow, Russia), p-benzoquinone (JSC NevaReactive, St. Petersburg, Russia), zinc nanooxide (20–40 nm, HPS Ltd., Moscow, Russia). Deionized water was used as a solvent. The Aqua Lab-UVOI-MF-1812 installation (JSC NPK Mediana-Filter, Moscow, Russia) was used for water purification.

2.2. Procedures

2.2.1. Preparation of Plant Extracts

The aqueous extracts of plum (Prúnus), gooseberry (Ribes uva-crispa), black currant (Ríbes nígrum), strawberry (Fragaria vesca) and sea buckthorn (Hippophae rhamnoides) were prepared using the following method: 2 g of each plant’s leaves was pestle-crushed in a mortar and placed in a heat-resistant glass, filled with 100 mL of deionized water. Then, the extracts were kept at 70 °C for one hour of constant stirring using a magnetic stirrer with controlled heating (IKA-Werke GmbH and Co., Ltd., KG, Staufen, Germany). The resulting suspensions were cooled to room temperature, and the extracts were filtered using White Ribbon filters (Melior XXI LLC, Moscow, Russia). For this purpose, 0.2 mL of the analyzed plant extract was added to 9.8 mL of stirred phosphate buffer solution (pH 7) containing 10 mmol L⁻¹ potassium hexacyanoferrate (III) and 0.1 mmol L⁻¹ potassium hexacyanoferrate (II). The potential changes were recorded using an MPA-1 analyzer (IVA Ltd., Yekaterinburg, Russia). The decrease in the potential was recorded using an MPA-1 analyzer (IVA Ltd., Yekaterinburg, Russia). The measurements were carried out in five parallels.

2.2.2. Determination of AOA Extracts

The AOA of the plant extracts was characterized by a potentiometric method using a standard red-ox system potassium hexacyanoferrate (III)/potassium hexacyanoferrate (II) [56]. The measurements were carried out using a three-electrode electrochemical cell consisting of a working planar platinum electrode, a TD-1400 thermal sensor (IVA Ltd., Yekaterinburg, Russia) and an EVL-1M3.1 silver/silver chloride (Ag/AgCl) reference electrode (JSC Gomel Plant of Measuring Equipment, Gomel, Belarus).

2.2.3. Preparation of ZnO NPs

The synthesis of ZnO NPs was performed as described below. Different amounts of zinc acetic acid dihydrate powder were added to 25 mL of plant extract so that the concentration of the precursor salt in the reaction mixture varied from 0.025 mol L⁻¹ to 0.5 mol L⁻¹. The acidity of the reaction mixture (pH 7, 9, 10) was varied by adding 2 mol L⁻¹ sodium hydroxide dropwise. The pH value was monitored using an EcoDigital digital pH meter (EcoHitek, Inc. N. Hollywood, CA, USA). The reaction mixture was kept at 70 °C for 2 h of continuous stirring. Completion of the reaction was assessed visually based on turbidity of the solution and precipitation. After that, the reaction mixture was evaporated at 100 °C, and then the precipitate was calcined in a muffle furnace EMP-011 (Averon, Yekaterinburg, Russia) at 500 °C for 2 h. The synthesized oxide nanoparticles were used for characterization and the study of synthetic MB dye photodegradation.

2.2.4. Characterization of ZnO NPs

The ZnO NPs suspensions were characterized by UV–vis spectroscopy using an ECO-VIEW UV 1200 spectrophotometer (Shanghai Mapada Instruments Co., Ltd., Shanghai, China) equipped with glass cuvettes with a 5 mm optical path length. The UV–vis spectrum of ZnO NPs was scanned in the wavelength range of 305–500 nm.

The size distribution of the nanoparticles and their morphological characteristics were determined from the transmission electron microscopy (TEM) images obtained using a DSR-2100 microscope (JEOL Ltd., Tokyo, Japan). The elemental analysis of the ZnO NPs was performed using energy-dispersive X-ray spectroscopy (EDX) and an Inka Energy TEM 250 detector (Oxford Instruments plc., Abingdon, UK). Selected area electron diffraction (SAED) was used to identify crystallographic structure (crystalline phases present in ZnO NPs and lattice parameters). The lattice parameter was calculated using the microscope constant and the radii of the obtained diffraction patterns.

X-ray phase analysis was carried out on a Bruker D8 Advance (Bruker AXS Inc., Ettlingen, Germany) X-ray diffractometer using Cu Kα radiation (filtered with a SolX energy-dispersive detector) in the reflection angle range 2θ = 25–75° at a voltage of 40 kV and a tube current of 40 mA, using incident-beam Soller slits. The measuring diameter was 500 mm, the step size was 0.025°, and the counting time at each step was ~1 s. Qualitative phase analysis was performed with the DiffracPlus^® EVA software package version 5.1 (Bruker AXS Inc., Madison, WI, USA) by matching reference patterns from the ICDD PDF2 X-ray diffraction database. The lattice parameter of the ZnO NPs was determined using the Rietveld method using the Topas software package version 4.2 (Bruker AXS Inc., Madison, WI, USA).

