1. INTRODUCTION
The textile industry is a significant contributor to environmental pollution, particularly in water systems, due to the discharge of untreated waste containing high concentrations of organic and inorganic compounds 1. Among these pollutants, dyes pose a serious environmental challenge as they are difficult to degrade naturally and are harmful to aquatic ecosystems. Pollution that occurs in the textile industry comes from synthetic dyes which are not environmentally friendly and are very dangerous to living creatures, which are toxic and carcinogenic 2. Synthetic dyes have complex aromatic structures that can provide photochemical, thermal, biological, and optical stability 3. This is what causes synthetic dyes to become an environmental problem so organic compounds need to be removed from waste before disposal 2. One of the dyes that is often used is methylene blue 4. Methylene Blue is an aromatic compound that is cationic heterocyclic. The concentration of methylene blue has a permissible threshold value in waters of around (5-10) mg/L 5. Methylene Blue has the molecular formula C16H18ClN3S which can easily dissolve in water 4,5, in terms of price, it is very economical and easy to obtain 3. Methylene blue is a synthetic dye that can be used as a dye for paper, cotton, silk, and wool. In the production process, methylene blue releases some products into the environment to be used as waste which will cause dangerous effects. For example, in humans, it will have an impact on health and cause nausea, diarrheal, shortness of breath, skin irritation, and haemolysis. Thus, it is necessary to remove methylene blue from wastewater 6.
Therefore, a waste processing technology that can accelerate the decomposition of dye waste is needed. One method that is effective, cheap, and easy to use is photodegradation. Photocatalysis is the process of chemical reactions occurring with the help of energy from photons. Here, the photon energy must be greater than the band gap of the semiconductor to generate electrons and holes 7. There are important things about a photocatalyst system, namely the desired band gap, stability, appropriate morphology, and high surface area 8. Many semiconductors have been employed as photocatalytic materials, such as TiO29, ZnO 10,11, Fe2O312, CdS 13, Nb2O514, and CeO215. ZnO is the material most widely used in the photocatalyst process due to its direct band gap of 3.37 eV and high exciton binding energy of 60 MeV. ZnO is also an inorganic semiconductor material that does not have toxic properties and can provide high mobility and good thermal stability.
Nanostructured ZnO can be obtained by various synthesis methods including hydrothermal 16, spray pyrolysis 11, precipitation 17, sol-gel 18, and sputtering 19. Spray pyrolysis is a particle synthesis method that has been studied intensively because of several advantages, such as simplicity, low cost, and faster deposition time 20. Several efforts have been made to modify the performance of ZnO-based photocatalysts via doping 21, annealing 22, heterojunction 23, and making a composite 24,25. Doping is a simple way to improve the optical and electrical properties of ZnO. Several dopant atoms have been successfully applied to the ZnO crystal system, such as Ti 26,27, Sn 28, Cu 29, Fe 30, Ni 31, and Al 32. However, the application of Ni-doped ZnO film for reusable photocatalytic materials is rarely studied. In this paper, the nanostructured films were deposited using the spray pyrolysis method with various Ni concentrations. The best sample in degradation efficiency is then tested to analyze the reusable capability.
2. EXPERIMENT DETAILS
The glass substrate with dimensions of 1x1 cm was washed using acetone, ethanol, and distilled water for 10 minutes (each solution) to remove organic contaminants. At the same time, spray solutions were also prepared. Zinc Acetate Dehydrate (ZAD) and Nickel Chloride Hexahydrate (NCH) precursors were used as the source of Zn (host materials) and Ni (doping). The precursor used is ZAD because it has the highest photocatalysis effectiveness with a hexagonal structure and can produce ZnO particles with a high level of purity 32. The precursor was dissolved in 40 mL of distilled water. To vary the concentration of Ni doping, the mass ratio between ZAD and (NCH) was weighed as presented in Table 1. The estimation of the weight percent of NCH is 0% (ZN-1 sample), 1% (ZN-2 sample), and 3% (ZN-3 sample). After that, the solution was stirred for 60 minutes at 800 rpm at room temperature until it became homogeneous. The homogeneous solution was placed into the spray tank. The commercial spray machine used in this study was an air compressor nebulizer (Elvasense NEB39). The distance between the nozzle and substrate was kept at around 2 cm. The spray rate was 0.19-0.40 mL/min with a 4.01-4.48 µm particle size. Before the spray pyrolysis, the substrate is first heated on a hotplate at 400°C for 10 minutes. This aims to distribute the temperature evenly over the entire surface during the spray process. Deposition time was 15 mins at 400°C.
