Photocatalytic degradations of organic pollutants in wastewater using hydrothermally grown ZnO nanoparticles

Materials

Zinc chloride (

Preparation of ZnO nanoparticles

Zinc oxide nanoparticles (ZnO NPs) were synthesized using a simple hydrothermal process followed by calcination. In a typical synthesis, 1.39 g of \mathrm{ZnCl_{2}}ZnCl2 and 1.42 g of HMTA were dissolved in 70 mL of deionized water under continuous stirring for 30 minutes to ensure complete dissolution. Subsequently, 5 mL of ammonia solution was added to adjust the pH of the solution, and stirring was maintained. The resulting solution was transferred to a Teflon-lined stainless-steel autoclave with a 100 mL capacity and heated at 100^\circ∘C for varying durations in an oven. After the reaction, a white precipitate was separated from the solution by vacuum filtration and dried at 60^\circ∘C for 4 hours to remove residual solvent. The dried precipitate was calcined at 500^\circ∘C for 2 hours at a heating rate of 5^\circ∘C/min to enhance the crystallinity of the ZnO nanoparticles. The resulting nanopowder was used for further characterization and photocatalytic studies. hree different samples were fabricated by varying the hydrothermal reaction time (i.e., 4 h, 6 h, and 8 h), and the corresponding ZnO samples were named ZnO-4h, ZnO-6h, and ZnO-8h, respectively.

Materials characterizations

The structural, morphological, and optical properties of the ZnO nanoparticles were characterized using various techniques. The crystal structure, crystallinity, and crystallite size were analyzed by X-ray diffraction (XRD) using Cu-K\alphaα radiation (wavelength = 0.154 nm) in the 2\thetaθ range of 5^\circ∘ to 80^\circ∘ using XRD, Rigaku, SmartLab SE basic. The morphological structure of the ZnO nanoparticles was examined using a scanning electron microscope (SEM), and their elemental composition was determined by energy-dispersive spectroscopy (EDS) coupled with SEM (SEM-EDS, JEOL JSM-6510). Optical absorption spectra were recorded in the wavelength range of 200–800 nm using a UV-Vis spectrophotometer (Shimadzu UV-1280).

Photocatalytic investigations

The photocatalytic performance of ZnO was evaluated by monitoring the degradation of methylene blue (MB) under dark and UV light irradiation. The experiments were conducted in a photoreactor chamber equipped with four Phillips UVC lamps (10 W, \lambdaλ\approx≈ 253.7 nm). For each experiment, 100 mL of aqueous MB solution (5 ppm concentration) was prepared, and 0.1 g of ZnO nanopowder was dispersed in the solution. The mixture was stirred at 250 rpm using a magnetic stirrer inside the photoreactor. To study adsorption and degradation, the experiment was conducted both in the dark and under UV light. Initially, the suspension was stirred in the dark for 1 hour to achieve adsorption equilibrium, and a 3 mL sample was withdrawn for measurement. Afterward, the suspension was exposed to UV irradiation for 5 hours, with 3 mL aliquots taken every hour. The absorbance of the MB solution at its maximum wavelength (\lambdaλ = 663 nm) was measured using a UV-Vis spectrophotometer (Shimadzu UV-1280). The concentration of MB was determined from the absorbance, and the degradation rate (k) was calculated using the Eq.1.

ZnO characteristics

The hydrothermally synthesized ZnO nanoparticles were systematically characterized and evaluated for their photocatalytic potential in degrading organic pollutants in wastewater. Figure 1 shows the SEM-EDS analysis of the ZnO prepared using various hydrothermal temperatures. SEM imaging revealed insignificant changes in morphology with increasing synthesis duration. ZnO-4h Fig. (1a) consisted of small, irregularly shaped particles, indicative of incomplete crystal growth. With extended synthesis times, ZnO-6h Fig. (1b) and ZnO-8h Fig. (1c) displayed progressively refined morphologies, culminating in the rod-like structures observed in ZnO-8h. The EDS analysis (shown in right side of each sample) confirmed the predominance of Zn and O across all samples, consistent with ZnO formation. It also shows that distinct variations in the Zn:O atomic ratios across the samples synthesized at different durations. ZnO-4h exhibited a Zn:O ratio of 1.05, indicating near-stoichiometric ZnO with minimal deviation from the ideal 1:1 ratio. This suggests that even with a shorter synthesis time, the hydrothermal process was effective in forming ZnO with relatively balanced stoichiometry. ZnO-6h displayed a Zn:O ratio of 0.92, reflecting a slightly oxygen-rich composition. This may be indicative of a higher concentration of oxygen-related surface defects, which are characteristic of intermediate synthesis times, where the material undergoes further crystal growth and atomic reordering 19,20. Meanwhile, ZnO-8h demonstrated a Zn:O ratio of 1.13, signifying a zinc-rich composition likely due to enhanced atomic ordering and reduced oxygen-related defects as a result of prolonged synthesis. The increasing Zn:O ratio with synthesis duration reflects the gradual refinement in material stoichiometry and the diminishing presence of defects associated with oxygen excess.

