INTRODUCTION

It is necessary to remove dye contaminants from the effluents before discharged into the environment, not only for aesthetic reasons but Environmental problems associated with harmful organic dyes pollutants in water are the driving forces for sustained fundamental and applied research in the area of environmental remediation[1],[2] Dye adversely impacts on biological oxygen demand (BOD) and chemical oxygen demand (COD). Textile dyes and their metabolites act as mutagenic, toxic, and carcinogenic agents. They appear to cross the food chains, leading to biomagnification, so that species have a higher level of contamination at a higher trophic level relative to their prey [3].In recent years, there has been a lot of interest in the heterogeneous photocatalytic destruction of refractory organic water contaminants by semiconductors [4], [5], [6]. Wide-bandgap semiconductor photocatalysts like ZnO and TiO₂ have been shown in earlier research to destroy a variety of organic contaminants when exposed to UV light, which opens up the possibility of eliminating harmful compounds [7], [8]. considerable attempts have been focused on developing photocatalysts that are active in the most abundant part of the solar spectrum, visible light (λ≥420 nm)[9][10]– [12]. This has inspired many researchers to learn more about ZnO’s characteristics in various photocatalytic processes.

Several methods have been reported in the review for the preparation of undoped and doped ZnO nanoparticles like sol-gel[13], solution combustion[14], spray atomization[15], controlled synthesis[16], and hydrothermal [17] Alternatives to these methods for producing ZnO nanoparticles in an environmentally acceptable way include plant extract or plant biomass.

ZnO/NiO nanocomposite from Clitoriaternatea flower extract [18],

ZnFe₂O₄@ZnO nanocomposites using Chrysanthemum spp. floral waste for photocatalytic dye degradation [19],

CuO–ZnO–C nanocomposites using marigold [20],

green synthesis of ZnO nanoparticles by plant biomass [21]

flower-like ZnO structures using Arabic and Karaya Gums [22].

Hibiscus rosasinesis flowers have been demonstrated to have protective effects against the tumor, cancer development, anti-implantation, and anti-spermatogenic properties[23], [24]. It has been found that the leaves and petals encourage hair development and help an ulcer heal[25]. According to a phytochemical investigation, the aqueous extract of Hibiscus rosasinesis flowers contains tannins, saponins, alkaloids, steroids, and flavonoids. These phytochemicals present in the Hibiscus rosasinesis flowers can function as a capping and reducing agent.

However, due to the rapid recombination rate of the photogenerated electron/hole pairs in this technique, the photocatalyst needs to be substantially enhanced before it can be commercialized. Therefore, to increase photocatalytic effectiveness, it is imperative to inhibit the recombination of photogenerated electron-hole pairs in semiconductors [26]. Doping of Nickel which is a good conductor of heat and electricity and loses its metallic character very slowly. The ionic radii of Ni²⁺ (0.69 Å) and Zn²⁺ (0.74 Å) are very close and they have the same valance with the same structure[27] which can be easily replaced by Zn²⁺ in ZnO lattice without changing the hexagonal structure of ZnO.

Based on the aforesaid facts and due to the simplicity, easy approach, versatility, and cost-effectiveness, the co-precipitation method was used for the preparation of Ni/ZnO with the addition of flower extract of Hibiscus rosasinesis which act as a capping agent in the current study. The synthesized nanoparticles were investigated on the structural, optical and photocatalytic properties basis. The photocatalytic activity ofZO, HFZO and HFNZOwas appraised for RA dye under Visible light irradiation.However, it would be interesting to study the time dependent photocatalytic response of HFNZO for the degradation of the RA for the first-time adsorption desorption isotherm has achieved within 15 min using flower extract as a surface modifier.