Furthermore, field-emission scanning electron microscopy (FESEM) was employed to determine the morphology of the film deposited by the spray pyrolysis method. X-ray diffraction (XRD) technique was used to determine the crystallinity of the samples. Ultraviolet-Visible (UV-Vis) spectroscopy was used to determine the optical bandgap of the sample and photocatalytic performance. The photocatalytic activity was analyzed using methylene blue or MB (C16H18ClN3S) with a concentration of 10 ppm as the pollutant sample. The film was soaked in MB solution. It was then exposed to a 50 W halogen lamp for 0, 1, and 5 hours. The distance between the sample and the halogen lamp is 12 cm. After being exposed to the specified time, the degradation results of methylene blue were measured using UV-Vis spectroscopy. To check the reusable photocatalytic performance, the film was immersed in alcohol before the subsequent photocatalytic measurement was performed. This measurement was repeated for 5 cycles.
| Samples | DI water (mL) | ZAD (gr) | NCH (gr) |
|---|---|---|---|
| ZN-1 | 40 | 4.3900 | 0 |
| ZN-2 | 40 | 4.3461 | 0.0439 |
| ZN-3 | 40 | 4.1710 | 0.1317 |
3. RESULTS AND DISCUSSION
3.1 Surface Morphology
Morphological of the film deposited by the spray pyrolysis was then characterized using the FESEM technique, as shown in Figure 1. The surface morphology of the pure ZnO film (ZN-1) is nanorice in the background. At the surface, the particles agglomerate into a flower-like structure with an estimated diameter of around 1.3 μm by sampling 50 points. For the Ni-doped ZnO films (ZN-2 and ZN-3 samples), the morphology changes significantly into nanoridges, while the presence of a flower-like structure cannot be found. Ni doping changes the surface energy of ZnO, which affects the aggregation and growth of particles 33. Ni dopant may reduce the surface energy of non-polar faces of ZnO. This condition enables the nanostructures to grow in the lateral direction and influences how particles agglomerate and grow, shifting from flower-like structures to nanoridges 33. In the ZN-2 sample, the nanoridges structure is observed, while the background structure cannot be observed due to the significant height of the hill. In the ZN-3 sample, the background structure is observed, from nanorice to nanoparticle structure, due to agglomeration. The average particle size of the ZN-3 sample is 1̃.5 μm.
3.2 Structural Properties
Figure 2 shows the diffraction patterns of ZN-1, ZN-2, and ZN-3 samples at 2θ of 10-80°. XRD is important to determine the crystal structure, lattice parameters, and possibility of other phase formations in the film. In the ZN-1 samples, crystalline planes of (100), (002), (101), (112), and (004) are observed at 2θ of 31.50°, 34.11°, 35.91°, 67.64°, and 72.61° respectively. It indicates the formation of hexagonal wurtzite structures (JCPDS no. 89-1397). No peaks contributed from Ni, NiO, or ZnNiO are observed, indicating the success of the inserting of Ni dopant into the ZnO host. In the ZN-2 and ZN-3 samples, the intensity of these planes is reduced significantly, indicating crystal deterioration after inserting the Ni dopant. The Ni2+ ion (0.69 Å) substitutes the Zn2+ sites (0.74 Å) in the ZnO lattice, causing lattice mismatch and inducing strain 33. This strain disrupts the regular periodic arrangement of atoms, reducing the crystallinity and leading to broader and less intense diffraction peaks.
3.3 Optical Properties
UV-Vis Spectrophotometer was employed to determine the optical properties of the samples (Figure 3). Absorbance spectra showed that the highest intensity was obtained in ZN-1 samples in the UV region (300–400 nm). By introducing Ni doping, both ZN-2 and ZN-3 show significantly reduced absorbance intensities. Ni doping can introduce defects or distortions into the ZnO lattice. This disruption may reduce the crystallinity or introduce non-radiative recombination centers, leading to lower optical absorption. On the other hand, the transmission spectra reveal that ZN-1 has the lowest transmission, while ZN-2 and ZN-3 exhibit higher transmission across the UV-Vis region. Higher transmission in Ni-doped samples suggests a decrease in light interaction within the film, possibly due to reduced optical density due to structural defects caused by Ni doping.
Furthermore, the band gap energy of ZN-1, ZN-2, and ZN-3 thin films was obtained by plotting the absorption data using the direct transition equation. The (αhν)² vs (hν) plot, known as the Tauc-plot method 34, is shown in Figure 4. Linear extrapolation of the curve to the zero-absorption line gives the energy band gap for the direct transition. The band gap energy values produced in the samples of ZN-1, ZN-2, and ZN-3 are 3.22 eV, 3.34 eV, and 3.34 eV. A smaller band gap means that electron-hole pairs are easily formed, which directly facilitates the photocatalysis process.
