Eco-Friendly Synthesis of ZnO-Based Nanocomposites Using Haloxylon and Calligonum Extracts for Enhanced Photocatalytic Degradation of Methylene Blue

Abstract

This study presents a green synthesis of zinc oxide (ZnO) nanoparticles (NPs) capped with Haloxylon (P1) and Calligonum (P2) extracts. The use of plant-derived biomolecules as natural capping agents offers an environmentally friendly strategy to tune surface chemistry and to enhance the photocatalytic behavior of ZnO NPs. ZnO/plant extracts nanocomposites were prepared via a hydrothermal route and systematically characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–Vis spectroscopy, and photoluminescence (PL), followed by evaluation of their photocatalytic performance against methylene blue (MB) under UV irradiation. XRD confirmed a wurtzite structure with crystallite sizes ranging from 8.95 to 10.93 nm, while PL spectra indicated an improved charge carrier separation in extract-capped ZnO. The characteristics and pollutant removal performance of the greenly synthesized ZnO composites were compared with those of a chemically synthesized ZnO nanoparticles reference sample. Adsorption tests under dark conditions revealed a strong difference between the materials: ZnO-P1 removed 48% of MB, whereas ZnO-P2 adsorbed only 7%, demonstrating a much higher affinity of the Haloxylon-derived surface groups toward MB. In comparison, the chemically synthesized ZnO exhibited an adsorption capacity of 54%, confirming that the Haloxylon-mediated surface provides a comparable efficient dye uptake prior to irradiation. After UV irradiation, all samples exhibited a photocatalytic activity with a total MB removal reached ~59% for the reference ZnO sample and ~53% for ZnO-P1 compared to about 13% for the ZnO-P2. Kinetic analysis also confirmed that ZnO-P1 possessed a high degradation rate constant, indicating a better intrinsic photocatalytic efficiency in addition to the strong adsorption contribution. The enhanced performance of plant-capped ZnO is attributed to phytochemical-induced surface defects, which facilitated charge separation and boosted the generation of reactive oxygen species (ROS). Overall, these results demonstrate that Haloxylon and Calligonum extracts are effective and sustainable capping agents, providing a low-cost, eco-friendly approach for designing ZnO nanocatalysts composites with promising applications in wastewater treatment and environmental remediation.

1. Introduction

Nanomaterials and their composites have attracted significant interest due to their size-dependent physicochemical properties, which differ markedly from those of bulk materials. They are widely used in applications such as photocatalysis reactions, environmental remediation, and energy-related processes because of their favorable optical and electronic characteristics [1,2,3]. Many physical and chemical methods, including laser ablation, pyrolysis, chemical or physical vapor deposition, sol–gel, and electroplating by lithography, are typically used to synthesize nanoparticles. The majority of these procedures are costly and involve the use of hazardous solvents [4]. Considerable work has recently been done to synthesize noble metal based nanoparticles using eco-friendly techniques [5]. These environmentally friendly techniques are quick, cheap, and effective, and they often produce crystalline nanoparticles with diameters between 1 and 100 nm and a range of morphologies [6]. Since it doesn’t require harmful reducing and stabilizing chemicals, biologically based production of nanoparticles is a green technique that is frequently used [7]. In order to address the drawbacks of traditional synthetic procedures like physical and chemical processes, biological methods have been developed as promising alternatives. Various biological materials and bioactive compounds have been used to create metal-based nanoparticles, including algae [8], enzymes [9], and plant extracts [10]. These stabilizers are all regarded as potent green nanofactories. Numerous biological applications have taken advantage of metal-microbe or plant extract interactions. As a result, research on biologically based methods for creating nanoparticles, including combinations of nanotechnology and biotechnologies, has become increasingly promising. The most straightforward, dependable, economical, and non-toxic of these biological processes is plant-mediated synthesis. This approach can be readily scaled up for large-scale synthesis and can overcome the drawback of maintaining clean microbial cultures, which is necessary for microbial approaches [11]. Metal nanoparticles have been synthesized from a variety of plant elements, including leaves, roots, callus, stems, seeds, bark, fruits, and peels. Numerous biomolecules found in plant extracts, including phenols, proteins, amino acids, flavonoids, enzymes, and terpenoids, can function as stabilizing for the creation of nanoparticles [12]. Among these metal oxide nanoparticles, zinc oxide (ZnO) has attracted attention because of its properties such as non-toxicity, low cost, availability, and high optical absorption in the visible and ultraviolet range [13]. The electronic properties of zinc oxides offer a vast potential of applications in solar energy conversion [14]. electronic devices [15], magnetic storage devices [16], biosensors [17], photocatalysis [18], and antibacterial activities [19]. Extensive research has focused on the green synthesis of ZnO nanoparticles using plant extracts as eco-friendly capping, stabilizing, and reducing agents. Numerous studies have reported that phytochemicals such as polyphenols, flavonoids, alkaloids, and terpenoids can effectively mediate ZnO nucleation and growth while enhancing surface functionality and stability. Various plant species including Lupinus albus L. [20], Polyalthia longifolia [21], Nyctanthes arbor-tristis [22], Hardwickia binata [23], and Plectranthus amboinicus [24] have been used in the environmentally friendly synthesis of ZnO NPs with improved biocompatibility, reduced toxicity, and tunable optical and catalytic properties. These studies collectively demonstrate the potential of plant-mediated synthesis as a sustainable alternative to conventional chemical routes. However, despite these advances, only limited attention has been given to exploring the influence of less common desert plants such as Haloxylon and Calligonum, whose unique phytochemical compositions may yield distinct physicochemical characteristics and enhanced photocatalytic performance. Combining these plant extracts with ZnO nanoparticles as a novel composite material for water treatment applications forms the main motivation of the present study and contributes to the ongoing efforts aimed at finding solutions to global water security challenges. In this context, developing nations are especially severely affected by a lack of access to clean drinking water because they lack the financial means to adequately address the rising contamination of the water supply. Furthermore, as the population grows, the situation worsens, and rampant industrialization adds to the strain on water supplies. The pursuit of higher agricultural yields, along with advancements in the pharmaceutical, chemical, and textile sectors, has led to the use of several complex molecules with novel functionalities. These compounds are then discovered in industrial effluents and enter watercourses [25]. Some of these chemicals are highly hazardous, posing a threat to human health as well as aquatic flora and animals. These chemicals can also build up in cultivated plants, and consuming them might cause digestive problems and organ abnormalities [26]. Therefore, it is advised to use easy, affordable, and efficient techniques to eliminate these contaminants. Advanced Oxidation Processes (AOPs) have been developed to address this issue [27]. AOPs have demonstrated remarkable efficacy in eliminating hazardous and tenacious pollutants. Their efficacy is primarily based on the in situ production of OH-, which has a higher oxidizing power than other more traditionally used oxidants like H₂O₂. Organic molecules in suspension can be mineralized in whole or partly by hydroxyl radicals. These radicals can be easily produced by utilizing the heterogeneous ZnO/UV photocatalysis technique, which has drawn a lot of attention lately. The interest in this process is due to its simplicity and its ability to eliminate a wide range of pollutants at ambient temperature [28]. Photocatalysis is a frequently used supplementary treatment technology for liquid effluent, demonstrating efficacy in degrading organic pollutants. Therefore, it represents a very promising substitute for traditional separation/degradation methods. However, implementing a photocatalysis process in a real-world application requires overcoming two challenges: optimizing the technical, energy, and economic costs of the ZnO nanoparticles recovery stage following photocatalysis, which necessitates a treatment station for filtration, and addressing the limit of semiconductor absorbance in the ultraviolet range.