The catalytic activity of the phytosynthesized ZnO NPs was evaluated based on degradation of the model synthetic MB dye when exposed to UV radiation using a Philips TL-D 18 W BLB (365 nm) lamp (Warsaw, Poland). For this purpose, 15–100 mg of ZnO NPs was dispersed in 50 mL 10 μmol L⁻¹ MB. Then, the beaker with the resulting mixture was placed on a magnetic stirrer and kept in dark with active stirring for 60 min to attain the adsorption–desorption equilibrium. The solution of the MB with the ZnO NPs was treated with UV radiation for 3 h. The MB degradation was measured by removing 5 mL samples every 15 min. The samples were centrifuged, and the liquid was separated from the solid phase and used to obtain optical spectrum in a wavelength range from 500 to 700 nm. The maximum absorbance of MB was noted at a wavelength of 664 nm. The photodegradation efficiency (η) was calculated using Equation (1)

$$\mathrm{\eta }={\displaystyle \frac{A_0-A}{A_0}}\cdot 100\%$$

(1)

where A₀ is absorbance of the initial solution of MB; A is absorbance of the MB solution after radiation with a UV lamp.

To evaluate the catalyst reusability, four cycles of the MB dye degradation under UV radiation were performed. First, 25 mg of ZnO NPs was added to 50 mL of a 10 μmol L⁻¹ MB solution. The mixture was stirred in the dark for 60 min to achieve the adsorption equilibrium and then exposed to UV radiation for 180 min. After each cycle, the catalyst was collected via centrifugation, washed with distilled water, and dried at 60 °C.

To investigate the active species formed in the system, containing the ZnO NPs catalyst and the MB dye, the experiments were carried out using the following scavenger reagents: ammonium oxalate (AO, 10 mmol L⁻¹), p-benzoquinone (PBQ, 1 mmol L⁻¹), silver nitrate (AgNO₃, 100 mmol L⁻¹), and isopropanol (IPA, 1:20, v/v) to capture holes (h⁺), superoxide radicals (∙O₂⁻), electrons (ē), and hydroxyl radicals (∙OH), respectively.

3. Results

3.1. “Green” Synthesis of ZnO NPs

The widespread plants of the Ural region, such as plum, gooseberry, black currant, strawberry and sea buckthorn, were selected for ZnO NPs phytosynthesis. The dried leaves of these plants were used for the extract preparation. The AOA of the plant extracts was evaluated using the potentiometric method. According to the obtained results, the AOA of the plant extracts increases in the following order: plum (1.6 mmol-eq L⁻¹) < gooseberry (2.08 mmol-eq L⁻¹) < black currant (6.09 mmol-eq L⁻¹) < strawberry (12.74 mmol-eq L⁻¹) < sea buckthorn (15.01 mmol-eq L⁻¹).

The mechanism of the reaction between a plant extract and zinc acetate dihydrate precursor salt for producing oxide nanoparticles involves several stages: dissociation of zinc acetate in the solution; chelation of the zinc cations formed during dissociation by polyphenols from the plant extract and formation of complex compounds; formation of the zinc oxide nanoparticles under high temperature exposure [57]. In this process, phytochemicals act as stabilizing and capping agents, promoting the synthesis of the nanosized particles [58]. Significant factors affecting the size characteristics of nanoparticles are the nature of the plant extract, the quantitative characteristic (AOA), as well as the acidity (pH) of the reaction mixture in which the nanoparticles are formed, and the concentration of the precursor salt Zn(CH₃COO)₂∙2H₂O.

3.1.1. Effect of Plant Extracts AOA

For spectrophotometric characterization and size estimation of ZnO NPs, the samples were obtained by resuspending 15 mg of the ZnO NPs powder in 5 mL of a 1:1 (v:v) water/dimethyl sulfoxide mixture. Figure 1 shows an image of five ZnO NPs suspension samples and their UV–vis absorption spectrum. As can be seen from Figure 1a, the ZnO NPs suspension obtained using the plum extract appears visually darker than the suspension obtained using the sea buckthorn extract. The suspensions of all ZnO NPs showed the characteristic absorbance peak at 373–378 nm (Figure 1b) [59]. Moreover, the darker suspension of the ZnO NPs obtained using the plum extract has a higher absorbance than the lightest suspension of the ZnO NPs prepared from the sea buckthorn extract. It can be assumed that the dark color of the suspension is due to the extract remains left unburned in the oven due to having the lowest AOA among the considered plants.

Figure 1. Photograph of ZnO NPs suspension samples synthesized using extracts of various plants: 1—plum, 2—gooseberry, 3—black currant, 4—strawberry, 5—sea buckthorn (a). UV–vis absorption spectrum of the corresponding ZnO NPs samples (b).

The effect of the plant extracts’ AOA on the wavelength of the maximum absorbance peak of ZnO NPs is presented in Figure 2. With an increase in the AOA of the extract from 1.6 mmol-eq L⁻¹ (plum) to 15.01 mmol-eq L⁻¹ (sea buckthorn), the wavelength corresponding to the maximum absorbance peak of the nanoparticles decreases. A shift in the absorbance peak to lower wavelengths indicates a decrease in the size of the synthesized ZnO NPs [60]. This suggests that an extract with the highest AOA facilitates the production of the smallest ZnO NPs.

