Enhanced Photocatalytic Activities of F Doped Zinc Oxide Nanoparticles by Coprecipitation Method

Zinc oxide nanoparticles (ZnO NPs) have gained significant attention due to their unique properties such as high surface area, tunable band gap, and excellent catalytic and electrochemical activity. The textile business produces a significant amount of organic dyes each year, which has detrimental effects on the environment. Doping of ZnO NPs is often favoured over undoped configurations due to the synergistic effects of piezoelectric, optical, electrical, magnetic, and photocatalytic properties that arise from the incorporation of metals or non-metals [1]. Fluoride (F⁻) serves as a non-metallic dopant in the ZnO structure owing to its ionic radius (1.36 Å), which allows potential substitution at oxygen sites (ionic radius 1.40 Å) or occupancy of oxygen vacancies within the ZnO lattice [2, 3]. A significant increase in oxygen vacancies in the ZnO lattice is achieved by substituting F⁻ for O. Doping of ZnO NPs with F can enhance their photocatalytic efficiency by modifying their electronic band structure, surface properties, and charge carrier dynamics, thereby improving their ability to generate reactive oxygen species and degrade organic pollutants under visible light [4]. Thus, F doped ZnO NPs were synthesized through the co-precipitation method for addressing environmental challenges.

In this investigation, F doped ZnO nanoparticles were synthesized by the co-precipitation method. Zinc acetate (0.4 M) was added to 0.2 M NaOH solution and 5 wt% NH₄F with constant stirring. The solution was stirred well at 80 °C until the precipitate was formed. Then it was filtered and washed continuously using distilled water. To dry the precipitate, it was then kept in an oven for 2 h at 120 °C. Finally, it was annealed at 500 °C for 4 h to enhance the crystallinity.

Figure 1(a) depicts the XRD pattern of F doped ZnO nanoparticles. It shows a polycrystalline hexagonal wurtzite structure (JCPDS card no. 36-1451). The obtained peaks correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 1) planes, confirming the purity of the samples. The crystallite size of the powders is estimated using Scherrer’s formula [5]:

where λ is the wavelength used, β is the full width at half maximum and θ is the Bragg angle. The average crystallite size of the sample, calculated using the Debye–Scherrer formula, is 11 nm. The lattice constants a and c are calculated using the formula [5]:

The volume of the unit cell (V) is estimated using the formula [5]:

The calculated lattice constants a and c and the cell volume V are found to be a = 3.231 Å, c = 5.19 Å and V = 47.153 ų. The acquired lattice parameters and cell volume are comparable to the standard values (a = 3.2498 Å, c = 5.20661 Å and V = 47.62 ų) of JCPDS card no. 36-1451. The observed lattice constants and cell volume decrease with respect to the bulk values, which is strong supporting evidence for the progressive increase in the substitutional incorporation of F⁻ in the ZnO lattice sites.

Figure 1(b) shows the UV-visible spectrum of F doped ZnO nanoparticles. ZnO is shown to absorb light at a wavelength of 377 nm. The measured band gap energy of F doped ZnO nanoparticles (3.41 eV) showed a blue shift of approximately 0.08 eV in comparison with the bulk band gap energy of 3.33 eV [6]. The inclusion of F is confirmed by the blue shift in wavelength in F doped ZnO nanoparticles [5].

The photocatalytic degradation of congo red (CR), rhodamine B (Rh B), and methylene blue (MB) dyes in the presence of F doped ZnO nanoparticles under the illumination of an ultraviolet (UV) light simulator was conducted for 160, 180, and 140 minutes, respectively. The nanocatalyst, F doped ZnO nanoparticles, demonstrated significant photocatalytic activity, as evidenced by the decrease in dye absorption. With increasing UV radiation exposure duration, the concentrations of CR, Rh B, and MB dyes gradually decreased. Figure 2(a) and (b) illustrate the degradation efficiency and rates of CR, Rh B, and MB dyes over time under UV light using these photocatalysts. The F doped ZnO catalyst demonstrates optimal degradation efficiency, resulting in 96% degradation of the MB dye in 140 min. According to Figure 2, MB demonstrates higher degradation efficiency than CR and Rh B due to differences in their physicochemical characteristics and structural composition. The maximum degradation efficiency can be ascribed to the introduction of impurity levels closer to the conduction band, which causes ZnO’s band gap to contract. Therefore, electrons can be excited from the valence band to the conduction band or the impurity level with a modest amount of energy, which means that the ability of ZnO sites to absorb UV light may have improved. Furthermore, F⁻ ions, which can act as electron trapping sites, promote charge separation by preventing the recombination of photogenerated charges, which improves the degradation efficiency.

F doped ZnO nanoparticles have been successfully prepared by the co-precipitation method. The influence of F doping on the optical and structural properties of ZnO nanoparticles was investigated. XRD shows that the crystallite size decreases with increasing F content. The photocatalytic activity was studied using CR, Rh B, and MB dyes under UV light. The photocatalytic decomposition of CR, Rh B, and MB dyes follows first-order reaction kinetics. The F doped ZnO catalyst demonstrates optimal degradation efficiency, resulting in 96% degradation of the MB dye in 140 min.