In the present work, we aim to contribute to ongoing research on developing highly efficient catalyst materials for environmental applications through low-cost and green synthesis methods. We report a novel green method for synthesizing ZnO based composites using two different plant extracts, in contrast to the conventional chemical synthesis methods. We also present a comprehensive characterization of the prepared nanocomposites along with an evaluation of their performance in the photodegradation of methylene blue (MB), a cationic thiazine dye, as a model pollutant.

2. Experimental Section

2.1. Materials

Analytical-grade chemicals were procured from Sigma-Aldrich (Sigma-Aldrich, Gillingham, UK) without any further purification, ensuring the purity and reliability needed for accurate experimental results. The main chemicals used include Zinc (II) acetate dihydrate (Zn[CH₃COO]₂·2H₂O, MW = 219.51 g/mol, purity ≥ 98%), which serves as a zinc source for the synthesis of ZnO NPs. L-cysteine (C₃H₇NO₂S, MW = 121.16 g/mol, purity ≥ 98%) was used as capping agent fo the chemically prepared ZnO reference sample. Sodium hydroxide (NaOH, MW = 40.00 g/mol, purity ≥ 98%) was used to adjust the pH during nanoparticle synthesis and to facilitate the precipitation of ZnO NPs. Haloxylon and Calligonum plant extracts were utilized for the nanocomposite synthesis, with methanol (CH₃OH, purity ≥ 99.8%) serving as the solvent. Additionally, an ultrapure water (resistivity > 18 MΩ·cm) was employed for the extraction process.

2.2. Plants Extraction

Haloxylon and Calligonum plants were collected from Hail region in Saudi Arabia. The selected leaves were thoroughly washed with deionized water, then air-dried in the shade at room temperature before being ground into a fine powder. A portion of 11.5 g of this powder was mixed with 200 mL of distilled water and boiled for 2 h at 100 °C. After cooling to room temperature, the mixtures were filtered, and the resulting aqueous filtrate was collected. This extract was subsequently employed in the synthesis of ZnO nanoparticles in the present study.

2.3. Preparation of Green ZnO Based Nanocomposites Using Plants Extract/L-Cysteine

ZnO nanoparticles were synthesized through a green aqueous route using Haloxylon and Calligonum plant extracts (Figure 1) [29]. A reference ZnO sample was also synthesized for comparison purposes using a common chemical method with L-cysteine as a capping agent. A zinc acetate precursor molarity was prepared in methanol, and the amount was added dropwise under constant stirring for 30 min. The pH of the mixture was adjusted to ~12 using sodium hydroxide (2 M) to promote the nucleation of ZnO NPs. The reaction was allowed to continue for 24 h at room temperature to ensure complete interaction between Zn²⁺ ions and phytochemicals. The precipitated nanoparticles were collected by centrifugation, washed thoroughly with distilled water and ethanol to remove residual organics and were kept in vacuum at room temperature. In this approach, the phytochemicals present in the plant extracts act as surface capping agents, stabilizing the ZnO nanoparticles without requiring thermal calcination [30].