Figure 2. Dependence of the wavelength of ZnO NPs absorbance peak on the AOA of plant extracts used for synthesis: 1—plum, 2—gooseberry, 3—black currant, 4—strawberry, 5—sea buckthorn.

To confirm the spectrophotometric data on the effect of the extracts’ AOA on the size characteristics of ZnO NPs, transmission electron microscopy (TEM) studies were performed, and TEM images of the synthesized ZnO NPs were obtained. As can be seen from Figure 3, the nanoparticles are predominantly spherical in shape. However, it should be noted that with a decrease in the extract AOA, from the sea buckthorn extract (Figure 3e) to the plum extract (Figure 3a), the number of large nanoparticles over 50 nm increases, while their shape becomes more elongated and single triangular-shaped nanoparticles are present.

Figure 3. TEM images and histograms of particle distribution by size of ZnO NPs synthesized using extracts of plum (a), gooseberry (b), black currant (c), strawberry (d), sea buckthorn (e).

According to the histograms of the particle distribution in Figure 3, the sizes of the ZnO NPs obtained using the sea buckthorn extract vary within a range of 8–52 nm, the strawberry extract 15–61 nm, the black currant extract 23–75 nm, the gooseberry extract 27–76 nm and the plum extract 18–90 nm. The average nanoparticle diameters were 21, 33, 46, 46, 50 nm for the sea buckthorn, strawberry, black currant, gooseberry and plum extracts, respectively (Table 1). An increase in the average diameter in the plant series sea buckthorn–strawberry–black currant–gooseberry–plum is associated with an increase in the proportion of large nanoparticles and a decrease in the proportion of small nanoparticles. There is an inverse relationship between the plant extract AOA and the average diameter of synthesized ZnO NPs, which is manifested in an increase in the average diameter of nanoparticles while the extract AOA decreases.

Table 1. Diameter and polydispersity index of ZnO NPs synthesized using different plant extracts.

Plant Extract Used for ZnO NPs Synthesis	Diameter Range, nm	Average Diameter Value, nm	PI*
plum	18–90	50 ± 13	0.07
gooseberry	27–76	46 ± 10	0.05
black currant	23–75	43 ± 12	0.08
strawberry	15–61	33 ± 9	0.07
sea buckthorn	8–52	21 ± 8	0.15

PI*—polydispersity index.

The observed trend can be explained by the fact that an increase in the AOA leads to a higher concentration of the phytochemical compounds (phenolic compounds, alcohols, amines, tannins, and others) in the extract, which possess reducing, capping, and stabilizing properties. This results in a greater number of nucleation centers, accelerates the rate of the nanoparticle formation, and promotes the development of stable phytocoatings on the nanoparticle surface. Stabilization is achieved through a combination of electrostatic and π–π stacking interactions, van der Waals forces, and hydrogen bonding. This mechanism is further supported by findings from other studies [44,50].

Table 1 gives the calculated polydispersity index (PI) for all synthesized ZnO NPs. A value of PI less than 0.1 indicates a narrow size distribution and uniformity of the nanoparticles. A higher PI value is observed for the nanoparticles with smaller diameters, which could be attributed to their tendency to aggregate due to their high surface energy.

Semi-quantitative analysis of ZnO NPs was performed via energy-dispersive X-ray spectroscopy (EDX) using a copper substrate (its contribution was not taken into account in the calculations). The EDX spectrum shows significant peaks of zinc at 1.0, 8.6 and 9.6 keV and oxygen at 0.5 keV (Figure 4). The mass content of zinc and oxygen was 78.01% and 18.57%, respectively. This indicates the high purity of the obtained nanoparticles. The presence of additional small peaks of calcium, potassium, and phosphorus is presumably due to their presence in the phytomolecules that are involved in the synthesis of ZnO NPs.

Figure 4. EDX spectrum of ZnO NPs synthesized using the extract of sea buckthorn.

The selected area electron diffraction (SAED) pattern of ZnO NPs synthesized using the sea buckthorn extract is presented in Figure 5a. The microdiffraction pattern is almost annular (pseudo-annular) because it was taken from a multitude of nanosized particles randomly oriented relative to the incident electron beam. The electron diffraction patterns were interpreted by measuring the radii of the rings formed by reflection from the nanoparticle crystallographic planes. The specific planes identified in the SAED pattern include (100), (101), (003), (110) and (103), which confirms the high crystallinity of the sample.

Figure 5. SAED patterns of ZnO NPs synthesized using the sea buckthorn extract (a), XRD patterns of ZnO NPs synthesized using different plant extracts: 1—plum, 2—gooseberry, 3—black currant, 4—strawberry, 5—sea buckthorn (b).

For a more accurate determination of the obtained samples’ structure, the X-ray diffraction (XRD) approach was employed. Figure 5b shows the XRD patterns of the ZnO NPs samples synthesized using the extracts of different plants. Nine specific distinct peaks are observed, corresponding to the (010), (002), (011), (012), (110), (013), (020), (112), and (021) crystalline planes. The interpretation of the XRD patterns and comparison with the ICDD database, PDF^® 036-1451 (Zincite) (Newtown Square, PA 19073, USA), confirmed that the ZnO NPs have a hexagonal wurtzite structure.

