A comparative analysis of solar photocatalytic degradation of MB, RhB, and CV dyes with ZnO nanoparticles

Abstract

This study reports the synthesis of ZnO nanoparticles via a simple co-precipitation method and their evaluation for solar-driven photocatalytic degradation of organic dyes. XRD analysis confirmed the formation of a hexagonal wurtzite phase with good crystallinity, while Raman spectroscopy further supported the structural integrity of ZnO. Transmission electron microscopy (TEM) revealed the morphology and particle-size distribution of the synthesized nanoparticles, and selected-area electron diffraction (SAED) indicated their polycrystalline nature. The photocatalytic performance of ZnO nanoparticles was investigated under natural sunlight using three model dyes: Methylene Blue (MB), Rhodamine B (RhB), and Crystal Violet (CV). UV–Visible spectroscopy showed a progressive decrease in dye concentration, following pseudo-first-order kinetics. The degradation efficiencies reached approximately 98% for MB, 93% for RhB, and 96% for CV within 160 min. XPS analysis suggests the presence of oxygen vacancy-related defect states, which may contribute to enhanced photocatalytic activity by influencing charge carrier dynamics. Overall, the study demonstrates the effectiveness of ZnO nanoparticles for solar-driven dye degradation under natural conditions. However, further investigations, including detailed optical characterization and stability studies, are required to fully assess their practical applicability in wastewater treatment.

Graphical abstract

Introduction

The swift advancement of industry and agriculture has undeniably improved human life; however, it has also led to severe environmental pollution, emerging as one of the most critical challenges confronting all living beings in the 21st century [1–6]. In particular, industrial wastewater poses a significant threat to aquatic biodiversity and disrupts ecological balance [3]. making it a pressing environmental issue that demands urgent attention. Water contamination continues to endanger both natural ecosystems and human health worldwide [4]. Simultaneously, the planet is grappling with an escalating energy and environmental crisis [7]. Among the many contributors to water pollution, the discharge of persistent organic pollutants (POPs) is recognized as a primary cause [5]. Among these, organic dyes are especially problematic due to their intense coloration, large volumes, toxicity, and resistance to biodegradation, making the treatment of dye-containing wastewater before environmental release a particularly challenging task [1, 8–11].

Currently, several methods are commonly employed for the treatment of dye-contaminated wastewater, including photocatalytic oxidation, physical coagulation, Fenton oxidation, and biodegradation [2, 12–16]. Among these, semiconductor-based photocatalytic oxidation has attracted significant research attention due to its notable advantages, such as chemical stability, non-toxicity, high efficiency, low energy consumption, and environmental friendliness [17–20]. In particular, solar-driven photocatalytic oxidation has emerged as a green and efficient approach [13] capable of degrading harmful pollutants by breaking down organic molecules into harmless products such as carbon dioxide, water, and other small molecules (6,8). In this context, the search for highly efficient, cost-effective, and stable photocatalysts remains a crucial focus for advancing photocatalytic wastewater treatment technologies [9, 11].

Semiconductor metal oxides have proven to be critical materials for the photocatalytic treatment of industrial wastewater, largely due to their resistance to light-induced corrosion [3]. Several oxides, including TiO₂, Fe₂O₃, ZnO, and CuO, have been extensively investigated for their ability to degrade organic dyes through photocatalysis [12, 17]. Among these, ZnO has garnered particular attention owing to its strong reduction capability and its versatility in forming diverse morphologies [20]. In addition to being an efficient photocatalyst, ZnO offers several attractive features such as chemical and physical stability, strong oxidative power, environmental benignity, and low cost [21–23]. Nevertheless, its practical application remains limited. This is primarily because ZnO possesses a relatively wide bandgap (~ 3.3 eV) [24, 25], restricting its ability to absorb a broad range of sunlight—only around 5% of the solar spectrum falls within the UV region it can utilize—thereby leading to reduced photocatalytic efficiency for organic pollutant degradation [26, 27].