2.4. Nanocomposite Catalysts Characterization

The ZnO-plant extracts nanocomposites and the chemically synthesized ZnO reference sample catalysts were characterized using a combination of analytical techniques. X-ray diffraction (XRD) patterns were recorded on a Panalytical X’Pert Pro diffractometer equipped with a Cu Kα radiation source (λ = 1.542 Å). Fourier transform infrared (FTIR) spectra of the ZnO nanostructures were obtained at room temperature in the 400–4000 cm⁻¹ range using a Perkin Elmer spectrophotometer (version 5.3) in transmission mode with KBr pellet disks. A double aberration-corrected transmission electron microscope (TEM) of the model Titan Themis Z from Thermo Fisher Scientific (TFS), with an accelerating voltage of 300 kV, was used to obtain the morphological images of the samples. Bright-field TEM (BF-TEM) mode was utilized to acquire images at different magnifications for investigating the morphology of the samples. The images were recorded using a scintillator-based complementary metal-oxide-semiconductor (CMOS) detector, model Ceta, from TFS. The samples for TEM investigation were obtained by pouring a drop of nanocrystal solution over carbon-coated copper grids, allowing the excess solvent to drain. At least 100 randomly chosen nanocrystals were measured using an image processing application (ImageJ, version 1.50) to determine their size and size-distribution data according to the TEM images. UV–Vis absorption spectra were recorded at room temperature in the 200–700 nm range using a SPECORD 210 Plus spectrophotometer with a quartz cuvette. Photoluminescence (PL) spectroscopy was carried out using a helium–cadmium laser source (λ = 325 nm) as the excitation source to investigate the emission properties and defect states of the ZnO samples.

2.5. Photocatalytic Degradation Experiments

The photocatalytic performance of the synthesized ZnO nanoparticles based nanocomposite catalysts were evaluated through the degradation of methylene blue (MB) dye, following a procedure previously reported in the literature [31]. Methylene blue was selected in this study due to its high molar extinction coefficient and well-defined optical properties, which make it a standard model pollutant for benchmarking photocatalytic activity. In the current study, water treatment experiments were conducted using MB-contaminated solutions with an initial concentration of ~2 × 10⁻⁵ M and a pH of approximately 7. For each test, 10 mg of ZnO nanoparticles composite were dispersed in 10 mL of MB aqueous solution. The absorption spectra of the samples were first recorded after 30 min of equilibration in the dark, followed by measurements at 30, 90, and 180 min under UV irradiation. The degradation efficiency and reaction rate were calculated using the following equations [32]:

$$\eta = {\displaystyle \frac{C_0-C_t}{C_0}}\times 100$$

(1)

$$Ln \left({\displaystyle \frac{C_0}{C_t}}\right)=kt$$

(2)

where k (min⁻¹) represents the apparent reaction rate constant, and C_t denotes the dye concentration (mg/L) at irradiation time t (min). The value of k was obtained by calculating the slope of the linear fitting of the experimental data using the plot of ln (C_t/C₀) versus t.

3. Results and Discussion

3.1. Nanoparticles Characterization

3.1.1. Morphological Study

Figure 2 shows the XRD patterns of the ZnO-P1 and ZnO-P2 nanocomposites. According to standard ZnO XRD patterns, the obtained hexagonal structure successfully matched the peaks in the reference card of hexagonal structure (JCPDS No79-2205) [33]. Additionally, the broad XRD diffraction bands in Figure 2 demonstrate the nanoscale nature of the synthesized materials. XRD diffraction patterns for ZnO-P1 NPs exhibit peaks at 2θ = 31.80°, 34.47°, 36.27°, 47.46° and 56.55°. These peaks correspond to the hexagonal diffraction planes (100), (002), (101), (102), and (110) of ZnO. Similarly, the XRD pattern of ZnO-P2 nanoparticles displays nearly identical diffraction peaks to those of ZnO-P1, confirming that the nanoparticles synthesized with the P2 plant extract also crystallize in the hexagonal phase. The diffraction peaks observed at 2θ values of 31.73°, 34.43°, 36.51°, 47.58°, and 56.58° are indexed to the (100), (002), (101), (102), and (110) planes of hexagonal ZnO, respectively. The pronounced intensity of these peaks confirms the high crystallinity of the synthesized ZnO nanoparticles. The average crystallite size was further estimated using the Debye–Scherrer equation [34]:

$$D={\displaystyle \frac{K\lambda }{\beta \cos \left(\theta \right)}}$$

(3)

where D is the crystallite size of ZnO in nm, K is the Scherrer shape factor (0.90), λ is the X-ray wavelength used (1.5406 Å), β is the full width at half maximum (FWHM) in radians and θ is the Bragg diffraction angle in degrees. The average particle sizes of ZnO-P1 and ZnO-P2 were estimated as 10.93 and 8.95 nm, respectively. Different plant extracts contain varying concentrations of phytochemicals, which can influence the nucleation and the growth of the nanoparticles. The difference in the crystallite size of the grown samples is most likely explained by the fact that the P1-Zn interaction is much weaker than that of P2-Zn [35]. In the case of P1, the relatively weak interaction with Zn results in a weaker binding affinity to the ZnO quantum dot surface, thereby promoting a faster crystal growth rate. Conversely, the stronger interaction between P2 and Zn enhances the binding force at the ZnO surface, which slows down the crystal growth and limits the formation of larger particles [35]. Furthermore, the lattice constant of the hexagonal ZnO nanocrystals can be determined using the following expression [34]:

$$d_{hkl}^2= {\displaystyle \frac{1}{\frac{4\left( h^2+hK+K^2\right)}{3a^2} +\frac{l}{c^2}}}$$

(4)

where d_hkl is the inter-reticular distance that is given for the hexagonal cubic structure; h, k, and l are Miller’s indices of; a and c are the lattice constants of the hexagonal phase of nanocrystals. The average calculated values of lattice parameters are presented in

Table 1. Structural properties of ZnO-P1 and ZnO-P2 nanocrystals.