Table 2 presents the crystal lattice parameters of the ZnO NPs synthesized using different plant extracts, which are close to bulk ZnO (a = 3.249 Å, c = 5.206 Å). Minor deviations from the reference values may be due to lattice distortion and the formation of defects induced by the synthesis conditions.

Table 2. The crystal lattice parameter of ZnO NPs.

Extract	a, Å	c, Å
plum	3.253	5.208
gooseberry	3.252	5.210
black currant	3.254	5.209
strawberry	3.251	5.206
sea buckthorn	3.252	5.208

3.1.2. Effect of Reaction Mixture pH

The pH of the reaction mixture has a significant effect on the morphology and size of the phytosynthesized ZnO NPs [54,61]. To study the effect of pH, ZnO NPs were synthesized at pH 5, 7, 9 and 10 using the sea buckthorn extract with the highest AOA. Figure 6 shows the UV–vis absorption spectrum of ZnO NPs synthesized in the reaction mixture with different pH, and the dependence of the absorbance peak wavelength on the pH of the reaction mixture. It can be seen from Figure 6a that the nanoparticles’ absorbance increases, while the wavelength of the absorbance peak of the ZnO NPs decreases with the increasing reaction mixture pH (Figure 6b). The shift in the peak absorbance to a low-wavelength region of the spectrum, observed in Figure 6b, indicates a decrease in the average diameter of the ZnO NPs along with an increase in the pH of the reaction mixture [62]. However, high pH values should be avoided. At pH > 10, the formation of nanoparticles is constrained, probably due to the amphoteric properties of zinc and the formation of soluble complexes.

Figure 6. UV-vis absorption spectrum of ZnO NPs synthesized using the sea buckthorn extract at different pH (a). Dependence of the wavelength of ZnO NPs absorption peak on the reaction mixture pH (b).

Figure 7 shows the TEM images of the ZnO NPs synthesized using an aqueous extract of sea buckthorn with different pH. The ZnO NPs were spherical in shape. The formation of a small number of large nanoparticles was noted. Their proportion decreased along with the increasing pH of the reaction mixture.

Figure 7. TEM images and histograms of ZnO NPs distribution by size synthesized using the sea buckthorn extracts at different pH: pH 5 (a), pH 7 (b), pH 9 (c), pH 10 (d).

Table 3 presents the values of diameter and polydispersity index for the ZnO NPs synthesized in different pH conditions. It was found that with an increase in the pH of the reaction mixture, the average diameter of ZnO NPs decreases. This pattern can be explained by the fact that in an alkaline medium, in addition to phytomolecules, OH⁻ anions concentrate around Zn²⁺ ions, forming a stronger insulating shell that eliminates the growth and aggregation of nanoparticles, which leads to the formation of small nanoparticles [63]. In addition, increasing pH may change the charge/conformation of the plant molecules, which may better stabilize the small cores of nanoparticles, thus reducing their agglomeration [64].

Table 3. Diameter and polydispersity index of ZnO NPs synthesized using the sea buckthorn extract at different pH.

pH Reaction Mixture	Diameter Range, nm	Average Diameter Value, nm	PI
5	8–52	21 ± 8	0.15
7	7–34	19 ± 5	0.09
9	8–31	17 ± 5	0.08
10	6–33	13 ± 4	0.10

It should be noted that changing the acidity from a slightly acidic environment (pH 5) to neutral (pH 7) and alkaline (pH 9 and 10) contributed to obtaining more homogeneous nanoparticles with a narrow size distribution. This observation is supported by the PI values of the nanoparticles, which do not exceed 0.1 at pH 7–10 during synthesis. Thus, increasing the pH from 5 to 10 promotes the formation of smaller and more uniformly sized ZnO NPs.

3.1.3. Effect of Precursor Salt Concentration

Figure 8 shows the absorption spectrum of the ZnO NPs obtained at different concentrations of a precursor salt, Zn(CH₃COO)₂∙2H₂O. It can be seen that with an increase in the precursor salt concentration, the maximum absorption increases at 373–378 nm (Figure 8a) and shifts to longer wavelengths, which indicates an increase in the nanoparticle size (Figure 8b), supported by the TEM use.

Figure 8. UV-vis absorption spectrum of ZnO NPs synthesized using the sea buckthorn extract at pH 10 and different concentrations of Zn(CH₃COO)₂∙2H₂O (a). Dependence of the wavelength of absorption peak of ZnO NPs on the precursor salt concentration (b).

Figure 9 presents the TEM images of the ZnO NPs synthesized at different concentrations of zinc acetate. The ZnO NPs were spherical in shape. The histograms illustrate that the smallest distribution in the nanoparticles size was observed at a concentration of 0.1 mol L⁻¹.

Figure 9. TEM images and histograms of ZnO NPs distribution by size synthesized using the sea buckthorn extract at pH 10 and different concentration of Zn(CH₃COO)₂∙2H₂O: 0.025 (a), 0.05 (b), 0.1 (c), 0.25 (d), 0.5 (e) mol L⁻¹.