EXPERIMENTAL

Materials and Method

The entire chemicals were purchased from Merck, India of analytical grade and Fresh flower of Hibiscus rosasinesis were collected from the Theni District. To eliminate salts and other associated pollutants, the freshly harvested Hibiscus rosasinesis flowers were washed and they were then chopped into small pieces. 10 g of flower with 100 ml of double-distilled water are heated at 100°C for 30 minutes. The heated filtrate was filtered through Whatman No. 1 and used for subsequent research which is stored in a refrigerator at 4°C [28], [29].

Using a magnetic stirrer, 5.5 g of zinc acetate dehydrate was dissolved in 50 mL of distilled water, and to the above mixture 5 mL of floral extract was added dropwise. The pH of the solution was then raised to 12 with 0.1 N NaOH for the formation of the precipitate. 0.02 molar solution of NiCl₄ was added to the aforementioned precipitate. The resulting suspension was agitated for two hours. The precipitate of flower extract modified Ni/ZnO (HFNZO) had been washed with distilled water and allowed to air dry. The HFNZO nanoparticle was kept in a hot air oven for an hour. The same process is carried out without the use of NiCl₄ for the preparation of flower extract-modified ZnO. For comparison, undoped ZnO was prepared in the same way without the addition of Ni and flower extract. Finally, the samples were coded by ZO, HFZO, and HFNZO for ZnO, Flower extract-modified ZnO, and flower extract-modified Ni-doped ZnO respectively.

Characterization

UV-VIS light spectra of the synthesized nanoparticles were recorded (l25 spectrophotometer, Perkin Elmer Germany). The same amount of different samples was considered for UV-VIS data analysis. Powder X-ray diffraction (Miniflex 600, Rigaku, Sustin, TX, USA) patterns of the synthesized materials were analyzed in the range of 2q from 20° to 80° using powder diffractometer, Model D8, BRUKER AXS, by Cu Ka radiation (a ¼ 0.15425 nm). The FT-IR studies of the prepared QFW adsorbent were characterized using a JASCO spectrophotometer (JASCO-FT-IR-460 plus) with KBr pelletization in a wide range wavelength ranging from 400 cm^-1 and 4000 cm^-1.  A scanning electron microscope (JEOL, JSM - 6380 A, Tokyo, Japan) was used to image the nanoparticles in both secondary and backscattered electron modes. The elemental analysis was detected by energy-dispersive X-ray spectroscopy (Jeol, JSM - 6380 A, Tokyo, Japan). The particle shape was determined by transmission electron microscopy (TEM, Tecnai G20, FEI, Eindhoven, The Netherlands). XPS analysis was done using a VG Micro tech Multi lab ESCA 2000 equipment with a CLAM MCD detector, Al Kα (hν = 1453.6 eV) radiation, and using 50 and 20 eV pass energy for the acquisition of the survey and high-resolution spectra, respectively. A pH meter from EUTECH was used to track pH.

Procedure for evaluation of photocatalytic activity

Roseaniline Hydrochloride (RA) is a cationic dye that was used as a model pollutant and used for photocatalytic studies for synthesized HFNZO [30]. The schematic diagram of the photoreactor was shown in Fig. 1 in which a cylindrical glass vessel, 300 mL of RA solution with a starting concentration of 20 µM was added. An inner jacket of water circulating was placed around it to keep the lamp cool. An air pump was used to continually bubble air into the aliquot, providing a steady source of dissolved oxygen from the bottom of the reactor. Because, the oxygen combines with produced hydroxyl radicals (˚OH) along with other oxidants, e.g., superoxide radical anion (O₂^-˚), can further mineralize organic compounds to end products (water and CO₂)[31][32].  Then the vessel was filled with the photocatalyst. Before being exposed to irradiation, the aqueous suspension was stirred continuously for 15 minutes in the dark to achieve adsorption isotherm and it is compared with 30 minutes as shown in Fig. 2. Both kinetic plots show the same R² value of 0.9862 and 0.9847 for 15 and 30 min respectively which confirms that the adsorption isotherm has been achieved within 15 min. 300W Xenon arc lamps with a UV-cut-off filter were used as a visible light source. 5 mL of aliquot samples were withdrawn from the reaction mixture at a regular time interval of 15 min. The sampled suspensions were then centrifuged at 4000 rpm for 20min to remove the catalyst and the residual dye concentration was analyzed by UV–vis spectrophotometer.