Sample	Crystallite Size D (nm)	Dominant Plans dhkl	Lattice Constant (Å)	Strain(ε)	Dislocation Density (δ) (lines/m2) × 1015	Stacking Fault (SF)
ZnO-P1	10.93	−101	a = 3.23 c = 5.21	0.0032	8.37	0.006
ZnO-P2	8.95	−101	a = 3.19 c = 5.15	0.0041	12.48	0.0077

. The Scherrer formula provides a lower limit for the size of the nanocrystals as it ignores the micro-strain and only considers the size effects resulting from the diffraction data. Consequently, the grown NCs may be further investigated by a few structural factors, such as the stacking fault (SF), dislocation density (δ), and lattice strain (ε). The following formulas are used to evaluate these parameters, and Table 1 shows their calculated values [36]:

$$\epsilon ={\displaystyle \frac{\beta cos\left(\theta \right)}{4}}; \delta ={\displaystyle \frac{1}{D^2}}; SF={\displaystyle \frac{2{\pi }^2}{45{\left(3tan\theta \right)}^{\frac{1}{2}}}}{\beta }_{hkl}$$

(5)

The disruption of lattice constants resulting from crystal defects such lattice dislocation is measured by the lattice strain (ε) [37]. Due to size restriction, the lattice expansion or contraction in the nanocrystals is the primary cause of the lattice strain. Accordingly, the number of defects in the nanocrystal size is described by the stacking fault (SF) and dislocation density (δ) [38]. Because of their tiny dimensions, the semiconductor nanocrystals are less organized, as indicated by the dislocation density value. ZnO-P2 has the highest possibility of dislocations compared to ZnO-P1, as seen from the table, which increases from 8.37 × 10¹⁵ to 12.48 × 10¹⁵ m⁻² when the crystallite size (D), decreases from 10.93 to 8.95 nm. This is may be due to the small dimensionality of ZnO-P2 NCs which are the lesser ordered as well as the nature of P2-Zn interaction [39]. Micro-strain and stacking fault both exhibited similar patterns, rising as the P1 to P2 ligand crossed. ZnO-P2 sample has more lattice defects and vacancies than ZnO-P1 samples, according to the increasing drift of micro-strain. The distortion of crystallographic planes is likewise reflected in the stacking fault (SF). The observed increase in SF from the P1 to the P2 samples suggests that ligand exchange intensified the structural imperfections. Therefore, it can be concluded that the proposed crystalline arrangements of the Haloxylon and Calligonum-capped ZnO quantum dots remain inconclusive. Compared with previously reported plant-mediated ZnO nanoparticles, which often exhibit sizes above 20 nm such as ~24.5 nm with Polyalthia longifolia, ~23.8 nm with Passiflora foetida, and ~24.6 nm with Vachellia erioloba extracts [21,40,41]. ZnO nanoparticles synthesized in this study using Haloxylon and Calligonum extracts are significantly smaller, with average crystallite sizes around 10 nm. This indicates that the choice of plant extract and specific synthesis conditions can strongly influence nucleation and growth, allowing the production of finer nanostructures. Despite the smaller size, the structural integrity and crystallinity of the prepared ZnO based samples in the current study are maintained, demonstrating that these plant-derived capping agents provide an effective and tunable green approach to controlling nanoparticle dimensions compared with previously reported systems. To further investigate the plant-extract-capped nanoparticles, we utilized high-resolution TEM (HRTEM).

Furthermore, TEM was employed to investigate the size distribution and morphology of ZnO nanoparticles synthesized using plant extracts and L-cysteine as capping agents. TEM images revealed that ZnO nanoparticles capped with Haloxylon extract (Figure 3a) are predominantly spherical with slight asymmetry and with an average particle size of 11.7 ± 0.5 nm. The presence of phytochemicals such as flavonoids and phenolic compounds likely contributed to the complexation and stabilization of Zn²⁺ species during nucleation, as well as to the capping of the resulting ZnO nanocrystals, thereby controlling their growth and preventing agglomeration [42]. Mild aggregation observed in the images can be attributed to the complex organic matrix surrounding the nanoparticles and the uneven surface coverage provided by the extract components. In comparison, ZnO nanoparticles synthesized with Calligonum extract (Figure 3b) displayed a slightly smaller mean size of 9.4 ± 0.5 nm, with shapes ranging from nearly spherical to oval. Biomolecules, including tannins and polysaccharides in the Calligonum extract, appeared to influence nucleation and growth kinetics, resulting in a narrower size and shape distribution of the particles in this sample [43]. TEM analysis also showed moderate clustering, suggesting that while plant-derived capping agents provide a degree of stability, some interparticle contact remains. Overall, TEM observations are consistent with XRD data, confirming the successful synthesis of nanosized ZnO particles using both plant extracts. The average crystallite sizes obtained from XRD (8.95–10.93 nm, Table 1) were slightly smaller than the particle sizes measured by TEM (9.4–11.7 nm). This discrepancy arises because XRD estimates the crystalline core size, whereas TEM measures the entire particle, including amorphous or polycrystalline surface layers. Despite the difference, both techniques consistently confirm the nanoscale regime and the same size trend (P2 < P1) [44]. These results highlight the crucial role of phytochemicals in tailoring nanoparticle size and morphology, while also demonstrating that Haloxylon and Calligonum extracts represent sustainable and eco-friendly alternatives for ZnO nanoparticle production. The reference ZnO particles synthesized using the non-green chemical method exhibited a particle size slightly smaller than those synthesized using plant extracts with an average size of about 8.44 ± 0.5 nm as seen from Figure 3c. The TEM characterization of ZnO nanoparticles synthesized in this study using Haloxylon and Calligonum extracts reveals nearly spherical particles with average sizes around 10 nm, which are significantly smaller than many previously reported plant-mediated ZnO nanoparticles. For instance, ZnO synthesized with Polyalthia longifolia extract exhibited particles in the range of 27.5 nm, while Punica granatum extract produced ZnO nanoparticles of ~25 nm, and Evolvulus alsinoides extracts typically yielded sizes around 100 nm [21,45,46]. These comparisons indicate that the proposed synthesis route using Haloxylon and Calligonum extracts effectively limit particle growth during synthesis, producing finer nanostructures.