To address these challenges, many strategies have been proposed, including doping with various metals, forming composites, or utilizing green synthesis approaches to improve the photocatalytic efficiency of ZnO. While these methods have shown promise, they often involve complex processes, expensive materials, or multiple preparation steps. Moreover, achieving high photocatalytic performance using pure ZnO under direct sunlight conditions remains a significant challenge.

While the photocatalytic degradation of individual organic dyes using ZnO nanoparticles has been widely investigated, comprehensive comparative studies evaluating structurally distinct dye classes under identical natural solar conditions remain scarce. Most existing literature focuses heavily on optimizing operational parameters for single-dye systems, rather than deconstructing how the intrinsic molecular stability and chromophore structures of different pollutants influence degradation macro-kinetics simultaneously. To bridge this gap, this study systematically investigates and compares the solar photocatalytic degradation of three model dyes representing entirely different chemical classes: MB (a thiazine dye), RhB (a xanthene dye), and CV (a triarylmethane dye). By deliberately fixing the operational parameters at a baseline of 0.5 g/L catalyst dosage and 10 ppm initial dye concentration, this work successfully isolates the role of localized adsorption behavior, molecular weight, and structural stability on the terminal degradation pathways. This comparative macro-kinetic approach provides critical insights into the broad-spectrum efficiency of synthesized ZnO nanoparticles under natural sunlight, establishing a clear baseline for practical multi-pollutant water remediation.

In the present study, ZnO nanoparticles were synthesized via a simple and cost-effective co-precipitation method using citric acid as a chelating agent to control particle growth and morphology. The photocatalytic performance of the synthesized ZnO nanoparticles was evaluated through a comparative study of the solar-driven degradation of (MB), (RhB), and (CV) dyes under natural sunlight conditions. Unlike many previous studies that focus on single-dye systems or artificial irradiation sources, this work provides a systematic comparison of multiple dyes under identical solar conditions, enabling a better understanding of dye-dependent degradation behavior. To precisely isolate how the intrinsic molecular stability and chromophore structure of each pollutant influence degradation macro-kinetics, the experimental parameters were deliberately fixed at a baseline of 0.5 g/L catalyst dosage and 10 ppm initial dye concentration. Furthermore, XPS analysis was employed to investigate surface defect states, particularly oxygen vacancies, and their possible role in photocatalytic activity.

This study aims to provide insight into the relationship between material properties and photocatalytic performance under realistic conditions, while also highlighting the potential of ZnO nanoparticles for environmental remediation. However, because specific reactive oxygen species (ROS) trapping via scavenger studies and full parameter optimization sweeps were not within the current scope, further investigations involving detailed mechanistic trapping and real wastewater systems are required to fully establish practical applicability.

Experimental details

Materials and synthesis

Zn nitrate hexahydrate (Zn(NO₃)₂·6 H₂O, 99.99% purity) and ammonium hydroxide (NH₄OH, 99% purity) were both purchased from Sigma-Aldrich.

Characterization techniques

The synthesized ZnO nanoparticles were characterized using various analytical techniques to investigate their structural, morphological, optical, and surface properties. The phase composition and crystal structure were examined using XRD with a RIGAKU Ultima IV diffractometer operated at 40 kV and 200 mA using Cu Kα radiation (λ = 1.5406 Å). Raman spectroscopy was employed to further analyze the vibrational properties and structural characteristics of the synthesized material. The morphology and particle size distribution of the nanoparticles were investigated using TEM with a JEOL JEM 2010 electron microscope operated at 200 kV. Surface chemical composition and elemental states were analyzed by XPS using an Omicron ultra-high vacuum system (ESCA 2000–125). UV–Visible absorption measurements for photocatalytic degradation studies were recorded using a Shimadzu UV-1900i spectrophotometer.