The values of average diameters and polydispersity indices of the nanoparticles obtained at different concentrations of the precursor salt are presented in Table 4. A slight decrease in the nanoparticle size is observed upon moving from 0.025 mol L⁻¹ to 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O and a significant increase at the concentration above 0.1 mol L⁻¹. This can be explained by the fact that with an increase in the concentration of zinc acetate from 0.025 mol L⁻¹ to 0.1 mol L⁻¹, the number of potential nanoparticle nuclei increases. At concentrations above 0.1 mol L⁻¹, the ratio of the precursor salt to the extract changes, which results in a decrease in the extract capping and stabilizing effect, aggregation of nanoparticles [65,66] and a visible increase in their size [66,67,68]. With an increase in the concentration of the precursor salt and an increase in the size of the nanoparticles, the polydispersity index decreases, which indicates the greater homogeneity of larger particles.

Table 4. Diameter and polydispersity index of ZnO NPs synthesized using the sea buckthorn extract at pH 10 and different precursor salt concentrations.

C(Zn(CH3COO)2∙2H2O), mol L−1	Diameter Range, nm	Average Diameter Value, nm	PI
0.025	6–42	17 ± 6	0.13
0.05	6–42	15 ± 5	0.10
0.1	6–33	13 ± 4	0.10
0.25	9–44	24 ± 7	0.07
0.5	14–55	29 ± 8	0.07

The obtained results are consistent with the results of other researchers, who showed that a high concentration of the precursor salt promotes faster nucleation and the formation of smaller nanoparticles, and when a threshold concentration is reached, the aggregation of nanoparticles and an increase in their size occur [69,70,71].

3.2. Determination of Band Gap of ZnO NPs

To determine the band gap width of the ZnO NPs synthesized under optimal conditions (sea buckthorn extract, pH 10, 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O), their UV–vis absorption spectrum was used (Figure 10a). The absorption coefficient (α) was calculated based on the UV–vis spectroscopy data using Equation (2):

α = (2.303A)/d

(2)

where A is optical density and d is cuvette thickness. The hν value was calculated from the values of absorption spectrum wavelengths using Equation (3):

hν = 1240/λ

(3)

where λ is the wavelength (nm) and 1240 is a numerical constant relating light energy to wavelength. For the direct transition, the following Tauc Equation (4) was used [72,73]

(αhν) = (hν − E_g)^1/n

(4)

where α is the absorption coefficient, hν is the incident photon energy, n is the exponent indicating the type of optical transition (direct allowed transition n = 2 for ZnO), and E_g is the optical band gap energy (eV).

Figure 10. UV–vis absorption spectrum of ZnO NPs synthesized using the sea buckthorn extract at pH 10 and 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O (a); Tauc plot for ZnO NPs (direct allowed transition n = 2) with extrapolation of linear section to energy axis (b).

Figure 10b shows the Tauc plot for the ZnO NPs synthesized using the sea buckthorn extract, pH 10, and a concentration of 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O. The linear section of (αhν)² vs. hν dependence was extrapolated to the x-axis, allowing us to estimate the optical band gap E_g = 2.85 eV [74,75]. The decrease in the band gap compared to the literature data (3.37 eV) [76,77,78,79] may be due to crystal lattice defects, such as the formation of oxygen vacancies in a semiconductor during its synthesis. A smaller band gap facilitates photodegradation reactions, allowing the use of not only UV but visible radiation as well [80,81].

3.3. Photocatalytic Degradation of Methylene Blue Dye

The catalytic properties of phytosynthesized ZnO NPs were evaluated using the photodegradation of MB dye under varying UV exposure times. To eliminate the influence of dye adsorption on the photodegradation process, the dye–nanoparticle mixture was first kept in the dark for 60 min and then exposed to UV radiation.

3.3.1. Photodegradation of Methylene Blue Under Different Conditions

Figure 11a–c show the absorption spectra of the MB dye obtained under different conditions. All spectra exhibit a maximum at λ_max = 664 nm, corresponding to the absorption of the MB dye. Figure 11a shows the absorption curves of the dye solution incubated for different periods in the dark in the presence of ZnO NPs. It can be seen that with longer UV exposure time, the absorption decreases slightly due to the adsorption of the dye on the surface of the ZnO NPs, caused by electrostatic interaction and the large specific surface area of the particles. From the absorption curves, it can be observed that the absorption maximum is virtually identical over 50 min of exposure in the dark, indicating the establishment of the adsorption equilibrium. Taking adsorption into account allows us to separate the physical sorption stage from the catalytic degradation of the pollutant under UV radiation. The efficiency of photodegradation of the dye solution when kept in the dark for 60 min in the presence of ZnO NPs is 10.2% (Figure 11d, curve 1, section from −60 to 0 min). When the dye solution is exposed to UV, but in the absence of ZnO NPs, insignificant decomposition of methylene blue also occurs (Figure 11b), but the efficiency of the photodegradation process decreases by half to 5.0%, as compared with the process in the dark in the presence of ZnO NPs (Figure 11d, curve 2). When the dye solution is exposed to UV for 120 min in the presence of ZnO NPs, the dye solution color completely vanishes and light absorption does not occur (Figure 11c). Thus, in case the system, containing MB and ZnO NPs, is exposed to UV radiation for 120 min, 79.5% of the dye is removed due to the photocatalytic process and 10.2% due to physical adsorption, which, in total, gives a degradation efficiency of 89.7% (Figure 11d, curve 1, section from 0 to 120 min). This suggests that the combined use of ZnO NPs and UV radiation helps to almost completely degrade the MB dye.