The XRD patterns of ZnO nanoparticles (see 2 ) synthesized under hydrothermal conditions for 4, 6, and 8 hours exhibit characteristic peaks at 2\thetaθ values corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), and (201), planes of the hexagonal wurtzite structure of ZnO. The observed 2\thetaθ values for these planes are approximately 31.8^\circ∘ (100), 34.4^\circ∘ (002), 36.3^\circ∘ (101), 47.5^\circ∘ (102), 56.6^\circ∘ (110), 62.8^\circ∘ (103), 66.4^\circ∘ (200), 68.0^\circ∘ (112), and 69.1^\circ∘ (201). These peaks match well with the standard JCPDS card (PDF\##36-1451), confirming the phase purity and successful formation of ZnO in all samples 21,22. As the synthesis duration increases, the intensity of the diffraction peaks becomes sharper and more pronounced, particularly for the ZnO-8h sample. This trend is corroborated by the crystallinity analysis, which shows a slight increase in the crystallinity percentage with extended synthesis times. The refined crystallinity values indicate 64.13%, 62.92%, and 64.51% for ZnO-4h, ZnO-6h, and ZnO-8h, respectively. The ZnO-8h sample exhibits the highest crystallinity, reflecting its superior atomic ordering and reduced defect density due to prolonged hydrothermal treatment.

The XRD results were further analyzed by calculating the crystallite sizes of ZnO nanoparticles using the Scherrer equation, which is based on the broadening of the diffraction peaks. The calculated crystallite sizes for ZnO-4h, ZnO-6h, and ZnO-8h are 79.6 nm, 92.8 nm, and 79.6 nm, respectively 23. The larger crystallite size observed for ZnO-6h suggests a temporary growth in particle dimensions during intermediate synthesis durations, while the comparable sizes of ZnO-4h and ZnO-8h indicate a stabilization in crystallite size with prolonged hydrothermal treatment. This observation aligns with the changes in peak sharpness and intensity, further highlighting the influence of hydrothermal reaction time on the structural properties of the ZnO nanoparticles.

ZnO photocatalytic degradations performance

The photocatalytic performance of ZnO nanoparticles synthesized for 4, 6, and 8 hours was assessed by monitoring the degradation of an organic pollutant under UV illumination. The UV-Vis absorbance spectra (Figures 3a–c) show a progressive decrease in the intensity of the characteristic pollutant absorption peaks, indicating effective degradation over time. All three ZnO samples demonstrated significant photocatalytic activity, with noticeable differences in their degradation efficiencies. The C/C₀ versus time plots (Figure 3d) highlight the temporal changes in pollutant concentration under UV light and dark conditions. Initially, a slight reduction in pollutant concentration was observed during the dark experiment, indicating minimal degradation likely due to adsorption onto the ZnO surface. This suggests that while adsorption equilibrium was largely established before UV exposure, some interactions between the pollutant and the catalyst surface contributed to the minor degradation under dark conditions. Upon UV illumination (Figure 3e), all ZnO samples achieved substantial degradation, with ZnO-6h exhibiting the most rapid and complete removal of the pollutant, followed by ZnO-4h and ZnO-8h. This ranking reflects the interplay of surface area, crystallinity, and defect density in influencing photocatalytic performance.

The pseudo-first-order kinetic analysis (Figure 3f) revealed linear relationships in the ln(C/C₀) versus time plots, confirming that the degradation process follows first-order kinetics. The calculated rate constants (k) were 0.016 min^-1 for ZnO-4h, 0.017 min^-1 for ZnO-6h, and 0.013 min^-1 for ZnO-8h. The highest rate constant observed for ZnO-6h underscores its superior photocatalytic efficiency, attributed to an optimal balance of crystallinity, surface properties, and defect chemistry that enhance charge carrier dynamics and active site availability. The variation in photocatalytic activity across the ZnO samples is closely linked to their structural, morphological, and compositional characteristics. ZnO-6h, with its intermediate morphology and high crystallinity, provides an ideal configuration for photocatalytic reactions. Additionally, the slightly oxygen-rich composition of ZnO-6h, as indicated by the Zn:O atomic ratio of 0.92 from EDS analysis, suggests the presence of oxygen vacancies. These vacancies act as active sites for pollutant degradation and improve charge separation by trapping photogenerated electrons, thereby reducing recombination rates. The superior performance of ZnO-6h relative to ZnO-8h, despite the latter’s higher crystallinity, highlights the critical role of a balanced defect density in providing reactive sites without compromising structural stability. In contrast, ZnO-4h, with a higher surface area but lower crystallinity, demonstrates slightly reduced activity due to limited charge separation efficiency. Overall, the exceptional photocatalytic performance of ZnO-6h is a result of its synergistic combination of structural integrity, sufficient surface area, and the beneficial role of oxygen vacancies in enhancing reactivity.