The functional groups involved in the synthesis of ZnO nanoparticles composites were identified using FTIR spectroscopy, as shown in Figure 4. For P1-capped ZnO NPs, a broad absorption band at 3302 cm⁻¹ is observed, corresponding to hydrogen-bonded O–H stretching vibrations of phenolic compounds and flavonoids present in the plant extract. Additional characteristic bands appeared at ~722 cm⁻¹ and 1107 cm⁻¹, which can be attributed to C–H and C–O stretching modes, respectively [47]. Moreover, distinct peaks at 1400 and 1601 cm⁻¹ are assigned to carboxylate groups, with the signal near 1595 cm⁻¹ corresponding to the asymmetric COO⁻ stretch and that at 1400 cm⁻¹ to the symmetric COO⁻ stretch, confirming the interaction of phytochemicals with Zn ions [48]. In the case of P2-capped ZnO NPs, a broad absorption band at 3390 cm⁻¹ indicates O–H stretching of phenolic compounds and flavonoids, while weak absorptions at ~780 and 1054 cm⁻¹ are assigned to C–H and C–O vibrations, respectively [49]. Overall, FTIR analysis confirms that the greenly synthesized ZnO nanoparticles are stabilized by the major phytoconstituents present in both P1 and P2 extracts. Similar vibrational bands can also be seen in the case of the reference sample including C–H, C–O, COO⁻ and O–H stretching modes. Earlier studies on plant-extract–synthesized ZnO have reported FTIR spectra with bands from organic functional groups (e.g., –OH, –C=O, –C–O) attributed to phytochemicals acting as reducing and stabilizing agents on the ZnO surface, in contrast to uncapped ZnO which lacks these features [50,51]. In particular, as-synthesized ZnO prepared with plant extracts shows clear FTIR evidence of surface organic species compared with commercial or uncapped ZnO, indicating the presence of the capping agents that modify the surface chemistry [52]. Integrated with the obtained data, the presence of strong and shifted carboxylate and carbonyl bands in Haloxylon- and Calligonum-derived ZnO samples confirms stronger coordination and distinct photophysical modifications compared to other green ZnO reports.

3.1.2. Optical Study

The optical properties of the ZnO nanoparticles were investigated by UV–Vis absorption spectroscopy in the 300–700 nm range (Figure 5) to examine their optical characteristics as well as the influence of the possible quantum confinement effects. All composite samples exhibited a noticeable blue shift in their absorption edges compared to the bulk ZnO band gap (~375 nm), confirming the formation of nanocrystalline structures [53]. Specifically, the absorption edge was observed at 364 nm for ZnO-P1 and at 347 nm for ZnO-P2, which can be attributed to quantum confinement effects relative to bulk ZnO (Eg ≈ 3.3 eV) [54]. These results are in good agreement with the TEM and XRD analyses, as the pronounced blue shift further substantiates the nanoscale dimensions of the synthesized particles. The pronounced blue shift observed in Calligonum-capped ZnO nanoparticles can be attributed to the combined effects of reduced particle size and strong surface coordination [53]. The functional groups present in Calligonum extract, such as hydroxyl and carboxyl groups, effectively bind to the nanoparticle surface, limiting growth and preventing aggregation [55]. This results in smaller crystallite sizes, enhancing the quantum confinement effect and leading to an increase in the bandgap energy, manifested as a blue shift in the absorption spectrum. In contrast, ZnO nanoparticles capped with Haloxylon extract exhibited a less pronounced spectral shift, which can be attributed to weaker surface coordination and slightly larger particle sizes. By comparison, the stronger surface passivation imparted by Calligonum extract enhances nanoparticle stability and minimizes defect states, thereby contributing to the more significant blue shift observed [56].The optical band gap energy (Eg) was then estimated graphically using the Tauc relation [57]:

$$\alpha h\nu = A {\left(h\nu -Eg\right)}^{1/2}$$

(6)

In this relation, α represents the absorption coefficient, A is a proportionality constant, h is Planck’s constant, and ν denotes the photon frequency.

The optical band gap (Eg) of the capped ZnO nanoparticles was determined from a Tauc plot by extrapolating the linear portion of the curve of (αhν)² versus hν to the energy axis at α = 0. The intercept of this extrapolation with the hν axis corresponds to the estimated band gap value. The calculated band gap energies of the capped ZnO nanoparticles were found to be 3.05 and 3.17 eV for Haloxylon and Calligonum-capped ZnO NPs, respectively. Although these values are lower than the typical band gap of bulk ZnO 3.3 eV), they are consistent with the behavior of ZnO nanostructures, which can exhibit band gap narrowing due to quantum confinement effects or the presence of surface defects [30]. The variation in band gap between the two samples reflects the influence of particle size, morphology, surface capping, and synthesis conditions on the electronic structure of ZnO nanoparticles. In addition, a direct comparison with the chemically synthesized ZnO (L-cysteine–assisted) has been incorporated to strengthen the assessment of the green-synthesized materials. The calculated band-gap energy of the chemical ZnO (3.58 eV) is noticeably higher than those of the plant-extract–capped ZnO nanoparticles. This band-gap narrowing in the green-synthesized samples is attributed to the presence of phytochemical species acting as surface modifiers, which enhance light absorption in the near-UV region. Despite its higher band gap, the chemically synthesized ZnO possesses significantly smaller particle size, which increases its specific surface area and the number of accessible active sites. This enhanced surface-to-volume ratio facilitates faster adsorptive interactions and more efficient charge-transfer processes at the interface.