Preparation of the synthesis of pure ZnO nanoparticles by the co-precipitation method

First, Zn nitrate hexahydrate (Zn(NO₃)₂·6 H₂O) was weighed according to the required calculations and dissolved in 40 mL of double-distilled water. The solution was stirred at room temperature for about 30 min. After that, the pH was checked, and ammonium hydroxide (NH₄OH) was added dropwise until the pH reached around 10. Then, the temperature was increased to 60 °C, and the solution was kept under continuous stirring for 2 h. After the reaction, the sample was centrifuged at 9000 rpm for 5 min and washed several times: first with double-distilled water, then with ethanol, and finally with double-distilled water to remove any remaining impurities.

The obtained product was then dried in an oven at 90 °C for 12 h. After drying, it was calcined in a muffle furnace at 600 °C for 2 h. Finally, the sample was ground using a mortar and pestle to obtain the ZnO nanoparticles (Fig. 1).

Fig. 1.

Schematic illustration of the synthesis of ZnO nanoparticles by the co-precipitation method. Zinc nitrate hexahydrate was dissolved in 40 mL of double-distilled water and stirred continuously for 30 min, followed by dropwise addition of NH₄OH to induce precipitation. The precipitate was collected by centrifugation at 9000 rpm, dried at 90 °C for 12 h in a hot air oven, and finally calcined at 600 °C for 2 h to obtain the ZnO nanoparticles

Results and discussions

XRD analysis

The phase composition and crystallinity of the synthesized ZnO nanoparticles were confirmed by XRD analysis, as presented in Fig. 2. The diffraction peaks located at 2θ values of 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.8°, 66.3°, 68.0°, 69.2°, 72.6°, and 77.0° correspond to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (204), and (202) planes, respectively. These peaks are in excellent agreement with the hexagonal wurtzite ZnO structure (space group: P6₃mc, JCPDS Card No. 36-1451) [28]. The absence of secondary phases confirms the high phase purity of the synthesized material. The high intensity and sharpness of the diffraction peaks reflect the good crystallinity of the ZnO nanoparticles, with the (101) plane showing the highest intensity, indicating a preferred orientation along this plane. The average crystallite size of the ZnO nanoparticles was estimated to be approximately 46.49 nm using the Debye–Scherrer equation applied to the most intense XRD peak (101). These findings are consistent with previous reports on the crystalline structure of ZnO nanoparticles synthesized via wet chemical routes [29].

Fig. 2.

XRD patterns of pure ZnO nanoparticles synthesized by the co-precipitation method. All diffraction peaks are indexed to the hexagonal wurtzite structure (JCPDS Card No. 36-1451), confirming phase purity and good crystallinity with no detectable impurity phases

Raman analysis

The structural quality and vibrational characteristics of the synthesized ZnO nanoparticles were thoroughly investigated through Raman spectroscopy, as illustrated in Fig. 3. The recorded Raman spectrum reveals distinct and well-resolved peaks at approximately 206, 329, 380, 435, and 577 cm⁻¹. These peaks correspond to the vibrational modes of hexagonal wurtzite ZnO, specifically the E₂(low), E₂(high)–E₂(low) (a multi-phonon scattering process), A₁(TO), E₂(high), and A₁(LO) modes, respectively. Among these, the most prominent peak observed at ~ 435 cm⁻¹ is assigned to the E₂(high) mode, which is associated with the vibration of oxygen atoms in the ZnO lattice. The sharpness and intensity of this peak are indicative of the excellent crystallinity and long-range order of the ZnO structure.

Fig. 3.