Where C₀(µM) is the initial concentration of RA and C (µM) is the concentration of RA after a certain irradiation time. 

RESULT AND DISCUSSION

Characterization

UV-Visible diffuse reflectance spectroscopy was measured at room temperature between 200-800 nm range of wavelength as shown in Fig. 3. (a). The band gap depends on various factors, including grain size, oxygen deficiency, surface roughness, and lattice strain[33] The smaller band gap will easily categorize a photocatalytic reaction of the nanoparticles and show good photocatalytic activity for the degradation of RA. Due to the smaller band gap, the electron is easily excited from the valence band to the conduction band. Subsequent findings indicate that doping of Ni and floral extract modification decreases the band gap [34]. This can be attributed to the smaller particle size and greater surface roughness.

The above equation [35] extrapolates the linear section of the α² versus hνgraphs to yield the optical band gap E_g of the nanoparticles.Fig. 3(b), 1(c), and 1(d) show the tauc plot of ZO, HFZO, and HFNZO, respectively. ZO, HFZO, and HFNZO were reported to have 3.0 eV, 2.32 eV, and 1.65 eV, respectively of their band gap which is red-shifted [36]. HFNZO results in a decreased band gap which has greater visible light photocatalytic activity than ZO and HFZO.

In Fig. 4. two prominent absorption peaks at 437 cm^-1 and 505 cm^-1, which correspond to the hexagonal primitive lattice of ZnO and are in good agreement with the XRD conclusion [37], were seen for all of the samples. The bonding between Zn and O is reflected by the peaks at 611 cm^-1. Ni-O stretching vibration is thought to be responsible for the peak at 735 cm^-1[38]. The peak at 1369 cm^-1 and 1507 cm^-1 was caused by the rocking of C-H and C=Cstretching, and the peaks at 2349 cm^-1 are ascribed to the stretching vibration of CO₂[39]. Variations from 1055 cm^-1 and 619 cm^-1 to 1125 cm^-1 and 635 cm^-1 imply that proteins, amines, and polyphenols functional group in the flower extract. ZnO nanoparticles may interact with phytochemicals in the flower extract, building a capping over the ZnO[40].

The phase structure of synthesized ZO, HFZO, and HFNZO nanoparticles was portrayed by XRD pattern and displayed in Fig. 5. According to JCPDS no. 65-3411, The diffraction edges of ZnO acquired at 31.82˚, 34.47˚, 36.30˚, 47.58˚, and 56.64˚ that can be indexed to reflections from the (100), (002), (101), (102), and (110) planes of hexagonal ZnO, respectively. In addition to hexagonal ZnO peaks, the HFNZO nanoparticles XRD pattern also revealed peaks from the (111), (200), (220), and (311) planes corresponding to Ni (JCPDS no 89-7129). Debye’s Scherrer equation [27] has been used to estimate the average crystallite size:

where K is the shape factor, and β is the most intense 2θ peak whole width at half height maximum (0.89). The incident angle and wavelength of X-rays are represented by θ and λ, respectively. ZO, HFZO, and HFNZO nanoparticles’ average particle sizes were determined to be 35.12 nm, 31.58 nm, and 24.85 nm, respectively[36], [38]