The photoluminescence (PL) spectra of ZnO nanoparticles synthesized with different capping agents were recorded to investigate how variations in surface chemistry and quantum confinement influence their optical properties. Significant differences were observed between Haloxylon and Calligonum-capped ZnO nanoparticles, reflecting variations in particle size, degree of crystallinity, and the extent of surface passivation.

The photoluminescence (PL) spectra were recorded in the wavelength range of 350–640 nm, and the results are presented in Figure 6. Multiple emission peaks are visible in the photoluminescence (PL) spectrum of ZnO nanoparticles capped with Haloxylon plant extract. These peaks may be deconvoluted into discrete transitions associated with both intrinsic and surface-related defects. The crystalline structure of the nanoparticles is reflected in the strong emission peak centered about 370 nm (~3.35 eV), which is attributed to the direct band-to-band recombination of excitons of ZnO [53]. However, the second emission peak centered around 377 nm (~3.29 eV) could be assigned to the nanocrystals’ size distribution, which causes another emission peak related to the direct band-to-band recombination of excitons [58]. The presence of such a sharp UV emission indicates that the ZnO nanoparticles have good crystallinity and relatively low defect density, since radiative recombination dominates over defect-mediated recombination. The Haloxylon phytochemicals acting as capping agents likely stabilized the nanoparticle surface, reducing non-radiative centers. Small changes in crystal size allow quantum confinement to slightly shift the bandgap, which results in an extra near-band-edge emission attributed to direct band-to-band excitonic recombination [59]. In addition, the subsequent emissions at 389 nm (~3.19 eV) and 396 nm (~3.13 eV) arise from shallow-level processes, including donor–acceptor pair (DAP) recombination and longitudinal optical (LO) phonon replicas of the near-band-edge emission, as well as possible free-to-acceptor transitions [60]. The band appearing at 410 nm (~3.02 eV) is attributed to band-edge emission, confirming the band gap value obtained from the absorption studies. In contrast, the band at 430 nm (~2.88 eV) is associated with intrinsic defect states, specifically the extended state of zinc interstitials (ex-I_Zn)—VB transition and/or zinc interstitials (I_Zn)—zinc vacancies (V_Zn) transition [61,62]. These defect-related emissions are strongly influenced by the phytochemical capping layer, which modifies surface states and introduces additional carrier traps, thereby shaping the optical properties of the ZnO-P1 nanoparticles [63]. Both intrinsic excitonic recombination and defect-mediated transitions contribute to the deconvoluted photoluminescence spectrum of the Calligonum plant extract-capped ZnO nanoparticles, exhibiting several emission bands ranging from the near-UV to the visible range (350–600 nm) [64]. The crystalline structure of the nanostructures is confirmed by the strong emission narrowed at 366 nm (~3.39 eV), which is consistent with the near-band-edge (NBE) excitonic process [65]. The subsequent emissions observed at 408 nm (~3.04 eV) can be ascribed to shallow defect levels, primarily involving zinc interstitials and singly ionized oxygen vacancies [62]. A broader emission band at 441 nm (~2.81 eV) is associated with zinc vacancies or oxygen interstitials [66]. The green emission band centered at ~497 nm, commonly referred to as the “green luminescence band” of ZnO-P2, is where the strongest contribution is seen. It is caused by deep-level states of oxygen vacancies (VO^••) and unsaturated surface states that are maintained by the phytochemical capping layer [67,68]. In addition to introducing new trap sites, the plant-derived ligands are more likely to promote surface passivation, altering the defect density and stabilizing radiative recombination pathways. Collectively, the presence and relative intensities of these emission features indicate that the plant extracts play a pivotal role in regulating the balance between shallow and deep defect states. This control is crucial not only for the optical and photocatalytic properties of the nanoparticles but also for tuning their growth dynamics and crystallinity [69]. The positions and Full width at half maximum (FWHM) of PL peaks in each sample are compiled in

Table 2. Photoluminescence Analysis of ZnO Nanoparticles with P1, P2 and reference sample Surface Modifications.

Nanoparticles	Peaks (nm)	FWHM (nm)
ZnO-P1	Peak 1 = 370	4.26
	Peak 2 = 377	10.57
	Peak 3 = 389	7.21
	Peak 4 = 396	9.97
	Peak 5 = 410	12.99
	Peak 6 = 430	60.81
ZnO-P2	Peak 1 = 366	50.43
	Peak 2 = 408	42.84
	Peak 3 = 442	83.57
	Peak 4 = 497	73.46
ZnO-Ref	Peak 1 = 376	47.4
	Peak 2 = 409	61.04
	Peak 3 = 452	33.33
	Peak 4 = 462	115.06

.

The reference ZnO sample also exhibits emission features corresponding to near-band-edge band-to-band transitions along with defect-related emissions arising from zinc vacancies, oxygen vacancies, and zinc interstitials. However, compared with ZnO-P1 and ZnO-P2, the reference ZnO shows fewer defect-related peaks, indicating a lower density of intrinsic defects and a more controlled surface passivation. These findings demonstrate that the optical properties of ZnO nanoparticles are significantly influenced by the capping agent utilized. Photoluminescence analysis clearly shows that the ZnO-P1 sample (Haloxylon extract) exhibits a higher number of defect-related emission peaks compared to ZnO-P2, indicating that its surface is less passivated. In contrast, the Calligonum-capped ZnO (ZnO-P2) displays fewer PL peaks, suggesting a relatively more passivated surface with a lower density of radiative defect states. A similar trend is observed for the L-cysteine–capped ZnO, which likewise shows reduced defect-related emissions, indicating effective surface passivation comparable to that of the Calligonum-capped nanoparticles. Although high defect density in ZnO can increase non-radiative recombination, in photocatalytic systems these surface and bulk defects are often beneficial because they act as adsorption sites and promote the formation of reactive oxygen species (ROS) under UV illumination [68]. Haloxylon capping leads to a richer defect profile that enhances charge carrier lifetime, interfacial redox activity and interaction with pollutant molecules, thereby improving photocatalytic performance despite less complete surface passivation. Therefore, the stronger defect-related emissions observed in ZnO-P1 imply a higher density of active sites, which can enhance dye adsorption through efficient interaction with pollutant molecules and facilitate photocatalytic reactions. Nonetheless, the nature and distribution of these defects rather than their number alone play a decisive role in determining the overall pollutant removal performance of the prepared ZnO nanoparticles.