Raman spectra of pure ZnO nanoparticles synthesized by the co-precipitation method. The dominant E₂(high) mode at ~ 438 cm⁻¹ confirms the hexagonal wurtzite crystal structure, consistent with phase-pure ZnO

The peak at 577 cm⁻¹, corresponding to the A₁(LO) mode, typically arises due to longitudinal optical phonon modes that become active in the presence of lattice imperfections or point defects such as oxygen vacancies and Zn interstitials. These intrinsic defects play a significant role in tuning the electronic structure and enhancing the photocatalytic performance of ZnO by facilitating charge separation and extending light absorption. The absence of extraneous peaks in the spectrum further confirms the sample’s phase purity, ruling out secondary phases or impurity-related vibrational modes. Overall, the Raman spectral features are in excellent agreement with the established vibrational fingerprints of wurtzite ZnO, reinforcing the structural integrity and defect-controlled nature of the synthesized material [29, 30].

XPS analysis

XPS analysis was carried out to investigate the surface chemical composition and electronic states of the synthesized ZnO nanoparticles. The survey spectrum, Fig. 4a, confirms the presence of Zn, O, and C, with no detectable impurity peaks, indicating the material’s high purity. The high-resolution Zn 2p spectrum Fig. 4d, exhibits two prominent peaks at binding energies of approximately 1022.49 eV and 1045.61 eV, corresponding to Zn 2p₃/₂ and Zn 2p₁/₂, respectively. The energy separation between these peaks (~ 23.12 eV) is consistent with the characteristic spin–orbit splitting of Zn²⁺ in ZnO, confirming the oxidation state of Zn in the synthesized nanoparticles.

Fig. 4.

X-ray photoelectron spectroscopy (XPS) analysis of pure ZnO nanoparticles synthesized by the co-precipitation method. a XPS survey spectrum confirming the presence of Zn, O, and C elements with characteristic peaks of Zn 2p₃/₂, Zn 2p₁/₂, O 1s, and C 1s. b High-resolution O 1s spectrum deconvoluted into two components: lattice oxygen at 530.9 eV and surface oxygen (hydroxyl/defect-related) at 532.08 eV. c High-resolution C 1s spectrum deconvoluted into three peaks: 285.69 eV assigned to adventitious C–C/C–H carbon from surface contamination, 287.21 eV attributed to C–O groups, and 289.58 eV corresponding to O–C=O species. d High-resolution Zn 2p spectrum showing two spin-orbit split peaks at 1022.49 eV (Zn 2p₃/₂) and 1045.61 eV (Zn 2p₁/₂) with a spin-orbit splitting of 23.12 eV, confirming the Zn²⁺ oxidation state characteristic of stoichiometric ZnO

The O 1s spectrum, Fig. 4b, can be deconvoluted into two distinct components. The peak centered at ~ 530.9 eV is attributed to lattice oxygen (O²⁻) in the ZnO crystal structure, while the higher binding energy peak at ~ 532.08 eV is associated with surface oxygen species and oxygen vacancy-related defect states. The presence of this higher energy component suggests the existence of oxygen vacancies or loosely bound oxygen on the surface of ZnO nanoparticles.

The C 1s spectrum Fig. 4c, shows a dominant peak at ~ 285.9 eV, which is typically assigned to adventitious carbon (C–C/C–H bonds) originating from atmospheric exposure during sample handling and is commonly used as a reference for charge correction. Deconvolution of the spectrum reveals two additional peaks at 287.21 eV and 289.58 eV. The peak at 287.21 eV is attributed to carbonyl carbon (C=O), while the higher binding energy peak at 289.58 eV corresponds to carboxylate or carbonate species (O–C=O). The presence of these surface oxygen-containing species, alongside oxygen-vacancy-related defect states, is expected to heavily influence the photocatalytic behavior of the ZnO nanoparticles. These vacancies serve as trapping sites for photogenerated charge carriers, thereby reducing electron–hole recombination. This delayed recombination mechanism likely plays a crucial role in the observed photocatalytic activity under natural sunlight conditions. However, further studies using complementary techniques would be required to quantitatively assess defect density and charge carrier dynamics.