3.4 Photocatalytic Performance
Photocatalytic tests were carried out for 0, 1, and 5 hours in 10 ppm methylene blue solution, as shown in Figure 5. The testing was carried out using a UV-Vis spectrophotometer to determine the change in the absorption spectrum, which directly correlates to the concentration of methylene blue. The decrease in methylene blue concentration is indicated by the decrease in the absorbance spectrum. Photocatalytic activity of the ZN-1, ZN-2, and ZN-3 samples is observed by increasing the illumination time. It is indicated by the reduction of the absorbance intensity for 1 and 5 h illumination. It has resulted from an electron excitation from the valence band to the conduction band. Electrons in the conduction band interact with oxygen so that they help in the electron transfer process to produce superoxide radical ions ·O2−, while methylene blue molecules will bind to holes in the valence band on the surface of the photocatalyst and produce hydroxyl radicals ·OH. This reaction aims to act as a strong oxidizer in the process of oxidizing organic compounds, such as methylene blue. Increasing the illumination time causes an increase in degradation. This is caused by the increasing number of photons hitting ZnO, thereby increasing the formation of hydroxyl radicals as a pollutant oxidizer.
Figure 6(a) shows the intensity of the main absorbance peak for all samples. In general, the increase in the irradiation time decreases the absorbance intensity. The ZN-3 sample shows the lowest absorbance intensity for 1 and 5 hours of irradiation times. This condition leads to the best degradation efficiency obtained from the ZN-3 sample, as shown in Figure 6(b). The degradation efficiency is calculated by
where I0 is the initial intensity (before photocatalysis) and I is the final intensity. The degradation efficiency of 1 hour illumination time is 30% (ZN-1), 60% (ZN-2), and 80% (ZN-3). By increasing the illumination time up to 5 hours, the degradation efficiency is 43% (ZN-1), 76% (ZN-2), and 95% (ZN-3). Although the overall bandgap may slightly increase, Ni doping often introduces mid-gap states or d-d transitions related to Ni ions. These states allow ZnO to absorb a wider spectrum of light, particularly into the visible range. This increased absorption contributes to more effective utilization of the incident light for ROS (reactive oxygen species) generation such as hydroxyl radicals and superoxide anions.
Figure 7 shows the C/C0 and ln C/C0 for various irradiation and doping concentrations. The C/C0 data shows how the pollutant concentration changes over time during photocatalysis. This value decreases by increasing degradation time. The plot of ln C/C0 provides a more quantitative understanding of the reaction mechanism. It determines the reaction kinetics, especially if the process follows pseudo-first-order kinetics. A fitting of the straight line indicates pseudo-first-order reaction kinetics. The slope (m) of the line corresponds to the reaction rate constant (k) as expressed in the following equation
ZnO films prepared via spray pyrolysis are considered excellent candidates for photocatalytic applications, especially for recycling purposes. Figure 8 is the reusable test for the ZN-3 sample for 5-hour irradiation times. The degradation efficiency is 95% (cycle 1), 85% (cycle 2), 72% (cycle 3), 82% (cycle 4), and 82% (cycle 5). The lowest degradation efficiency in cycle 3 may be caused by the cleaning process that only used distilled water rinse. Films deposited via spray pyrolysis adhere well to substrates, ensuring long-term stability during cyclic use. ZnO in the form of a thin film is easy to handle and recover after the reaction. Unlike powders, it does not require filtration or centrifugation, making recycling straightforward. This technique is low-cost, scalable, and allows for uniform deposition over large areas, making it suitable for industrial-scale applications.
4. CONCLUSION
This study demonstrates that Ni doping significantly enhances the photocatalytic performance and reusability of ZnO-based materials. The incorporation of Ni induces a morphological transformation from flower-like structures to nanoridges and reduces crystalline quality, as observed in FESEM and XRD analyses. Despite a slight increase in the bandgap from 3.22 eV to 3.34 eV, the photocatalytic efficiency improves substantially, achieving up to 80% degradation after 1-hour illumination. Furthermore, the materials exhibit excellent stability, maintaining high photocatalytic efficiency even after five recycling cycles. These results highlight the potential of Ni-doped ZnO as a low-cost, reusable, and effective photocatalytic material for environmental applications, paving the way for sustainable water and air purification technologies.