3.2. Photocatalytic Activity of MB Using ZnO Based Nanocomposite Catalysts

The photocatalytic performance of the synthesized ZnO nanoparticles composites was evaluated through the degradation of methylene blue (MB) under UV irradiation. Prior to illumination, all samples—including the reference chemically synthesized ZnO—were subjected to a 30-min dark adsorption step to establish adsorption–desorption equilibrium. In all catalysts, the MB absorption peak at ~664 nm showed a gradual decrease with longer irradiation time, indicating effective photodegradation [70]. As shown in Figure 7, the extent of MB removal during this stage strongly depends on the capping agent used during synthesis. After 30 min of adsorption, the MB concentration decreased by approximately 48% for ZnO-P1, 7% for ZnO-P2, and around 54% for the reference ZnO, confirming that the chemically-capped nanoparticles possess the highest adsorption capacity, followed by the Haloxylon synthesized ZnO, while the Calligonum-capped sample exhibits minimal adsorption. This figure also demonstrates that the extent of MB degradation depends on the capping agent used in nanoparticles synthesis. Upon UV exposure, all samples demonstrated additional MB removal due to photocatalytic degradation. After 180 min of irradiation, ZnO-P1 and ZnO-P2 reduced the dye concentration to approximately 53% and 13% of the initial value, respectively. In contrast, the reference chemically synthesized ZnO achieved a total removal of nearly 59. Both ZnO-Ref and ZnO-P1 samples show a comparable photocatalytic activity, although ZnO-P1 exhibits a higher defect density and a wider defect range. However, the slightly higher MB photocatalytic activity of ZnO-P1 in comparison to ZnO-P2 may arise from its defect-rich PL signatures, i.e., multiple shallow and deep emission bands that prolong carrier lifetimes, enhance dye adsorption at defect/surface sites, and promote ROS generation [71]. The presence of these multiple defect states is highly beneficial for photocatalysis, since shallow traps can temporarily capture photogenerated electrons or holes, delaying their direct recombination and thereby prolonging carrier lifetimes.

In parallel, deeper and surface-associated states act as active adsorption and reaction sites, facilitating the transfer of charge carriers to dissolved oxygen or surface-bound dye molecules, which accelerates the production of reactive oxygen species (OH*, O₂*⁻) [72]. Compared to ZnO-P2, whose PL profile is dominated by a broad visible emission with fewer resolved components, ZnO-P1 presents a more favorable balance between carrier trapping and recombination suppression. This synergy between efficient carrier separation and surface-mediated redox reactions explains why ZnO-P1 achieves enhanced photocatalytic activity despite similar band-gap characteristics [73]. Additionally, the reduced MB adsorption ability of the sample ZnO-P2 in comparison to the superior adsorption performance demonstrated by the sample ZnO-P1 can be attributed to the large contribution of defects, especially I_Zn to the PL spectrum of the ZnO-P2 sample. The presence of I_Zn defects introduces positive charges to the nanoparticle surface and reduces its ability to adsorb the cationic dye due to repulsion interactions [74,75]. Figure 8 presents the relation between Ln C/Co and irradiation time for the prepared samples. The observed linearity of the fitted experimental data confirms that the MB photodegradation by the prepared samples follows pseudo-first-order kinetics [76]. The obtained reaction constant values from the slopes of these curves show that the sample P1 exhibits a better K-value compared to the sample P2 that has a slightly lower reaction value indicating its less ability to photocatalytically degrade MB dye. This can also be seen from Figure 8d, which illustrates the MB degradation efficiency of the prepared samples. The P2 sample shows the lowest overall removal, with only about 13% total degradation after 180 min of UV irradiation. In contrast, the P1 sample synthesized using Haloxylon extract achieves a significantly higher degradation efficiency of approximately 53%. For comparison, the chemically synthesized ZnO sample exhibits a total MB removal of around 59% under the same conditions. This comparison clearly shows that the green-synthesized Haloxylon-capped ZnO not only outperforms the Calligonum-capped sample but also provides photocatalytic activity comparable to the chemically produced ZnO reference. Investigations of MB removal showed that both adsorption and photocatalysis contributed to the overall performance of the materials. Dark adsorption tests revealed a pronounced difference between the two nanocomposites where Haloxylon-capped ZnO exhibited a much higher adsorption capacity than Calligonum-capped ZnO. During subsequent UV irradiation, all samples displayed photocatalytic activity, and kinetic analysis confirmed that Haloxylon-capped ZnO also possessed enhanced intrinsic photocatalytic degradation efficiency. Therefore, the superior total MB removal observed for ZnO-P1 arises from a synergistic combination of strong adsorption and improved photocatalytic charge separation, rather than from photocatalysis alone. These results demonstrate the effectiveness of the used green route synthesis in preparing ZnO nanoparticles with improved MB degradation performance. The enhanced photocatalytic activity of Haloxylon-capped ZnO nanoparticles compared to their Calligonum-capped nanocatalysts can be attributed to a more favorable interplay between surface chemistry and dye–nanoparticle interactions. Phytochemical analyses show that Haloxylon extracts are rich in phenolic acids, flavonoids, tannins, and other bioactive molecules bearing hydroxyl and carbonyl groups [77]. These functional moieties strongly bind to the ZnO surface, creating abundant active sites for methylene blue adsorption and facilitating intimate dye–catalyst contact.