TEM analysis

The morphology and particle size distribution of the synthesized ZnO nanoparticles were analyzed using TEM, as shown in Figs. 5 and 6. The TEM images reveal that the ZnO nanoparticles predominantly exhibit polyhedral shapes with slight irregularities. The particles appear relatively well-dispersed, with limited aggregation, and show reasonably defined edges. Although the particles display a relatively narrow size distribution, some variation in particle dimensions is observed. Based on the particle size distribution histogram, the average particle size (D₀) was estimated to be approximately 109.15 nm, with a standard deviation (σ) of 38.56 nm.

Fig. 5.

TEM images of pure ZnO nanoparticles synthesized by the co-precipitation method at increasing magnifications: a 200 nm, b 100 nm, c 50 nm, and d 20 nm scale bars. The images reveal predominantly quasi-spherical and irregular polygonal morphology with some particle agglomeration

Fig. 6.

Particle size distribution histogram of pure ZnO nanoparticles (ZnO-CP) synthesized by the co-precipitation method, fitted with a Gaussian curve. The average particle size is 109.15 nm with a standard deviation of 38.56 nm

It is important to note that the particle size obtained from TEM is larger than the crystallite size (~ 46.49 nm) calculated from XRD analysis. This difference suggests that individual particles observed in TEM may consist of multiple smaller crystallites, indicating a polycrystalline or aggregated nature. The relatively larger particle size indicates that surface area alone may not be the dominant factor influencing photocatalytic performance. In this context, XPS analysis confirms the presence of oxygen vacancy-related defect states, which may enhance charge carrier separation by reducing electron–hole recombination, thereby contributing to the observed photocatalytic activity. Overall, the TEM and XRD results together confirm that the co-precipitation method successfully produces nanocrystalline ZnO with controlled morphology and structural characteristics [29, 31].

SAED pattern

The crystallinity and phase purity of the synthesized ZnO nanoparticles were further verified through selected area electron diffraction (SAED) analysis, as illustrated in Fig. 7. The SAED pattern displayed a series of sharp, continuous diffraction rings, characteristic of a polycrystalline structure. These rings were successfully indexed to the (100), (101), (102), (103), and (200) planes of the hexagonal wurtzite phase of ZnO, aligning well with the XRD results (JCPDS Card No. 36-1451). The clarity and definition of the rings indicated high crystallinity and a fine crystallite size in the material. Significantly, no additional diffraction features related to secondary phases or impurities were observed, reinforcing the conclusion that the synthesized ZnO nanoparticles possess excellent phase purity. These observations are in good agreement with previous reports on ZnO nanostructures synthesized by wet chemical methods [29, 31].

Fig. 7.

SAED pattern of pure ZnO nanoparticles synthesized by the co-precipitation method. Concentric diffraction rings indexed to the (100), (101), (102), (103), and (200) planes of hexagonal wurtzite ZnO confirm the polycrystalline nature of the sample

Photocatalytic activity of pure ZnO nanoparticles

The photocatalytic activity of the synthesized ZnO nanoparticles was evaluated by investigating the degradation of three organic dyes: (MB), (RhB), and (CV) under natural sunlight irradiation. These dyes are commonly used as model organic pollutants in photocatalytic studies due to their distinct absorption characteristics and well-known degradation behavior.

For the photocatalytic experiments, aqueous dye solutions (10 ppm) were prepared using double-distilled water. A fixed amount of ZnO nanoparticles (0.5 g/L) was added to each dye solution as the photocatalyst. The reaction mixtures were then exposed to direct sunlight under continuous stirring using a wrist-action shaker to maintain uniform dispersion and reaction homogeneity throughout the experiment, as shown in Fig. 8.

Fig. 8.

Photocatalytic degradation setup for methylene blue (MB), rhodamine B (RhB), and crystal violet (CV) dyes using ZnO nanoparticles under direct sunlight irradiation. Dye concentration: 10 mg/L; catalyst loading: 300 mg/L

ZnO nanoparticles for the photocatalytic degradation of (MB) dye

The photocatalytic performance of the synthesized ZnO nanoparticles prepared via the co-precipitation method was evaluated through the degradation of (MB) under natural sunlight irradiation over 160 min, as shown in Fig. 9. The UV–Visible absorption spectra exhibit a gradual decrease in the characteristic absorption peak of MB at 664 nm, indicating progressive dye degradation during solar irradiation. Approximately 98% degradation efficiency was achieved after 160 min.