Figure 11. UV–vis absorption spectrum of MB at different conditions: without UV at dark (a); with UV without ZnO NPs (b); with UV in the presence of ZnO NPs (c); and MB photodegradation efficiency in the presence (curve 1) and in absence (curve 2) of ZnO NPs depending on the UV exposure time (d). Reaction mixture composition: 15 mg ZnO NPs synthesized using the sea buckthorn extract in 50 mL 10 μmol L⁻¹ MB.

3.3.2. Effect of Synthesis Conditions on the Photocatalytic Activity of ZnO NPs

To study the effect of the green synthesis conditions (AOA, pH, precursor salt concentration) on the photocatalytic activity of the ZnO NPs, the absorbance of the MB solution, exposed to ZnO NPs and UV radiation for different periods of time, was recorded at 664 nm, and the photodegradation efficiency was assessed.

Figure 12a shows a graph of the photodegradation efficiency of the MB solution under UV radiation and the ZnO NPs obtained using extracts with different AOA. The most effective nanocatalyst, ZnO, was synthesized using the sea buckthorn extract. The dye photodegradation efficiency with this catalyst was 90.93% in 120 min. The lowest efficiency, 80.74%, was found for the ZnO NPs obtained using the plum extract. As shown previously, the AOA affects the nanoparticle size. Thus, the ZnO NPs obtained from the sea buckthorn extract are the smallest, while the ZnO NPs synthesized using the plum extract are the largest. Apparently, the size of the nanoparticles affects the catalytic properties, meaning smaller sizes result in more effective photocatalysts due to the increased surface area, greater number of active sites, and more efficient separation of electron–hole pairs.

Figure 12. MB photodegradation efficiency using ZnO NPs synthesized with different plant extracts (a), pH of reaction mixture (b) and the precursor salt concentration (c).

The photodegradation of MB using the ZnO NPs catalysts synthesized at different pH values showed that nanoparticles obtained at pH 10 exhibited the highest efficiency (Figure 12b). This may be due to the excess negative charge on the nanoparticles’ surface, which ensures strong electrostatic interactions between the positively charged dye and the catalyst [53], allowing for dye degradation in a shorter time with similar efficiency. Thus, 91.58% of the dye was degraded in just 90 min using the ZnO NPs obtained at pH 10, compared to only 86.4% of the dye degraded in the same time using the ZnO NPs synthesized at pH 5.

The results (Figure 12c) of the photocatalytic study of the ZnO NPs, synthesized using different precursor salt concentrations, revealed that the nanoparticles obtained using 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O exhibited the highest efficiency. At this precursor concentration, the ZnO NPs were the smallest relative to other samples (Table 4), which ensured a larger surface area of the nanocatalyst and improved interaction with the MB dye molecules.

Thus, the highest efficiency of the MB photodegradation was achieved using the ZnO NPs synthesized with the use of the sea buckthorn extract characterized by the highest AOA, pH 10, and 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O.

3.3.3. Choosing Optimal Conditions for the Methylene Blue Photodegradation

To select the optimal conditions that ensure the maximum efficiency of the MB photodegradation process, the effect of the ZnO NPs’ amount and the pH solution on the decomposition of the dye was considered. When studying the amounts of the catalyst, weighed samples of 5 mg, 25 mg, 50 mg, and 100 mg ZnO NPs were taken. These samples were placed in 50 mL of 10 μmol L⁻¹ MB with pH 7. The results of this experiment are shown in Figure 13a. With a small amount of the catalyst equal to 5 mg, the MB photodegradation occurs slowly under UV radiation, since the solution does not have enough active catalyst sites that generate radicals for the pollutant decomposition reactions. With 25 mg of the catalyst in the solution, the efficiency of the dye photodegradation increases sharply, since a greater number of active sites, available on a larger surface area, leads to more active radical generation and an increase in the MB decomposition efficiency. However, above 25 mg of ZnO NPs, the photodegradation was less efficient. This might be due to the fact that the amount of the catalyst above a certain limit could cause its aggregation, which reduces the surface area and increases the turbidity and opacity of the solution. Because of that, UV radiation is scattered and less absorbed by the catalyst [82].

Figure 13. MB photodegradation efficiency using different amounts of ZnO NPs (a) and solution pH (b).

When implementing the photodegradation of the MB dye in real wastewater, it is important to understand the efficiency of the MB degradation over a wide pH range. The results of the study of the MB photodegradation in the solutions with pH values ranging from 4 to 11 are presented in Figure 13b. As can be seen from the results, the degradation of MB dye after exposure to UV light is less efficient in acidic and neutral environments than in alkaline ones. It is known that the pH value for the zero point of charge (pH_zpc) for ZnO is 8.5. Therefore, at pH ˂ pH_zpc, the photocatalyst surface is saturated with positively charged H⁺ ions, which repel positively charged MB molecules, impairing adsorption and, consequently, the accessibility of pollutant molecules to radicals generated during the catalyst activation by UV radiation. At pH ˂ pH_zpc, the negative charge of the ZnO surface facilitates the photodegradation process. In addition, photolysis in an alkaline solution results in the enhanced formation of ∙OH radicals from OH^– ions, which promote the process of dye degradation [75], and the destruction of the dye structure occurs. However, at a very high pH 10–11, the efficiency of the photodegradation decreases, apparently due to the dimerization of the dye itself, which manifests itself in a change in the absorption spectrum and deterioration of the dye molecules’ reactivity. Thus, almost complete (96.4%) degradation of the MB dye is achieved at solution pH 9, exposure to UV radiation, and 25 mg of the loaded ZnO NPs catalyst, synthesized using the sea buckthorn extract at a medium pH 10 and a concentration of 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O.