To better understand the photocatalytic process, the degradation mechanism of methylene blue in the presence of capped ZnO nanoparticles can be described in two main stages: adsorption and photocatalysis. In the initial stage, MB molecules adsorb onto the nanoparticle surface, ensuring close dye–catalyst interaction. Upon UV irradiation, ZnO absorbs photons with sufficient energy, promoting electrons from the valence band to the conduction band, leaving behind positively charged holes. This excitation generates electron–hole pairs, which are the key drivers of the photocatalytic reaction. The capping agents not only enhance the colloidal stability of ZnO nanoparticles but also play a crucial role in suppressing charge carrier recombination, thereby improving photocatalytic efficiency. The photogenerated holes oxidize water molecules or hydroxide ions to form highly reactive hydroxyl radicals (OH*), while the excited electrons reduce adsorbed oxygen into superoxide radicals (O₂*⁻). These reactive oxygen species subsequently attack MB molecules, breaking them down into less complex and non-toxic intermediates. Several reports have highlighted the plant-extract-mediated synthesis of metal oxide nanostructures and have evaluated their photocatalytic activity toward methylene blue (MB) degradation. For instance, TiO₂ nanoparticles synthesized using Aloe vera extract showed only ~50% MB degradation after 180 min under UV light, and the presence of organic residues on the catalyst surface further reduced the efficiency to ~10% under certain conditions [78]. Similarly, ZnO nanoparticles prepared with Lupinus albus extract achieved ~47.55% MB degradation under UV irradiation [79]. Finally, non-plant synthesized systems such as pristine MoS₂ also demonstrated low MB degradation (~23–44%), supporting the concept that certain catalyst designs strongly correlate with the photodegradation efficiencies [80]. However, the obtained MB degradation efficiency in this study for the nanocomposite obtained by the green synthesis of ZnO nanoparticles with Haloxylon is clearly better than the above given examples and demonstrates its high potential as an efficient photocatalyst material under green synthesis conditions.

The underlying mechanism can be described by the following key reactions [81]:

(a)

Adsorption phase

ZnO-P + MB → ZnO-P-MB

(7)

(b)

Photon absorption and electron–hole generation

ZnO-P + hν → e_BC⁻ + h_BV⁺

(8)

(c)

Reduction of dissolved oxygen to superoxide radicals

e_CB⁻ + O₂ → O₂*⁻

(9)

(d)

Production of superoxide and hydroxyl radicals

e⁻ + O₂ → O₂*⁻

(10)

H₂O + h⁺ → OH* + H⁺

(11)

h⁺ + OH⁻ → OH*

(12)

(e)

Degradation phase

O₂*⁻/OH* + MB→ Intermediates + CO₂ + H₂O

(13)

The photocatalytic degradation of MB dye is primarily governed by the action of hydroxyl (OH*) and superoxide (O₂*⁻) radicals. Bioactive compounds present in plant extracts, such as phenols and flavonoids, can further contribute to the generation of these reactive species [82]. The superior photocatalytic performance of ZnO-P1 nanoparticles is largely attributed to the rich organic composition of Haloxylon extract, which contains phenolic acids, flavonoids, tannins, and other bioactive molecules [77]. These functional groups coordinate strongly with the ZnO surface, promoting efficient electron transfer while reducing the recombination probability of photogenerated electron–hole pairs under UV irradiation. In addition, biomolecules anchored to the nanoparticle surface act as electron-trapping sites, thereby enhancing the production of reactive oxygen species (OH*, O₂*⁻) that drive the oxidative degradation of pollutants. During the breakdown of methylene blue, hydroxyl radicals preferentially attack the C–S⁺=C bond and other vulnerable sites within the dye molecules, leading to their stepwise mineralization [83]. Therefore, based on these findings, ZnO nanoparticles capped with Haloxylon extract emerge as a highly promising photocatalyst for organic pollutant removal. Beyond dye degradation, the natural biomolecules from phenolic acids, flavonoids, tannins, and other bioactive molecules also render these capped nanoparticles a sustainable platform for extended applications, including antibacterial activity and gas sensing.

4. Conclusions

In this work, ZnO nanocomposites capped with Haloxylon (P1) and Calligonum (P2) extracts were successfully synthesized through an eco-friendly route and compared with a a chemically synthesized ZnO reference sample. The structural, optical, and morphological analyses confirmed the formation of crystalline, surface-functionalized ZnO nanoparticles whose properties were influenced by the choice of capping agent. The photocatalytic studies revealed that MB removal by the green-synthesized nanocomposites results from both adsorption and photodegradation. Haloxylon-capped ZnO (P1) exhibited significantly higher dark adsorption and more effective charge separation, leading to greater overall MB removal compared with P2. The findings demonstrate that plant-based capping agents not only enable green synthesis and stabilization of ZnO nanoparticles but also allow tuning of their adsorption behavior and photocatalytic response with a pollutant removal performance comparable to that of the samples prepared using conventional chemical methods. Overall, this study highlights the potential of biologically capped ZnO nanocomposites as environmentally sustainable materials for wastewater treatment and underscores the importance of capping-agent selection in optimizing photocatalytic performance.