Fig. 9.

Photocatalytic degradation of methylene blue (MB) dye using pure ZnO nanoparticles (ZCP) synthesized by the co-precipitation method under direct sunlight irradiation. a Photograph showing the visual decolorization of MB solution at irradiation intervals of 0–160 min; b schematic illustration of the photocatalytic experimental setup; c UV-Vis absorption spectra recorded at 20 min intervals showing progressive decrease in the characteristic MB absorption peak; d degradation efficiency (%) vs. irradiation time, reaching a maximum of 98% after 160 min; e linear fit of ln(C/C₀) vs. irradiation time confirming pseudo-first-order kinetics with a rate constant of k = 0.0247 min⁻¹ (R² = 0.9445); f C/C₀ vs. irradiation time showing continuous reduction in MB concentration over 160 min

The photocatalytic degradation kinetics were analyzed using a pseudo-first-order model. The plot of ln(C₀/C) versus irradiation time shows good linearity (R² = 0.94), with an apparent rate constant (k_app) of 0.025 min⁻¹. A relatively rapid degradation was observed during the initial 60 min of irradiation, followed by a slower degradation rate at longer irradiation times. This behavior may be associated with a decrease in the available active dye concentration and progressive occupation of surface-active sites during the reaction process. The observed photocatalytic activity may be related to the good crystallinity of ZnO and the presence of oxygen vacancy-related defect states identified through XPS analysis, which can influence charge carrier dynamics and photocatalytic behavior under natural sunlight conditions.

ZnO nanoparticles for photocatalytic degradation of (RhB) dye

The photocatalytic activity of the synthesized pure ZnO was thoroughly assessed for the degradation of (RhB) dye under natural sunlight irradiation. The temporal evolution of the degradation process was monitored by recording UV–Visible absorption spectra at regular intervals. A notable decrease in the characteristic absorption peak of RhB centered around 554 nm was observed with increasing exposure time, signifying the progressive breakdown of the dye molecules. Pure ZnO demonstrated remarkable photocatalytic efficiency, achieving a degradation of approximately 93% within 160 min of sunlight exposure, as shown in Fig. 10.

Fig. 10.

Photocatalytic degradation of Rhodamine Bar (RhB) dye using pure ZnO nanoparticles synthesized by the co-precipitation method under direct sunlight irradiation. a Photograph showing the visual decolorization of RhB solution at irradiation intervals of 0–160 min; b schematic illustration of the photocatalytic experimental setup; c UV-Vis absorption spectra recorded at 20 min intervals showing progressive decrease in the characteristic RhB absorption peak; d degradation efficiency (%) vs. irradiation time, reaching a maximum of 93% after 160 min; e linear fit of ln(C/C₀) vs. irradiation time confirming pseudo-first-order kinetics with a rate constant of k = 0.0149 min⁻¹ (R² = 0.9095); f C/C₀ vs. irradiation time showing continuous reduction in RhB concentration over 160 min

The degradation efficiency (η) was quantified using the following expression:

1

where C₀ is the initial concentration of RhB and C_t is the concentration at time t. A significant decrease in the ratio C_t/C₀ with increasing time indicated enhanced photodegradation performance.

To understand the reaction kinetics, the photodegradation data were fitted to the pseudo-first-order kinetic model, given by:

where k_app is the apparent rate constant (min⁻¹), and t is the irradiation time (min). The linear fit to this equation yielded a rate constant k_app = 0.0175 ^min−1 with a correlation coefficient R² ≈ 0.9095, suggesting that the degradation followed first-order kinetics to RhB concentration.