The morphological structure of ZO, HFZO, and HFNZO were studied by SEM. The results are displayed in Fig. 4. The irregular plate with aggregation is seen in Fig. 6(a) with nanoparticles having almost spherical shapes [5]. Such aggregation and flaky agglomeration could be due to a high surface energy of the nanoparticles and also perhaps due to densification of the narrow space between nanoparticles. In Fig. 6 (b) it seems that the aggregation, as well as the flake thicknesses, becomes less for ZnOnanoparticlesand after additional flower extract Hibiscus rosasinesis. On doping with Ni, the SEM is revealed to be needle-shaped. It has become clear that the metal and added floral extract has a significant effect on the nanoparticle’s shape. The new morphology of zinc oxide nanoparticles made them suitable for photodegradation. The EDX spectra obtained at various sites are depicted in Fig. 6(a, b, and c). They only contain the predicted elements (Zn, Ni, and O) with no other contaminant elements. For HFNZO, the typical atomic percentage ratio of Zn, Ni, and O is around 6.95:2.39:80:60.

To further elucidate the size and structures of nanoparticles and nanoneedles TEM was carried out. Fig. 7 shows the TEM image of HFNZO indicates the TEM image of doped and flower extract-modified ZnO nanoparticles of sizes varying from 22 to 28 nm. The varying size is due to the agglomeration of particles. The difference in the crystal growth velocities in different directions causes changes in surface morphology. The addition of Ni ions in the precursor solution induces a thermodynamic barrier. It has been observed that doping plays an important role in producing nanoneedles which is corroboration with the earlier reports[41].

Fig. 8 shows the XPS results for the chemical compositions and valence states properties of the HFNZO nanoparticles. Two sharp peaks are observed corresponding to the spin-orbit of Zn 2p3/2 and Zn 2p1/2 at binding energies 1021.1 eV and 1044.4 eV respectively. The broad peak is assigned to O²⁻ ions at a binding energy of 531.2 eV in the Zn–O bonding of the hexagonal structure of ZnO. The convoluted peak at 532.9 eV is attributed to O⁻ and O²⁻ ions in oxygen-deficient regions in the HFNZO surface[42], [43].In the Ni 2p core-level spectrum ∼861.2 and 883eV show that these peaks are attributed to Ni in the form of oxide and hydroxide phases.

Visible Light Photocatalytic Evaluation

The photocatalytic activity of HFNZO was measured in an aqueous solution of RA at 20 µM at a catalyst concentration of 0.050 g/L, and after irradiation for 90 min. In Fig. 6, the absorption spectra of RA under visible light irradiation with HFNZO are shown. As the irradiation period increases, the strength of the absorption peak at λ=540 nm steadily declines until it practically vanishes at 90 min, showing the degradation of RA by the photocatalytic activity of HFNZO. In the absence of a photocatalyst, the least amount of RA degradation results from chemical reactions rather than adsorption. A pseudo-first-order kinetic model was developed to compare the reaction rates of various nanoparticles in equation (4),

where C₀ was the original RA concentration at time t=0 min, C was the initial MB concentration at time t of illumination, and k was the rate constant. In Fig. 9, a linear relationship between the variables -ln (C/C₀) and irradiation time ‘t’ was shown. We assume that the amount of RA following desorption-adsorption equilibrium is the starting concentration C₀ given the experimental setup[34], [44]. The rate constants k of HFNZO, HFZO, and ZO are determined to be 3.93×10⁻² s⁻¹, 2.279×10⁻² s⁻¹, and 1.648×10^-3 s^-1, respectively, based on the kinetic model mentioned above. Therefore, surface modification with plant extract and dopants, especially Ni²⁺, dramatically increases ZnO reactivity toward visible light.

The photocatalytic activity of the HFNZO sample is greater than that of pure ZO. New impurity levels between the conduction and valence bands are generated in the lattice of ZnO when Ni²⁺ is substituted, and electrons can be promoted from the valence band to these impurity levels[49]. The photocatalytic activities may then attract more photogenerated electrons and holes as a result.  Since the band gap of the HFNZO sample is less than that of pure ZnO, it may increase the absorption of visible light. In addition to this result, Ni²⁺ ions can act as charge carrier (e^- or h⁺) shallow trapping sites in photocatalytic reactions, as reported in [50]:

Ni²⁺ serves as an electron and hole trap and its ability to separate the arrival time to the solution phase (Eqs. (3) and (5)) or the substrate result in a reduction in the electron-hole pair combining rate and an increase in photocatalytic activity.