To evaluate the stability and reusability of the ZnO NPs, four photodegradation cycles were performed using the MB dye at a concentration of 10 μmol L⁻¹, pH 9, and 25 mg of the loaded catalyst, synthesized under optimal conditions. After each cycle, the ZnO NPs were separated via centrifugation, washed three times with distilled water, and dried at 60 °C. As can be seen from Figure 14, the photodegradation efficiency for the four consecutive cycles was 96.4%, 94.7%, 92.2%, and 89.6%.

Figure 14. MB photodegradation efficiency with repeated use of ZnO NPs synthesized using the sea buckthorn extract and 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O at pH 10.

A slow decrease in the photodegradation efficiency is observed, apparently due to the blocking of the active catalyst sites by the adsorbed intermediates, which are not completely removed during catalyst washing [83,84]. However, overall, the material still retains a high photocatalytic activity even after the fourth cycle, allowing it to be reused.

3.3.4. Radical Scavenger Test

To identify the species responsible for the photodegradation of the MB dye, the following scavengers were used: isopropanol to absorb hydroxyl radicals (∙OH), benzoquinone for superoxide radicals (∙O₂⁻), AgNO₃ for electrons (ē), and ammonium oxalate for holes (h⁺). Table 5 presents the results of how different scavengers affected the MB degradation when the ZnO NPs photocatalyst was exposed to UV light for 120 min. A slight decrease in the MB photodegradation was observed with the addition of ammonium oxalate, benzoquinone and AgNO₃ (84%, 78%, and 72%, respectively).The role of holes (h⁺), superoxide radicals (∙O₂⁻), and electrons (ē) was insignificant in the degradation mechanism of MB. However, a significant decrease in the photodegradation efficiency was observed with the addition of isopropanol—6%, which is an indicator of the importance of hydroxyl radicals (∙OH) in the degradation mechanism of the MB dye [85].

Table 5. Efficiency of MB degradation under the influence of UV radiation in the presence of active particle scavengers.

Scavenger	None	AO	PBQ	AgNO3	IPA
Degradation efficiency (%)	96	84	78	72	6

The mechanism of photocatalysis is based on the generation of electron–hole pairs in a semiconductor affected by the light of a specific wavelength. The ZnO semiconductor is excited by a photon (hν) with equal to or greater than its band gap energy. Electrons (ē) in the valence band absorb photon energy exceeding the band gap energy, which ensures a transition to the conduction band and the formation of an unfilled vacancy, the so-called “hole” (h⁺). Some of the generated ē/h⁺ pairs recombine and decay. The remaining pairs are transferred to the ZnO surface and undergo oxidation and reduction reactions. The ē electrons in the conduction band react with oxygen to form superoxide anion free radicals (∙O₂⁻). This leads to a protonation reaction and hydrogen peroxide radicals (HO₂∙) are formed, which, in turn, continue to react to form hydrogen peroxide (H₂O₂). Hydrogen peroxide reacts with ∙O₂⁻, forming a hydroxyl free radical (∙OH), while reactions with ē and hν also result in the formation of ∙OH. The “holes” in the valence band react with water and hydroxide ions to form ∙OH. These ∙OH hydroxyl free radicals are strong oxidizing agents, capable of oxidizing pollutants to carbon dioxide and water [82]. As the scavenger test showed, it is hydroxyl radicals that play the most significant role in the degradation of the MB. The mechanism of MB photodegradation using a ZnO nanocatalyst can be described by the following set of reactions (Equations (5)–(12)), ultimately leading to the formation of CO₂ and H₂O and decolorization of the solution:

ZnO + hν → ZnO (ē + h⁺)

(5)

ē + O₂→ ∙O₂⁻

(6)

O₂⁻+ H⁺→ HO₂∙ + ē→ HO₂⁻ + H⁺→ H₂O₂

(7)

H₂O₂ + ∙O₂⁻→ ∙OH+ OH⁻

(8)

H₂O₂ + ē→ ∙OH + OH⁻

(9)

h⁺ + H₂O → ∙OH

(10)

h⁺ + OH ⁻→ ∙OH

(11)

∙OH + MB (dye) → H₂O + CO₂

(12)

This mechanism of the dye photocatalytic degradation is consistent with other authors’ findings [86,87].