2

Additionally, the Langmuir–Hinshelwood (L–H) model, often employed to describe heterogeneous photocatalysis, can also be applied to explain the degradation behavior further [32–34].

3

where r is the degradation rate, k is the surface reaction rate constant, K is the adsorption equilibrium constant, and C is the dye concentration. Under the condition of low dye concentration (KC≪1), the equation simplifies to:

r = kappC 4

This supports the application of the pseudo-first-order model used in this study. The high degradation efficiency and kinetic performance of pure ZnO highlight its potential as an efficient photocatalyst for the removal of organic pollutants like (RhB) under sustainable sunlight irradiation.

ZnO nanoparticles for the photocatalytic degradation of (CV) dye

The photocatalytic degradation of (CV) dye was investigated using ZnO by the Co-Precipitation method under sunlight irradiation. The time-dependent UV–Visible absorption spectra revealed a steady decrease in the absorption peak intensity at around ~ 583 nm, signifying efficient photocatalytic degradation. ZnO-CP demonstrated a high degradation efficiency of approximately 96% within 160 min. The kinetic study fitted well with a pseudo-first-order model, providing an apparent rate constant (kₐₚₚ) of 0.0187 min¹ with a strong linear correlation (R² ≈ 0.91). The variation of C/C₀ and degradation percentage with irradiation time confirmed the remarkable photocatalytic activity of ZnO-CP towards CV dye under solar light, as shown in Fig. 11.

Fig. 11.

Photocatalytic degradation of Crystal Violet (CV) dye using pure ZnO nanoparticles synthesized by the co-precipitation method under direct sunlight irradiation. a Photograph showing the visual decolorization of CV solution at irradiation intervals of 0–160 min; b schematic illustration of the photocatalytic experimental setup; c UV-Vis absorption spectra recorded at 20 min intervals showing progressive decrease in the characteristic CV absorption peak; d degradation efficiency (%) vs. irradiation time, reaching a maximum of 96% after 160 min; e linear fit of ln(C/C₀) vs. irradiation time confirming pseudo-first-order kinetics with a rate constant of k = 0.0187 min⁻¹ (R² = 0.9163); f C/C₀ vs. irradiation time showing continuous reduction in CV concentration over 160 min

Comparative photocatalytic degradation of three dyes (MB), (RhB), and (CV)

Figure 12 presents the comparative photocatalytic degradation performance of ZnO nanoparticles against three model organic dyes: (MB), (RhB), and (CV) under natural sunlight irradiation. A gradual increase in degradation efficiency was observed with increasing irradiation time for all dyes.

Fig. 12.

The combined photocatalytic degradation efficiency of Methylene Blue (MB), Rhodamine B (RhB), and Crystal Violet (CV) dyes under direct sunlight using the ZnO photocatalyst was assessed over 160 min

After 160 min of solar irradiation, the ZnO nanoparticles achieved degradation efficiencies of approximately 98% for MB, 93% for RhB, and 96% for CV. During the initial stage of irradiation, CV showed a relatively faster degradation rate compared to MB and RhB, which may be associated with differences in molecular structure, adsorption behavior, and interaction with the photocatalyst surface. The overall degradation trend followed the order: MB > CV > RhB. The differences in degradation efficiency among the dyes may be related to variations in molecular stability, chromophore structure, and photocatalytic reaction pathways.

These results indicate that the synthesized ZnO nanoparticles exhibit effective photocatalytic activity toward different classes of organic dyes under natural sunlight conditions. However, further investigations involving real wastewater systems and long-term stability studies are required to fully assess their practical applicability (Table 1).

Table 1.