Choosing the appropriate catalyst loading for optimal degradation is desirable to avoid the usage of excess catalysts. A series of tests were performed with a catalyst dosage ranging from 0.030 to 0.060 g/L, a dye concentration of 20 µM, and a 90-minute irradiation period. Fig. 10 shows the degradation efficiency of RA for different photocatalyst loadings. The results indicate that the dye degradation dramatically increases from about 59% to nearly 95% with an increase in catalyst loading from 0.030 to 0.050 g/L. This is because increasing the catalyst loading increases the active sites on the surface of the catalyst, which in turn increases the amount of RA species that can bind to the surface [51]. As an outcome, greater catalyst loading increases degrading efficiency. The degradation rate reduces as catalyst loading is raised from 0.050 to 0.060 g/L, indicating an increase in total active surface area and the availability of additional active sites for photoreaction. As a result, the suspension’s turbidity increased, which has the effect of blocking out visible light. For this concentration of RA (20 mg/L), the catalyst efficiency has been brought to an optimal point. A further increase in catalyst dosage over 0.050 g/L caused a decrease in dye degradation. This condition might be brought on by an excess of photocatalyst particles that prevents and blocks light from passing through. On the other hand, particle aggregation is significant at high catalyst concentrations, which reduces the active sites on the catalyst surface.

The impact of initial dye concentration on the degradation of RA is shown in Fig. 11. Initial RA dye concentrations ranged from 10 to 30 µM. With an increase in MB (10–20 µM), the photodegradation of RA rises and then falls with an increase in concentration (20–30 µM). The concentration has a negative effect on the photodegradation efficiency. The equilibrium adsorption of dye on the catalyst surface, which leads to a reduction in the number of active sites, was thought to explain why degradation decreased as dye concentration increased. Hydroxyl radicals, which are thought to be the dye’s major oxidizing agent, are produced as a result of this occurrence[52][53]. The initial concentration is inversely correlated with the number of organic compounds that are adsorbed on the catalyst surface and the degree of coloration of the solution. As a result, there will be fewer active sites for OH- adsorption, which will lower the amount of OH- generated[54]. A further increase in RA concentration may result in a reduced photon penetration path into the aqueous solution of RA and a decrease in the catalytic activity.

The reusability and stability of a photocatalyst are very important things for its practical application. The catalyst is recovered after each cycle by centrifugation, dried in a hot air oven for 4 hours then reused. The results of the recycle photocatalyst for HFNZO are depicted in Fig. 12. As shown in the figure, HFNZO showed 98%photocatalytic activity in the first two cycles and from Table 1 literature review photocatalyst. After the third reuse, there is a decrease in the degradation due to the loss of catalyst during washing. The recycling experiments imply that HFNZO might be effectively used for the degradation of organic pollutants in water[52].

CONCLUSION

In this study, HFNZO nanoparticles were successfully synthesized to remove methylene blue dye from an aqueous solution using Fresh Hibiscus rosasinensis flower extract as a surface modifier. The photocatalyst of HFNZO is successful for dye RA when subjected to visible light irradiation. Most importantly, the system could show a 98% photodegradation efficiency for the model pollutants, due to slow electron-hole recombination rate. The results showed that these HFNZO nanoparticles possessed well crystallinity and significant optical properties. These nanoparticles have good nanostructures and resulting in a remarkable photocatalytic performance in the photocatalytic degradation of RA dye under visible light within 90 min at a dye concentration of 20 µM with a nanoparticles dosage of 0.050 g/L. The results also indicate the possibility that employing the photocatalyst in its current state will aid in the degradation of organic dyes that have better stability, conformity, and optical characteristics, which will help to address environmental issues.

CONFLICT OF INTEREST

The authors hereby declare that there is no conflict of interest.