3.3.5. Comparison of Synthesized ZnO NPs with Other Nanocatalysts

The synthesized and commercial ZnO NPs were compared under conditions of photocatalytic degradation of the MB dye with a concentration of 10 μmol L⁻¹, pH 9, and 25 mg of the loaded catalyst. The results of this experiment, presented in Figure 15a, show that the ZnO NPs synthesized by using the sea buckthorn extract (curve 1) are more efficient and are capable of degrading 96.4% of the MB within 2 h, while the commercial nanocatalyst (curve 2) degraded only 80% of the dye in the same period.

Figure 15. MB photodegradation efficiency using ZnO NPs synthesized with the sea buckthorn extract and 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O at pH 10 (curve 1) and commercial ZnO NPs (curve 2) (a), plots of ln(C₀/C) vs. radiation time for pseudo-first-order reaction kinetics (b).

Many studies describe the process of MB degradation over time involving ZnO NPs by the pseudo-first-order kinetic model. In this model, the degradation rate of the MB dye depends primarily on its concentration, while the concentration of active radicals formed on the surface of the ZnO catalyst remains virtually constant [88,89,90,91,92]. This model is described by Equation (13):

ln [C] = ln [C₀] − kt

(13)

where C₀ and C are the initial concentration and concentration in a certain period of time (mol L⁻¹), k is the rate constant (min⁻¹), t is the photodegradation time (min).

It was suggested that in our case, the dye degradation process follows a pseudo-first-order model as well. To confirm this, the dependence ln(C₀/C) = f(t) was plotted for the ZnO NPs synthesized using sea buckthorn extract. For comparison, the same dependence was plotted for a commercially available ZnO nanocatalyst. As can be seen from Figure 15b, in both cases, a high degree of linear approximation is observed (R² = 0.9785 for the phytosynthesized ZnO NPs; R² = 0.9909 for the commercially available ZnO NPs). This proves the applicability of the pseudo-first-order kinetic model for MB degradation. The slope of the linear plot ln(C₀/C) = f(t) corresponds to the reaction rate constant and is 0.0229 min⁻¹ for the phytosynthesized ZnO NPs and 0.0106 min⁻¹ for the commercially available ZnO NPs. A twofold increase in the constant value when using the synthesized nanoparticles shows that they have higher catalytic activity compared to commercially available ZnO NPs. The established value of the MB degradation rate constant for the phytosynthesized ZnO NPs is consistent with the values reported in other studies [88,92] and, in some cases, even exceeds them [93,94,95], which further confirms the good catalytic properties of ZnO NPs. When comparing the efficiency of the MB photodegradation in the presence of the ZnO nanocatalysts synthesized using extracts from different plants, as presented in Table 6, it can be noted that the nanocatalyst synthesized using the sea buckthorn extract demonstrates similar and, in many cases, even superior efficiency in comparison with other samples.

Table 6. Characteristics of phytosynthesized ZnO nanocatalysts in the decomposition of dyes.

Plant Extract	ZnO NPs Size, nm	Dye	Degradation Time, min	Photodegradation Efficiency, %	Ref.
Amphipterygium adstringens (Cuachalalate) bark extract	10−20	methylene blue	180	90	[88]
Allium caliphalum Wendelbow leaf extract	30−72	methylene orange	140	74	[89]
Terminalia chebula fruit extract	15−17	rhodamine B	300	70	[90]
Myrtus communis plant extract	150−200 (spheres), 10,000 (nanowires)	methylene blue	10–50	93	[91]
Coriandrum sativum plant extract	11	methylene blue	110	20	[92]
Thymus vulgaris leaf extract	20−30	methylene blue	60	95	[96]
Rubus coreanus fruit extract	23	malachite green	240	90	[97]
Hippophae rhamnoides leaf extract	20	malachite green	180	89	[98]
Suaeda japonica plant extract	100	methylene blue	180	54	[99]
Hippophae rhamnoides (sea buckthorn) leaf extract	13	methylene blue	120	96	This work

4. Conclusions

An eco-friendly synthesis of ZnO nanoparticles was carried out using leaf extracts from five plants with varying antioxidant activity. Based on the results of a study on the effect of synthesis parameters on the morphology and size of ZnO nanoparticles, it was found that increasing the antioxidant activity of plant extracts facilitates the production of the smallest and most catalytically active ZnO nanoparticles, which suggests that this parameter is essential when selecting a plant extract for the phytosynthesis of nanoparticles of a given size and properties. The smallest spherical ZnO nanoparticles with an average diameter of 13 nm and polydispersity index of 0.1 were obtained using the sea buckthorn extract with the highest antioxidant activity, pH 10 of the reaction mixture and 0.1 mol L⁻¹ Zn(CH₃COO)₂∙2H₂O as a precursor salt. The chemical composition of these ZnO nanoparticles is as follows: zinc—78.01%, oxygen—18.57%. The band gap is 2.85 eV, which is significantly less than 3.37 eV for bulk ZnO. The nanoscale semiconductor synthesized with the sea buckthorn extract demonstrated better catalytic properties and a high potential for removing organic pollutants in water. The photodegradation efficiency of the hazardous synthetic methylene blue dye in the presence of ZnO nanoparticles was 96.4% after 120 min of UV radiation. Thus, the scientific approach to the targeted selection of a plant extract proposed in this work can be considered a practical guide for controlling the process of phytosynthesis and obtaining highly effective “green” nanocatalysts with given properties for environmental remediation applications.