Comparison of recent reports on Methylene blue, Rhodamine, and Crystal Violet dyes degradation using Zinc Oxide nanoparticles

Year	Photocatalytic system	Photocatalytic dye	Degradation (%)	Light source	References
2024	GO/g-C3N4/ZnO ternary composite	Methylene Blue	90%	Visible light	ACS Omega. [35]
2025	Pristine ZnO nanoparticles	Methylene Blue	90%	Visible light	Scientific reports. [36]
2024	Pure ZnO nanoparticles	Methylene Blue	72.3%	UV Light	Wiley Aiche Journal. [37]
2023	N-doped ZnO and N-ZnO/carbon dot nanocomposites	Methylene Blue	83.4%	UV Light	ACS Omega. [38]
2024	Pure ZnO nanoflakes	Rhodamine B	72.3%	UV light	ACS Omega. [39]
2024	ZnO nanoparticle thin films	Methylene Blue	74%	UV laser and UV light	Scientific reports. [40]
2024	ZnO nanoparticles synthesized with ethanol solvent	Methylene Blue	95–98%	UV light,	RSC Advances. [41]
2024	Nano-ZnO synthesized by zinc acetate	Methylene Blue	98%	UV irradiation,	PMC. [42]
2024	Pristine ZnO nanoparticles	Rhodamine dye	80%	Visible light LED	Scientific reports. [43]
2025	Pure ZnO NPs	Methylene blue, Rhodamine, and Crystal Violet dyes	MB- (98%) RhB- (93%) CV- (96%)	Direct Sunlight	This Work

Photocatalytic degradation mechanism

Under natural sunlight irradiation, ZnO nanoparticles absorb photons with energy sufficient to excite electrons from the valence band (VB) to the conduction band (CB), generating electron–hole (e⁻/h⁺) pairs. The photogenerated electrons react with dissolved oxygen molecules to produce superoxide radicals (•O₂⁻), while the holes react with water molecules or hydroxide ions to generate hydroxyl radicals (•OH).

These reactive oxygen species play a significant role in the degradation of organic dye molecules such as (MB), (RhB), and (CV), ultimately leading to their decomposition into smaller intermediate products.

In addition, oxygen vacancy-related defect states identified through XPS analysis may contribute to improved charge carrier separation by reducing electron–hole recombination, thereby enhancing the photocatalytic activity of ZnO under sunlight irradiation.

Conclusion

In this work, pure ZnO nanoparticles were successfully synthesized using a co-precipitation method and characterized to evaluate their structural and photocatalytic properties. XRD analysis confirmed the formation of hexagonal wurtzite ZnO with good crystallinity and phase purity, while Raman spectroscopy further supported the structural integrity of the material. TEM revealed polyhedral-shaped nanoparticles with some size variation, and SAED patterns confirmed their polycrystalline nature.

The photocatalytic performance of the synthesized ZnO nanoparticles was investigated using (MB), (RhB), and (CV) under natural sunlight. UV–Visible analysis showed a gradual decrease in dye concentration, following pseudo-first-order kinetics. The degradation efficiencies reached approximately 98% for MB, 93% for RhB, and 96% for CV within 160 min of irradiation. XPS analysis suggests the presence of oxygen vacancy-related defect states, which may contribute to improved photocatalytic performance by influencing charge carrier dynamics. These findings indicate that ZnO nanoparticles synthesized via a simple co-precipitation route can effectively facilitate dye degradation under solar irradiation. However, this study is limited to model dye systems, and additional investigations, such as reusability studies, real wastewater analysis, and detailed mechanistic and optical characterization, are required to fully establish the practical applicability of the material.

Author contributions

Adil Hamid: Conceptualization, synthesis of ZnO nanoparticles, structural characterization (XRD, Raman, TEM, SAED), photocatalytic experiments, data analysis, and writing the original draft.Mohit Sahni: Supervision, guidance on experimental design, validation of results, and reviewing and editing the manuscript.

Funding

Open access funding provided by Sharda University. This research was supported by internal funding from Sharda University.

Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

This article does not contain any studies involving human participants or animals performed by any of the authors.

Consent to participate

Not applicable.

Consent to publish

Not applicable.